International Building Code Illustrated Handbook 2015 International Code Council

Chapter 23 of the International Building Code® (IBC®) originated as an amalgamation of the wood design provisions in the 1997 Uniform Building Code (UBC), the 1996 BOCA National Building Code (NBC), and the 1997 Standard Building Code (SBC), with the addition of seismic provisions from the 1997 National Earthquake Hazard Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures.¹ The provisions in the IBC were selected from these source documents based on their technical merit, and new provisions were added where they did not already exist. The use of seismic design categories instead of seismic zones was the most significant change for code users who were most familiar with the UBC. Seismic design categories are determined based on a combination of use of the structure, as defined by the Risk Category (formerly called Occupancy Category), the soil type, and the potential seismic hazard at the site. See Table 1604.5 for Risk Category descriptions based on the use of the structure. The seismic hazard at the site is a combination of the seismic risk associated with potential ground motion (mapped spectral response accelerations) and the site soil classification (Site Class). See the discussion of Section 1613 for a more detailed analysis of earthquake load effects, site class, and determination of seismic design category.

During the code development process from the 2000 IBC to the current 2015 edition, the most significant change to Chapter 23 consisted of the elimination of substantial code language in Sections 2305 and 2306, in favor of the referenced standards, National Design Specification (NDS) for Wood Construction² and Special Design Provisions for Wind and Seismic (SDPWS).³ Both the NDS and SDPWS are dual-format standards that include allowable stress design (ASD) and load and resistance factor design (LRFD) procedures. The NDS and SDPWS are referenced in Sections 2305, 2306, and 2307 without amendments. The most significant change in the 2015 IBC is the substantial reorganization of the conventional construction provisions of Section 2308, which is discussed in more detail later.

In June 2010, the American Wood Council (AWC) was re-chartered, evolving from its predecessor groups. Prior to the founding of the new AWC, the forest products industry was represented by the American Forest & Paper Association (AF&PA), which grew out of the National Forest Products Association (NFPA) and the American Paper Institute (API). The 2015 IBC references the 2015 edition of the National Design Specification (NDS) for Wood Construction (ANSI/AWC NDS-2015) and 2012 Supplement, which is an AWC standard developed by the AWC’s Wood Design Standards Committee.

Section 2301 General

2301.1 Scope. Chapter 23 covers materials, design, construction, and quality of wood buildings and structures. The chapter is formatted into eight major sections that follow a logical format: general requirements (2301), definitions (2302), minimum standards and quality for wood-related construction materials (2303), general construction requirements for wood structures (2304), general design requirements for lateral-force-resisting systems used in wood structures, allowable stress design of wood structures (2305), load and resistance factor design of wood structures (2307), prescriptive conventional construction requirements for light wood-frame structures (2308), and the new Section 2309 which references the AWC WFCM.

On a historical note, the 2003 IBC referenced the 1997 National Design Specification (NDS) for Wood Construction, published by the American Forest and Paper Association (AF&PA)² for allowable stress design (ASD), and ASCE 16-95, Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction,⁴ for strength design of wood structures. One significant change to the 2006 IBC is that both Sections 2306 and 2307 reference the 2005 edition of the AF&PA NDS, which is a dual-format standard that includes both ASD and LRFD procedures. The 2009 IBC also referenced the 2005 NDS and Supplement. For lateral design of wood structures, the 2009 IBC references the 2008 edition of the Special Design Provisions for Wind and Seismic (SDPWS). The 2012 IBC references the 2012 edition of the NDS and 2012 Supplement and the 2008 edition of the SDPWS. The 2015 IBC references the latest 2015 edition of the NDS and SDPWS.

Wood products addressed in Chapter 23 include boards, dimension lumber, posts, timbers, glued-laminated members, wood structural panels including plywood, composite panels, oriented strand board, particleboard, fiberboard, hardboard, prefabricated wood I-joists, structural composite lumber, laminated veneer lumber, and parallel strand lumber.

The chapter contains material specifications, quality requirements, and design provisions, as well as empirical and prescriptive provisions for wood-frame construction.

Wood and wood-based products are also regulated in other chapters of the code. For example, Chapter 14 contains provisions for weather coverings for walls, as well as provisions for veneers and exterior trim. Wood and wood-based products used in fire-resistance-rated assemblies must also comply with Chapter 7. Wood and wood-based products used as interior finish on walls, ceilings, and floors must comply with Chapter 8. Wood roof coverings are regulated in Chapter 15.

2301.2 General design requirements. There are three basic design methods permitted for wood structures—allowable stress design in accordance with Section 2306, load and resistance factor design in accordance with Section 2307, and prescriptive conventional light-frame construction in accordance with Section 2308. Lateral design requirements for wood structures are covered in Section 2305. Regardless of the design method used, the general design and construction requirements in Section 2304 apply as well. As an alternative to the prescriptive conventional construction provisions of Section 2308, an exception allows wood-frame buildings to be designed in accordance with the AF&PA Wood Frame Construction Manual (WFCM) which is now referenced in Section 2309. The 2015 IBC references the 2015 edition of the WFCM.

The first two design methods, ASD under Section 2306 and LRFD under Section 2307, are engineering methods, and the third method under Section 2308 is a prescriptive method. Engineering methods require the structure to be designed and detailed to resist all the applicable loads prescribed in Chapter 16 and ASCE 7. In contrast, the prescriptive provisions are essentially a collection of rules that anyone may follow without calculating any loads. If all of the rules are followed, the resulting structure is deemed to comply with the intent of the code. See Section 2308 for further discussion of the prescriptive conventional wood-frame construction provisions.

The three design methods are discussed in more detail below.

1. Allowable stress design. Allowable stress design uses the load combinations of Section 1605.3 for the determination of strength. The designer has two options for ASD load combinations: the basic load combinations in Section 1605.3.1, or the alternative basic load combinations in Section 1605.3.2. It should be noted that all deformations and drifts from seismic load effects are determined using strength level forces in accordance with the requirements of Section 12.12 of ASCE 7. The seismic load effect, E, is not multiplied by 0.7 (or divided by 1.4) for determination of deformation and drift. The so-called “special seismic load combinations” were deleted from the 2009 IBC. They are now called “Seismic Load Effect Including Overstrength Factor” in Section 12.4.3 of ASCE 7. These load combinations are used for collector design for all structures assigned to Seismic Design Categories C through F, except for wood-frame buildings braced entirely by light-frame shear walls. In this case, the collectors must be designed to resist the diaphragm forces in ASCE 7 Section 12.10.1.1. See Exception 2 in Section 12.10.2.1 of ASCE 7. Refer to the discussion of Section 1605 and ASCE 7 Section 12.4.3 for general application of the seismic load combinations with overstrength factor.

2. Load and resistance factor design. Beginning with the 2006 IBC, the LRFD procedure is included as part of the 2005 edition of the NDS, so Section 2307 now references the NDS. The applicable load combinations for LRFD and strength design are in Section 1605.2. Note that wood structures designed using the LRFD procedure are subject to the same general and lateral force design provisions as structures designed using ASD. Refer to the NDS for specific requirements, strength properties, and design values related to the LRFD procedure.

3. Conventional light-frame wood construction. The prescriptive conventional construction provisions are in Section 2308. Perhaps the most important aspect of the prescriptive conventional construction provisions is the restrictions and limitations in Section 2308.2. A building or any portion of a building that does not conform to the limitations must be designed to resist all applicable loads of Chapter 16 in accordance with one of the engineering methods. See also Sections 2308.1.1 and 2308.8 for design of portions and elements. As with the engineering methods, buildings designed by the prescriptive conventional construction provisions of Section 2308 are also required to comply with the general construction requirements in Section 2304. For clarification, a new definition of conventional light-frame construction was added to the 2009 IBC. The term light-frame construction is also defined in Section 202.

4. AWC WFCM. Section 2309 permits the use of the 2015 Wood Frame Construction Manual (WFCM) for design of wood-frame buildings in Risk Category I or II subject to the limitations and load restrictions of WFCM Section 1.1.3. The section imposes a mean roof height of 33 feet and a maximum building width or length of 80 feet. Other limitations on floor, roof, and wall framing systems are provided in Chapters 2 and 3 of the standard. Structural elements that exceed the imitations must be designed in accordance with accepted engineering practice in accordance with other provisions of Chapter 23 and the NDS.

5. Log structures. The IBC references ICC 400 for the design and construction of log structures. Because ICC 400 gives base values and references AF&PA NDS-05 for design, either ASD or LRFD procedures can be used. Section 2303.1.10 references ASTM D 3957 for determining stress grades for structural log members and requires identification by grade stamps or certificates of inspection.

2301.3 Nominal sizes. Nominal lumber sizes, for example, 2 inches by 4 inches, are the sizes usually referred to or specified in this chapter. Actual or net dimensions, which are less than nominal dimensions, are used in structural calculations to determine member section properties, actual stresses, and strength properties. The nominal and actual sizes of dimension lumber are established by Department of Commerce (DOC) Voluntary Product Standard PS 20, American Softwood Lumber Standard. The edition referenced in previous editions of the IBC is DOC PS 20-99. The most current edition is DOC PS 20-05, which is referenced in the 2012 and 2015 IBC. The PS 20 standard is available from the National Institute of Standards and Technology (NIST). The nominal size, dressed size, and section properties for sawn lumber and glued laminated timber are given in Tables 1A, 1B, 1C, and 1D of the NDS Supplement. See Section 202 for the definition of nominal size lumber.

Section 2302 Definitions

The specific terms related to wood construction listed in Section 2302 are defined in Section 202 to clarify their meaning. Many of the terms are to clarify terms used to describe elements of the lateral-force-resisting systems.

New definitions for prefabricated wood I-joist, structural composite lumber, laminated veneer lumber, parallel strand lumber, and some modifications to the definition of composite panels, oriented strand board, and plywood were made to coordinate the terms in the IBC with the NDS. Some changes were made to the definition of treated wood in the 2009 IBC. Treated wood is a general term with two specific types defined: fire-retardant-treated wood and preservative-treated wood. Also, the definition of termite-resistant wood was expanded to include Alaska yellow cedar and Western red cedar.

Some specific terms are discussed below.

COLLECTOR. The collector collects shear from the (floor or roof) diaphragm and delivers it to vertical lateral-force-resisting elements such as shear walls. The term drag strut is a colloquial expression that means collector. Collectors can also be used to transfer forces within a diaphragm. The IBC recognizes both horizontal and sloped (or nearly horizontal) diaphragms and as such the definition of collector applies to both horizontal and sloped diaphragms. The word horizontal was used to differentiate diaphragms from vertical lateral-force-resisting elements such as a shear wall. In the legacy codes, the terms horizontal diaphragm and vertical diaphragm were often used to describe diaphragms and shear walls. The newer terminology in the IBC is to simply use the terms diaphragm and shear wall.

CONVENTIONAL LIGHT-FRAME CONSTRUCTION. For clarification, a new definition of conventional light-frame construction was added to the 2009 IBC. It is a type of construction whose primary structural elements such as walls, floors, and roof are formed by repetitive wood-framing members constructed in accordance with Section 2308. The term light-frame construction is also defined in Section 202.

CROSS-LAMINATED TIMBER. A new definition in the 2015 IBC for a wood-based product called Cross-Laminated Timber (CLT) was added to Chapter 2 and the new manufacturing standard ANSI/APA PRG 320 is referenced in Chapter 23 and added to Chapter 35. First developed in Europe 20 years ago, CLT has been used extensively in Europe as large-section structural timber. A new North American product manufacturing standard, ANSI/APA PRG 320-2011, Standard for Performance-Rated Cross-Laminated Timber, provides requirements and test methods for qualification and quality assurance for performance-rated cross-laminated timber, which is manufactured from solid-sawn lumber or structural composite lumber.

