International Building Code Illustrated Handbook 2015 International Code Council

Chapter 18 was completely reorganized in the 2009 International Building Code® (IBC®) to facilitate proper application of the provisions. Some notable elements of the restructured Chapter 18 format include categorization of foundations into two main types of shallow and deep foundations, and the collection of the common provisions for piles and piers, such as installation, under specific sections for deep foundations. The reorganization also consolidated other previously scattered requirements, such as material strength requirements for concrete and grout and stresses, in two new tables rather than being scattered in various sections of the code. Overall the reorganization of Chapter 18 was a big improvement over previous editions of the IBC and has resulted in better understanding of the intent of the code. Relatively few significant changes were made to the 2012 IBC. Changes were made to Sections 1803.5.11 and 1803.5.12 regarding seismic requirements for geotechnical reports in areas of high seismic risk and to Section 1810.3.11.2 related to seismic design of pile caps. A few minor changes were made to Chapter 18 of the 2015 IBC, which are discussed below.

Satisfactory performance of the building structural system is critical in the overall performance of any building during its life cycle. The IBC structural provisions in Chapters 16, 17, 18, 19, 20, 21, 22, and 23 work together to provide for the overall performance and safety of the structural system. Section 1604.4, Analysis, requires that all structural systems have a continuous load path from the point of origin to the resisting element. The resisting element for buildings and other structures is the foundation that ultimately transfers the loads to the supporting soil. Therefore, the satisfactory performance of the foundation system is critical to the satisfactory performance of the overall structure. Most building structures are designed assuming a fixed unyielding base that is not subjected to large total or differential settlements or displacements. Shallow foundations on firm soils will generally perform satisfactorily if the requirements of these provisions are followed.

Foundation design, however, becomes a significant factor for large structures, embedded structures such as a tall building constructed over a multilevel basement garage, structures on soft soils, structures supporting rotating or reciprocating equipment, and structures sensitive to differential displacements. It is important to have a good knowledge of the behavior of the various foundation types, including their limitations. In addition, for structures subject to high-wind forces or seismic ground motion, special consideration must be given to the lateral load path, and, in the case of deep foundation supported structures, the ability of the deep foundation to survive the displacements and curvatures imposed on the pile by seismic ground motion.

Sufficient understanding of the behavior and limitations of the various deep and shallow foundation systems is necessary to determine that the foundation and the supported structure will provide the intended serviceability. It is important to determine whether the estimated total and differential settlements of the foundation are compatible with the selected structure type. For example, a stiff bearing-wall structure with openings may be more sensitive to differential settlements than a more flexible light-frame structure. When considering seismic ground motion, sufficient knowledge of ground-shaking effects on the foundation is important, particularly in soft soils.

Section 1801 General

1801.1 Scope. The requirements of this chapter apply to all building and foundation systems of any type and any location. For example, buildings located in beach-front properties that would be subject to wave run up and inundation during hurricanes can still be designed using applicable loads from ASCE 7 (Minimum Design Loads for Buildings and Other Structures) and ASCE 24 (Flood Resistant Design and Construction) with proper geotechnical investigation and engineering analysis. ASCE 7 Chapter 5 and ASCE 24 are referenced in Section 1612.4 for the design and construction of buildings and structures in flood hazard areas, including coastal high-hazard areas and coastal A zones.

1801.2 Design basis. Bearing pressures, stresses, and lateral pressures used in this chapter are allowable pressures or stresses, not strength level values. These allowable foundation pressures are to be used with the allowable stress design load combinations set forth in Section 1605.3 unless noted otherwise. Site excavations and grading are covered in Chapter 33.

Section 1802 Definitions

1802.1 Definitions. Specific definitions applicable to Chapter 18 are referenced in this section and include the following terms: Deep Foundation, Drilled Shaft, Socketed Drilled Shaft, Helical Pile, Micropile, and Shallow Foundation. Definitions of terms commonly used in foundation design can be found in various foundation engineering references.^2–6 All chapters of the IBC now show, in italics, any term that is defined if the definition is applicable to the topic being presented. All definitions are located in Chapter 2 since the 2012 IBC.

For example, terms such as Approved, Building Official, and Allowable Stress Design are shown in italics in the text of the code.

Section 1803 Geotechnical Investigations

1803.1 General. A geotechnical investigation must be conducted when required by the building official. In general, the investigation should be required unless the foundation is designed and constructed in accordance with the presumptive allowable foundation pressures and lateral-bearing pressures set forth in Section 1806. Some minimal knowledge of soil classification at the bearing elevation of the foundation is required by Section 1806. A registered design professional should be used as required by the professional practice laws of the state in which the jurisdiction is located. The practice of geotechnical or soils engineering is a branch of civil engineering and is generally regulated by the various states.

1803.2 Investigation required. A foundation and soils investigation is required for any of the adverse subsurface conditions listed in Sections 1803.5.2 through 1803.5.12.

In certain cases, an exception allows the building official some flexibility in requiring a foundation and soils investigation when the soil conditions of the site are already known from other soils reports. For example, if the site is located in an area where there are reasonably uniform and horizontal soil strata and a soils report is available for the adjacent parcels, then a new soils report should not be necessary. This exception does not apply to some very specific conditions such as analysis of lateral pressure, liquefaction potential, and ground stabilization techniques in Seismic Design Categories (SDCs) D, E, and F. Other conditions where the exception does not apply and a geotechnical investigation is mandated are excavation near foundations, compacted fill materials, and controlled low-strength materials (CLSMs).

1803.3 Basis of investigation. The investigation cannot be solely based on theoretical analysis and research; rather, analysis and observation through tests of materials by borings, test pits, or other subsurface explorations in appropriate locations is needed. The number and types of tests, equipment used, type of site inspections, and other issues relevant to the scope of the investigation are the responsibility of and must be under the supervision of a registered design professional experienced in soils exploration. Additional studies shall also be made as necessary for conditions such as slope stability, soil strength, position and adequacy of load-bearing soils, the effect of moisture variation on soil-bearing capacity, compressibility, liquefaction, and expansiveness.

1803.4 Qualified representative. Whenever the allowable bearing capacity is in doubt or a geotechnical investigation is necessary, exploratory borings are necessary to determine the soil characteristics and the load-bearing capacity. The investigation procedures must be outlined and identified by the registered design professional in accordance with the accepted engineering practice and acceptable standards. The apparatus and equipment used in the investigation must be calibrated and perform as expected for accurate results. Special inspection requirements for soil-related activities are provided in Chapter 17 of the IBC. Qualified representatives must be present on site during the boring or sampling operations. Qualifications of investigative or inspection agencies and individuals are typically established by their experience, certification, and accreditation through an approved accreditation body such as the International Accreditation Service (IAS).

1803.5 Investigated conditions. Those cases where geotechnical investigations are required have been provided in Sections 1803.5.2 through 1803.5.12. It should be emphasized that the exception discussed in Section 1803.2 that allows the building official to waive the investigation can be used in most of these conditions except those that are critical or are site specific and cannot be determined based on existing geotechnical investigations of surrounding areas (lateral pressure, liquefaction potential, and ground stabilization techniques in SDCs D, E, and F, excavation near foundations, compacted fill materials, and CLSMs). Soil materials are required to be classified in accordance with the Unified Soil Classification System found in ASTM D 2487 (Practice for Classification of Soils for Engineering Purposes) (Unified Soil Classification System). IBC Table 1610.1 provides the Unified Soil Classification for some backfill materials. This table establishes the minimum soil lateral loads for the design of foundation walls and retaining walls.

1803.5.2 Questionable soil. Where the classification, strength, or compressibility is uncertain, or where bearing capacity of soil in excess of the presumptive value is claimed, the code allows the building official to obtain a geotechnical report. In this section, two cases trigger a foundation soils investigation and report:

1. Where the design load-bearing value is greater than the presumptive allowable foundation pressures and lateral-bearing pressures set forth in Section 1806.

2. Where the type of soil, the bearing capacity, or the stiffness of the soil is questionable, such as in areas subject to liquefaction from strong ground shaking, in areas containing soft or sensitive clays such as bay muds, or in areas with unconsolidated or improperly consolidated fills.

1803.5.3 Expansive soil. Expansive soils are those that shrink and swell appreciably because of changes in soil moisture content. Reference should be made to Section 1808.6 for mitigation methods and design for expansive soils. Frost heave is not considered in this section. Expansive soils are present or prevalent in all 50 states of the United States and in many other countries around the globe. Soils must meet all four of the criteria to be classified as expansive, not just a high Plasticity or Expansion Index.

The section allows two different ways to identify expansive soils. The first option involves meeting four criteria: (1) Plasticity Index (PI) of 15 or greater determined by ASTM D 4318; (2) more than 10 percent of the soil particles pass a No. 200 sieve determined by ASTM D 422; (3) more than 10 percent of the soil particles are less than 5 micrometers in size as determined by ASTM D 422; and (4) an Expansion Index greater than 20 according to ASTM D 4892. The second option is to determine if the Expansion Index (according to ASTM D 4892) is greater than 20. If the Expansion Index is determined, the first three tests need not be conducted. In other words, Expansion Index > 20 is both a necessary and sufficient condition by itself.

1803.5.4 Ground-water table. This section may cause significant changes to the conventional approach to foundation design and construction for light-frame buildings with subsurface floors, either a basement or a hillside building with a floor cut into the hillside. A foundation and soils investigation is required to show that the ground-water table is at least 5 feet below the elevation of the lowest floor level unless waterproofing is provided in accordance with Section 1805.

1803.5.5 Deep foundations. The purpose of a geotechnical investigation is to define the general subsurface stratifications of soil and rock materials, determine the soil and rock profiles, and locate the ground-water table. Such information will help in selecting the type of deep foundations and in estimating deep element lengths. Furthermore, a geotechnical investigation is often required to render data on specific soil properties, such as shear strength, relative density, compressibility of the soil, and other such findings that will help in analyzing subsurface conditions for determining design loads, type of deep foundations, driving criteria, suitable bearing strata, and probable durability of foundation materials relative to the particular soil conditions found at the construction site. It is generally not economical or feasible to use deep foundations without a geotechnical investigation and report.

Subsurface information is normally obtained by means of test borings that yield suitable samples of soils and rock and give the depths from which they are obtained. Sometimes, certain in situ tests are also conducted, but these should not be made without first making test borings.

This section outlines the kinds of information to be derived from a geotechnical investigation and report. In addition to the items listed in the code provision, the investigation could include other valuable and applicable data that would help in the evaluation, such as:

• Information on existing construction at the site or at adjacent sites, including the type and condition of these structures, age, types of foundations used, performance data, and so on.

• Information on the existence of deleterious substances in the soils or other conditions that could seriously affect the durability and structural performance of the piles.

• Information on the geologic conditions at the site, which could include such items as the existence of mines, earth cavities, underground streams or other adverse water conditions, history of seismic activity, and so on.

1803.5.6 Rock strata. If a rock stratum is being used for bearing, the characteristics of the layer must be known in sufficient detail to classify the rock per Section 1803.5.1.

The language of this section was updated in the 2015 IBC to address evaluation of rock materials for foundation support and be more consistent with current geotechnical engineering practice. The existing wording of the section suggested that it is possible to provide “assurance of the soundness of rock” during the geotechnical evaluation phase, which is not necessarily the case. Experience has shown that even at sites where rigorous evaluation of rock conditions is undertaken, it is often determined during construction that rock conditions between the locations sampled can vary significantly. Often the actual rock conditions at foundation locations are exposed or better defined (through excavation, proof-drilling, etc.) during construction, and interpretations of the conditions exposed during the construction process are necessary to complete the design of the foundation system. The modifications to the language in the 2015 IBC are intended to express the characteristics necessary to assess the rock strata and estimate load-bearing capacity based on observations and testing.

