Prev Next

16
Structural Provisions

This chapter is a summary of structural design requirements, which begin with Chapter 16 of the code. Chapter 16 sets forth general design criteria for structural loads to be accommodated by the structural system of a building. Detailed criteria for building materials are contained in the code chapters devoted to specific materials, such as wood (Chapter 23), concrete (Chapter 19), and steel (Chapter 22). Chapter 17 of the code governs the testing and inspection of construction materials. Chapter 18 of the code applies to the requirements for soils, site grading, and foundation design.

Structural Design

The structural design requirements contained in Chapter 16 apply to all buildings and structures. The chapter focuses on the engineering principles that underlie the requirements and design of structural systems to accommodate anticipated loads, such as the weight of the building, the weight of occupants and materials in the building, and loads imposed by nature such as wind, snow, floods, and earthquakes.

Chapter 16 contains numerous tables and criteria that are to be applied in specific situations based on building use, occupancy, construction type, and geographic location. Detailed analysis must be undertaken by the designer in concert with appropriate engineering consultants as needed to supplement the designer's training and expertise to prepare a code-compliant design.

Definitions

The terms in Chapter 16 are defined in Chapter 2. Other basic structural terms may not be defined, but also need to be understood. Terms include:

Figure depicts snow, wind, earthquake, and dead loads in the structure of a building.

Figure depicts essential facilities (left), live load in the structure of a building (top), and stress-strain curve (bottom).

Construction Documents

Construction documents must have sufficient information to allow the AHJ to review the documents for code compliance. The documents are to show the size, section dimensions, and relative locations of structural members. Design loads are to be indicated to verify compliance with structural design requirements for various types of loading. Light-frame buildings, such as houses and small commercial buildings constructed under the conventional framing provisions of § 2308, have a separate, shorter set of design requirements for floor and roof live loads, ground snow load, basic wind speed, seismic design category, flood design data, soil bearing values, and rain load data.

The construction documents are to show the loads and information for specified items. The detailed requirements for these items will be elaborated on later in this chapter.

Figure depicts the braced and moment frames in the structure of a building.

Figure depicts shear wall that resist lateral forces applied parallel to the plane of the wall (top), and wall, load bearing system (bottom).

General Design Requirements

Buildings are to be designed in accordance with one of several defined and approved structural design methods, such as the strength design method, the load and resistance factor design method, the allowable stress method, and so forth. The structure is to support the factored loads in load combinations as defined in the code.

Serviceability

The structure is to be designed to limit deflections and lateral drift under anticipated loading. Load effects on the structure are to be determined by application of a rational analysis taking into account equilibrium of the structure, general stability, and short-term and long-term material properties.

Risk Category

Table 1604.5 classifies buildings into risk categories by importance. Factors noted in the structural chapters are for typical buildings. Values for loads and strengths are to be increased by the factors based on the anticipated consequences of the failure of the structure being categorized.

Calculation formulas apply different factors for each category, taking into account the relative importance of each facility. Thus “essential facilities” may have factors of up to 1.5 applied to loading criteria to provide additional strength to such structures in the event of such events as hurricanes, earthquakes, or floods. Note that these factors are applied to external loading, not other effects such as fires or loads imposed by use. These other load factors are addressed in live load tables.

Anchorage

Roofs are to be anchored to walls and columns, and walls and columns are to be anchored to foundations, to resist the anticipated uplift and sliding forces that result from the application of the prescribed loads, whether from dead loads or live loads.

Load Combinations

Various combinations of dead loads, live loads, seismic loads, and wind loads are to be applied in the design of structural systems. The load factors for each combination depend on the type of analysis used. Various combinations of loading are to be examined and the design is to resist the most critical effects of the combinations specified. We will examine the detailed descriptions for each load category.

Figure depicts the various combinations of dead loads, live loads, seismic loads, and wind loads applied in the design of structural systems.

Where partitions may be installed and later moved, a uniformly distributed partition live load of 15 psf (0.74 kN/m²) is to be assumed unless the specified floor live load is 80 psf (3.8 kN/m²) or greater.

To the right is a summary of examples from Table 1607.1. The numbers alongside the occupancy correspond to the location of the examples in the complete table.