DIAPHRAGM, UNBLOCKED. A diaphragm is a horizontal or nearly horizontal (sloped) structural element that transmits lateral forces to the vertical resisting elements (shear walls or frames) of the lateral-force-resisting system. An unblocked diaphragm has edge nailing at the supported edges only. In an unblocked diaphragm, the continuous panel joint is unblocked. In a blocked diaphragm, all sheathing panel edges are supported by framing members or solid blocking members. Blocked diaphragms have continuity between the sheathing panel edges and therefore have significantly less deflection than unblocked diaphragms because of the stiffness developed by the continuity at the blocked panel edges.

The definition of a diaphragm existed in Chapter 23 of the 2000 IBC but not in Chapter 16. In the 2003 IBC, the definition of a diaphragm was deleted from Chapter 23 and added to Chapter 16 along with the various types and elements of diaphragms. Chapter 23 only has a definition for unblocked diaphragm. See Section 1602 for additional terms pertaining to diaphragms. It is important to note that the definition of diaphragm also includes horizontal bracing systems.

In terms of distribution of seismic or wind forces, the stiffness of the diaphragm relative to the stiffness of the vertical lateral-force-resisting elements (shear walls or frames) is the important parameter by which to classify the diaphragm. Diaphragms can be flexible, rigid, or semi-rigid. Refer to the discussion of Section 12.3 of ASCE 7 for further discussion of diaphragm classifications.

ENGINEERED RIM BOARD. A new definition in the 2015 IBC for engineered wood rim board was added to Chapter 2 and two new standards are referenced in Chapter 23 and added to Chapter 35. Engineered rim board is an important structural element in many engineered wood floor applications where both structural load path through the perimeter member and dimensional change compatibility are important design considerations. Two new standards address products intended for engineered wood rim board applications. Both ANSI/APA PRR 410 and ASTM D7672 address the fundamental requirements for testing and evaluation of engineered rim board. ASTM D7672 is applicable to determination of product-specific rim board performance (i.e., structural capacities) for engineered wood products that may be recognized in manufacturer’s product evaluation reports. The PRR 410 standard also includes performance categories for engineered wood products used in engineered rim board applications. Under PRR 410, products are assigned a grade based on performance category (e.g., categories based on structural capacity) and bear a mark in accordance with the grade.

GRADE (LUMBER). References Department of Commerce (DOC) standard PS 20, American Softwood Lumber Standard, listed under DOC in Chapter 35. Voluntary Product Standard PS 20 is published by the U.S. Department of Commerce and establishes standard sizes and requirements for development and coordination of the lumber grades of the various species, the assignment of design values when called for, and the preparation of grading rules applicable to each species. The PS 20 standard is available from the National Institute of Standards and Technology (NIST), which administers the Department of Commerce Voluntary Product Standards program or the American Lumber Standards Committee website at www.alsc.org.

NATURALLY DURABLE WOOD. Although naturally durable wood is not listed in Chapter 23, it is defined in Chapter 2. Naturally durable wood is the heartwood of a durable listed species. Naturally durable wood includes decay-resistant woods, which are redwood, cedar, black locust, and black walnut, and termite-resistant woods, which are redwood and Eastern red cedar. The definition of termite-resistant wood was expanded in the 2009 IBC to include Alaska yellow cedar and Western red cedar, which were recently determined to be termite resistant in a study involving the Formosan subterranean termite. Note that only the heartwood of redwood and red cedar are both decay and termite resistant.

PERFORMANCE CATEGORY. A new term “performance category” in the 2012 IBC reflects the latest versions of the DOC PS 1 and PS 2 standards, which use terminologies of bond classification to reference glue type and performance categories to reference the thicknesses tolerance consistent with the nominal panel thicknesses in the IBC. The performance category value is the “nominal panel thickness” or “panel thickness.” See Section 2303.1.4.

PREFABRICATED WOOD I-JOIST. This definition was added to the 2006 IBC for structural members manufactured of sawn or structural composite lumber flanges and wood structural panel webs bonded together with exterior exposure adhesives in the form of an “I” cross-sectional shape.

SHEAR WALL. A shear wall is a vertical lateral-force-resisting element that is designed to resist lateral seismic and wind forces parallel to the plane of a wall. The definitions of perforated shear wall and perforated shear wall segment were consolidated under the shear wall definition in the 2006 IBC. A perforated shear wall is a wood structural panel sheathed wall with openings that has not been specifically designed and detailed for force transfer around the openings. Perforated shear wall segments are sections of the shear wall with full-height sheathing that meet the height-to-width ratio limits specified in SDPWS. Substantial portions of Section 2305 were deleted in the 2009 IBC and the section references the SDPWS, which is also referenced in the 2012 and 2015 IBC. The detailed requirements for design of shear walls in wood-frame structures are now in the SDPWS standard and not the IBC.

STRUCTURAL COMPOSITE LUMBER. The definition of structural composite lumber (SCL) was added to the 2006 IBC. The two main types are laminated veneer lumber (LVL), which is composed of thin wood veneer sheet elements, and parallel strand lumber (PSL), which is composed of wood strand elements. LVL is the most widely used SCL product. Both LVL and PSL have wood fibers that are primarily oriented along the length of the member. Some common examples of structural composite lumber are laminated veneer lumber (LVL) and parallel strand lumber (PSL) such as Microllam® LVL Beams or Parallam® PSL Beams manufactured by Weyerhaeuser, VERSALAM® LVL manufactured by Boise Cascade, and LP® SolidStart® LVL manufactured by Louisiana Pacific (LP). See also discussion of Section 2303.1.9.

TREATED WOOD. Includes both fire-treated wood and wood treated to resist decay and termites. The definition was reorganized in the 2009 IBC so that treated wood is now a general term with two specific types defined: fire-retardant-treated wood and preservative-treated wood. Additional required information for treated wood was added, the definition of preservative-treated wood was revised, and the definition for fire-retardant-treated wood was added. The ability of the wood to extinguish itself once the source of ignition is consumed or removed is an important element of the material. The definition of preservative-treated wood was not really a definition but a reference to a code section. In addition, preservative-treated wood will not reduce susceptibility to all insects, only those that actually eat the wood. Section 2303.2 requires testing of fire-retardant-treated wood. The test must be continued 20 minutes beyond the 10 minutes required to establish the flame spread without any significant progressive combustion.

WOOD STRUCTURAL PANEL. Wood structural panels are manufactured from veneers, wood strands, or wafers, or a combination thereof, bonded together with waterproof synthetic resins. The terms composite panels, oriented strand board (OSB), and plywood were added in the 2003 IBC under the definition of wood structural panel for clarification. When used for structural purposes such as siding, roof and wall sheathing, subflooring, diaphragms, and built-up members, wood structural panels must conform to the requirements for their type in DOC PS 1 or PS 2. DOC PS 1 covers plywood and DOC PS 2 covers wood-based structural panels. DOC Voluntary Product Standards are developed under procedures published by the Department of Commerce in Title 15 Code of Federal Regulations Part 10. The purpose of these standards is to establish nationally recognized requirements for products and to provide all concerned interests with a basis for common understanding of the characteristics of the products. The National Institute of Standards and Technology (NIST) administers the Voluntary Product Standards program. The DOC PS 1 and PS 2 standards are available from the NIST global standards information program website at www.gsi.nist.gov.

Section 2303 Minimum Standards and Quality

2303.1 General. Structural lumber, wood structural panels, and other wood products are highly variable in strengths and other mechanical properties. The code requires that these materials (defined in the first paragraph of this section) conform to the applicable standards and grading rules specified in the code. Furthermore, the code requires that they be identified by a grade mark or be accompanied by a Certificate of Inspection issued by an approved agency. The grade mark is also required to be placed on the material by an approved agency (see labeling—Chapter 17). The proper use of a wood structural member cannot be determined unless it has been properly identified as to species and grade. Counterfeit grade stamps do occasionally appear on lumber in the field, and it is important that designers and enforcement personnel be familiar with the grade-approved stamps. Examples of grade marks are shown in Figure 2303-1.

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Figure 2303-1 Typical lumber grade stamps.

Joist hangers are subject to the applicable requirements of this chapter as well as the requirements in Chapters 17 and 22.

2303.1.1 Sawn lumber. Lumber references the voluntary standard, American Softwood Lumber Standard, PS 20. The 2015 IBC references the 2005 edition of DOC PS 20. The current edition is PS 20-10 and is available from the American Lumber Standard Committee at www.alc.org. The NDS references ASTM D 1990, ASTM D 245, ASTM D 2555, the Wood Handbook,⁵ and PS20 for the classification, definition, methods of grading, and development of design values for lumber. The NDS also references the various standard grading rule documents such as NLGA, NELMA, NSLB, SPIB, WCLIB, and RIS.

In the 1991 NDS, changes in design values for dimension lumber were based on the In-Grade Testing program conducted by the North American forest products industry. The program was carried out over an eight-year period and involved the destructive testing of 70,000 pieces of lumber from 33 different species groups. A new test method standard, ASTM D4761, was also developed for the mechanical test methods used in the program. In addition, the standard practice, ASTM D1990, was developed for procedures used to establish design values for visually graded dimension lumber from test results obtained from in-grade test programs.

2303.1.1.1 Certificate of inspection. Certification is an acceptable alternative to a grade mark from both U.S. and Canadian grading agencies certified by the ALSC. The code allows certain types of structural lumber to have a certificate of inspection instead of a grade mark. A certificate of inspection is acceptable for precut, remanufactured, or rough-sawn lumber and for sizes larger than 3 inches (76 mm) nominal in thickness. It is industry practice to place only one label (grade mark) on a piece of lumber, which may be removed on precut and remanufactured lumber. Each piece of lumber is graded after it has been cut to a standard size. The grade of the piece is determined based on its size, number, and location of strength-reducing characteristics. Therefore, one log of timber may produce lumber of two or more different grades. It is also industry practice not to label lumber having a nominal thickness larger than 3 inches, or rough-sawn material where the label may be illegible. A certificate of inspection from an approved agency is acceptable instead of the label for these types of lumber. The certificate should be filed with the permanent records of the building or structure. If defects exceeding those permitted for the grade allegedly installed are visible, then a certified grader would be able to determine that the wood is definitely not of a suitable grade. To determine if the wood in question is definitely of a suitable grade, the grader must inspect all four faces of the piece.

2303.1.1.2 End-jointed lumber. Approved end-jointed or edge-glued lumber is presumed to be equivalent to solid-sawn lumber of the same species and grade. The NDS permits the use of end-jointed lumber of the same species and grade. When finger-jointed lumber is marked “STUD USE ONLY” or “VERTICAL USE ONLY,” such lumber is limited to use where bending or tension stresses are of short duration. The use of the term approved is intended to convey the need for quality control during the production of these glued products, and also to establish the qualification tests for the type of end joint used. Joints are tested for strength and for durability, and adhesive manufacturers test their products for durability. End-joined lumber can be manufactured in different ways. Finger joints or butt joints are typical methods of joinery. Adhesives used in finger-jointed lumber are of two basic types, depending on whether they are to be used for members with long-duration bending loads like floor joists or short-duration bending and tension loads like wall studs. To add elevated-temperature performance requirements for end-jointed lumber adhesives intended for use in fire-resistance-rated assemblies, end-jointed lumber manufactured with adhesives must meet the new requirements and be designated as “Heat Resistant Adhesive” or “HRA” on the grade stamp.

An example of end-jointed lumber is shown in Figure 2303-2, which illustrates a finger-jointed end joint.

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Figure 2303-2 Finger-jointed end joint.