1803.5.7 Excavation near foundations. The intent of this section is clear in that lateral and subjacent support of any foundation must not be removed unless an investigation is conducted to identify the potential problems that might be created and how to provide the needed lateral support. For example, the area shown in Figure 1803-1 should not be excavated without conducting an investigation to address support for the foundation.

images

Figure 1803-1 Lateral support.

The language of this section was modified in the 2015 IBC to add basic requirements for providing adequate underpinning, and excavations have been added. The new language is intended to provide specific guidelines to identify responsibilities and basic requirements for providing safe and adequate underpinning and excavations. New Section 1804.2 was also added to provide specific requirements when underpinning is chosen to provide support for adjacent structures.

1803.5.8 Compacted fill material. When compacted fill of more than 12 inches (305 mm) in depth is used for shallow foundation support, the geotechnical investigation required in Section 1803 must also contain the seven items listed in this section.

1803.5.9 Controlled low-strength material (CLSM). CLSM was introduced into the 2003 IBC as an acceptable backfill material that need not be compacted. Prior to the 2003 IBC, CLSM would need to be approved under the alternative materials, design, and methods of construction provisions in Section 104.11. It is a self-compacted, cementitious material used as backfill instead of compacted backfill. It has also been referred to as “flowable fill” and “lean mix backfill.” CLSM has compressive strength of 1200 psi or less. Most CLSM applications require unconfined compressive strengths of 200 psi or less to allow for future excavation of CLSM. CLSM is composed of water, portland cement, aggregate, and fly ash. It is a fluid material with typical slumps of 10 inches or more and has a consistency similar to a milk shake. Although there is no referenced standard for CLSM in the IBC, there is a national report promulgated by the ACI Committee 229, entitled “ACI 229R-99 Controlled Low-Strength Materials.” The ACI standard was reapproved in 2005 and a new edition is expected to be published in 2012.

1803.5.10 Alternate setback and clearance. This alternate procedure allows the building official to approve alternate setbacks and clearances from slopes, provided that the intent of Section 1808.7 is met. This section gives the building official the authority to require a geotechnical investigation by a qualified geotechnical engineer to establish that the intent of Section 1808.7 is met and specifies the minimum parameters to be investigated.

1803.5.11 Seismic Design Categories C through F. For all structures in SDCs C through F, a geotechnical investigation is required to evaluate liquefaction, slope stability, total and differential settlement, and surface displacement caused by faulting or lateral spreading. Significant ground motion can occur even in areas with moderate seismic risk. Liquefaction typically occurs at sites with loose sands and high water tables. Surface rupture generally occurs with large-magnitude earthquake events. Lateral spreading generally occurs adjacent to waterways where a saturated soil has a free edge.

1803.5.12 Seismic Design Categories D through F. In addition to the investigation required for SDCs C through F by Section 1803.5.11, the investigation must evaluate the additional lateral pressures on basement or retaining walls from ground shaking, the detrimental effects and consequences of liquefaction or soil strength loss, and mitigation measures. The potential for liquefaction and soil strength loss must be evaluated using the peak acceleration based on a site-specific study and including the effects of soil amplification.

Liquefaction causes loss of bearing capacity with resulting large differential or total settlements. Structures with high height-to-width ratios that have liquefaction occur under a portion of the structure are subject to overturning. Many of the structures in various Japanese earthquakes were damaged by liquefaction.

Mitigation measures for liquefaction include:

1. In situ densification of the loose sands subject to liquefaction.

2. Use of pile foundations penetrating through the liquefying layers. The pile capacity must be developed through bearing or skin friction in the soils below the liquefying layers.

3. Use of rigid raft foundations that can minimize the effects of settlement caused by loss of bearing capacity.

Soil strength loss is associated with “sensitive” or “quick” clays that are sensitive to remolding effects. These soft clays typically occur in marine (or former marine) environments and are often called “bay muds.” The clays lose significant strength when remolded. Remolding and subsequent strength loss occurs when a pile foundation through these clays is subjected to strong ground shaking. As a consequence, lateral support for the pile is lost.

Mitigation measures for soils susceptible to strength loss such as “sensitive” or “quick” clays include:

1. Replacement of the soft clay.

2. In situ consolidation of the soft clays by preloading or water removal.

3. Use of pile foundations penetrating through the soft clay layers. The pile capacity must be developed through bearing or skin friction in the soils below the soft clay layers.

When using pile foundations, the effects of the layers of liquefaction or soils susceptible to strength loss on the curvature (and hence, moment) demands on the pile must be investigated. Often these soil-induced curvatures place a much higher moment demand on the piles than would be determined from conventional lateral-force P-y analyses. The curvatures are significantly increased at the interface between the soft soils and stiffer soils. The curvature demands increase as the ratio of stiffness of the stiff layer to the soft layer increases.

The requirement that geotechnical reports address earthquake loads on foundation walls and retaining walls in SDCs D, E, and F (see Item 1) has been modified in the 2012 IBC so that it only applies to those walls supporting more than 6 feet of backfill. In the previous editions of the IBC, there was no exemption based on the height of the wall or the amount of soil supported by the wall. This was deemed to be overly restrictive for light-frame foundation walls, small retaining walls, and swimming pools. Evidence from recent earthquakes and recent experimental research results, including work recently completed at the University of California–Berkeley, has demonstrated that retaining wall structures must move in order to develop the failure wedge postulated in the so-called Mononobe and Okabe method. However, the postulated condition can only occur when the wall has already failed due to other causes. The current body of field evidence does not provide any evidence for the existence of this mechanism of failure. It was determined that the requirement in the 2009 IBC and ASCE 7-05 imposed an unjustifiable burden on the permit applicant to investigate a site for small retaining structures such as foundation walls, retaining walls, and swimming pools that support no more than 6 feet of backfill.

1803.6 Reporting. The objective of a geotechnical investigation is to produce all the necessary information for design and construction of a structure’s foundation. To ensure that the report of the foundation and soils investigation meets this objective and provides enough data to ensure compliance with the code and a safe foundation, the report must contain at a minimum the items enumerated in this section. See also report requirements for compacted fill in Section 1803.5.8 and for CLSM in Section 1803.5.9. Other data such as results of consolidation tests, compaction curves, sieve analyses, Atterberg Limits tests, Plasticity and Expansion Index tests, and so on should be included in the report where available.

Section 1804 Excavation, Grading, and Fill

1804.1 Excavations near foundations. Excavations should not reduce the lateral support provided by any foundation without first protecting the foundation against detrimental movements. See discussion under Section 1803.5.7.

The language of this section was modified and new provisions for underpinning are included in the 2015 IBC to add basic requirements for providing safe and adequate underpinning because the existing language in the code did not specifically address excavations adjacent to structures. Although Section 3307, Protection of Adjacent Property, requires adjoining public and private property including footings, foundations, party walls, etc., to be adequately protected from damage during construction, remodeling, and demolition work, there were no specific details provided. Failures to perform proper preconstruction investigations and monitoring procedures have led to construction failures during underpinning and excavation operations. Improperly constructed excavations have resulted in doors and windows that do not open, cracking of bearing walls and support members, failures of some critical structural members, and even collapse resulting in fatalities. Because the term “detrimental” is used to discuss settlement in other provisions of Chapter 18 as well as other chapters of the IBC, the term is added here as well. The term “remove support” was changed to “reduce support,” because removal of support could lead to failure. As indicated in Section 1803.5.7, underpinning is only one way of providing support; thus, new Section 1804.2 provides requirements where underpinning is chosen to provide support.

1804.3 Placement of backfill. Backfill should be performed in accordance with an approved soils report. If no soils report is needed, the backfill should be placed in maximum 6-inch layers free from any rocks or cobbles larger than 4 inches and compacted to a minimum 90-percent relative density (Modified Proctor per ASTM D 1557) using the appropriate compaction equipment. The 2003 IBC introduced the use of CLSM as an acceptable backfill material that need not be compacted. See further discussion under Section 1803.5.9.

1804.4 Site grading. The general requirement is that surface water must drain away from foundations. Minimum slope is 5 percent for a distance of 10 feet. An alternate method is permitted if physical obstructions or lot lines prohibit the 5-percent slope for a minimum of 10 feet horizontally. In this case, swales or impervious surfaces must have a minimum 2-percent slope where located within 10 feet of the building foundation.

1804.5 Grading and fill in flood hazard areas. In general, grading is not permitted in designated flood hazard areas. Changes in the configuration or shape of floodways by grading or fill can divert erosive flows and increase wave energies that could increase forces and adversely affect adjacent buildings and structures. The restrictions in the code are consistent with provisions related to fill in ASCE 24, Flood Resistant Design and Construction. The exceptions permit grading in flood hazard areas, provided an engineering analysis demonstrates that the proposed grading will not increase flood levels or otherwise adversely affect the design flow. The last exception allows grading in flood hazard areas that are not designated floodways, provided the overall effect of the encroachment does not increase the design flood elevation by more than 1 foot at any point.

1804.6 Compacted fill material. See discussion under Section 1803.5.8.

1804.7 Controlled low-strength material (CLSM). See discussion under Section 1803.5.9.

Section 1805 Dampproofing and Waterproofing

This section covers the requirements for waterproofing and dampproofing those parts of substructure construction that need to be provided with moisture protection. Sections 1805.1 through 1805.3 identify the locations where moisture barriers are required and specify the materials to be used and the methods of application. The provisions also deal with subsurface water conditions, drainage systems, and other protection requirements.

Dampproofing requirements are outlined in Section 1805.2, and waterproofing requirements are covered in Section 1805.3. Although both terms are intended to apply to the installation and the use of moisture barriers, dampproofing does not furnish the same degree of moisture protection as does waterproofing.

Dampproofing generally refers to the application of one or more coatings of a compound or other materials that are impervious to water, which are used to prevent the passage of water vapor through walls or other building components, and which restrict the flow of water under slight hydrostatic pressure. Waterproofing, on the other hand, refers to the application of coatings and sealing materials to walls or other building components to prevent moisture from penetrating in either a vapor or liquid form, even under conditions of significant hydrostatic pressure. Hydrostatic pressure is created by the presence of water under pressure. This pressure can occur when the ground-water table rises above the bottom of the foundation wall, or the soil next to the foundation wall becomes saturated with water caused by uncontrolled storm water runoff.

1805.1 General. This section is an overall requirement specifying that waterproofing and dampproofing applications are to be made to horizontal and vertical surfaces of those below-ground spaces where the occupancy would normally be adversely affected by the intrusion of water or moisture. Moisture or water in a floor below grade can cause damage to structural members such as columns, posts, or load-bearing walls, as well as pose a health hazard by promoting growth of bacteria or fungi. Moisture can adversely affect any mechanical and electrical appliances that may be located at that level. It can also cause a great deal of damage to goods that may be located or stored in that lower level. These vertical and horizontal surfaces include foundation walls, retaining walls, underfloor spaces, and floor slabs. Waterproofing and dampproofing are not required in locations other than residential and institutional occupancies where the omission of moisture barriers would not adversely affect the use of the spaces. An example of a location where waterproofing or dampproofing would not be required is in an open parking structure, provided the structural components are individually protected against the effects of water. Waterproofing and dampproofing are not permitted to be omitted from residential and institutional occupancies where people may be sleeping or services are provided on the floor below grade. A person waking in a flooded basement may find themselves in a very hazardous situation particularly if the possibility exists of an electrical charge in the water caused by electrical service at that level.

Section 1805.1.1 addresses the type of problem faced when a portion of a story is above grade, whereas Section 1805.1.2 limits any infiltration of water into crawl spaces so as to protect this area from potential water damage and prevent ponding of water in the crawl space. These sections reference other applicable sections of Chapter 18.