Loads on Handrails and Guards

Roof Loads

Figure depicts the roof load in the structure of a building (left), landscaped roofs in the structure of a building (right)..

Interior Walls and Partitions

Snow Loads

Snow loads are to be determined in accordance with a reference standard: Chapter 7 of ASCE 7. Snow loads in the contiguous United States are shown in Figure 1608.2. Snow loads are determined based on historical data and are correlated to geographic location and to elevations.

For example, the snow load in north central Kansas is 25 psf (1.19 kN/m²).
In northeast Arizona, the load varies from zero up to the 3000' (914 m) elevation; 5 psf up to the 4500' elevation (0.24 kN/m² load up to the 1372 m elevation); 10 psf up to the 5400' elevation (0.48 kN/m² load up to the 1645 m elevation); and 15 psf up to the 6300' elevation (0.72 kN/m² load up to the 1920 m elevation).
In heavy snow areas, such as the Sierra Nevada and the Rocky Mountains, the snow load is to be determined by case studies that are based on 50-year recurrence data and must be approved by the building official.

Roofs are to be designed in accordance with ASCE 7 to accommodate snow loads under varying conditions, such as:

Figure indicates the snow loads on flat roofs (top), snow loads on sloped roofs (middle), and drifting snow on low roofs and snow sliding from higher sloped roofs onto lower roofs (bottom).

Wind Loads

Buildings and portions of buildings are to be designed to withstand, at a minimum, the wind loads included in the code in accordance with Chapters 26–30 of ASCE 7. The wind is assumed to come from any horizontal direction, and no reduction is to be taken for the effect of shielding by other structures. This is in keeping with the principle that the code applies to the building in question and is affected neither positively nor negatively by adjacent buildings. There are, however, portions of § 1609 where adjacent site and topographic conditions may impact wind loads. See § 1609.4.

Figure depicts total wind load in the structure of a building (top), structural members and systems as well as building cladding to resist overturning, uplift, or sliding caused by wind forces (bottom).

Wind Speed

Wind speeds are designated as “ultimate design” or “nominal design” wind speeds and are used for either strength design or allowable stress designs respectively. The ultimate design wind speeds are indicated in Figures 1609.3 (1), (2), and (3), and vary based on the building's risk category and location. The ultimate design wind speeds for a Risk Category II building vary from 110 mph (49 m/s) on the West Coast of the U.S. to 180 mph (80 m/s) in hurricane-prone areas in southern Florida. These wind speeds would convert to a nominal design wind speed, or what was previously called the “basic wind speed,” of 85 mph (38 m/s) for the West Coast and 139 mph (61m/s) for southern Florida when using allowable stress design (ASD).

Exposure Category

The exposure category reflects the how ground surface irregularities affect design wind pressure. The exposure conditions vary from the most protected to the least protected wind exposures. Where buildings have multiple exposures, the condition that results in the highest wind force shall apply. The exposures are determined by applying a “surface roughness” category to wind calculations over each 45° (0.79 rad) sector from which wind can impact the building. The factors roughly increase with each surface roughness category:

Soil Lateral Load

Rain Loads

Figure depicts the rain load in the structure of a building (bottom).

Flood Loads

§ 1612 requires that, in flood hazard areas established under the Federal Energy Management Agency Flood Insurance Study Program, all new buildings as well as major improvements or reconstruction projects must be designed to resist the effects of flood hazards or flood loads. Determination of whether a building falls under this requirement is based on locally adopted flood-hazard maps. These identify the anticipated flooding areas and elevations of flood waters for given anticipated return periods such as 50 or 100 years. The elevation and location of the building site must be compared to the flood-hazard maps to determine if this section is applicable.

Earthquake Loads

§ 1613 contains the provisions for the seismic design of building structures. Earthquake design must be investigated for every structure and included to varying degrees based on the location of the building and the anticipated seismicity of the location. Earthquake design can be quite complex and involve detailed calculations. Certain basic types of structure, notably those wood-frame residences and light commercial buildings using the Conventional Light-Frame Construction provisions of § 2308, are deemed to comply with the seismic requirements of the code. Other more complex buildings must undergo seismic analysis based on ASCE 7. This analysis takes into account several basic factors. While we will not go into the design calculation bases in detail, it is worth understanding the basic criteria that are to be addressed by seismic analysis and design. The 2006 edition of the code greatly shortened the earthquake section of Chapter 16 by adopting references to ASCE 7. Our discussion is conceptual, with the assumption that those seeking greater detail will refer to the design criteria contained in ASCE 7.