2303.1.2 Prefabricated wood I-joists. A new definition for prefabricated wood I-joist was added to the 2006 IBC. Wood “I”-joists are structural members manufactured of sawn or structural composite lumber flanges and wood structural panel webs bonded together with exterior exposure adhesives in the form of an “I” cross-sectional shape. The shear, moment, and stiffness capacities of prefabricated wood I-joists must be established in accordance with ASTM D 5055. This standard also specifies that application details, such as bearing length and web openings, are to be considered in determining the structural capacity. Wood I-joists are manufactured of sawn or structural composite lumber flanges and structural panel webs, and are bonded together with exterior adhesives to form an “I” cross section. Wood I-joists are structural members typically used in floor and roof construction. The standard requires I-joist manufacturers to employ an independent inspection agency to monitor the procedures for quality assurance. The standard specifies that proper installation instructions accompany the product to the job site. The instructions are required to include weather protection, handling requirements and, where required, web reinforcement, connection details, lateral support, bearing details, web hole-cutting limitations, and any special conditions.

2303.1.3 Structural glued laminated timber. Glued laminated timbers are manufactured in accordance with ANSI/AITC A 190.1, which references several other AITC standards. ASTM D 3737 is the standard method for establishing allowable stresses for glued laminated timber. See AITC Timber Construction Manual⁶ for more information on and construction of glue laminated timber.

2303.1.4 Structural glued cross-laminated timber. This new section in the 2015 IBC references the new manufacturing standard ANSI/APA PRG 320 for a wood-based product called Cross-Laminated Timber (CLT). A new definition was added to Chapter 2 and the new standard was added to Chapter 35. CLT products were developed in Europe 20 years ago where they have been used extensively as large-section structural timber. The new North American product manufacturing standard, ANSI/APA PRG 320-2011, Standard for Performance-Rated Cross-Laminated Timber, provides requirements and test methods for qualification and quality assurance for performance-rated cross-laminated timber. CLT products are manufactured from solid-sawn lumber or structural composite lumber.

2303.1.5 Wood structural panels. Wood structural panels must conform to the Department of Commerce voluntary product standards PS-1 or PS-2. PS-1 is the product standard for Construction and Industrial Plywood, and PS-2 is the Performance Standard for Wood-Based Structural-Use Panels such as oriented strand board (OSB). Plywood, oriented strand board, and composite panels are all considered wood structural panels and are covered under US DOC PS-1 and PS-2. Other than in the definition of wood structural panel in Section 2302, the code does not differentiate between the different types of wood structural panels such as composite panels, oriented strand board, or plywood. For example, the shear wall and diaphragm Tables 2306.2(1), 2306.2(2), and 2306.3(1) refer to wood structural panels. Wood structural panels include all-veneer plywood; composite panels consisting of a combination of veneer and wood-based material; and mat-formed panels that contain wood fiber only, such as OSB and waferboard. The primary distinction between OSB and waferboard is that the wood fibers in OSB are generally all oriented in the same direction, whereas the wood fibers in waferboard are oriented randomly in all directions within the plane of the board. Plywood is defined as panels made by cross laminating three or more wood veneers and joining the veneers together with glue. DOC PS-1 has been developed as a guide and specification for the manufacturing of plywood intended for industrial and construction uses. DOC PS-2 is a consensus standard that has been developed as a specification of product performance for various grades of wood structural panels. Provisions in DOC PS-1 and DOC PS-2 define the requirements for structural-grade panels and give the requirements for sheathing and single floor-grade wood structural panels.

A new APA standard for wood siding was added to the 2012 IBC, ANSI/APA PRP 210-8 Standard for Performance-Rated Engineered Wood Siding. Based on APA’s PRP-108 Performance Standards and Policies for Structural-Use Panels, ANSI/APA PRP-210 provides requirements and test methods for qualification and quality assurance for performance-rated engineered wood siding intended for use in construction applications as exterior siding. There were previously no American National Standards covering these products.

A new term, “performance category,” in the 2012 IBC reflects the latest versions of the DOC PS 1 and PS 2 standards, which use terminologies of bond classification to reference glue type and performance categories to reference the thicknesses tolerance consistent with the nominal panel thicknesses in the IBC. The performance category value is the “nominal panel thickness” or “panel thickness.”

The user is encouraged to obtain the referenced standards for additional technical information on wood structural panel products. It must be emphasized that the proper fastening of wood structural panels to the supporting structural frame is very important. The nailing schedules contained in the code, referenced standards, and the manufacturer’s recommendations must be strictly observed for good performance. The correct nail size and spacing is necessary to achieve the design strength and performance of the wood structural panel system.

Wood structural panels manufactured in accordance with DOC PS 1 and DOC PS 2 are inspected and labeled to certify compliance by an approved agency. Examples are the American Plywood Association and Timber Engineering Company (TECO). The label identifies the grade and span ratings of the product. The inspection agencies maintain a continuous monitoring program designed to produce products that meet or exceed the applicable product standard. A number of tests, including deflection measurements, are required.

Besides the grades cited above, wood structural panels are also classified by exposure type:

1. Exterior—exterior type with a 100-percent waterproof glue line. Only the higher grades of veneers are allowed in exterior grades. Exterior-rated panels are suitable for continuous exposure to weather.

2. Exposure 1—interior type made with waterproof exterior glue. Exposure 1–rated panels are suitable for extended construction exposure. The lower grades of veneers or strands used in the backs and interiors of Exposure 1 panels can affect the glue-line performance and cause delamination/deterioration during continuous exposure to weather.

3. Exposure 2—interior type made with interior glue. Exposure 2–rated panels are not suitable for exposure to weather.

Exposure ratings for APA structural wood panels designated in APA trademarks are based on bond classification. The exposure ratings are Exterior, Exposure 1, Exposure 2, or Interior. Exterior panels have a fully waterproof bond and are designed for applications subject to permanent exposure to the weather or to moisture. Exposure 1 panels have a fully waterproof bond and are designed for applications where long construction delays may be expected prior to providing protection, or where high moisture conditions may be encountered in service. Exposure 1 panels are made with the same exterior adhesives as those used in Exterior panels. However, because other compositional factors may affect bond performance, only Exterior panels should be used for permanent exposure to the weather. Exposure 2 panels (identified as Interior type with intermediate glue under PS 1) are intended for protected construction applications where only moderate delays in providing protection from moisture may be expected. Interior panels or panels that lack further glue-line information in their trademarks are manufactured with interior glue and are intended for interior applications only.

Plywood is manufactured from more than 70 species of wood, which are divided into five groups in accordance with their stiffness and strength characteristics. Construction and industrial-panel grades are generally identified under PS-1 in terms of face veneer grade or by a name indicating an intended use, such as APA-Rated Sturd-I-Floor. The plies may be of any species listed, except for panels designated Structural I and other special-use panels, which use only Group I species. The veneer grade defines the appearance in terms of natural, unrepaired growth characteristics (knots) and the number and size of repairs that may be made during manufacturing. The highest grades are N and A. The lowest grade is D. Grade D veneers may only be used for backs and inner plies of interior-use panels. Panels are also marked as sanded, unsanded, and touchsanded.

OSB is manufactured from several species of wood. PS-2 sets forth performance requirements in terms of strength, stiffness, and durability. OSB is manufactured to meet the strength, stiffness, and durability requirements instead of being manufactured to a prescriptive recipe, as is plywood.

Wood structural sheathing and subflooring panels are classified as:

• Rated Sheathing—Exterior-Rated Sheathing—Exposure 1–Rated Sheathing—Exposure 2

• Structural I-Rated Sheathing—Exterior

• Structural I-Rated Sheathing—Exposure 1

Wood structural panels intended for single-floor construction have limited voids in the inner plies in addition to the solid face veneer to prevent indentation caused by small, concentrated loads.

Single-floor panels intended for use under carpets and resilient flooring are classified as:

• Rated Sturd-I-Floor Exterior

• Rated Sturd-I-Floor Exposure 1

• Rated Sturd-I-Floor Exposure 2

Rated Single Floor Exposure 2 Underlayment panels for use over subflooring are classified as:

• Underlayment Interior

• Underlayment Exposure 1

• C-C Plugged Exterior

Siding is manufactured as panels or as lapped siding and includes:

• 303-OL-MDO Exterior

• 303 Siding Exterior

Note that wood structural panels permanently exposed to weather, such as siding grades, must be exterior type. Panels that are interior type bonded with exterior glue (Exposure 1) are not allowed for siding applications but are allowed on the exposed underside of roof overhangs.

2303.1.6 Fiberboard. Fiberboard is a smooth textured panel made up of natural fibers such as wood or cane. Fiberboard is used primarily as an insulating board and for decorative purposes, but may also be used as wall or roof sheathing under the provisions of this section. Unlike particleboard, the cellulosic components of the fiberboard are broken down to individual fibers and molded to create the bond between the fibers. Other ingredients may be added during processing to provide or improve certain properties such as strength or water resistance, or to achieve specific surface finishes for decorative products. Fiberboard is used in most locations where panels are necessary, including wall sheathing, insulation of walls and roofs, roof decking, doors, and interior finish. Although fiberboard sheathing board may be used for shear walls [see Table 2306.3(2)], fiberboard may not be used for diaphragms. Certification of fiberboard products is performed by an approved agency. The material is generally labeled to indicate an intended use, strength values, and flame resistance where applicable.

Fiberboard may be used as roof or wall insulation, but is not intended for prolonged exposure to sunlight, wind, rain, and snow. Where fiberboard is used as roof insulation, it must be protected with an approved roof covering to prevent water saturation and subsequent delamination, and to avoid decay and destruction of the roofing bond caused by moisture. The section in the IBC pertaining to insulating roof decking was deleted from the 2006 IBC because fiberboard insulating roof decking is no longer manufactured.

Fiberboard is permitted without any fire-resistance treatment in the walls of all types of construction. When used in fire walls and fire separation walls, the fiberboard must be attached directly to a noncombustible base and protected by a tight-fitting, noncombustible veneer that is fastened through the fiberboard to the base. This will prevent the fiberboard from contributing to the spread of fire.

Fiberboard used in building construction must comply with ASTM C 208, Specification for Cellulosic Fiber Insulating Board. For several decades, the fiberboard industry supported parallel ASTM and ANSI standards. During the last revision of ASTM C 208, the differences were resolved and the Board of Directors of the American Fiberboard Association voted to discontinue support of the ANSI standard in favor of ASTM C 208. Thus, the previous ANSI/AHA A 194.1 standard was deleted in the 2006 IBC because the standard was withdrawn by ANSI and the fiberboard manufacturers no longer support it.

When used as structural sheathing, fiberboard must be identified by an approved agency. See SDPWS Table 4.3A for nominal unit capacities for wood-based panel shear walls, including structural fiberboard sheathing.

2303.1.7 Hardboard. Hardboard is used as exterior siding and in interior locations for paneling and underlayment. The code references three Composite Panel Association standards for hardboard panels.

2303.1.8 Particleboard. Particleboard is a generic term for construction panels and products manufactured from cellulosic materials, usually wood, in the form of discreet pieces and particles as distinguished from fibers. The particles are combined with synthetic resins and other binders and bonded together under heat and pressure.

Particleboard is used as underlayment, siding, and for shear walls. Particleboard used structurally for siding or shear walls must be stamped (labeled) M-S Exterior or M-2 Exterior. The “M” stands for medium density; the “2” designates the strength grade (grades range from 1 to 3); and “S” designates “special grade.” Particleboard designated M-S is medium density and has physical properties between an M-1 and M-2 designation. Both must be made with exterior glue. See SDPWS Table 4.3A for nominal unit capacities for wood-based panel shear walls, including particleboard.