1805.1.1 Story above grade plane. The provisions of this section require that where a basement is deemed to be a story above grade plane, the section of the basement floor that occurs below the exterior ground level and the walls that bound that part of the floor are to be dampproofed in accordance with the requirements of Section 1805.2.

The use of dampproofing, rather than waterproofing, is permitted here because high hydrostatic pressure will not tend to develop against the walls if the basement is a story above grade plane and the ground level adjacent to the basement wall is below the basement floor elevation for not less than 25 percent of the basement perimeter.

Any water pressure that may occur against the walls below ground or under the basement floor would be relieved by the water drainage system required in this section. The drainage system would be installed at the base of the wall construction in accordance with Section 1805.4.2 for a minimum distance along those portions of the wall perimeter where the basement floor is below ground level.

Because of the relationship of grade to the basement floor and the inclusion of foundation drains, the potential for hydrostatic pressure buildup is not significant. Therefore, a ground-water table investigation, waterproofing, and a basement floor gravel base course are not required.

The objective of this section is to prevent moisture migration in basement spaces. In story-above-grade plane construction that meets the requirements of this section, the basement floor would be only partly below ground level (sometimes a small part) and the need for section-required moisture protection is unnecessary. Dampproofing of the floor slab would be required, however, in accordance with Section 1805.2.1.

1805.1.2 Under-floor space. Essentially, the requirements of this section are designed to prevent any ponding of water in under-floor areas such as crawl spaces. Crawl spaces are particularly susceptible to ponding of water as they are usually uninhabitable spaces that are infrequently observed. Water can build up in these spaces and remain for an extended period of time without being noticed by the building occupants. This is also to prevent water from ponding under the structure if it is flooded. Under-floor spaces of Group R-3 buildings located in flood hazard areas that meet the requirements of FEMA/FIA-TB-11 need not comply with the requirements in this section.

Stagnant water collected under a building can result in a serious health concern. Water buildup in a crawl space can also damage the structural integrity of the building. Wood exposed to water can deteriorate and rot, and concrete and masonry exposed to water can deteriorate with a loss of strength.

Steel exposed to water or high humidity can eventually rust to the extent that effective structural capability is jeopardized. Water buildup in a crawl space can also damage mechanical or electrical appliances located in the space by causing corrosion of electrical parts or metal skins and deterioration of insulation used to protect heating elements.

Where it is known that the water table can rise to within 6 inches of the outside ground level, or where there is evidence that surface water cannot readily drain from the site, then the finished ground surface in under-floor spaces is to be set at an elevation equal to the outside ground level around the perimeter of the building unless an approved drainage system is provided. For the drainage system to be approved, it must be demonstrated that the system will be adequate to prevent the infiltration of water into the under-floor space. This is done by determining the maximum possible flow of water near the foundation wall and footing, and designing the drainage system to remove that flow of water as it occurs, thereby preventing the buildup of water at the foundation wall.

To prevent the ponding of water in the under-floor space from a rise in the ground-water table, or from storm water runoff, the finished ground level of an under-floor space is not to be located below the bottom of the foundation footings.

Dampproofing, waterproofing, or providing subsoil drainage is not necessary if the ground level of the under-floor space is as high as the ground level at the outside of the building perimeter, as the foundation walls do not enclose an interior space below grade.

Compliance with Sections 1805.2, 1805.3, and 1805.4 would be required where the finished ground surface of the under-floor space is below the outside ground level.

1805.1.3 Ground-water control. After completion of building construction, it is necessary to maintain the water table at a level that is at least 6 inches below the bottom of the lower floor to prevent the flow or seepage of water into the basement. Where the site consists of well-draining soil and the highest point of the water table occurs naturally at or lower than the required level, there is no need to provide any kind of a site drainage system specifically designated to control the ground-water level. Where the soil characteristics and the site topography are such that the water table can rise to a level that will produce a hydrostatic pressure against the basement structure, a site drainage system may be installed to reduce the water level. When ground-water control in accordance with this section is provided, waterproofing in accordance with Section 1805.3 is not required.

There are many types of site drainage systems that can be employed to control ground-water levels. The most commonly used systems may involve the installation of drainage ditches or trenches filled with pervious materials, sump pits and discharge pumps, well point systems, drainage wells with deep-well pumps, sand-drain installations, and so on. This section requires that all such systems be designed and constructed using accepted engineering principles and practices based on considerations that include the permeability of the soil, amount and rate at which water enters the system, pump capacity, capacity of the disposal area, and other such factors that are necessary for the complete design of an effective drainage system.

1805.2 Dampproofing. Where a ground-water table investigation has established that the high water table will occur at such a level that the building substructure will not be subjected to significant hydrostatic pressure, then dampproofing in accordance with this section and a subsoil drain in accordance with Section 1805.4 are sufficient to control moisture in the floor below grade.

Wood foundation systems specified in Section 1807.1.4 are to be dampproofed as required by the American Forest and Paper Association Permanent Wood Foundation (AF&PA PWF) standard. AF&PA PWF-2007 replaces the previous AF&PA Technical Report No. 7. The new PWF Design Specification was written as part of an effort to update design recommendations and procedures in the wood industry’s design aides, such as Technical Report 7: The Permanent Wood Foundation System (1987) and the Permanent Wood Foundation System: Design, Fabrication and Installation Manual (1987).

1805.2.1 Floors. Floors requiring dampproofing in accordance with Section 1805.2 are to employ materials specified in Section 1805.2.1. The dampproofing materials must be placed between the floor construction and the supporting gravel or stone base, as shown in Figure 1805-1. Even if a floor base is not required, dampproofing should be placed under the slab.

images

Figure 1805-1 A foundation drainage system.

The installation is intended to provide a moisture barrier against the passage of water vapor or seepage into below-ground spaces.

The dampproofing material most commonly used for underslab installations consists of a polyethylene film not less than 6 mil in thickness, which is applied over the gravel or stone base required in Section 1805.4.1. Care must be taken in the installation of the material over the rough surface of the base and during the concreting operation so as not to puncture the polyethylene. Joints must be lapped at least 6 inches.

Dampproofing materials can also be applied on top of the base concrete slab if a separate floor is provided above the base slab, because the dampproofing is provided to prevent moisture infiltration of the interior space, and not the concrete slab.

Materials commonly used for dampproofing floors are listed in Table 1805-1.

Table 1805-1. Dampproofing Materials

images

1805.2.2 Walls. Walls requiring dampproofing in accordance with Section 1805.2 are first to be prepared as required in Section 1805.2.2.1, then coated with any of the bituminous materials listed in Table 1805-1 or by other approved materials and methods of application. Approved materials are those that will prevent moisture from penetrating the foundation wall.

Coatings are applied to cover prepared exterior wall surfaces extending from the top of the wall footings to slightly above ground level so that the entire wall that contacts the ground is protected. Surfaces are usually primed to provide a bond coat and then dampproofed with a protection coat of asphalt or tar pitch.

Dampproofing materials for walls may be any of the materials specified in Section 1805.3.2 for waterproofing. Table 1805-1 gives a list of bituminous materials that can be used, including the applicable standards that may be used as the basis of acceptance of such materials. Included in Table 1805-1 is ASTM D 1668 for glass fabric that is treated with asphalt (Type I), coal-tar pitch (Type II), or organic resin (Type III).

Surface-bonding mortar complying with ASTM C 887 may be utilized. This specification covers the materials, properties and packaging of dry, combined materials for use as surface-bonding mortar with concrete masonry units that have not been prefaced, coated, or painted. Because this specification does not address design or application, the manufacturer’s recommendations should be followed. This standard covers proportioning, physical requirements, sampling, and testing. The minimum thickness of the coating is ⅛ inch.

Acrylic-modified cement coatings may be utilized at the rate of 3 pounds per square foot. These types of materials have been used successfully as dampproofing materials for foundation walls. Surface-bonding mortar and acrylic-modified cement are limited in use to dampproofing. The ability of these two types of products to bridge nonstructural cracks, as required in Section 1805.3.2 for waterproofing materials, is not known. Therefore, their use is limited to dampproofing and they are not permitted to be used as waterproofing. Dampproofing may also include other materials and methods of installation acceptable to the building official.

1805.2.2.1 Surface preparation of walls. Before applying dampproofing materials, the concrete must be free of any holes or recesses that could affect the proper sealing of the wall surfaces. Air trapped beneath the dampproofing coating or membranes can cause blistering. Rocks and other sharp objects can puncture membranes. Irregular surfaces can also create uneven layering of coatings, which can result in vulnerable areas of dampproofing. Surface irregularities commonly associated with concrete wall construction can be sealed with bituminous materials or filled with portland cement grout or other approved materials.

Unit masonry walls are usually parged (plastered) with a ½-inch-thick layer of portland cement and sand mix (1:2½ by volume) or with a Type M mortar proportioned in accordance with the requirements of ASTM C 270 and applied in two ¼-inch-thick layers. In no case is parging to result in a final thickness of less than ⅜ inch. The parging is to be coved at the joint formed by the base of the wall and the top of the wall footing to prevent the accumulation of water at that location.

The moisture protection of unit masonry walls provided by the parging method may not be required where approved dampproofing materials such as grout coatings, cement-based paints, or bituminous coatings can be applied directly to the masonry surfaces.

1805.3 Waterproofing. Waterproofing installations are intended to provide moisture barriers against water seepage that may be forced into below-ground spaces by hydrostatic pressure.

Where a ground-water table investigation has established that the high water table will occur at such a level that the building substructure will be subjected to hydrostatic pressure, and where the water table is not lowered by a water control system, as described in the discussion to Section 1805.4.2, all floors and walls below ground level are to be waterproofed in accordance with Sections 1805.3.1 and 1805.3.2.

1805.3.1 Floors. Because floors required to be waterproofed are subjected to hydrostatic uplift pressures, such floors must, for all practical purposes, be made of concrete and designed and constructed to resist the maximum hydrostatic pressures possible. It is particularly important that the floor slab be properly designed, as severe cracking or movement of the concrete would allow water seepage into below-ground spaces. The ability of the waterproofing materials to bridge cracks is limited. Concrete floor construction is to comply with the applicable provisions of Chapter 19.

Materials used for waterproofing below-ground floors are to conform to the requirements of Section 1805.3.1.

Below-ground floors subjected to hydrostatic uplift pressures are to be waterproofed with membrane materials placed as underslab or split-slab installations, including such materials as rubberized asphalt, butyl rubber, and neoprene, or with polyvinyl chloride or polybutylene films not less than 6 mil in thickness, lapped at least 6 inches. All membrane joints are to be lapped and sealed in accordance with the manufacturer’s instructions to form a continuous, impermeable waterproof barrier. There are many proprietary membrane products available that are specifically made for waterproofing floors and walls (i.e., polyethylene sheets sandwiched between layers of asphalt), which may be used when approved by the building official. Products that have an ICC-ES evaluation report are acceptable in most jurisdictions if the requirements of the evaluation report are followed. ICC-ES reports are intended to address the technical aspects and requirements of new and innovative products that are approved by the building official under the alternative materials and methods of construction provisions of Section 104.11. All ICC-ES evaluation reports are posted online at www.icc-es.org. A sample ICC-ES report is shown in Figure 1805-2.

images

Figure 1805-2 Sample ICC-ES evaluation report.

1805.3.2 Walls. Walls that are required to be waterproofed in accordance with Section 1805.3 must first be prepared as required in this section and then waterproofed with the required membrane-type installations.

The walls must be designed to resist the hydrostatic pressure anticipated at the site, as well as any other lateral loads to which the wall will be subjected, such as soil pressures or seismic loads. As with the floors required to be waterproofed, it is particularly important that the walls required to be waterproofed be properly designed to resist all anticipated loads, as cracking and other damages would allow water seepage into below-ground spaces. Concrete or masonry construction must comply with the applicable provisions of Chapters 19 and 21, respectively.

