How to determine structural loads—Part 4: Seismic forces in accordance with the 2006 IBC


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    According to Section 1613.1 of the 2006 International Building Code (IBC), the effects of earthquake motions on structures and their components are to be determined in accordance with the 2005 edition of the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-05), excluding Chapter 14 and Appendix 11A. This section of the IBC also contains four exemptions to seismic design requirements, which are essentially the same as those in ASCE/SEI Section 11.1.2.

    Table 1 provides a summary of the chapters of ASCE/SEI 7-05 that contain earthquake load provisions that are referenced by the 2006 IBC. This article focuses on Chapters 11, 12, and 22.

    Table 1: Summary of chapters in ASCE/SEI 7-05 that are referenced by the 2006 IBC for earthquake load provisions

    Chapter      Title
    11           Seismic Design Criteria
    12            Seismic Design Requirements for Building Structures
    13           Seismic Design Requirements for Nonstructural Components
    15           Seismic Design Requirements for Nonbuilding Structures
    16           Seismic Response History Procedures
    17           Seismic Design Requirements for Seismically Isolated Structures
    18            Seismic Design Requirements for Structures with Damping Systems
    19           Soil Structure Interaction for Seismic Design
    20           Site Classification Procedure for Seismic Design
    21           Site-Specific Ground Motion Procedures for Seismic Design
    22           Seismic Ground Motion and Long Period Transition Maps
    23           Seismic Design Reference Documents

    Seismic ground motion values

    The provisions discussed below must be understood to correctly determine the appropriate seismic ground motion values.

    Mapped acceleration parameters—IBC Figures 1613.5(1), 1613.5(2), and ASCE/SEI Figures 22-1 and 22-2 contain contour maps of the conterminous United States giving SS and S1, which are the mapped maximum considered earthquake (MCE) spectral response accelerations at periods of 0.2 seconds and 1.0 seconds, respectively, for a Site Class B soil profile and 5-percent damping. Similar maps for specific regions of the conterminous United States, Alaska, Hawaii, Puerto Rico, and U.S. Commonwealths are given in IBC Figures 1613.5(3) through 1613.5(14) and ASCE/SEI Figures 22-3 through 22-14.

    In lieu of the maps, MCE spectral response accelerations can be obtained from the Ground Motion Parameter Calculator from the U.S. Geological Survey website: Accelerations are output for a specific latitude-longitude or zip code.

    Site class—Six site classes are defined in IBC Table 1613.5.2 and ASCE/SEI Table 20.3-1. A site is to be classified as one of these six site classes based on one of three soil properties (soil shear wave velocity, standard penetration resistance or blow count, and soil undrained shear strength) measured over the top 100 feet of the site. Steps for classifying a site are given in IBC 1613.5.5.1 and ASCE/SEI 20.3. Methods of determining the site class where the soil is not homogeneous over the top 100 feet are provided in IBC 1613.5.5 and ASCE/SEI 20.4.

    When soil properties are not known in sufficient detail to determine the site class in accordance with code provisions, Site Class D must be used, unless the building official requires that Site E or F must be used at the site.

    Site coefficients and adjusted MCE spectral response acceleration parameters — Once the mapped spectral accelerations and site class have been established, the MCE spectral response acceleration for short periods SMS and at 1-second period SM1 adjusted for site class effects are determined by IBC Eqs. 16-37 and 16-38, respectively, or ASCE/SEI Eqs. 11.4-1 and 11.4-2, respectively.

    Design spectral response acceleration parameters—Five-percent damped design spectral response accelerations at short periods SDS and at 1-second period SD1 are determined by IBC Eqs. 16-39 and 16-40, respectively, or ASCE/SEI Eqs. 11.4-3 and 11.4-4, respectively.

    The design ground motion is 2/3 = 1/1.5 times the soil-modified MCE ground motion; the basis of this factor is that it is highly unlikely that a structure designed by the code provisions will collapse when subjected to ground motion that is 1.5 times as strong as the design ground motion.

    Flowchart 1 can be used to determine the required seismic ground motion values.

    Seismic Design Category
    All buildings and structures must be assigned to a Seismic Design Category (SDC) in accordance with IBC 1613.5.6 or ASCE/SEI 11.6. In general, a SDC is a function of occupancy or use and the design spectral accelerations at the site.