Although similar in characteristics to medium-density Grade 1 particleboard, the particleboard intended for use as floor underlayment is designated “PBU” and has stricter limits on permitted levels of formaldehyde emission than those placed on Grade M particleboard. The particleboard intended for use as floor underlayment is not commonly manufactured with exterior glue, which could emit higher levels of formaldehyde than that permitted for “PSU”-grade floor underlayment by ANSI A 208.1.

Particleboard underlayment is often applied over a structural subfloor to provide a smooth surface for resilient-finish floor coverings or textile floor coverings. The minimum ¼-inch thickness is suitable for use over panel-type subflooring. Particleboard underlayment installed over board or deck subflooring that has multiple joints should have a thickness of ⅜ inch. Joints in the underlayment should not be located over the joints in the subflooring.

All particleboard underlayment with thicknesses of ¼ inch through ¾ inch should be attached with minimum 6d annular threaded nails spaced 6 inches on center on the edges and 12 inches on center for intermediate supports. See Table 2304.9.1.

2303.1.9 Preservative-treated wood. The applicable American Wood Preservers Association (AWPA) standards are cited. Different preservative treatments are used depending on whether the wood is above ground or in contact with the ground. See commentary in Section 2304.11. ICC publishes a book containing all of the AWPA standards referenced in both the 2009 IBC and the IRC.

2303.1.9.1 Identification. All wood required to be preservative treated by Section 2304.11 must be stamped (labeled) with the information listed in the section. There are no exceptions.

2303.1.9.2 Moisture content. Preservative treatments used in above-ground locations are water-borne salts. These salts may leach unless the wood is dried below a moisture content of 19 percent (i.e., dry) and covered with a protective material.

2303.1.10 Structural composite lumber. Structural composite lumber (SCL) is covered in the code because of its increasingly widespread use. Structural properties and strength capacities for SCL are set forth in manufacturers’ literature and evaluation reports by ICC. A new definition for structural composite lumber was added to the 2006 IBC for structural members manufactured using wood elements bonded together with exterior adhesives.

LVL is the most widely used SCL product. It is produced by bonding thin wood veneers together. The grain of the veneers is parallel to the long direction of the member. LVL members have enhanced mechanical properties and dimensional stability and offer a broader range in product width, depth, and length than conventional sawn lumber. LVLs are used for a variety of applications such as headers and beams, hip and valley rafters, rim board, scaffold planking, studs, flange material for prefabricated wood I-joists, and truss chords. Some common examples of laminated veneer lumber products are Microllam® LVL Beams manufactured by Weyerhaeuser, VERSALAM® LVL manufactured by Boise Cascade, and LP® SolidStart® LVL manufactured by Louisiana Pacific (LP).

PSL members consist of long veneer strands in parallel formation and bonded together with adhesive to form the finished structural section. PSL members are commonly used for long-span beams, heavily loaded columns, and beam and header applications where high bending strength is needed. A common example of a parallel strand lumber product is Parallam® PSL Beams manufactured by Weyerhaeuser.

Another type of SCL is laminated strand lumber (LSL) and oriented strand lumber (OSL), which are similar to PSL, but are manufactured from flaked wood strands that have a high length-to-thickness ratio. The primary difference between OSL and LSL is the length of strand used to fabricate them. OSL strands are shorter (up to 6 inches) than LSL strands (approximately 12 inches). The strands are combined with adhesive, and are oriented and formed into a large mat or billet and pressed. Although their strength and stiffness properties are somewhat lower than LVL and PSL members, they have good fastener-holding strength and mechanical connector performance. LSL and OSL members are used in a variety of applications, such as beams, headers, studs, rim boards, and millwork. Examples of LSL and OSL products are Ainsworth Engineered Durstrand LSL/OSL and LP® SolidStart® LSL.

Reports prepared by approved agencies or evaluation reports published by the ICC Evaluation Service may be accepted as part of the evidence and data needed by the building official to form the basis of approval of a material or product not specifically covered in the code. Such research reports supplement the resources of the building official and eliminate the need for the official to conduct a detailed analysis on every new product. Note that evaluation reports are approved under the alternative materials, design, and methods of construction provisions of Section 104.11.

2303.1.11 Structural log members. A new Section 2303.1.10 was added to the 2006 IBC to provide structural capacity and grading requirements for logs used as structural members. In the past, the design of log structures could be challenging for both designer and building official because the building code contained no specific provisions that addressed structural capacity and grading requirements for logs used as structural members. All log structures require engineering, and the structural design values were approved under the alternative materials, design, and methods of construction provisions in Section 104.11. This new section provides acceptable methods for establishing structural capacities of logs based on ASTM D3957 Standard Practices for Establishing Stress Grades for Structural Members Used in Log Buildings and specifies the requirement for a grading stamp or alternative means of identification. Between publication of the 2006 IBC and the 2009 IBC, the International Code Council (ICC) developed a new standard for log structures known as Standard on the Design and Construction of Log Structures (ICC 400-2007). The 2012 edition of ICC 400 is new standard is now referenced in Section 2301.2 of the 2015 IBC. The goal of the new standard is to provide technical design and performance criteria that will facilitate and promote the design, construction, and installation of safe, reliable structures constructed of log timbers. It is intended to be used by design professionals, manufacturers, constructors, and building and other government officials, and continue as a referenced standard in future building codes. Because ICC 400 gives base values and references the NDS for design, either ASD or LRFD can be used.

2303.1.12 Round timber poles and piles. This new section referencing the ASTM standards for round timber poles and piles was added to the 2006 IBC to coordinate the requirements with Chapter 6 of the NDS. Chapter 6 of the 2015 NDS has been updated to address changes to the ASTM standards for developing and adjusting round timber pile and round construction pole design values.

2303.1.13 Engineered wood rim board. Engineered rim board is an important structural element in many engineered wood floor applications where both structural load path through the perimeter member and dimensional change compatibility are important design considerations. This new section in the 2015 IBC references two new standards for products intended for engineered wood rim board applications and a new definition for engineered wood rim board was added to Chapter 2. The new standards ANSI/APA PRR 410 and ASTM D7672 that address the fundamental requirements for testing and evaluation of engineered rim board were added to Chapter 35. ASTM D7672 is applicable to determination of product-specific rim board performance (i.e., structural capacities) for engineered wood products that may be recognized in manufacturer’s product evaluation reports. The PRR 410 standard also includes performance categories for engineered wood products used in engineered rim board applications. Under PRR 410, products are assigned a grade based on performance category (e.g., categories based on structural capacity) and bear a mark in accordance with the grade.

2303.2 Fire-retardant-treated wood. Fire-retardant-treated wood (FRTW) is defined as plywood and lumber that has been pressure impregnated with chemicals to improve its flame-spread characteristics beyond that of untreated wood. The principal objective of impregnating wood with fire-retardant chemicals is to produce a chemical reaction at certain temperature ranges. This chemical reaction reduces the release of certain intermediate products that contribute to the flaming of wood, and also results in the increased formation of charcoal and water. Some chemicals are also effective in reducing the oxidation rate for the charcoal residue. Fire-retardant chemicals also reduce the heat release rate of the FRTW when burning over a wide range of temperatures. This section gives provisions for the treatment and use of FRTW. However, the fire-retardant chemicals are generally quite corrosive and corrosion-resistant fasteners may be required with FRTW. See Section 2304.9.5.

The effectiveness of the pressure-impregnated fire-retardant treatment is determined by subjecting the material to tests conducted in accordance with ASTM E 84, with the modification that the test is extended to 20 minutes rather than 15 minutes. Under this procedure, a flame-spread index is established during the standard 10-minute test period. The test is continued for an additional 20 minutes. During this added time period, there must not be any significant flame spread. At no time must the flame spread more than 10½ feet past the centerline of the burners. These criteria have been correlated with large-scale fire tests. Changes to this section in the 2009 IBC included the addition of three new subsections that clarify the meaning of the phrase pressure process or other means during manufacture and provide testing requirements of treatments not impregnated by a pressure process in accordance with Section 2303.2.

2303.2.1 Pressure process. Treatment using a pressure process requires a minimum pressure of 50 psi.

2303.2.2 Other means during manufacture. This section requires treatment using other means during manufacture to be an integral part of the manufacturing process.

2303.2.3 Testing. This section requires added testing of treatments not impregnated by a pressure process. Requiring equivalent performance from all sides of the wood product eliminates any concern over the orientation when it is installed. Only the front and back faces of wood structural panels need to be tested.

2303.2.4 Labeling. Each piece of FRTW must be stamped (labeled). The labeling must show the performance of the material, including the 20-minute ASTM E84 test. The labeling must state the strength adjustments, and conformance to the requirements for interior or exterior application.

The FRTW label must be distinct from the grading label to avoid confusion between the two. The grading label gives information about the properties of the wood before it is fire-retardant treated. The FRTW label gives properties of the wood after FRTW treatment. It is imperative that the FRTW label be presented in such a manner that it complements the grading label and does not create confusion over which label takes precedence.

The requirements for labeling fire-retardant-treated lumber and wood structural panels were expanded in the 2003 IBC, giving a list of specific requirements for the labeling.

2303.2.5 Strength adjustments. Several factors can significantly affect the physical properties of FRTW. These factors are the pressure treatment and redrying processes used, and the extremes of temperature and humidity that the FRTW will be subjected to once installed. The design values for all FRTW must be adjusted for the effects of the treatment and environmental conditions, such as high temperature in attic installations and humidity. The design adjustment values must be based on an investigation procedure, which includes subjecting the FRTW to similar temperatures and humidities. The procedure must be approved by the building official. The FRTW tested must be identical to that which is produced. The building official reviewing the test procedure should consider the species and grade of the untreated wood and conditioning of wood, such as drying before the fire-retardant-treatment process. A fire-retardant wood treater may choose to have its treatment process evaluated by the ICC Evaluation Services.

The FRTW is required to be labeled with the design adjustment values. These design adjustment values can take the form of factors that are multiplied by the original design values of the untreated wood to determine its allowable stresses, or new allowable stresses that have already been factored down in consideration of the FRTW treatment.

Two subsections were added to the 2003 IBC that prescribe specific strength adjustment requirements for treated wood structural panels and lumber. The effects of treatment and redrying after treatment and exposure to high temperatures and high humidities on the flexural properties of treated plywood and the design properties of treated lumber are required to be determined in accordance with ASTM standards D 5516 and D 5664. The section requires the manufacturer to publish allowable maximum loads and spans for service as floor and roof sheathing and the modification factors for roof framing for its particular treatment process. The section references the ASTM standard D 6841 to be used to evaluate the ASTM D 5664 test data.

2303.2.5.1 Wood structural panels. This section references the test standard developed to evaluate the flexural properties of fire-retardant-treated plywood that is exposed to high temperatures. Note that while the section title refers to wood structural panels, the referenced standard is limited to softwood plywood. Therefore, judgment is required in determining the effects of elevated temperature and humidity on other types of wood structural panels. The product manufacturer is required to publish the allowable maximum loads and spans for floor and roof sheathing for the particular type of treatment.

2303.2.5.2 Lumber. This section references the test standard developed to determine the necessary adjustments to design values for lumber that has been fire-retardant treated including the effects of elevated temperatures. The lumber manufacturer is required to publish the modification factors to design values.

2303.2.6 Exposure to weather, damp, or wet locations. Some fire-retardant treatments are soluble when exposed to the weather or used under high-humidity conditions. When an FRTW product is to be exposed to weather conditions, it must be further tested in accordance with ASTM D 2898. The material is then subjected to the ASTM weathering test and retested after drying. There must not be any significant differences in the performance recorded before and after the weathering test.

2303.2.7 Interior applications. When an FRTW product is intended for use under high-humidity conditions, it must be further tested in accordance with ASTM D 3201. The material must demonstrate that when tested at 92-percent relative humidity, the moisture content of the FRTW does not increase to more than 28 percent. The label must show the test results.