Table 1805-2 lists materials commonly used for the installation of moisture barriers in wall construction and the related standards that may be used as a basis for acceptance of such materials.

Table 1805-2. Waterproofing Materials

images

Asphalt and coal-tar products are not compatible and should not be used together.

Waterproofing installations are to extend from the bottom of the wall to a height not less than 12 inches above the maximum elevation of the ground-water table. The remainder of the wall below ground level (if the height is small) may be either waterproofed as a continuation of the installation or must be dampproofed in accordance with the requirements of Section 1805.2.

This section requires that waterproofing must consist of two-ply hot-mopped felts. The practice of the waterproofing industry is to select the number of plies of membrane material based on the hydrostatic head (height of water pressure against the wall). As a general practice, if the head of water is between 1 and 3 feet, two plies of felt or fabric membrane are used; between 4 and 10 feet, three-ply construction is needed; and between 11 and 25 feet, four-ply construction is necessary.

Waterproofing installations may also use polyvinyl chloride materials of not less than 6-mil thick, 40-mil polymer-modified asphalt, or 6-mil polyethylene. These materials have been widely recognized for their effectiveness in bridging nonstructural cracks. Other approved materials and methods may be used, provided that the same performance standards are met. All membrane joints must be lapped and sealed in accordance with the manufacturer’s instructions.

1805.3.2.1 Surface preparation of walls. Before applying waterproofing materials to concrete or masonry walls, the wall surfaces must be prepared in accordance with the requirements of Section 1805.2.2.1, which requires the sealing of all holes and recesses. Surfaces to be waterproofed must also be free of any projections that might puncture or tear membrane materials that are applied over the surfaces.

1805.3.3 Joints and penetrations. This section requires that all joints occurring in floors and walls and at locations where floors and walls meet, as well as all penetrations in floors and walls, be made watertight by approved methods. Sealing the joints and penetrations in the waterproofing is of primary importance to ensure the effectiveness of the waterproofing. If the joints or penetrations are not sealed properly, they can develop leaks, which become a passageway for water to enter the building. Because the remainder of the foundation is wrapped in waterproofing, moisture can actually become trapped in the foundation walls or floor slab, and serious damage to these structural components can occur.

Methods may involve the use of construction keys between the base of the wall and the top of the footing, or, if there is a hydrostatic pressure, floor and wall joints may require the use of manufactured waterstops made of metal, rubber, plastic, or mastic materials. Floor edges along the walls and floor expansion joints may employ any of a number of preformed expansion joint materials, such as asphalt, polyurethane, sponge rubber, self-expanding cork, cellular fibers bonded with bituminous materials, and so on, which all comply with applicable ASTM standards or other approved specifications. A variety of sealants may be used together with the preformed joint materials. Gaskets made of neoprene and other materials are also available for use in concrete and masonry joints. The National Roofing Contractor’s (NRCA) Roofing and Waterproofing Manuals provide details for the reinforcement of membrane terminations, corners, intersections of slabs and walls, through-wall and slab penetrations, and other locations.

Penetrations in walls and floors may be made watertight with grout or manufactured fill materials and sealants made for the purpose.

1805.4 Subsoil drainage system. Subsoil drainage systems are required to drain the area adjacent to basement walls to eliminate hydrostatic loads.

This section covers subsoil drainage systems in conjunction with dampproofing (see Section 1805.2) to protect below-ground spaces from water seepage. Such systems are not used where basements or other below-ground spaces are subject to hydrostatic pressure, because they would not be effective in disposing of the amount of water anticipated if hydrostatic pressure conditions exist. Ground-water tables may be reduced to acceptable levels by methods described in the discussion of Section 1805.1.3.

The details of subsoil drainage systems are covered in the requirements of Sections 1805.4.1 through 1805.4.3.

1805.4.1 Floor base course. This section requires that floors of basements, except for story-above-grade-plane construction, must be placed on a gravel or stone base not less than 4 inches thick. Not more than 10 percent of the material is to pass a No. 4 sieve to provide a porous installation and provide a capillary break. Material that passes a No. 4 sieve would be silt or clay that does not permit the free movement of water through the floor base, but allows for upward migration by capillary action.

This requirement serves three purposes. The first is to provide an adjustment to the irregularities of a compacted subgrade so as to produce a level surface upon which to cast a concrete slab. The second purpose is to provide a capillary break so that moisture from the soil below will not rise to the underside of the floor. Finally, where required, the porous base can act as a drainage system to expel underslab water by means of gravity, or the use of a sump pump or other approved method.

The exception allows for the omission of the floor base when the natural soils beneath the floor slab consist of well-draining granular materials such as sand, stone, or mixtures of these materials. Some caution, however, is justified in the use of this exception. If the granular soils contain an excessive percentage of fine materials, the porosity and the ability of the soil to provide a capillary break may be considerably diminished. The exception should be applied only if the natural base is equivalent to the floor base otherwise required by this section.

1805.4.2 Foundation drain. This section describes in considerable detail the materials and features of construction required for the installation of foundation drain systems.

This type of drain system is suitable where the water table occurs at such elevation that there is minimal hydrostatic pressure exerted against the basement floor and walls and where the amount of seepage from the surrounding soil is so small that the water can be readily discharged by gravity or by mechanical means into sewers or ditches. The objective is to combine the protection afforded by the dampproofing of walls and floors (see Section 1805.2) and that given by the perimeter drains to maintain below-ground spaces in a dry condition.

A foundation drain system usually consists of the installation of drain tiles made of clay or concrete or of drain pipes of corrugated metal or nonmetallic pipes surrounded by crushed stone or gravel and a filter membrane material (filter fabric). The foundation drain is set adjacent to the wall footing and extends around the perimeter of the building. Drain tiles are placed end to end with open joints to permit water to enter the system. Metallic and nonmetallic drains are made with perforations at the invert (bottom) section of the pipe and are installed with connected ends. Where drain tile or perforated drain pipe is used, the invert must not be set higher than the basement floor line so that water conveyed by the drain does not seep into the filter material and then create a hydrostatic pressure condition against the foundation wall and footing. The inverts should not be placed below the bottom of the adjacent wall footings to avoid carrying away fine soil particles whose loss, in time, could possibly undermine the footing and cause settlement of the foundation walls.

Tile joints or pipe perforations should be covered with an approved filter membrane material to prevent them from becoming clogged and to prevent fine particles that may be contained in the surrounding soil from entering the system and being carried away by water.

The filter material around the drain tiles or pipes (not to be confused with filter membrane material) should consist of selected gravel and crushed stone containing not more than 10 percent of material that passes a No. 4 sieve. The filter materials should be selected to prevent the movement of particles from the protected soil surrounding the drain installation into the drain. Filter material is to be placed in the excavation so that it will extend out from the edge of the wall footing a distance of at least 12 inches, with the bottom of the fill being no higher than the bottom of the base under the floor (see Section 1805.4.1) and the top of the footing.

Requiring the bottom of the foundation drain to be no higher than the bottom of the floor base is necessary so that if the water table rises into the floor base, it will also be able to rise unobstructed into the foundation drain. The foundation drain will then drain the water away from the building, as required by Section 1805.4.3. The top of the filter fill material must be covered with an approved filter membrane to allow water to pass through to the perimeter drain tile or pipes without allowing fine soil materials to enter the drainage system.

Drain tiles or pipes are to be installed in the filter bed and should be seated on at least 2 inches of filter material and covered with at least 6 inches of filter material to maintain good water flow into the drain tile or pipe.

1805.4.3 Drainage discharge. This section references the International Plumbing Code® (IPC®) for requirements for installing piping systems for the disposal of water from the floor base and the foundation drains. Chapter 11 of the IPC considers the piping materials, applicable standards, and methods of installation of subsurface storm drains to facilitate water discharge either by gravity or by mechanical means.

Where the soil at the site consists of well-drained granular materials such as gravel or sand-gravel mixtures to prevent the occurrence of hydrostatic pressure against the foundation walls and under the floor slab, the use of a dedicated drainage system as prescribed in the IPC is not required, because the site soils would permit natural drainage.

Section 1806 Presumptive Load-Bearing Values of Soils

1806.1 Load combinations. The presumptive bearing values in the code are allowable stress values, not strength level values. The format of IBC Table 1806.2 shown in Table 1806-1 comes from the legacy 1997 UBC. However, some of the footnotes from the UBC version have been moved into the text of the code. Each of the legacy model codes used a different approach to generate their allowable values. Hence, there was a disparity between the values shown in the model codes. The tabular values for allowable foundation pressure cannot be increased for width or depth as was allowed by the UBC. Note, however, that the allowable foundation pressures are permitted to be increased by one-third with the alternative ASD load combinations of Section 1605.3.2 for combinations including wind or earthquake so as to be consistent with previous editions of the legacy model codes.

Table 1806-1. Presumptive Load-Bearing Values

images

1806.2 Presumptive load-bearing values. The presumptive allowable bearing and lateral pressures must be used unless a geotechnical investigation substantiates higher values. The term “unprepared fill” refers to fill that was not placed and compacted in accordance with an approved soils report.

The classifications in Table 1806.2 are from the Unified Soil Classification System. Most foundation-bearing strata can be classified into one of the classifications in the table. The allowable bearing pressures and lateral-bearing values are based on long experience with the behavior of these materials. However, the presumptive value for CL and CH may not be conservative for “soft” clays, depending on the degree of consolidation of the clay. The selection of an allowable bearing pressure should take into account the strength of weaker underlying soil strata so that the pressure in any weaker stratum does not exceed the allowable pressure, particularly in clay soils. Because of this, it is important to know the soil profile and classifications of the different strata.

1806.3 Lateral load resistance. Sections 1806.3.1 through 1806.3.4 deal with lateral load calculations when using the presumptive load-bearing values of Table 1806.2.

The classifications for lateral bearing in Table 1806.2 are from the Unified Soil Classification System. The lateral-bearing values are based on long experience with the behavior of these materials. The limitation on frictional resistance for silts and clays is intended to provide structural stability and improve serviceability.

The formulae for lateral bearings employing posts and poles (found in IBC Section 1807.3, embedded posts and poles) were originally for outdoor advertising structures. For these structures, deflections of ½ inch at the surface do not affect serviceability. Thus, the allowance of two times the tabular value for these structures is permitted.

Section 1807 Foundation Walls, Retaining Walls, and Embedded Posts and Poles

1807.1 Foundation walls. Foundation walls of materials such as concrete and masonry, rubble stone, or wood are addressed in Section 1807. Lateral soil loads that must be considered in foundation wall design are covered in Section 1610. If a drainage system is not placed behind the wall to drain ground water away from the wall, hydrostatic pressures, which can easily exceed the lateral pressures from the retained soil, will occur. For example, the active lateral pressure from a well-graded granular soil may be in the range of 30 to 35 pounds per square foot, whereas the hydrostatic pressure is 62.4 pounds per cubic foot. Hence, the equivalent fluid pressures on a wall that was designed as drained, but constructed without an effective drainage system, could be subjected to pressures approximately three times the design pressure.

Unbalanced backfill height and its method of measurement are presented in Section 1807.1.2 and rubble stone foundation walls in Section 1807.1.3. Because rubble masonry uses rough stones of irregular shape and size, a larger thickness, as compared to hollow unit masonry or concrete, is required for adequate bonding of the stone and mortar. Rubble stone foundation walls are not permitted in SDC C, D, E, or F.

1807.1.4 Permanent wood foundation systems. The requirements set forth in AF&PA Technical Report PWF must be rigidly followed. The wood foundation system is an assembly similar to a fire assembly—no substitution of materials or methods is allowed. All lumber and plywood must be treated in accordance with AWPA U1 (Commodity Specification A, Use Category 4B, Section 5.2) and must be identified and labeled in accordance with Section 2303.1.8.1. ICC in partnership with AWPA publishes all 24 AWPA standards that are referenced in the IBC.