    Occupancy categories are defined in IBC Table 1604.5 and ASCE/SEI Table 1-1. An importance factor I is assigned to a building or structure in accordance with ASCE/SEI Table 11.5-1 based on its occupancy category.

    The SDC is determined twice: first as a function of SDS by IBC Table 1613.5.6(1) or ASCE/SEI Table 11.6-1 and second as a function of SD1 by IBC Table 1613.5.6(2) or ASCE/SEI Table 11.6-2. The more severe of the two governs. The SDC may be determined by IBC Table 1613.5.6(1) or ASCE/SEI Table 11.6-1 alone provided all of the four conditions listed under IBC 1613.5.6.1 or ASCE/SEI 11.6 are satisfied.

    Conditions under which SDC E and SDC F are to be assigned are also given in IBC 1613.5.6 and ASCE/SEI 11.6.

    Equivalent Lateral Force Procedure
    The provisions of the Equivalent Lateral Force Procedure are contained in ASCE/SEI 12.8. This analysis procedure can be used for all structures assigned to SDC B and C, as well as for some types of structures assigned to SDC D, E, and F. Requirements on the type of procedure that can be used to analyze the structure for seismic loads are given in ASCE/SEI 12.6 and are summarized in Table 12.6-1. The permitted analytical procedures depend on the SDC, the occupancy of the structure, characteristics of the structure (height and period), and the presence of any structural irregularities.

    Seismic base shear—The seismic base shear V is determined as a function of the design response accelerations SDS and SD1, the response modification coefficient R (Table 12.2-1), the importance factor I, the fundamental period of the structure T (ASCE/SEI 12.8.2), and the effective seismic weight W (ASCE/SEI 2.7.2).

    Vertical distribution of seismic forces—The seismic base shear V is distributed over the height of the building in accordance with ASCE/SEI 12.8.3. For structures with a fundamental period less than or equal to 0.5 seconds, V is distributed linearly over the height, varying from zero at the base to a maximum value at the top. When T is greater than 2.5 seconds, a parabolic distribution is to be used. For a period between these two values, a linear interpolation between a linear and parabolic distribution is permitted or a parabolic distribution may be utilized.

    Horizontal distribution of forces—The seismic design story shear Vx in story x is the sum of the lateral forces acting at the floor or roof level supported by that story and all of the floor levels above, including the roof. The story shear is distributed based on the lateral stiffness of the diaphragm.

    For flexible diaphragms, Vx is distributed to the vertical elements of the seismic force-resisting system based on the area of the diaphragm tributary to each line of resistance.

    For diaphragms that are not flexible, Vx is distributed based on the relative stiffness of the vertical resisting elements and the diaphragm. Inherent and accidental torsion must be considered in the overall distribution (ASCE/SEI and Where Type 1a or 1b irregularity is present in structures assigned to SDC C, D, E, or F, the accidental torsional moment is to be amplified in accordance with ASCE/SEI (see Figure 12.8-1).

    Story drift determination—Design story drift Δ is determined in accordance with ASCE/SEI 12.8.6 and is computed as the difference of the deflections δx at the center of mass of the diaphragms at the top and bottom of the story under consideration (see Figure 12.8-2).

    The deflections δx at each floor level are obtained by multiplying the deflections δxe (the deflections determined by an elastic analysis using the code-prescribed forces applied at each floor level) by the deflection amplification factor Cd in Table 12.2-1 and dividing by the importance factor I. Limits on the design story drifts are given in ASCE/SEI 12.12.

    P-delta effects—Member forces and story drifts induced by P-delta effects must be considered in member design and in the evaluation of overall stability of a structure where such effects are significant. Equation 12.8-16 can be used to evaluate the need to consider P-delta effects. Equation 12.8-17 is used to check if the structure is potentially unstable; this equation must be satisfied even where computer software is utilized to determine second-order effects.

    Flowchart 3 contains the requirements of the Equivalent Lateral Force Procedure.

    David A. Fanella, Ph.D., S.E., P.E., is associate principal and Director of New Structures in the Chicago office of Klein and Hoffman, Inc. He can be reached at

    How to determine structural loads Parts 1, 2, and 3 discussed snow, rain, and wind loads and were printed in July, August, and November. More information, including additional flowcharts and worked-out design examples, can be found in the 2008 ICC publication Structural Load Determination Under 2006 IBC and ASCE/SEI 7-05.