2303.2.8 Moisture content. FRTW contains water-borne salts that are subject to leaching. The FRTW must be dried to the specified moisture contents after treating to minimize leaching. In addition, FRTW chemicals are quite corrosive to metal fasteners. Where the moisture content of the treated wood is too high, the corrosivity of the treated wood is even higher and contributes to greater corrosion of metal fasteners.

For wood that is kiln dried after treatment (KDAT), the kiln temperatures cannot exceed that used to dry the lumber and plywood that was submitted for the tests required by Section 2303.2.2.1 for plywood and Section 2303.2.2.2 for lumber.

2303.2.9 Types I and II construction applications. Use of FRTW in Type I or II construction is limited to nonload-bearing partitions and exterior walls.

2303.3 Hardwood and plywood. Hardwood plywood and decorative plywood are not used for structural purposes. This section references the American National Standard for Hardwood and Decorative Plywood.

2303.4 Trusses. Metal-plate-connected trusses are typically constructed out of nominal dimension lumber with the metal-plate connectors placed on either the narrow or wide dimension of the lumber (4-inch by 2-inch lumber for floor trusses versus 2-inch by 4-inch lumber for roof trusses). The NDS specifies the allowable design stresses for lumber, whereas the Truss Plate Institute (TPI) National Design Standard for Metal-Plate-Connected Wood Trusses specifies how the allowable metal-plate design values and maximum stresses in the truss elements are to be determined.

This section was revised in the 2006 IBC to more clearly define the requirements pertaining to metal-plate-connected wood trusses to achieve consistency with current design practice. Additional editorial and organizational changes were made in the 2009 IBC by the Wood Truss Council of America and the Structural Building Components to improve the organization and language of the provisions. The section clarifies the requirements pertaining to metal-plate-connected wood trusses to be consistent with the current industry practice and eliminates confusion regarding truss submittal requirements. The provisions include general design requirements; specific and detailed requirements for truss design drawings; requirements for truss member permanent bracing; a definition of the truss designer and truss design drawing seal and signature requirements; provisions for truss placement diagrams; specific requirements for the requirements for the truss submittal package; and specific truss anchorage requirements. The truss submittal package is part of the construction documents, which are part of the submittal documents required by Section 107.1. The term construction documents is defined in Section 202.

Adequate bracing for trusses is critical. Lateral bracing requirements (e.g., brace points, bracing size, or strength and stiffness) should be specified by the truss designer. Methods of permanent bracing are described in Section 2304.1.2. Temporary bracing should be left in place until permanent bracing is installed. All lateral bracing must be installed as assumed in the truss design so that the truss will have the same structural capacity for which it was designed. In any case, the individual truss member continuous lateral bracing locations are to be shown on the truss design drawings. See Section 2303.4.1.1, Item 14.

Permanent bracing must be installed in compliance with the truss industry’s permanent bracing standard details that follow sound engineering practice. These details are usually provided by the component manufacturers to the building design professional as the projects are being designed. The Building Component Safety Information (BCSI 1-08) Guide to Good Practice for the Handling, Installing & Bracing of Metal-Plate-Connected Wood Trusses is a booklet produced by the Truss Plate Institute (TPI) and the Wood Truss Council of America (WTCA). It is the truss industry’s new and improved guide for job-site safety and truss performance and replaces the HIB-91 booklet from TPI. In 2008, the WTCA changed its name to Structural Building Components Association (SBCA). The BCSI 1-08 publication is available from the SBCA website at www.sbcindustry.com.

The truss design drawings and specifications must be prepared by a registered design professional and must be provided to and approved by the building official prior to installation in the structure. In general, the regulations that govern registered design professionals are professional practice acts regulated by state law.

Note that the items listed in the code are the minimum requirements. The intent of adding the terms truss design drawings and truss placement diagram in the 2006 IBC was to minimize confusion that exists in the construction industry between a variety of terms that may mean the same thing, such as construction documents, shop drawings, and so on. The term truss placement diagram is used by the truss industry and is very specific. These terms are intended to provide better clarity where truss submittals are concerned.

The truss design drawings are required to show permanent bracing, and the truss designer generally is the most knowledgeable concerning the required strength, stiffness, and location of the bracing necessary to prevent buckling of the truss members. Bracing is necessary to resist buckling of the compression webs and chord under maximum gravity loads, as well as the uplift condition of dead load plus wind. Bracing may be necessary on the bottom chord at the first interior panel point to resist the wind uplift. The truss designer should specify either the strength and stiffness of braces, or the member size and grade of the braces (e.g., 2 by 4 DF #2) at the specified locations.

The truss designer should furnish complete calculations substantiating the size of all members and connector plate sizes. The truss calculations should indicate the combined stress index for members subjected to combined stresses from bending and axial compression and tension. The combined stress index should be less than one. The calculations are generally performed with a computer program; therefore, documentation may be required by the building official that substantiates the basis of the computer program used.

The anchorage requirements in Section 2303.4.4 was added in the 2006 IBC to clarify that the transfer of all design loads through the building structure and the connections of the trusses to the supporting structure to resist code-prescribed loads is the responsibility of the registered design professional of the building.

In the 2006 IBC, several sections pertaining to trusses were relocated from Sections 2308 to 2303.4 for clarification and consolidation, most notably Section 2303.4.5 pertaining to alterations to trusses. Truss members cannot be altered without concurrence by a registered design professional. In most cases, altering pre-engineered trusses requires additional engineering. Additional loading, such as installation of new mechanical equipment, requires engineering to verify that the trusses can adequately support additional loads.

There were changes to the inspection and quality-assurance requirements for metal-plate-connected wood trusses in the 2009 IBC. Section 2303.4.6 requires metal-plate-connected wood trusses to meet the quality assurance requirements of the 2007 TPI 1 standard and be inspected at the job site under Section 110.4, which gives the building official the authority to accept reports from approved inspection agencies. Trusses not manufactured under TPI 1 or in accordance with a referenced standard that provides quality control under the supervision of a third-party quality control agency must comply with Sections 1704.2.5 and 1705.5, which require an approved fabricator or special inspection during fabrication.

The 2009 and 2012 IBCs reference ANSI/TPI 1–2007, National Design Standard for Metal-Plate-Connected Wood Trusses. The 2015 IBC references the new 2014 edition of the Truss Plate Institute TPI 1 standard.

2303.4.1 Design. Wood trusses must be designed in accordance with accepted engineering practice, which is generally governed by state laws that regulate professional engineering. Truss members are permitted to be joined by any acceptable method.

2303.4.1.1 Truss design drawings. The section prescribes what information is to be provided on truss drawings. Truss design drawings are to be approved by the building official prior to installation, and the design drawings are to be on the job site. Note that where required by the building official or state law, truss deign drawings do need to be stamped and signed by the truss engineer.

2303.4.1.2 Permanent individual truss member restraint. Where permanent bracing or restraint is required it should be indicated on the truss design drawings and meet the methods prescribed in the section.

2303.4.1.3 Trusses spanning 60 feet or greater. Trusses spanning 60 feet or more require an RDP to design the temporary and permanent bracing. Note that a similar requirement in Section 2211.3.3 applies to cold-formed steel trusses spanning 60 feet or more.

2303.4.1.4 Truss designer. In general, the truss designer is a specialty engineer that works for the truss manufacturer and is not the same as the RDP responsible for the overall building design. Section 2303.4.1.4.1 specifies what documents need to be stamped and signed by the truss engineer.

2303.4.2 Truss placement diagram. The truss placement diagram is used in the field to facilitate proper installation of the trusses. Note that unlike truss design drawings, truss placement diagrams are not required to be stamped and signed by the truss engineer.

2303.4.3 Truss submittal package. The section describes what documents are required to be included in the truss submittal package.

2303.4.4 Anchorage. The requirement in this section is an essential ingredient in providing a complete and continuous load path as required by Section 1604.4. The design of the anchorage required to transfer loads (gravity and lateral) from each truss to the supporting structure is the responsibility of the RDP. This is most critical for lateral loads where forces must be transferred from the roof diaphragm to the collectors and supporting shear walls, but it is also important for gravity loads. To illustrate a simple case, if the trusses are spaced at 24 inches on center and the supporting studs are spaced at 16 inches on center, some trusses will occur between studs. Depending on the roof load, such as where high snow loads occur, the double top plate may not be adequate to span between studs and support the truss reaction. In this case, the simple solution is for the RDP to require a stud under each truss. Another example is multi-ply girder trusses with very high reactions. In this case, the simple solution is for the RDP to require a stud under each ply in the girder truss.

2303.4.5 Alterations to trusses. Obviously truss members should not be cut, notched, drilled, spliced, or altered in any way without approval of the engineer. Any addition of loads from HVAC equipment, piping, additional roofing, and so on should be reviewed and approved by the engineer to ensure that the trusses are capable of supporting additional loading.

2303.4.6 TPI 1 specifications. The design, manufacture, and quality assurance of metal-plate-connected wood trusses are to be in accordance with the TPI 1 standard, National Design Standards for Metal-Plate-Connected Wood Truss Construction. The 2015 references the 2014 edition of TPI 1, which is available from the Truss Plate Institute at www.tpinst.org.

2303.4.7 Truss quality assurance. The TPI 1 standard includes quality-assurance procedures for truss manufacturers. Trusses not manufactured in accordance with the TPI 1 standard or another approved standard that provides quality control under the supervision of a third-party quality control agency require special inspection unless the manufacturer is an approved fabricator in accordance with Section 1704.2.5.

2303.5 Test standard for joist hangers and connectors. In previous editions of the IBC, this section referenced Section 1711, which was deleted entirely in the 2015 IBC because the standard referenced (ASTM D1761-06) no longer contains provisions for testing of joist hangers and these provisions were relocated to ASTM D7147. The section now directly references ASTM D7147, which includes sampling and evaluation criteria as well as further refinements regarding quality of test materials, adjustments for variation in test materials, and limits on design values with materials other than those tested. In addition, because ASTM D7147 is specific to joist hangers used in wood construction and contains provisions that go beyond testing, the reference was appropriately relocated to Chapter 23. The remainder of Section 1711 is now found in Section 1504 because it is applicable to performance requirements for roof coverings and assemblies.

2303.6 Nails and staples. This section references ASTM F1667 for driven fasteners such as nails, spikes, and staples. Bending yield strength requirements for nails are provided. The bending yield strength requirements are those used in the NDS lateral strength tables, the IBC and IRC fastener schedules, as well as model code evaluation reports. These strengths are standardized within the nail industry for engineered fasteners and are set forth in ASTM F 1667 Specification for Driven Fasteners: Nails, Spikes and Staples. A code change to the 2006 IBC added the shank length and diameter in parentheses in the tables for the various types of nails used in wood connections with the intent of emphasizing the nail size, not just the penny weight designation. See a more complete discussion of fasteners for wood construction under Section 2304.9.1.

2303.7 Shrinkage. Because wood shrinkage is a concern, two new sections were added to the 2003 IBC pertaining to shrinkage. Section 2303.7 requires the designer to consider the effects of cross-grain dimensional changes (shrinkage) in the vertical direction that can occur in lumber that was fabricated green. See Sections 2.7 and 2.11 of PS 20 for definitions of dry and green lumber: Dry lumber is lumber of less than nominal 5-inch thickness that has been seasoned or dried to a maximum moisture content of 19 percent. Green lumber is lumber of less than nominal 5-inch thickness that has a moisture content in excess of 19 percent. For lumber of nominal 5-inch or greater thickness (timbers), green is defined in accordance with the provisions of the applicable lumber grading rules. See also Section 2304.3.3 for specific requirements related to shrinkage.