All hardware and fasteners must be corrosion resistant. Metals in contact with the preservative salts will corrode at a much faster rate than normal because of the influence of the salts. Hence, only corrosion-resistant fasteners made of silicon bronze, copper, or Type 304 or 316 stainless steel may be used, except that hot-dipped galvanized nails may be used when installed under the specific conditions set forth in the technical report for surface treatment of the nails and moisture protection of the foundation.

1807.1.5 Concrete and masonry foundation walls. Foundation walls must be designed within the applicable provisions of IBC Chapters 19 and 21. However, if the foundation wall is laterally supported at the top and bottom, such as a basement wall laterally supported by a floor diaphragm at the top and a basement floor slab at the base, the wall may be constructed in accordance with the prescriptive provisions of Section 1807.1.6 and its associated tables. These tables allow the use of unreinforced and plain (lightly reinforced) concrete or masonry walls that have been used in low or very low seismic risk areas.

Walls in moderate-to-high seismic risk areas will be subjected to ground shaking and ground displacements of unknown magnitude, and the walls will have an additional lateral load caused by the seismic ground motion. In the 2000 IBC, there were no specific requirements based on regional seismic considerations. Code changes were made to the 2003 IBC that were designed to address these concerns by the addition of a new section that has seismic requirements based on SDC. Specific seismic requirements for concrete and masonry foundation walls are now covered in Sections 1807.1.6.2.1 (concrete) and 1807.1.6.3.2 (masonry), based on the SDC of the building.

Additionally, if the prescriptive provisions are used, sufficient soil investigation should be done to properly classify the retained soils as indicated in the tables in accordance with the Unified Soil Classification Method (see IBC Section 1803.5.1).

1807.1.6 Prescriptive design of concrete and masonry foundation walls. The requirements and provisions of Sections 1807.1.6.1 through 1807.1.6.3.2 are applicable to the prescriptive design of concrete and masonry foundation walls that are laterally supported at the top and bottom. The minimum wall thicknesses are specified in the appropriate sections, based on the thickness of the supported wall, soil loads, unbalanced backfill height, and overall height of the wall. Rubblestone walls cannot be less than 16 inches thick where permitted (Section 1807.1.3). These minimum thickness provisions are to facilitate support of the wall above. These thickness provisions are empirical and have been used successfully in low or very low seismic risk areas. Additional seismic requirements for concrete and masonry foundation walls are covered in Sections 1807.1.6.2.1 (concrete) and 1807.1.6.3.2 (masonry), based on the SDC of the building.

1807.1.6.2 Concrete foundation walls. This section specifies the material requirements for walls constructed in accordance with the prescriptive tables. Note the effective depth, “d,” in Section 1807.1.6.2(3). Placement of reinforcing at the prescribed “d” is critical to develop adequate flexural strength necessary to resist the combined vertical and lateral soil loads.

The concrete section contains seven specific requirements to prescriptively select a foundation wall.

Concern has been expressed that the prescriptive foundation wall provisions do not impose a limitation on the maximum axial loads that the walls should support. To resolve this concern, a conservative maximum unfactored axial load of images for concrete and images for masonry are included in the requirements. The maximum unfactored axial load is based on a compressive stress on the outside face of the wall that is due to the axial load and bending moment induced by the backfill that is well below that permitted by ACI 318 or TMS 402/ACI 530/ASCE 5. Although this axial load limitation has merit, it requires a calculation to determine actual maximum axial load acting on a given wall. Table 1807-1 shows the maximum unfactored allowable axial load for the typical concrete foundation wall.

Table 1807-1. Maximum Permissible Axial Load for Concrete Walls Based on in Pounds per Foot of Wall

images

1807.1.6.2.1 Seismic requirements. The 2000 IBC had no specific seismic requirements for the prescriptive foundation wall provisions. This was of particular concern in the western states where earthquakes are relatively frequent and destructive. In the 2003 IBC, these concerns were addressed by adding specific seismic-related requirements in a new section, which covered seismic requirements based on SDC. These same specific seismic requirements for concrete and masonry foundation walls are now covered in Sections 1807.1.6.2.1 and 1807.1.6.3.2, based on the SDC of the building. The requirements are summarized below for concrete foundation walls.

Seismic requirements for concrete foundation walls constructed in accordance with Table 1807.1.6.2 are as follows:

1. SDCs A and B—One #5 bar is required at a minimum around window and door openings, which must extend beyond the corners of the openings or be anchored so as to develop f_y in tension at the corner of openings (reference Section 1909.6.3).

2. SDCs C, D, E, and F—The prescriptive tables are not allowed to be used except as permitted for plain concrete members in accordance with Section 1908.1.8, which modifies ACI 318, Section 22.10. The modification states that structural plain concrete members are not permitted in SDC C, D, E, or F except for structural plain concrete basement, foundation, or other walls below the base in detached one- and two-family dwellings three stories or less in height constructed with stud-bearing walls. Additional restrictions apply to dwellings in SDC D or E, where the walls cannot exceed 8 feet in height, cannot be less than 7.5 inches thick, and can retain no more than 4 feet of unbalanced fill. The last requirement states that the walls must be reinforced in accordance with ACI Section 22.6.6.5.

1807.1.6.3 Masonry foundation walls. This section contains requirements for masonry foundation walls, both plain and with reinforcement. Masonry is required to be solid in order to distribute the concentrated force, or hollow units must be solidly grouted as noted in Footnote c of the plain masonry foundation wall Table 1807.1.6.3(1). The masonry section contains 10 specific requirements for prescriptive selection of a masonry foundation wall. See Section 2104.2 if corbelling is necessary or desired to match the width of a masonry cavity wall above the foundation wall.

Table 1807-2 shows the maximum unfactored allowable axial load for the typical masonry foundation wall. Also see the discussion under Section 1807.1.6.2, Concrete foundation walls.

Table 1807-2. Maximum Permissible Axial Load for Masonry Walls Based on in Pounds per Foot of Wall

images

1807.1.6.3.1 Alternative foundation wall reinforcement. The code permits equivalent cross section of reinforcing, provided the spacing does not exceed 72 inches and the bar size does not exceed #11. If alternative reinforcement is used, it is preferable to reduce bar size and spacing rather than increase bar size and spacing. In development of the reinforcing, the bar size must be small enough that the reinforcing, and any splices, can be adequately developed. Development refers to the embedment of the reinforcing to adequately develop the bond between the reinforcing and the grout. A good rule of thumb to prevent splitting of concrete masonry is that the bar size number should not exceed t − 1, where t is the nominal thickness of the wall in inches.

1807.1.6.3.2 Seismic requirements (masonry). See discussion under Section 1807.1.6.2.1, Seismic requirements. Table 1807-3 summarizes requirements for masonry foundation walls based on seismic design category and gives applicable sections of the MSJC Code.

Table 1807-3. Seismic Requirements for Masonry Foundation Walls

images

Seismic requirements for masonry foundation walls constructed in accordance with Tables 1807.1.6.3(1) through 1807.1.6.3(4) are as follows:

1. SDCs A and B—No additional requirements apply.

2. SDC C—Additional requirements cover discontinuous members that are part of the lateral-force-resisting system, such as columns, pilasters, and beams that support reactions from walls or frames, but no specific requirements for foundation walls. Refer to Section 1.18.4.3 of TMS 402/ACI 530/ASCE 5 for other requirements.

3. SDC D—Must conform to the requirements of SDC C, as well as Section 1.18.4.4 of TMS 402/ACI 530/ASCE 5.

4. SDCs E and F—Must conform to the requirements of SDCs C and D, as well as Section 1.18.4.5 of TMS 402/ACI 530/ASCE 5.

1807.2 Retaining walls. Although the legacy codes 1999 BOCA/NBC and 1999 SBC, and ASCE 7-98 contained some requirements for retaining walls, they were very limited in scope. The legacy 1997 UBC had more detailed requirements for retaining walls, which are essentially the same as the provisions in the IBC. The IBC requires retaining walls to be designed to resist overturning, sliding, and excessive foundation-bearing pressure with a safety factor of at least 1.5 against lateral sliding and overturning using allowable stress design loads. (See discussion under Section 1610 and Table 1610.1 for soil lateral loads.) A keyway incorporated into the retaining wall base extending into the soil is considered to enhance the ability of the retaining wall against sliding.

There has been considerable debate in the structural engineering community whether both the passive pressure resisting the slide and the active pressure acting on the driving side of the key should be considered in the analysis. While some believe considering soil lateral pressure on both sides of the keyway is too conservative, there are others who believe not considering the active pressure will be too unconservative. Hence, the code now requires the consideration of all lateral pressures as is required to do in a free-body diagram. See Figure 1807-1.

images

Figure 1807-1 Retaining wall keyway in soil.

1807.3 Embedded posts and poles. The design criteria for the use of poles or posts embedded in the ground, or in concrete footings in the ground and unconstrained at the ground surface, were developed for the Outdoor Advertising Association of America, Inc. (OAAA). The research was conducted at Purdue University from 1938 to 1940, and continued in 1947 at the University of Notre Dame. The results of this research were used by OAAA for the design of outdoor advertising structures, which had previously used trussed A-frame supporting systems. Charts and a monograph were developed, which the association used for the design of poles as cantilever uprights for support of its outdoor advertising structures. These data were subsequently submitted through one of the ICC legacy organizations, International Conference of Building Officials’ (ICBO), code change process and were incorporated into the 1964 edition of the Uniform Building Code (UBC).

The criteria relate to lateral bearing and apply to a vertical pole considered a column embedded in either earth or in a concrete footing in the earth and used to resist lateral loads. In order for the pole to meet the conditions of research that resulted in the code formula, the code requires that the backfill in the annular space around a column that is not embedded in a concrete footing be either of 2,000 psi concrete or of clean sand thoroughly compacted by tamping in layers not more than 8 inches in depth.

The original design criteria established for the Outdoor Advertising Association of America, Inc., resulted in a ½-inch lateral pole deformation at the surface of the ground. These criteria were also based on field tests conducted in a range of sandy and gravelly soils and silts and clays.

The IBC employs allowable lateral-bearing stresses in IBC Table 1806.2, which are considerably lower than those developed for the Outdoor Advertising Association of America. Consequently, Section 1806.3.4 permits a doubling of the lateral-bearing values for isolated poles and poles supporting structures that can safely tolerate the ½-inch movement at the ground surface.

1807.3.1 Limitations. The limitations imposed by this section are intended for both structural stability and serviceability. The limitation of the frictional resistance for silts and clays is consistent with the UBC, which also limited the sliding resistance to one-half the dead load.

The limitations on the types of construction that use the lateral support of poles are based on the brittle nature of the materials. To prevent excessive distortions that would cause the cracking of these brittle materials, the code limits the use of the poles unless some type of rigid cross-bracing is provided to limit the deflections to those that can be tolerated by the materials.

Wood poles must be treated in accordance with AWPA U1. Sawn timber posts are Commodity Specification A, Use Category 4B, and round timber posts are Commodity Specification B, Use Category 4B.

1807.3.2 Design criteria. See IBC Section 1806.3 for allowable values of lateral bearing. Note that the two-time increase allowed per Section 1806.3.4 may only be used for structures where deflection of ½ inch at the surface is tolerable, for example, signs, flagpoles, and light poles.

1807.3.2.1 Nonconstrained. See Section 1807.3 discussion for the empirical basis of the formula. This formula should be used only for minor foundations of moderate size, which will fit within the constraints of the data from which the formula was developed. For large-sized piers, that is, more than 2 feet in diameter, a more appropriate method should be used. See Winterkorn et al.²

1807.3.2.2 Constrained. The term pavement means a rigid pavement such as reinforced concrete that will form a fulcrum for the column. Columns in flexible pavements such as asphalt concrete must use the formula in Section 1807.3.2.1 for unconstrained conditions.