Chapter 16 requires that all structures be designed and constructed to resist the effects of earthquake motions and be assigned a Seismic Design Category based on anticipated earthquake acceleration, risk category, and seismic design of the building as outlined in § 1613.3.5 and Tables 1613.3.5 (1 and 2).

Seismic forces are produced in a structure by ground motions that cause a time-dependent response in the structure. The response generated by the ground motions depends on:

the magnitude, duration, and harmonic content of the ground motions
the dynamic properties of the structure (size, configuration, and stiffness)
the type and characteristics of the soil supporting the structure

Site Ground Motion

§ 1613 provides procedures for determining design-earthquake ground motions.

The magnitude of earthquake ground motions at a specific site depends on the proximity of the site to the earthquake source, the site's soil characteristics, and the attenuation of the peak acceleration. The dynamic response of a structure to earthquake ground motions can be represented by a graph of spectral response acceleration versus period.

A graphical representation where spectral response acceleration is plotted on the y-axis and period in seconds on the x-axis. The curve representing normalized response spectra depends on the soil type.

Figures 1613.2.1 (1) through (8) map the maximum spectral response accelerations for the United States and its territories, now including Guam and American Samoa, at short (0.2 second) and longer 1-second periods. The spectral response accelerations are given in percentages of gravity (g), assuming Site Class B (rock).

Structural Design Criteria

All structures require lateral-force-resisting and vertical-force-resisting systems having adequate strength, stiffness, and energy dissipation capacity to withstand the anticipated or design earthquake ground motions.

These ground motions are assumed to occur along any horizontal direction of a structure.
Continuous load paths are required to transfer forces induced by earthquake ground motions from points of application to points of resistance.

Seismic Design Categories (SDC)

The seismic design section requires that each structure be assigned a Seismic Design Category (A, B, C, D, E or F) based on the occupancy category and the anticipated severity of the earthquake ground motion at the site. This classification is used to determine permissible structural systems, limitations on height and irregularity, which components must be designed for seismic resistance, and the type of lateral force analysis required.

Risk Groups

Risk Groups are taken from the classification of buildings in § 1604.

Determination of Seismic Design Category

All structures must be assigned a Seismic Design Category based on its Seismic Use Group and the design spectral response acceleration coefficient, based on either its short or 1-second period response acceleration, whichever produces the most severe SDC.

The design criteria provide the design requirements for buildings in Seismic Design Category A because earthquakes are possible but rare. These structures must have all of their parts interconnected and be provided with a lateral-force-resisting system to resist 0.01 of the design lateral force.

Building Configuration

Perhaps of greatest interest to architects and designers is the idea of building configuration, which classifies buildings into regular and irregular configurations. Irregularity in a building, either in its plan or its section configuration, can impact its susceptibility to damage in an earthquake.

Plan Irregularities

Plan irregularities include:

Figure depicts the torsional irregularity in the structure of a building.

Torsional irregularity existing when the maximum story drift at one end of a structure is 120–140% greater than the average of the story drifts at the two ends of the structure

Reentrant corners where the projections are greater than 15% of the plan dimension in the given direction

Discontinuous diaphragms, especially when containing cutouts or open areas greater than 50% of the gross enclosed diaphragm area

Out-of-plane offsets creating discontinuities in lateral-force-resisting paths

Figure depicts the nonparallel systems in which the vertical lateral-force-resisting systems are not parallel or symmetric about the major orthogonal axes of the lateral-force-resisting systems.

Nonparallel systems in which the vertical lateral-force-resisting systems are not parallel or symmetric about the major orthogonal axes of the lateral-force-resisting systems

Vertical Irregularities

Vertical or sectional irregularities include:

Figure depicts the soft story having a lateral stiffness significantly less than that in the story above.

Soft story having a lateral stiffness significantly less than that in the story above

Figure depicts the weight or mass irregularity caused by the mass of a story being significantly heavier than the mass of an adjacent story.