Section 2304 General Construction Requirements

2304.1 General. The requirements of Section 2304 are general and apply to all design methods, ASD, LRFD, and the conventional construction provisions.

2304.2 Size of structural members. Net dimensions of structural lumber are set forth in NDS. Refer to Tables 1A, 1B, 1C, and 1D of the NDS Supplement for nominal and dressed lumber sizes and section properties of sawn lumber and glued laminated timber members.

2304.3 Wall framing. Interior and exterior walls must be framed in accordance with conventional construction provisions unless a specific design is provided. Even most engineered wood-frame buildings have portions, such as walls, that are framed in accordance with the prescriptive conventional construction provisions of the code.

2304.3.1 Bottom plates. Sill plates must be at least nominal 2x even if design calculations show that a smaller size is acceptable. This requirement applies both to engineered (designed) structures and those constructed in accordance with the prescriptive conventional construction provisions.

2304.3.2 Framing over openings. Windows, doors, air-conditioning units, and other service equipment require openings to be provided in wood-stud walls and partitions. Loads imposed above these openings must be transferred by a structural element above the opening to supports on both sides of the opening and then to a load-bearing wall or partition. In most wood-frame structures, these structural elements are composed of two pieces of 2-inch dimensional lumber plus a spacer or plate, or solid 4x or 6x members. These elements must be fastened in accordance with Table 2304.9.1. Based on the span and the loading, some openings may require trusses, laminated veneer lumber, glued-laminated members, or even steel beams. Headers may be engineered or selected from Tables 2308.9.5 and 2308.9.6. Although these tables only have multiple 2x members, 4x and 6x headers are more common in the western regions of the United States. Header tables are also published in a number of other technical documents. For example, Western Wood Products Association (WWPA) publishes Tech Note No. 6, Design Load Tables for Solid Sawn Lumber Beams and Headers—Single 4x and 6x. All other headers not designed by span tables must be engineered in accordance with Section 2301.2. In all cases, headers and their supports must be adequate to support the imposed loads (see Figure 2304-1).

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Figure 2304-1 Headers over wall openings.

2304.3.3 Shrinkage. This new section was added to the 2003 IBC and imposes requirements for considering wood shrinkage. Wood-framed walls and bearing partitions cannot support more than two floors and a roof unless an analysis shows that there will be no adverse effects on the structure that are due to excessive shrinkage or differential movement caused by shrinkage. The provisions in this section were originally in the UBC to take into account the cumulative effects of shrinkage for horizontal wood-frame members (joists and stud wall plates) and the possible adverse effects on plumbing and mechanical systems. There have been cases reported in multistory wood-frame projects where shrinkage was not considered in the building design, and shrinkage of the framing caused plumbing breaks in the stud walls. Although some designers are familiar with the need for shrinkage analysis in multistory wood-frame buildings, many designers may not be aware of the problem. Four- and five-story wood-frame construction is growing in popularity, and in some areas of the country designers may not be aware of problems associated with multistory wood-frame buildings.

In addition, new Section 2303.7 applies to areas where green lumber is used in construction, and in the past has been considered a West Coast issue. The basis of this requirement is that green lumber shrinks more than dry lumber; therefore, shrinkage should be considered in buildings more than two stories in height. See discussion under Section 2303.7 for discussion of dry and green lumber.

2304.4 Floor and roof framing. The framing of floors, joists, and roof rafters must be in accordance with the conventional construction provisions or be engineered. Even some engineered wood-frame buildings have portions, such as floor systems, that are framed in accordance with the prescriptive conventional construction provisions of the code.

2304.5 Framing around flues and chimneys. Specific wording in the code requires wood framing to be a minimum of 2 inches clear from flues, chimneys, and fireplaces, and 6 inches clear from flue openings. This air space is required because the resistance of wood to ignition is lowered when it is subjected to low levels of heat for extended periods of time. The separations specified in this section and in the International Mechanical Code (IMC) are intended to prevent the possibility of the wood being subjected to low heat for extended periods of time. Note that 2 inches of masonry will not provide the same level of thermal resistance as 2 inches of air space. Therefore, adding 2 inches to the required thickness of a fireplace or chimney wall will not serve as an acceptable alternative to the 2-inch clear air space requirement. The openings between the wood and the masonry must be adequately fire blocked in accordance with Section 718.

2304.6 Exterior wall sheathing. In addition to protecting the building envelop from wind loads, exterior wall sheathing is intended to:

1. Protect the interior of the building from the weather. As required by Section 1405.2, exterior wall coverings must provide weather protection for the structure. Although not explicitly required by the code, where wood structural panel products are used for exterior siding, such as plywood, particleboard, or hardboard, joints should occur over framing members and be protected with continuous wood battens, tongue-and-groove joints, caulking, flashing, vertical or horizontal shiplaps, or other methods approved by the building official that will make the joints resistant to water penetration. Alternatively, these panels may be applied over lumber and wood structural panel sheathing, in which case the double layers provide for an overlapping of joints and prevent direct penetration of water.

2. Be of sufficient strength to span between the studs or other structural members. The strengths or thicknesses of most of the exterior wall-covering materials listed in Table 2304.6 are based on a stud spacing of 16 inches on center. However, there are a few cases where the thickness is based on a 24-inch spacing.

3. Have sufficient durability to withstand, for the life of the building, the weathering effects of the elements to which they are exposed. Thus, where wood structural panels are used, the code requires that they be rated as exterior type. Other materials must be of a weather-resistant type of material, such as cedar shingles, or the material must be finished with a weather-resistant finish, such as exterior paint.

Provisions for other wall coverings are prescribed in Section 1405, and exterior plaster provisions are set forth in Section 2512.

2304.6.1 Wood structural panel sheathing. Wood structural panel sheathing used as an exterior wall finish must have an Exterior type durability designation. For example, T1-11 siding is an APA 303 siding designed to be used as exterior finish exposed to outdoor conditions and is identified as Exterior. Wood structural panels used under an exterior finish such as roof or wall covering must have an Exterior or Exposure 1 designation. Exposure 1 panels are allowed to be exposed on the underside at eaves. See Section 2303.1.4.

Recent high-wind events including Hurricane Katrina and several thunderstorms associated with tornados have shown that failure of wall sheathing in winds as low as 60 mph has led to significant damage resulting from breaching of the wall envelope. A code change in the 2009 IBC added Table 2304.6.1, giving maximum wind speed for exterior wood structural panel wall sheathing or siding that must resist wind loads perpendicular to the wall. The table gives maximum wind speeds (Section 1609.3) based on the exposure category of the building site (Section 1609.4). For a given wind speed and exposure category, the table gives the minimum nail size, wood structural panel span rating, panel thickness, stud spacing, and nailing schedule. The limitations of the table are: the building must be enclosed (ASCE 7 Section 26.2); the mean roof height must be not greater than 30 feet (ASCE 7 Section 26.2), and topographic factor must be equal to 1.0 (ASCE 7 Section 26.8). Buildings that do not meet these limitations must have the exterior sheathing designed to resist component and cladding wind pressures in accordance with ASCE 7. The table was developed by comparing the component and cladding wind pressures given in ASCE 7-05 Figure 6-3 with the wood structural panel capacities based on U.S. DOC PS 2 standard, engineering calculations, and the APA Panel Design Specification referenced in Section 2306.1. Nail head pull through and withdrawal capacity was also considered in addition to panel stiffness and bending strength. Panel-fastener capacity was based on tributary area to a single critical nail.

2304.7 Interior paneling. Interior paneling must be installed in accordance with Table 2304.9.1 and comply with DOC PS-1 or PS-2. Wood structural panels installed in accordance with this section are considered interior wall finishes and cannot be used to resist lateral forces. Prefinished hardboard paneling must conform to the requirements of AHA A135.5, and hardwood plywood must conform to HPVA HP-1.

2304.8 Floor and roof sheathing. Structural floor and roof sheathing meeting the requirements given in the tables in Section 2304.8 are deemed to comply with the code.

2304.8.1 Structural floor sheathing. Structural floor sheathing serves three purposes:

1. Provides support for and distributes superimposed gravity loads to the floor joists or trusses.

2. Provides lateral support for the top of the floor joists.

3. Serves as the shear-resisting element when the floor acts as a diaphragm to distribute lateral wind or seismic loads to the shear walls or foundation.

The sheathing may either be engineered to meet the loading requirements, or be selected from the tables in the code. The tables are prescriptive and therefore deemed to comply with the minimum code requirements.

The maximum span for wood structural panels is limited by both strength and deflection. Table 2304.8(3) limits deflection to L/180 under the tabulated uniform dead and live load, or L/240 under the tabulated uniform live load only. Deflections for combination subfloor/underlayment grades in Table 2304.8(4) are based on a deflection of L/360 caused by a total uniform load of 100 psf except for 1⅛-inch panels on joists spaced at 48 inches on center, which are rated at a deflection of L/360 at a uniform load of 65 psf. When using the tables, all the conditions set forth in the table must be met regarding loads, type, grade, species group, plies, blocking, direction of span, and other requirements. Sheathing systems that do not meet all the conditions must be engineered.

Wood structural panels in Table 2304.8(3) are designated by the panel span rating (also called the panel identification index), instead of thickness. The rating consists of two numbers such as ³²/₁₆. The first number is the allowable span for use as roof sheathing, and the second is the allowable span for use as floor sheathing. Panels intended only for single floor grades are designated by one number, for example, 16, but the panels may be used for roof sheathing with the spans listed in the table. The single floor grades have higher-grade faces than Structural I panels and may be preferable for aesthetic reasons as roof sheathing where visible from the underside.

Several thicknesses of structural panel are included under a single panel span rating. This is because different grades of interior and face veneers or strands may be used in different combinations. For example, a five-ply plywood panel will have a higher load rating than a four-ply panel with the same thickness. Conversely, the five-ply panel may be thinner than the four-ply panel, yet have the same panel span rating. For example, a ¹⁵/₃₂-inch-thick four-ply panel will support the same total load as a ⁷/₁₆-inch-thick five-ply panel.

The ⁵/₁₆-inch wood structural panels were deleted from Table 2304.7(3) in the 2009 IBC because ⁵/₁₆-inch thickness is a very small fraction of the panels currently produced. Although ⁵/₁₆-inch wood structural panel has been the minimum panel thickness specified for many applications over the years, the building industry has shifted away from producing them due to manufacturing efficiencies and marketplace demand. The ⅜-inch wood structural panel is currently the minimum thickness in the table.

2304.8.2 Structural roof sheathing. Roof sheathing must be designed or conform to the provisions of Table 2304.8(1), 2304.8(2), 2304.8(3), or 2304.8(5). Wood structural panel roof sheathing must be bonded by exterior glue. See the discussion under Section 2304.8.1, Structural floor sheathing.

2304.9 Lumber decking. Section 2304.9 was revised to incorporate the lumber deck design provisions of American Institute of Timber Construction (AITC) 112-93 Standard for Tongue-and-Groove Heavy Timber Decking directly into the body of the code. The reference to AITC 112-93 in Section 2306.1 was deleted from the 2006 IBC because AITC no longer maintains the standard. The revisions in the 2006 IBC along with the addition of new Section 2306.1.4 incorporated the pertinent provisions of AITC 112-93 directly into the body of the code, eliminating the need to reference AITC 112-93. The title of Section 2304.9 was revised to indicate that the provisions cover all decking, including mechanically laminated and solid-sawn decking. The capacity of lumber decking is arranged according to the various layup patterns described in Section 2304.9.2. Section 2306.1.4 gives the design capacity of lumber decking for flexure and deflection according to the formulas given in Table 2306.1.4. In addition to the technical changes that were made to the section in the 2006 IBC, several editorial changes were made to improve the language in the 2009 IBC.