1807.3.2.3 Vertical load. There is no requirement to consider a combined lateral and vertical load. The vertical loads for which the formulae were derived were less than 0.1FA_g. If there are vertical loads greater than 0.1FA_g, these formulae should not be used.

1807.3.3 Backfill. Backfill in accordance with the requirements is necessary to achieve the strength predicted by the formulae. The required backfill was used as part of the research conducted to develop the formulae. Note that the sand should be compacted to a relative density of at least 85 percent.

Section 1808 Foundations

1808.1 General. The provisions of Section 1808 apply to all foundations. Specific requirements for shallow foundations and deep foundations are located in Sections 1809 and 1810, respectively. The two general types of foundations are shallow foundations and deep foundations. Section 202 defines a shallow foundation as an individual or strip footing, a mat foundation, a slab-on-grade foundation, or a similar foundation element. A deep foundation is defined as a foundation that does not meet the definition of a shallow foundation.

1808.2 Design for capacity and settlement. Footings should be designed for approximately equal settlements to minimize differential settlements. For footings on sands, this may require unequal footing pressures to affect equal settlements. For example, see Terzaghi et al.¹ Expansive soils are addressed in Section 1808.6.

1808.3 Design loads. Footings are to be designed using full dead load (including overlying fill materials), floor and roof live loads, snow load, wind or seismic forces, and any other loads required by Section 1605 that will produce the most severe loading. Live loads acting at the foundation may be reduced based on the reduced probability of simultaneous occurrence of maximum live loads. This section specifically permits live load reduction as specified in Sections 1607.10 and 1607.12 for the foundation design.

1808.3.1 Seismic overturning. When strength design loads are used to proportion the foundations, the seismic overturning effects are permitted to be reduced in accordance with ASCE 7 Section 12.13.4. This maximum recognizes that the seismic forces determined in accordance with the ASCE 7 standard are based on strength design, not allowable stress design (ASD). Foundations proportioned in accordance with ASD procedures have historically performed satisfactorily. Because of expected deviation from the results from the equivalent lateral force method, which assumes a fixed base of the building, overturning effects at the foundation are permitted to be reduced 25 percent for structures other than inverted pendulum or cantilever column systems when designed by the equivalent lateral force procedure. Overturning effects at the foundation are permitted to be reduced 10 percent for structures designed by the modal analysis method because of the higher degree of accuracy of the procedure. Note that these reductions cannot be used with the alternative basic ASD load combinations of Section 1605.3.2.

1808.3.2 Surcharge. This new section in the 2015 IBC adds requirements pertaining to surcharge loads that could affect an adjacent structure. Although Chapter 33 covers requirements during construction, Chapter 18 had no specific provisions related to the effects of permanent loads that could surcharge a neighboring structure. Fill or other surcharge loads are not permitted to be placed adjacent to a building or structure unless it is capable of withstanding the additional loads caused by the surcharge load. Existing footings or foundations that could be affected by an excavation must be protected from detrimental lateral or vertical movement and settlement. The exception allows minor grading for landscaping with limited grading heights using walk-behind equipment which does not induce high forces against an adjacent foundation or wall when approved by the building official.

1808.4 Vibratory loads. Footings supporting equipment should be designed to minimize the transmission of vibratory loads to the soils. The dynamic interaction of the footing, equipment, and soil mass should be analyzed, and the footing “tuned” to minimize the transmission. As a rough rule of thumb, footings for rotating or reciprocating equipment should have a mass that is at least four times the mass of the equipment.

Vibratory loads from equipment foundations that are transmitted to the soil can cause significant and damaging settlements. The transmitted vibration will cause densification of granular materials, particularly loose or medium dense sands. The reduction in volume can cause large settlements depending on the initial density of the sands. In saturated granular materials, such as loose or medium dense sands with a high water table, the transmitted vibrations can cause a buildup of pore pressure and liquefaction of the sands, with resulting loss of bearing capacity and settlements. In saturated clays, the vibrations can enhance the drainage of water from the pores and increase long-term settlements.

1808.5 Shifting or moving soils. For example, loose sands.

1808.6 Design for expansive soils. The requirements to mitigate the effects of expansive soils are set forth in this section. In addition to mitigation by foundation design, the effects of expansive soils may also be mitigated by removal of the expansive soils or stabilization by chemical means, pre-saturation, or dewatering. Expansive soils are cohesive soils, typically high plasticity clays, with a high Plasticity Index and a high Swell Index.

1808.6.1 Foundations. The large volume changes in expansive soils caused by changes in the soils’ water content can cause significant differential deflections in a building if not uniform. In a typical building on expansive soils, the soils at the perimeter of the building will have seasonal moisture changes, whereas the soils at the interior of the building will remain at a fairly constant moisture content. The perimeter foundations will rise and fall with the seasonal volume changes in moisture content, whereas the soil at the interior footings or slab will not have any volume changes, because of constant moisture content. The resulting differential displacements between the interior and exterior footings can cause significant structural distress. Hence, the requirements that the foundation be designed to resist the differential volume changes and to minimize racking or differential displacements in the structure.

1808.6.2 Slab-on-ground foundations. The slab-on-ground or raft foundation design methods cited in this section result in a raft that has sufficient stiffness to bridge differential displacements caused by the volume changes in the supporting soil.

Design moments, shears, and deflections are to be determined in accordance with WRI/CRSI Design of Slab-on-Ground Foundations or PTI Standard Requirements for Analysis of Shallow Concrete Foundations on Expansive Soils. Once the design moments, shears, and deflections are determined from the applicable standard, then conventionally reinforced (nonprestressed) foundations on expansive soils must be designed in accordance with WRI/CRSI Design of Slab-on-Ground Foundations, and post-tensioned foundations on expansive soils must be designed in accordance with PTI Standard Requirements for Design of Shallow Post-Tensioned Concrete Foundations on Expansive Soils.

The code also permits alternative methods of analysis, provided the methodology is rational and the basis for the analysis and design parameters are available for peer review.

1808.6.3 Removal of expansive soil. Removal of the expansive soil is an acceptable mitigation method and is the preferred method if the stratum of expansive soil is near the surface and reasonably thin. This method may also be the least expensive method if the expansive soil is at the surface.

1808.6.4 Stabilization. Expansive soils may be stabilized so that the moisture content does not change; hence, there will be no volume changes to cause differential displacements. Stabilization can be by chemical methods, by pre-saturating the soils to a maximum swell and capping the expansive layer to keep the moisture content constant, or by dewatering to a minimum shrinkage and providing drainage to keep the moisture content constant.

1808.7 Foundations on or adjacent to slopes. The provisions of this section apply only to buildings placed on or adjacent to slopes steeper than one vertical to three horizontal.

1808.7.1 Building clearance from ascending slopes. This setback requirement is intended to provide protection to the structure from shallow slope failure (sloughing) and protection for erosion and slope drainage. The setback space also provides access around the structure and helps to create a light and open environment. IBC Figure 1808.7.1 depicts the criterion for the setback or clearance. Figure 1808-1 also depicts the criteria set forth in this item for determination of the toe of the slope when the slope exceeds 1:1.

images

Figure 1808-1 Buildings adjacent to ascending slope exceeding 1:1.

1808.7.2 Foundation setback from descending slope surface. The setback requirement at the top of slopes is intended to provide vertical and lateral support for the foundations and minimize the possibility of shallow bearing failure of the foundation because of lack of lateral support. The setback also provides space for drainage away from the slope without creating too steep a drainage profile, which could cause erosion problems. The setback space also provides access around the structure and helps to create a light and open environment. IBC Figure 1808.7.1 depicts the criterion for the setback or clearance. Figure 1808-2 herein depicts the criteria set forth in this item for determination of the toe of the slope when the slope exceeds 1:1.

images

Figure 1808-2 Buildings adjacent to descending slope exceeding 1:1.

It is possible to locate a structure closer to the slope than indicated in IBC Figure 1808.7.1. The footing of the structure may be located on the slope itself, provided that the depth of embedment of the footing is such that the face of the footing at the bearing plane is set back from the edge of the slope at least H/3.

1808.7.3 Pools. Figure 1808-3 depicts the criteria for the design of swimming pool walls near the top of a descending slope. The wall must be sufficient to resist the hydrostatic water pressure without support from the soil to protect against failure of the pool wall should localized minor slope movement or sloughing occur. The pool setback should be established as one-half of the setback required by IBC Figure 1808.7.1.

images

Figure 1808-3 Swimming pool adjacent to descending slope.

1808.7.4 Foundation elevation. Figure 1808-4 depicts the criteria from this section for the elevation of the exterior foundations relative to the street, gutter, or point of inlet of a drainage device.

images

Figure 1808-4 Footing elevation on graded sites.

The elevation of the street or gutter shown is that point at which drainage from the site reaches the street or gutter. This requirement is intended to protect the structure from water encroachment in the case of heavy or unprecedented rains. This requirement may be modified if the building official finds that positive drainage slopes are provided to drain water away from the building and that the drainage pattern is not subject to temporary flooding from clogged drains, landscaping, or other impediments.

1808.7.5 Alternate setback and clearance. This alternative procedure allows the building official to approve alternative setbacks and clearances from slopes, provided that the intent of Section 1808.7 is met. This section gives the building official the authority to require a geotechnical investigation by a qualified geotechnical engineer to establish that the intent of Section 1808.7 is met. The parameters for such geotechnical investigation are established in Section 1803.5.10 and include consideration of material, height of slope, slope gradient, load intensity, and erosion characteristics of the slope material.

1808.8 Concrete foundations. Footings may be designed, or the requirements of Table 1809.7 may be used, for structures with light-framed walls of conventional construction, where frost heave or expansive soils are not a problem. Table 1809.7, which originated with the UBC, is based on anticipated dead and live loads from the floors and roof and an assumed soil classification of ML, MH, CL, or CH.

1808.8.1 Concrete or grout strength and mix proportioning. A new Table 1808.8.1 provides the minimum specified compressive strength, images for concrete or grout to be used in specific foundation types. Where the previous editions of the IBC required a minimum 2,500 psi compressive strength for footings, the new table now provides values for various SDCs as well as for piles and shafts. The minimum 2,500 psi is still the correct value for most footings of structures in SDC A, B, or C and for the footings of residential light-frame and utility structures, one or two stories in height, in SDCs D, E, and F. The minimum specified concrete strength of 2,500 psi is set to provide a material of adequate strength and durability. Concrete of lower strength may not have adequate durability, particularly in freeze-thaw areas.

The slump requirements stated are for cased piles. Slump requirements must be adjusted for other conditions. For example, concrete placed in uncased drilled holes needs to be in the 6- to 8-inch range so that the concrete flows readily into the irregularities in the drilled hole. Use of superplasticizers will provide the desired slump while keeping the water to cementitious material ratio low.

1808.8.2 Concrete cover. Cover requirements are set to provide protection and minimize steel corrosion. All concrete cover requirements of Chapter 18 are organized in Table 1808.8.2.

1808.8.3 Placement of concrete. Holes should be free from debris, loose soils, or water. Placement of concrete through water should be avoided because of the increased risk of segregation and dilution of the concrete paste. When concrete is placed under water, by tremie or other approved method, the mix must be different from the standard mix used for ordinary concrete foundations. The mix must be proportioned so that it is plastic with high workability and will flow without segregation. The desired consistency can be obtained by using rounded aggregates, high sand contents, entrained air, and superplasticizers. Higher cement contents are necessary to compensate for the increase in the water to cementitious materials ratio caused by dilution from placement through water. Minimum cement content should be 600 pounds per yard. Placement from the top of the deep foundations can cause segregation of concrete mix also, and proper measures such as the use of a funnel hopper (elephant trunk) should be taken to avoid the potential for segregation.