Weight or mass irregularity caused by the mass of a story being significantly heavier than the mass of an adjacent story

Figure depicts geometric irregularity caused by one horizontal dimension of the lateral-force-resisting system.

Geometric irregularity caused by one horizontal dimension of the lateral-force-resisting system that is significantly greater than that of an adjacent story

Figure depicts in-plane discontinuity in vertical lateral-force-resisting elements.

In-plane discontinuity in vertical lateral-force-resisting elements

Figure depicts weak story caused by the lateral strength of one story being significantly less than that in the story above.

Weak story caused by the lateral strength of one story being significantly less than that in the story above

Earthquake Loads: Minimum Design Lateral Force and Related Effects

The design criteria define the combined effect of horizontal and vertical earthquake-induced forces as well as the maximum seismic load effect. These load effects are to be used when calculating the load combinations of § 1605.

Redundancy

Redundancy provides multiple paths for a load to travel from a point of application to a point of resistance. The design criteria assign a redundancy coefficient to a structure based on the extent of structural redundancy inherent in its lateral-force-resisting system.

Deflection and Drift Limits

The design criteria specify that the design story drift not exceed the allowable story drift obtained from the criteria specifications. All portions of a building should act as a structural unit unless they are separated structurally by a distance sufficient to avoid damaging contact when under deflection.

Equivalent Lateral Force Procedure

The equivalent lateral force procedure for the seismic design of buildings assumes that the buildings are fixed at their base.

Seismic Base Shear

The basic formula for determining seismic base shear (V) is:
V = CsW

where:

Cs = the seismic response coefficient determined from the design criteria and W = the effective seismic weight (dead load) of the structure, including partitions and permanent mechanical and electrical equipment.
The seismic response coefficient is equal to a design spectral response coefficient amplified by an occupancy importance factor and reduced by a response modification factor based on the type of seismic-force-resisting system used.

Vertical Distribution of Seismic Forces

The design criteria specify how the seismic base shear is to be distributed at each story level.

Horizontal Shear Distribution

Seismic design story shear is the sum of the lateral forces acting at all levels above the story. The design criteria specify how the seismic design story shear is distributed according to the rigidity or flexibility of horizontal diaphragms and torsion.

Overturning

A structure must be able to resist the overturning moments caused by the lateral forces determined to impact the structure.

Dynamic Analysis Procedure for the Seismic Design of Buildings

The static analysis contained in the design criteria may only be used for buildings with a lower Seismic Design Category, or buildings with a regular configuration that meet height limitations and are assigned a higher Seismic Design Category. There are basically three types of dynamic analysis procedures that may be used for the seismic design of all buildings: modal response spectra analysis, linear time-history analysis, and nonlinear time-history analysis.

Seismic-Force-Resisting Systems

There are several basic types of seismic-force-resisting systems:

Figure depicts the bearing wall systems for seismic-force-resistance.

Bearing wall systems

Figure depicts the building frame systems for seismic-force-resistance.

Building frame systems

Figure depicts the moment-resisting frame systems for seismic-force-resistance.

Moment-resisting frame systems

Figure depicts the dual systems with special moment and intermediate moment frames for seismic-force-resistance.

Dual systems with special moment frames
Dual systems with intermediate moment frames

Figure depicts the inverted pendulum systems for seismic-force-resistance.

Inverted pendulum systems

Detailing of Structural Components

There are requirements for the design and detailing of the components making up the seismic-force-resisting system of a building.

After a building is analyzed by calculations to determine its dynamic and static responses to seismic loads, it must be detailed to implement the design requirements of its seismic-force-resisting system.

This analysis must determine the worst-case forces based on the direction of seismic load, and that maximum force is to be used as the design basis.

Among details to be considered are:

Seismic Design of Architectural, Mechanical, and Electrical Components

The seismic design of such non-structural elements as architectural, mechanical, and electrical components must also be considered. These components are typically not part of the structural system either for resisting conventional gravity loads or seismic forces. However, these components, when a permanent part of the building, must be seismically restrained. Failures of these systems result in a great deal of physical damage and are often a factor in casualties from earthquakes. Also, life safety systems, such as fire sprinklers and electrical systems, need to be functional after an earthquake.