2304.9.1 General. The general provisions specify the requirements for square ends, beveled ends, and the orientation of tongue-and-groove decking on roofs. Section 2304.9.2 provides specific requirements for the various layup patterns described in the subsections. Sections 2304.9.3 through 2304.9.5 cover specific requirements for mechanically laminated decking, 2-inch sawn tongue-and-groove decking, and 3- and 4-inch sawn tongue-and-groove decking, respectively.

2304.10 Connections and fasteners. Connectors and fasteners must comply with the applicable provisions of Sections 2304.10.1 through 2304.10.7. The section covers fasteners used to connect wood members, sheathing fasteners, fasteners for joist hangers, and framing hardware, and specific requirements for fasteners used in treated wood. Other fastening methods such as clips, staples, and glue are allowed where approved.

2304.10.1 Fasteners requirements. Table 2304.10.1 in the IBC is comparable to the nailing tables in the legacy model codes. Power-driven fasteners, along with the typical sizes used, and staples are included in the table. The size designations in the table are common to all fastener manufacturers. The code intends that Table 2304.10.1 provide minimum requirements for the number and size of fasteners used to connect wood-framing members. This table accommodates the builder of non-designed (conventional) construction, but also provides minimum fastening requirements for designed construction. Details such as end and edge distances and nail penetration are required to be in accordance with the applicable provisions of the NDS. Where required, corrosion-resistant fasteners must be either hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze, or copper.

A code change to the 2006 IBC added the shank length and diameter in parentheses to the tables for the various types of nails used in wood connections. It has been reported that improper nail sizes have been used in wood-frame building construction because the pennyweight system of specifying nail sizes is not universally understood. Code users sometimes focus on pennyweight (8d - 8 penny, 16d - 16 penny, etc.) and do not pay sufficient attention to the specific type of nail such as common, box, cooler, sinker, finish, and so on. A typical example is substitution of box nails for common nails of the same pennyweight. The specific type of nail is critical because there can be significant differences in strength properties of connections nailed with nails of the same pennyweight but of different nail type. Specifying the nominal dimensions of nails in the fastening tables may help avoid confusion and reduce misapplications in nailed connections. The code change proponent expected some reluctance on the part of some in the building construction community to completely abandon the pennyweight system of designating nail sizes, so the code continues to maintain the pennyweight designations. Because nominal dimensions are not as subject to misinterpretation, the shank length and diameter in parentheses will help prevent confusion and misapplication of the various types of nails used in wood connections.

2304.10.2 Sheathing fasteners. Fasteners should be driven flush with the surface of the sheathing, but not overdriven. Overdriving of fasteners can significantly reduce the shear capacity and ductility of the diaphragm or shear wall. For three-ply material, the strength reduction is significant if the fastener is overdriven through one ply.

If no more than 20 percent of the fasteners around the perimeter of the panel are overdriven by ⅛ inch or less, then no reduction in shear capacity need be considered. If more than 20 percent of the fasteners around the perimeter are overdriven by any amount, or if any fasteners are overdriven by more than ⅛ inch, then additional fasteners must be driven to maintain the desired shear capacity, provided that the additional fasteners will not split the substrate. For every two fasteners overdriven, one additional fastener should be driven. Panels with more than 20 percent of the fasteners overdriven greater than ⅛ inch should be replaced.

Also, if the actual panel thickness is greater than the design panel thickness needed to resist the design shear, the panel shear capacity may be adequate without the driving of additional fasteners. For example, if the design required a ¹⁵/₃₂-inch-thick panel, but a ¹⁹/₃₂-inch-thick panel with all fasteners overdriven by ⅛ inch is used for the sheathing, the panel is adequate because the net thickness is ¹⁵/₃₂ inch.

2304.10.3 Joist hangers and framing anchors. Most joist hangers and other framing hardware have reports issued by building product evaluation services. Such ICC Evaluation Services and detailed reports are issued for the various products. There are acceptance criteria for the performance of the hardware in terms of the strength limit states of the metal, wood, and fasteners, as well as deflection limit states. For example, ICC AC 13, Acceptance Criteria for Joist Hangers and Similar Devices, is used to evaluate devices used to support or attach wood members, such as joists, rafters, purlins, beams, girders, plates, posts, studs, and headers to wood, metal, concrete, or masonry where the attachment is by means of mechanical fastenings such as nails, spikes, lag screws, wood screws, bolts, and so on. The term “device” refers to one or more pieces or units so arranged as to transfer load vertically or laterally, within safe limits, from the end of a supported member (such as a joist) to a supporting member (such as a header, beam, or girder).

Test requirements for joist hangers were previously found in Section 1711.1. The section was deleted entirely in the 2015 IBC because ASTM D1761-06 no longer contains provisions for testing of joist hangers and the provisions were moved to ASTM D7147. The ASTM D7147 standard includes sampling and evaluation criteria as well as further refinements regarding quality of test materials, adjustments for variation in test materials, and limits on design values with materials other than those tested. In addition, because ASTM D7147 is specific to joist hangers used in wood construction and contains provisions that go beyond testing, the reference was appropriately relocated to Section 2304.10.3. The remainder of Section 1711 is now found in Section 1504, which is more appropriate because it is applicable to performance requirements for roof coverings and assemblies.

2304.10.4 Other fasteners. Fasteners not specifically cited in the code may be used but are subject to building official approval. See Section 1703.4 for requirements related to research reports and acceptance of evaluation reports.

2304.10.5 Fasteners in preservative-treated and fire-retardant-treated wood. The water-borne salts in preservative-treated and fire-retardant-treated wood are corrosive. Fasteners must be corrosion resistant when used with these materials. Corrosion-resistant fasteners are made of type 304 or 316 stainless steel, silicon bronze, copper, or steel that has been hot-dipped or mechanically deposited zinc-coated galvanized with a zinc coating of not less than 1.0 ounce per square foot.

The 2006 IBC introduced an alternative method for mechanically galvanizing, which is preferable to hot dipping for some types of fasteners, as indicated in the exception to this section. Class 55 was added to provide an equivalent amount of zinc as would be provided by the hot-dip process in accordance with ASTM A 153. According to the American Galvanizers Association, mechanically plating to a thickness of 55 microns provides an equivalent coating to 1 ounce per square foot of hot-dipped galvanized zinc, which is what is provided for fasteners by ASTM A 153. Class 55 provides 55 microns of thickness. The section references ASTM A 53 or B 695 for coating weight requirements.

Electro-galvanized steel fasteners do not qualify as corrosion resistant; the zinc coating typically is about 0.1 ounce per square foot. Electro-galvanized nails are suitable only for occasionally wet locations such as for nailing composition shingles.

In the 2009 IBC, the section was subdivided into three categories of fasteners in treated wood: fasteners in preservative-treated wood, fasteners in fire-retardant-treated wood in exterior or damp locations, and fasteners in fire-retardant-treated wood in interior locations.

A significant change in the 2009 IBC is the exception for fasteners used in preservative-treated wood in an interior dry environment. There is no documented evidence of any detrimental fastener corrosion when plain steel fasteners are used in SBX/DOT (sodium borate) or zinc borate preservative-treated wood in interior, dry environments. In this case, the exception permits plain carbon steel fasteners to be used.

In the 2012 IBC, the phrase “including nuts and washers” was added to each of the sections to clarify that the corrosion-resistance requirements apply to the fastener as well as nuts and washers.

2304.10.6 Load path. The code requires the load path to be continuous from the point of origin to the resisting element, which generally means from the roof to the foundation. A continuous load path for both gravity and lateral loads is necessary for adequate performance of the structure in response to superimposed vertical and lateral loads. This is especially critical in the case of high wind and load effects from earthquake ground motion. For example, visualize what happens to the wind suction load on a low-slope roof. The upward force that is not offset by the dead load of the roof elements must be resisted by dead load elsewhere. Similarly, where the structure is subjected to lateral load effects from earthquake ground motion, the inertial forces from the floor and roof diaphragms must be effectively transferred to the vertical lateral-force-resisting elements, for example, wood-framed shear walls and then to the supporting foundation. A positive, properly detailed continuous load path is essential to ensure the transfer of all gravity and lateral loads from the roof and floors through the structural system down to the foundation and supporting soil.

The required minimum thickness of steel straps used to splice discontinuous framing members was modified in the 2015 IBC to be consistent with the standard thickness in the new AISI Product Data Standard, S201. AISI S201 provides criteria, material, and product requirements for structural and nonstructural members utilized in cold-formed steel framing applications where the specified minimum base steel thickness is between 18 mils (0.0179 inch) and 11 mils (0.1180 inch). The minimum thickness specified in the code represented galvanized 20 gage steel, but the term “gage” is no longer a steel thickness designation and is now designated as 33 mils. According to the AISI Product Standard S201, the base metal thickness that corresponds to 33 mils is 0.0329 inch, which is the minimum base metal thickness of 0.0329 inch for steel straps used to splice discontinuous framing members.

2304.10.7 Framing requirements. Columns must be provided with full end bearing to transfer their loads or connections must be designed to resist the full compressive load. Connections must also be able to resist lateral and net uplift loads.

2304.11 Heavy timber construction. The provisions contain general provisions for column continuity, transfer of loads from beams to columns, other connection criteria, and requirements for structural anchorage and continuity. Minimum element size requirements for beams and columns used in Type IV construction are found in Section 602.4.

2304.11.1 Columns. Columns in heavy timber construction must be continuous throughout all stories by being connected by concrete or metal caps, base plates, timber splice plates, or other approved methods.

2304.11.1.1 Column connections. Girders and beams in heavy timber construction are required to be fitted around columns, and adjoining ends must be adequately tied to each other to transfer horizontal loads across the joints.

2304.11.2 Floor framing. Wall pockets or hangers are required where wood beams, girders, or trusses in heavy timber construction are supported by masonry or concrete walls. Beams supporting floors are required to bear on girders or be supported by ledgers, blocks, or hangers.

2304.11.3 Roof framing. Roof girders and alternative roof beams in heavy timber construction are to be anchored to supporting members with steel or iron bolts designed to resist vertical uplift of the roof.

2304.11.4 Floor decks. Floor decks and floor covering in heavy timber construction cannot extend closer than ½ inch to walls with the space covered by a molding fastened to the wall.

2304.11.5 Roof decks. Where supported by a wall, roof decks in heavy timber construction must be anchored to walls by steel or iron bolts to resist uplift forces. This section in the 2000 IBC requires roof decks supported by a wall to be anchored to walls at intervals not exceeding 20 feet, but did not specify the type or purpose of the anchorage. Code changes to the 2003 IBC added that the anchors must be capable of resisting uplift forces determined in accordance with Chapter 16, and the anchors must be steel or iron bolts of sufficient strength to resist vertical uplift of the roof.

2304.12 Protection against decay and termites. The provisions from the legacy model codes were reorganized in the IBC for clarity. The provisions are grouped into the following areas:

1. Wood used above ground

2. Laminated timbers exposed to weather

3. Wood with ground or fresh water contact

4. Supports for appurtenances

5. Termite protection

6. Retaining walls and cribs

The provisions of this section are intended to protect against decay and termite infestation. The provisions are based on the extensive material on biodeterioration of wood in the Wood Handbook published by the U.S. Forest Products Laboratory.

2304.12.1 Locations requiring water-borne preservatives or naturally durable wood. Wood used in the five locations specified in this section must be naturally durable or preservative treated with water-borne preservatives. Wood used above ground, if preservative treated, is usually treated with a water-borne preservative such as ammoniacal copper arsenate (ACA) or chromate copper arsenate (CCA) in accordance with AWPA U1 (Commodity Specifications A or F). The retention rates are lower than those required for ground contact.