When depositing concrete from the top of a deep foundation, the IBC requires concrete to be chuted directly into smooth-sided pipes and tubes or through a centering funnel hopper. The main purpose of the centering funnel for drilled piles is to prevent the concrete from encountering the soil at the perimeter of the hole, which is generally not a problem for pipes and tubes. The term smooth sided is included in the code to prevent possible segregation from the ridges or corrugations if nonsmooth pipes or tubes are used.

1808.8.4 Protection of concrete. Concrete footings should not be placed during rain, sleet, snow, or freezing weather without protection against either freezing or increase in water content at the surface from rain while plastic. If concrete placement is undertaken under such conditions without adequate protection, numerous complications can be expected. There have been many cases where a project was forced to come to a halt while concrete core samples, testing, analysis, and investigation had to be performed on the hardened concrete to determine suitability of the deposited concrete and of the structural elements. See ACI 306R and ACI 306.1 for cold-weather concrete operations.

1808.8.5 Forming of concrete. The soil should have sufficient strength and cohesion that the shape, dimensions, and vertical sides of the excavation can be maintained without sloughing prior to and during the concreting operations. Excavations in loose granular materials must be formed.

1808.8.6 Seismic requirements. Specific requirements for foundations in SDC C, D, E, or F are contained in Section 1905, which contains the modifications to ACI 318. In SDC D, E, or F, concrete must have a specified compressive strength of not less than 3,000 psi, except that 2,500 psi concrete strength is permitted in Group R or U occupancies of light-frame construction two stories or less in height.

Buildings in SDCs D, E, and F are required to comply with ACI 318, Sections 21.12.1 through 21.12.4, except for detached one- and two-family dwellings of light-frame construction two stories or less in height. ACI 318 Section 21.12.1 covers foundation requirements in general, and Sections 21.12.2 through 21.12.4 cover requirements for footings, mat foundations, pile caps, grade beams, and slabs on grade.

Note that plain concrete is either unreinforced or lightly reinforced concrete that contains less reinforcing than required to meet the minimum reinforcement requirements set forth in ACI 318, Section 10.5.

Section 1809 Shallow Foundations

1809.1 General. Foundations are divided into two major categories of shallow foundations and deep foundations. Shallow foundations are the individual or strip footings, the bottom of which is typically close to the surface such as mat foundations, slab on grade, or similar foundation types. Shallow foundations are regulated in Sections 1890.2 through 1809.13. Deep foundations are those that are not classified as shallow foundations.

1809.2 Supporting soils. Because the supporting soil for shallow foundations is close to the surface, in order to minimize differential settlement, shallow foundations must be constructed on undisturbed native soil, compacted fill material, or CLSM. Where constructed on fill, the material must be properly placed and compacted to achieve adequate density in accordance with Section 1804.6. CLSM must be placed and tested in accordance with Section 1804.7.

1809.3 Stepped footings. Footings are required to be stepped when the slope of the bearing surface exceeds 1 in 10. No recommendations or restrictions are provided. Figure 1809-1 schematically represents a satisfactory stepped foundation. The figure shows a recommended horizontal overlap of the top of the foundation wall beyond the step in the foundation to be larger than the vertical step in the foundation wall at that point. This is recommended to keep any crack propagation approximately at a 45-degree angle. To keep this cracking to a minimum, it is also recommended that the height of each step not exceed 1 to 2 feet. Other measures to protect against cracking, such as special reinforcing details, may be needed. See Section 2308.6.8.3 for specific requirements pertaining to stepped foundations for conventional construction.

images

Figure 1809-1 Stepped foundations.

1809.4 Depth and width of footings. Footings should always be placed a minimum of 12-inches deep on either firm, undisturbed earth or properly compacted fill, and be at least 12 inches in width.

1809.5 Frost protection. To prevent frost heave during winter and subsequent settlement upon thawing, foundations and building supports should be placed on a stratum with adequate load-bearing resistance that is below the frost line. Frost heave occurs because of the increased soil volume from the freezing of pore water in the soil. Clay soils, particularly saturated clays, are most susceptible to frost heave. Well-drained sands and gravels will not be susceptible to significant movement. If the foundations are built on soils that can freeze, the resulting frost heave, which is rarely uniform, can cause serious damage from differential settlements.

The frost line is defined as the lowest depth below the ground surface to which a temperature of 32°F extends. The factors governing the depth of the frost line are air temperature, the length of time the air temperature is below freezing (32°F), and the soil’s thermal conductivity. Frost lines vary significantly throughout the country from no penetration in southern Florida to 100 inches in the northern regions of Michigan and Maine. Data on frost penetration are available from the U.S. Department of Commerce Weather Bureau. See Figure 1809-2.

images

Figure 1809-2 Frost penetration depths.

The code offers three options for ensuring adequate frost protection for shallow foundations. The first option—the most common and simplest to accomplish—is to construct the bottom of the footing below the frost line for the particular locality. The second option is to construct the footing in accordance with the referenced standard, ASCE 32, Design and Construction of Frost-Protected Shallow Foundation. The third option, which is often encountered in areas where bedrock is prevalent, is to construct the footing on solid rock.

Note the exception where frost-protected foundation is not required: free-standing buildings classified in Risk Category I (see Section 1604.5), floor area of 600 square feet or less for light-frame construction or 400 square feet or less for other than light-frame construction, and eave height of 10 feet or less. Note that the term light-frame construction is defined in Section 201 as a system that uses repetitive wood or light-gauge steel-framing members.

The code prohibits footings from bearing directly on frozen soil unless the soil is permanently frozen. Permafrost may not meet this condition as permafrost is considered soil that remains in a frozen state for more than two years in a row.

1809.6 Location of footings. This restriction is intended to minimize the influence of vertical and lateral loads from footings at a higher elevation on footings at a lower elevation. See Figure 1809-3.

images

Figure 1809-3 Isolated foundation.

1809.7 Prescriptive footings for light-frame construction. Footings of concrete or masonry unit may be designed, or the requirements of Table 1809.7 may be used, for structures with light-frame walls of conventional construction, where frost heave or expansive soils are not a problem. IBC Table 1809.7 shown herein as Table 1809-1, which originated with the UBC, is based on anticipated dead and live loads from the floors and roof and an assumed soil classification of ML, MH, CL, or CH.

Table 1809-1. Prescriptive Footings Supporting Walls of Light-Frame Construction^a,b,c,d,e (Formerly Table 1809.7)

images

1809.8 Plain concrete footings. In compliance with ACI 318, Section 22.7.4, the edge thickness of plain concrete footings in other than light-frame construction must not be less than 8 inches. In accordance with ACI 318, Section 22.4.7, the thickness or depth used to compute footing stresses (flexure, combined axial load and flexure, or shear) should be 2 inches less than the actual thickness of the footing for footings cast against soil. This is done to allow for unevenness of excavation and contamination of the concrete adjacent to the soil.

The edge thickness of plain concrete footings can be reduced to 6 inches for R-3 occupancies, provided that the edge distance (projection) of the footing beyond the face of the stem wall does not exceed the thickness (6 inches depth = 6 inches extension). Figure 1809-4 illustrates this condition. (R-3 occupancies are described in Section 310.1.) For lightly loaded walls, this dimensional limitation should keep the flexural stresses in the footing below the limit of images and the shear stresses below images . These stresses should be checked for heavily loaded walls.

images

Figure 1809-4 Plain concrete footing.

1809.9 Masonry-unit footings. Masonry footings were widely used until the middle of the last century when they were replaced by steel or wood grillage footings, which in turn were replaced by more economical plain or reinforced concrete footings. Masonry footings were often built of stone cut to a specific size, or rubble masonry of random-sized stones bonded with mortar. Although seldom used, masonry footings may be constructed of hard-burned brick set in cement mortar to support lightweight buildings.

1809.9.1 Dimensions. Type M mortar is suitable for unreinforced masonry below grade or with earth contact. Type S should be used for reinforced footings. Projections of the footing beyond a wall or pier should not exceed one-half of the footing depth to keep the shear and flexural stresses in the footing within safe limits. For example, a footing with a 12-inch depth should project no more than 6 inches beyond the face of the wall.

1809.9.2 Offsets. The stepping back, or racking, of successive courses of the foundation wall supported by a masonry footing must not exceed 1½ inches for a single course or 3 inches for a double course. Where wide footings are necessary for bearing, the wall must be stepped back to keep the footing projection within the limits of Section 1809.9.1. See Figure 1809-5.

images

Figure 1809-5 Brick footing wall offsets.

1809.10 Pier and curtain wall foundations. Pier foundations may not be used to support structures assigned to SDCs D, E, and F because seismic detailing requirements for higher SDCs have not been developed. These foundation elements have been popular particularly in the Eastern United States, and under this section are allowed only for the support of light-frame construction of not more than two stories above grade plane. “Story above grade plane” is defined in Section 202 as any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is:

1. More than 6 feet above grade plane; or

2. More than 12 feet above the finished ground level at any point.

1809.11 Steel grillage footings. Steel grillage footings were used extensively during the latter half of the last century, but the development of reinforced concrete made the use of steel grillage footings obsolete, except for underpinning work.

A typical grillage consists of two or more tiers of steel beams, with each tier placed at right angles to the tier below. The beams in each tier are usually held together by a system of bolts and pipe spacers. For construction of new grillage footings, the beams should be clean and unpainted and the entire grillage system filled with and encased in concrete with at least 4 inches of cover. The grillage should be placed on a concrete pad at least 6 inches thick to distribute the load evenly to the soil.

1809.12 Timber footings. Use of timber footings is allowed only for Type V structures. (Type V structures are described in Section 602.5 as that type of construction in which the structural elements, exterior walls, and interior walls are of any material permitted by this code. Buildings of Type V construction are limited in area and height in accordance with Chapter 5 of the code.) Timber must be pressure treated to American Wood Protection Association (AWPA) U1 (Commodity Specification A, Use Category 4B) standards, except for foundations permanently below the ground-water table. The pressure preservative treatment protects the timber from decay, fungi, and harmful insects. The AWPA Use Category System is based on the end use hazard, similar to other international standards for wood treatment. The Use Category System (UCS) is used to specify the wood treatment based on the desired wood species and the environment of the intended end use. There are six use categories, which describe the exposure conditions that wood may be subject to in service. ICC in partnership with AWPA publishes all 24 AWPA standards that are referenced in the IBC. Stronger preservatives are necessary to prevent marine borers when timber foundations are used in coastal brackish or marine environments.

Preservative treatment by the pressure process within the limitations of the AWPA standards should not significantly affect the strength of the wood. Part of the process, however, involves the conditioning of the wood prior to treatment by steaming or boiling under vacuum. This conditioning can cause reduction in strength. This strength loss is recognized in the AF&PA National Design Specification for Wood Construction by use of the untreated factor, C_u, which provides an increase in the tabular design values for untreated timber poles and piles. Note that the NDS states that load duration factors greater than 1.6 are not allowed for structural members that are pressure treated with water-borne preservatives. This restriction would apply to impact loads that have a duration factor of 2.0.

Untreated timber may be used when the footings are completely embedded in soil below the ground-water table. Experience has long shown that timber permanently confined in water will stay sound and durable indefinitely. Wood submerged in fresh water cannot decay, because the necessary air is excluded. Because ground-water levels can sometimes change appreciably, untreated timber should only be used at depths sufficiently below the water table so that small drops in the water level will not expose the timbers to air.