2304.12.1.1 Joists, girders, and subfloor. The code requires 18 inches of clearance to joists and 12 inches clearance to wood girders from exposed ground if they are not of naturally durable or preservative-treated wood. See Figure 2304-2.

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Figure 2304-2 Under-floor clearance.

2304.12.1.2 Wood supported by exterior foundation walls. Framing, including sheathing (not siding), must have 8 inches of clearance from exposed earth if it is not naturally durable or preservative treated. See Figure 2304-3. This section was retitled in the 2006 IBC to clarify that the section applies to wood framing resting on exterior foundation walls. Note that Section 2304.12.1.5 allows a 6-inch clearance for wood siding.

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Figure 2304-3 Clearance between wood framing, wood siding, and earth.

2304.12.1.3 Exterior walls below grade. These requirements were put in a separate subsection in the 2006 IBC to clarify that they apply to wood framing attached to the interior side of exterior concrete or masonry foundation walls.

2304.12.1.4 Sleepers and sills. Concrete and masonry slabs that are in direct contact with the earth are very susceptible to moisture because of absorption of ground water. This can occur on interior slabs as well as at the perimeter. This section is intended to prevent the use of untreated wood that may decay under such conditions. Concrete that is fully separated from the ground by a vapor barrier is not considered to be in direct contact with earth.

2304.12.1.5 Wood siding. Wood siding must have 6 inches of clearance between the siding and earth unless made of naturally durable or preservative-treated wood. Note that the clearance for wood sheathing under the siding is 8 inches as required by Section 2304.12.1.2. In other words, siding can extend 2 inches below the foundation plate, or framing, whereas the sheathing must be terminated at the sill plate. See Figure 2304-3.

A code change in the 2009 IBC added a minimum 2-inch clearance between wood siding and an adjacent concrete slab such as a patio or walk. The previous code language should result in wood materials being at least 2 inches from the surface of typical 4-inch-thick concrete walk or porch slab if the required minimum of 6 inches distance from the ground is maintained. However, it is not unusual to see less than 2-inch clearance between wood siding and a concrete slab. Without specifying a minimum clearance, water that collects on the concrete can lead to decay in the wood. The IBC now requires a minimum 2-inch clearance between wood siding and a concrete slab in addition to the 6-inch clearance between the siding and the ground.

2304.12.2 Other locations. Wood used in the five locations specified in this section must be naturally durable or preservative treated in accordance with AWPA U1. Preservative-treated wood used in interior locations require a protective finish to be applied after the wood is dried unless water-borne preservatives are used.

2304.12.2.1 Girder ends. A ½-inch airspace is required around the ends of wood girders to reduce moisture that can contribute to decay of the member.

2304.12.2.2 Posts or columns. This section is an amalgamation of the requirements from the three model legacy codes. Figure 2304-4 illustrates the requirements.

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Figure 2304-4 Posts and columns.

2304.12.2.3 Supporting member for permanent appurtenances. Balconies, porches, and other appurtenances that are exposed to weather conditions may not have protective overhangs. Water can collect in the joints and on the surfaces, creating alternating cycles of wetting and drying conducive to deterioration and decay. In this case, the wood-framed structural supports must be of naturally durable or preservative-treated wood. The exception can be applied to buildings located in areas where climatic conditions are favorable enough to preclude the use of naturally durable or preservative-treated wood. For example, very dry desert areas such as Death Valley, California, and Yuma Valley, Arizona, have extremely low annual precipitation where the exception could be used with the approval of the building official.

2304.12.2.4 Laminated timbers. The portions of glue-laminated beams directly exposed to weather are subject to decay and should be of preservative-treated material or manufactured from naturally durable wood.

2304.12.2.5 Supporting members for permeable floors and roofs. Where wood framing is used to support floors and roofs that are moisture permeable, such as a concrete slab over a patio or a patio slab over a garage, the framing must be of pressure-treated wood or approved naturally durable species, unless the slab is separated from the wood framing by a waterproof membrane. Although these wood-framing members are not necessarily in direct contact with the ground, their exposure to moisture is similar to that of wood in direct contact with the ground. Therefore, the wood framing must be of naturally durable wood, or it must be preservative treated in accordance with AWPA C2, C9, or C22.

2304.12.3 Wood in contact with the ground or fresh water. Note specifically the limiting adjective fresh, which means this section only applies to wood in contact with the ground or fresh water. The water-borne preservatives used for fresh water are not suitable for brackish or salt water, where attack can also come from marine borers. The preservative retention rates are higher for wood in ground contact than for above-ground uses. The first paragraph of this code section allows wood in direct contact with the earth to be naturally durable. However, this only applies to wood in contact with the ground, not posts or columns. Posts and columns are required by Section 2304.12.3.1 to be preservative treated.

2304.12.3.1 Posts or columns. Posts or columns embedded in concrete or embedded in earth, such as columns in a pole-supported structure, have no opportunity to dry and are subject to decay. Hence, they must be of preservative-treated wood.

2304.12.4 Termite protection. Where termites are a significant hazard such as in some southern states, floor framing must be preservative treated, naturally durable, or have some other approved method of termite protection. Section 2603.8. has specific restrictions on the use of foam plastics in areas where the probability of termite infestation is very heavy, based on the termite infestation probability map shown in IBC Figure 2603.8. (See Figure 2304-5.) In areas where the probability of termite infestation is very heavy, naturally durable termite-resistant wood or preservative-treated wood must be used.

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Figure 2304-5 Termite infestation probability map.

2304.12.5 Wood used in retaining walls and cribs. Wood used in retaining walls, crib walls, bulkheads, and other walls that retain or support the earth must be of preservative-treated wood specified as ground-contact-treated wood.

2304.12.6 Attic ventilation. Refer to the discussion under Section 1203.2 in regard to attic ventilation.

2304.12.7 Under-floor ventilation. Refer to the discussion under Section 1203.3 in regard to under-floor ventilation.

2304.13 Long-term loading. Wood structural members can exhibit long-term creep, which increases deflection, particularly where a high dead load is present. Early editions of the IBC did not permit wood members to permanently support the dead load of masonry or concrete (with some exceptions), because additional deflection produced by long-term creep can cause severe cracking in the masonry or concrete. The provision in the IBC was a carryover from the 1997 UBC, which was intended to address concerns with long-term creep in wood members permanently supporting concrete and masonry. The section consisted of a restriction against wood members being used to permanently support dead load of masonry or concrete followed by a list of exceptions to the restriction. The restriction was deleted in the 2006 IBC and replaced by a new section that references the design method for limiting long-term deflections in Section 3.5.2 and Appendix F of the NDS. Under sustained loading, wood members exhibit additional time-dependent deformation (creep), which generally develops over long periods of time. The tabulated modulus of elasticity design values, E, in the NDS are intended to be used to estimate the immediate deformation under load. Where dead loads or sustained live loads represent a relatively high percentage of total design load, creep is an appropriate design consideration, which is addressed within the NDS. The total deflection under long-term loading is estimated by increasing the initial deflection associated with the long-term load component by 1.5 for seasoned lumber or 2.0 for unseasoned or wet lumber or glued laminated timber. See Section 3.5.2 and Appendix F of the NDS for design provisions related to time-dependent deformations known as creep.

Section 2305 General Design Requirements for Lateral-Force-Resisting Systems

Prior to the 2009 IBC, Section 2305 contained extensive design requirements for lateral-force-resisting systems used in wood-frame buildings such as diaphragms, chords, collectors, shear walls, and so on. The provisions are primarily based on a combination of 1997 UBC Section 2315 and 1997 NEHRP Sections 12.1 through 12.4. In the 2006 IBC, the 2005 edition of AF&PA Special Design Provisions for Wind and Seismic (SDPWS) was added as a permissible alternative to the detailed requirements contained in Section 2305. The SDPWS standard provides complete requirements for design and construction of wood members, fasteners, and assemblies that resist lateral forces from wind and seismic ground motion.

In the 2009 IBC, nearly all of Section 2305 was deleted, and the 2008 edition of the AF&PA SDPWS is a mandatory referenced standard for lateral design of wood structures. The 2012 IBC references the 2008 SDPWS, and the 2015 IBC references the current 2015 edition of the AWC SDPWS. Section 2305.1 states, “Structures using wood-framed shear walls or wood-framed diaphragms to resist wind, seismic or other lateral loads shall be designed and constructed in accordance with AF&PA SDPWS and the provisions of Sections 2305, 2306 and 2307.” Section 2306 contains requirements based on allowable stress design and Section 2307 contains requirements based on load and resistance factor design. Both Sections 2306 and 2307 reference the 2015 edition of the NDS, which contains both ASD and LRFD design procedures. Section 2305 now has only three subsections: Section 2305.1 is general requirements; Section 2305.1.1 contains a general requirement regarding openings in shear walls that originated with the UBC; Section 2305.2 contains provisions for calculating deflection of stapled diaphragms; and Section 2305.3 contains provisions for calculating deflection of stapled shear walls. All other requirements pertaining to lateral design of wood-framed structures are in the 2015 AWC SDPWS. For a complete history of the code change that deleted substantial portions of Section 2305, refer to Code Change S82-06/07 in the ICC publication, 2009 IBC Code Changes Resource Collection.

2305.1 General. The section references the AWC SDPWS for design and construction of wood-frame shear walls or wood-frame diaphragms to resist wind, seismic, or other lateral loads. Provisions in Sections 2305, 2306, and 2307 are also required to the extent that they apply.

2305.1.1 Openings in shear panels. This provision originated with the UBC and predates the perforated shear wall design method. Although the section requires that openings in shear walls that materially affect their strength be detailed and have their edges reinforced, the meaning of the phrase materially affect their strength is not defined in the code. It should also be noted that the perforated shear wall design method in SDPWS Section 4.3.5.3 does permit openings in shear walls without force transfer design, provided certain requirements, adjustments, and restrictions are met.

2305.2 Diaphragm deflection. The SDPWS does not address deflection calculations for stapled diaphragms. The code change that deleted most of Section 2305 from the 2009 IBC retained Equation 23-1 and the parameters necessary to calculate deflection of diaphragms fastened with staples. Stiffness properties (apparent shear stiffness, kip/inch) for diaphragms constructed of wood structural panels and lumber are given in SDPWS for purposes of complying with diaphragm classification, drift, and stiffness compatibility requirements specified in ASCE 7. See Section C4.2.2 of the SDPWS Commentary for detailed discussion of calculating diaphragm deflection.

Although there was no rational method to calculate the deflection of unblocked diaphragms, monotonic racking tests indicate that the deflection of an unblocked diaphragm may be on the order of three times the deflection of a blocked diaphragm. The 1999 edition of the SEAOC Blue Book⁷ (Recommended Lateral Force Requirements and Commentary of the SEAOC Seismology Committee) suggests that the deflection of an unblocked diaphragm at its tabulated allowable shear capacity will be about 2.5 times the calculated deflection of a blocked diaphragm of similar construction and dimensions at the same shear capacity. In the SDPWS, apparent shear stiffness values are tabulated for each combination of nailing and sheathing thickness for typical blocked and unblocked diaphragms in order to simplify calculation of diaphragm deflection. See Section C4.3.2 of the SDPWS Commentary for detailed discussion of calculating shear wall deflection.

2305.3 Shear wall deflection. As noted above, the SDPWS does not address deflection of stapled shear walls. The code change that deleted most of Section 2305 from the 2009 IBC retained Equation 23-2 and the parameters necessary to calculate deflection of shear walls fastened with staples. Stiffness properties (apparent shear stiffness, kip/inch) for shear walls constructed of wood structural panels and other materials are given in SDPWS for purposes of complying with drift and stiffness compatibility requirements specified in ASCE 7.

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