1809.13 Footing seismic ties. Interconnection of individual spread footings is required for structures in SDCs D, E, and F sited on soils in Site Class E or F. The footings must be interconnected with ties capable of transmitting a force equal to the larger footing load multiplied by the short period response acceleration, S_DS, divided by 10 or 25 percent of the smaller footing design gravity load, whichever is smaller. The intent of this requirement is to minimize differential movement or spreading between the footings during ground shaking, and have the individual footings act as a unit. If slabs on grade, or beams within slabs on grade, are used to meet the tie requirement, the load path from footing to slab or beam/slab and across joints in the slab or beam/slab should be checked for continuity. The slab or beam must be reinforced for the design tension load. In addition, the slab should be checked for buckling under the required compression load using an assumed slab width of no more than six times the slab thickness.

Section 1810 Deep Foundations

1810.1 General. Foundations are divided into two major categories of shallow foundations and deep foundations. Shallow foundations are the individual or strip footings, the bottom of which is typically close to the surface such as mat foundations, slab on grade, or similar foundation types. Deep foundations are those that are not classified as shallow foundations. Deep foundations are regulated in Sections 1810.1 through 1810.4 and are those typically referred to as piles, piers, or caissons that transfer load from a superstructure such as a building to the underlying soils or rock that provides support for the superstructure. Deep foundations are generally used when the loads are too high to be supported by shallow footings or mats. Load resistance is provided by skin friction between soil and the sides of the pile and by end bearing at the tip. Settlement has to be limited within acceptable levels and may control the pile capacity for design purposes.

Deep foundations can be subjected to compression, tension, and lateral loads. The loads can be static and dynamic forces resulting from soil pressure, wind pressure acting on the building, or earthquake load effects from ground motion. In many cases, there are groups of piles, and group interaction must be considered. Closely spaced deep foundation elements have lower capacities.

Deep foundations can be constructed of cased or uncased concrete, precast-prestressed concrete, timber, steel pipe or H sections, or other special types such as micropiles and helical piles. Piles can be either driven in place or drilled in place. The installation method for a particular project depends on a variety of factors such as the geotechnical engineer’s experience with local conditions, settlement limitations, acceptable noise levels, and so on.

The term caisson refers to deep foundations that are required when very heavy loads are supported such as high-rise building towers. Caissons are large-diameter concrete elements installed with a casing that can be left in place. The caisson bottom can be expanded, called a bell bottom, to provide more bearing area or it may include a rock socket. Trump Tower in Chicago is a 92-story building supported on 230 caissons bearing on rock and hardpan. The core caissons have a record-breaking 500-ksf bearing capacity with rock sockets up to 10 feet in diameter.¹

Within Section 1810 provisions are found for seismic design and detailing for various types of deep foundations in SDCs C through F. These mostly reflect the concern of the NEHRP Code Resource Development Committee that significant ground motions can occur in SDC C. Additional requirements are imposed on deep foundation elements in SDCs D through F, including a requirement that the upper portion of piles be detailed as special moment-resisting-frame columns, to prevent failure of the piles under severe ground motions. These provisions intend to include the deep foundation element bending and curvatures resulting from horizontal ground movement during an earthquake in the structural design. The reinforcement in the deep element, required to resist the curvature effect, increases ductility of the foundation such that bending or shear failure is precluded.

1810.1.1 Geotechnical investigation. See Section 1803.5.5 for discussion on geotechnical investigations.

1810.1.2 Use of existing deep foundation elements. This section allows the reuse of existing deep foundations where sufficient information is submitted to the building official to demonstrate they are adequate. This introduces flexibility for both the building designer and the building owner to make use of existing materials where it makes sense to do so.

Deep foundations remaining after structures that are demolished should not be used for the support of new loads, unless evidence shows them to be adequate. This is because of the lack of soil data or detailed information on the piling material, and because of the unavailability of the pile-driving records or pier construction records made during the construction of these older buildings or structures. As such, the true condition of the deep foundation elements is unknown and, over time, they may have deteriorated, or their load capacity may have been reduced. Such deep foundation elements may be used, however, if they are load tested or in the case of piles, if they are retracted and redriven to verify their capacities. Only the lowest allowable load capacity as determined by test data or redriving information should be used in the design.

1810.1.3 Deep foundation elements classified as columns. (Column action.) The code requires that deep foundation elements standing unbraced in air, water, or material not capable of providing adequate lateral support shall be designed in accordance with the column formulas of the code. Obviously, water and air do not provide lateral support. On the other hand, most soils do provide lateral support, although exceptionally loose and unconsolidated fills, liquified sands, and remolded clays are inadequate to provide lateral support. Cast-in-place elements can be designed as a “pedestal” under conditions where unsupported height to least horizontal dimension is three or less.

1810.1.4 Special types of deep foundations. Deep foundations such as pile or pier types are basically classified in accordance with the structural material used, such as concrete, steel, or wood. They can also be categorized in accordance with the method of construction or installation. There are many variations of foundation types used in the construction of deep foundations, including some special or proprietary types beyond the scope of the code. Section 1810 includes only those basic pile types commonly used in the construction practices of today.

Special deep foundation types that are not specifically included in the provisions of the code are not precluded from use, provided that adequate information covering test data, calculations, structural properties, load capacity, and installation procedures is submitted and accepted by the building official.

1810.2 Analysis. Sections 1810.2.1 through 1810.2.5 are related to the analysis methodology for deep foundations and apply to all deep foundations in all locations except where a specific analysis is called for in higher seismic active areas. The discussion of lateral support, stability, settlement, lateral loads, SDCs D through F, and group effects discussed below is related to analysis procedures and requirements.

1810.2.1 Lateral support. This section provides needed guidance to the designer and building official on what constitutes adequate lateral support for deep foundations. It specifies that any soil other than fluid soil is allowed to be considered to provide lateral support to prevent buckling of deep foundation elements. Liquefaction causes loss of lateral-bearing capacity with resulting loss of support for deep foundation elements. Loss of lateral support can also occur from the soil strength loss associated with sensitive or quick clays that are sensitive to remolding effects. These clays lose significant strength when remolded, as might occur when a pile foundation is moved through muds by seismic-induced displacements.

Portions of deep foundation elements standing unbraced in air, water, or material not capable of providing adequate lateral support are permitted to be considered laterally supported at a depth of 5 feet in stiff soils and at a depth of 10 feet in soft soils, as determined by the geotechnical investigation.

1810.2.2 Stability. A group of deep foundation elements such as piles designed to support a common load or to resist horizontal forces must be braced or rigidly tied together to act as a single structural unit that will provide lateral stability in all directions. Deep foundation elements connected by a rigid, reinforced concrete pile cap are deemed to be sufficiently braced to meet the intent of this provision.

This section clarifies that for pile or pier groups to be considered to provide lateral stability, they must meet the radial spacing requirements defined herein. Three or more deep foundation elements are generally used to support a building column load or other isolated, concentrated load. In a three-element group, lateral stability is assured by requiring that the elements are located such that they will not be less than 120 degrees apart as measured from the centroid of the group in a radial direction.

For stability of deep foundation elements in a group supporting a wall structure, the elements are braced by a continuous, rigid footing and are alternately staggered in two lines at least 1 foot apart and symmetrically located on each side of the center of gravity of the wall. Other approved deep foundation element arrangements may be used to support walls, provided the elements are adequately braced and lateral stability of the foundation construction is assured.

Exception 1 allows isolated cast-in-place deep foundation elements without lateral bracing where the minimum dimension is 2 feet, and the length must be less than or equal to 12 times the least dimension of the pier.

Exception 2 allows one- and two-family dwellings of lightweight construction, such as R-3 buildings, not exceeding two stories above grade plane or 35 feet in height, a single row of piles located within the width of the wall.

1810.2.3 Settlement. The purpose of a settlement analysis is to provide the data needed to design a deep foundation system that will maintain the stability and structural integrity of the supported building or structure. The load-bearing stratum of every soil must support the loads transferred through the deep foundation system, as well as the weight of all soil above. The capability of the strata underlying the deep foundation element to support additional loads without detrimental settlement can often be determined by analytical procedures. For example, serious settlements in a pile foundation system, particularly differential settlements, can cause great structural damage to the supported structure and the foundation itself.

Although the settlement analysis of an individual deep foundation element is complex, the analysis of a group of elements is significantly more complicated because of the overlapping soil stresses caused by closely spaced elements. Analytical procedures vary with the type of deep foundation element and especially with the soil conditions. Settlement analysis would generally include cases involving point-bearing piles on rock, and in granular soils and hard clay. It would also involve friction piles in sand and gravel soils, and in clay materials.

Load tests are often used to aid in the analysis. In the case of pile foundations in clay soils, however, there are no practical ways to determine long-term settlement from load tests, and therefore only approximations of settlement may be derived from laboratory tests.

1810.2.4 Lateral loads. Deep foundation moments, shears, and deflections must be based on nonlinear soil-deep foundation element interaction. If using deep foundations in soils with lenses of soft clays, lenses subject to liquefaction, or soils susceptible to strength loss from remolding, the effects of these layers on the curvature and, hence, moment demands on the deep foundations should be investigated. Often these soil-induced curvatures place a much higher moment demand on the deep foundations than would be determined from conventional lateral force P-y analysis. If the ratio of the depth of embedment of the deep foundation element to the element diameter or width is less than or equal to six, the deep foundation element may be assumed to be rigid. See Winterkorn et al.² for methods of analysis of rigid piers.

1810.2.4.1 Seismic design categories D through F. In addition to the requirements for SDC C, moments, shears, and deflections must be based on nonlinear soil-pile interaction. If using pile foundations in soils with layers of soft to medium stiff clays, layers subject to liquefaction, or soils susceptible to strength loss from remolding (see Site Class E and F), the effects of these layers on the curvature and, hence, moment demands on the pile must be investigated. Often these soil-induced curvatures place a much higher moment demand on the piles than would be determined from conventional lateral force P-y analysis. In addition, at the interfaces of the layers described above and stiffer layers, plastic hinging may occur. Hence, confinement reinforcement per the concrete special-moment frame provisions must be provided at these interfaces and at the pile-to-cap connection for concrete piers and piles. See ASCE 7 Chapter 12. The curvature capacity requirements are considered to have been met without the analysis outlined in this section under two conditions. One condition is for precast-prestressed concrete piles where the transverse reinforcement detailing complies with Section 1810.3.8.3.3. These provisions were developed specifically for precast-prestressed piles to meet the curvature requirements. The other condition is for cast-in-place concrete deep foundation elements that meet a minimum longitudinal reinforcement ratio of 0.005 the full length of the element and detailed in accordance with ACI 318, Sections 21.6.4.2 through 21.6.4.4.

1810.2.5 Group effect. Where deep foundation elements are spaced far apart, they are considered as individual elements and analyzed and designed accordingly based on various requirements of the code. As spacing between the elements is reduced, the loads and stresses from one element may affect surrounding deep foundation elements. This is the spacing at which group action must also be considered. The spacing for group action consideration in analysis of lateral loads is a center-to-center spacing of less than eight times the least horizontal dimension of an element, and for axial loads is a center-to-center spacing of less than three times the least horizontal dimension of a deep element. See Figure 1810-1.

images

Figure 1810-1 Group effect.

The last sentence of this section was added in the 2015 IBC to clarify that group effects on uplift of deep foundations should be evaluated where the spacing of elements is less than three times their least horizontal dimension. Although the language seemed clear without the change, Section 1810.3.3.1.6 is not specific in regard to the spacing that necessitates evaluation of group effects for uplift. Cross referencing Section 1810.3.3.1.6, Uplift capacity of grouped deep foundation elements, helps further clarify the intent.

Determination of the proper spacing of a pile group in relation to the type of pile foundation employed and the soil conditions encountered is a matter of design. The spacing of piles must be such that the loads transferred to the load-bearing strata do not exceed the safe load-bearing values of the supporting strata as determined by test borings, field load tests, or other approved methods.

Prev Next