Seismic Design Provisions of ASCE 7-10: Changes from ASCE 7-05

July 2011 » Features » CODES
Susan Dowty, S.E.

The ASCE 7 Standard Minimum Design Loads for Buildings and Other Structures is the document the International Building Code (IBC) relies upon for its structural provisions. ASCE 7-05, the standard referenced in the 2006 and 2009 IBC, has been replaced by ASCE 7-10 in the 2012 IBC. Major revisions were made to the wind design, seismic design, and other provisions of ASCE 7-05. Changes in the seismic design provisions are the subject of this article, with emphasis on Chapter 11, Seismic Design Criteria and Chapter 12, Seismic Design Requirements for Building Structures.

Ground motion maps
The seismic ground motion values of ASCE 7 are spectral response accelerations (for 5 percent of critical damping) at periods of 0.2 sec (SS) and 1.0 sec (S1) corresponding to the Maximum Considered Earthquake (MCE) for Site Class B (soft rock). Four significant changes were made to these seismic ground motion maps in ASCE 7-10.

Change 1: USGS updates
USGS has updated some source zone models, used Next Generation Attenuation (NGA) relationships – to the exclusion of the old attenuation relationships in the western United States – and used new attenuation relationships in addition to old relationships in the central and eastern United States. If only this one change had been made, ground motion – particularly long-period ground motion – would have decreased significantly (by 50 percent or more) in many parts of the United States.

Change 2: Risk-targeted ground motion
The probabilistic portions of the MCE ground motion maps in ASCE 7-05 provide ground motion values that have a 2 percent probability of being exceeded in 50 years. While this approach provides for a uniform likelihood (except in deterministic areas) that the ground motion would not be exceeded, it does not provide for a uniform probability of failure for structures designed for that ground motion. Adjustments to the ground motion values that result in structures with uniform collapse probabilities (1 percent in 50-year collapse risk target) have been made in the ASCE 7-10 maps. The Risk-Targeted Maximum Considered Earthquake (MCE) ground motion is designated MCER ground motion.

Relative to the probabilistic MCE ground motions in ASCE 7-05, the risk-targeted ground motions are smaller (by as much as about 30 percent) in the New Madrid region, near Charleston, S.C., and in coastal Oregon, with relatively little (less than 15 percent) change almost everywhere else.

Change 3: Maximum-direction ground motion
The procedure used to develop the statistical estimate of ground motion in the past resulted in the geometric mean (geo-mean) of two orthogonal components of motion at a site. The geo-mean was calculated as the square root of the product of the two horizontal response spectral accelerations at each period of interest. The quantities mapped in ASCE 7-10 correspond to ground motion in a direction that produces the maximum structural response.

The switch from —€œgeo-mean—€ ground motion to maximum-direction ground motion has resulted in increases in short-period ground motion by a factor of 1.1 and in long-period ground motion by a factor of 1.3.

Change 4: Modified deterministic ground motions
In regions of high seismicity, such as in many areas of California, the seismic hazard is typically controlled by large-magnitude events occurring on a limited number of well-defined fault systems. Probabilistic ground motions can be significantly larger than deterministic ground motions, based on characteristic magnitudes of earthquakes on these known active faults. For these regions, MCE ground motions were, and MCER ground motions are, determined directly by deterministic methods based on a conservative estimate of the ground shaking associated with characteristic earthquakes of well-defined fault systems.

Deterministic ground motions should account for uncertainties associated with near-fault ground motions, particularly at longer periods. This desirability led to a more statistically appropriate estimate of 5 percent damped spectral response accelerations than those based on 150 percent of median ground motions, as used in ASCE 7-05. Eighty-fourth-percentile ground motions are used in ASCE 7-10, which effectively requires increasing median ground motions by 180 percent.

Observations
1) On a regional basis, the changes from ASCE 7-05 to ASCE 7-10 cause only a slight increase or decrease in design ground motions, on average. Notable exceptions are short-period ground motions in the central and eastern United States, which are substantially lower. Other exceptions are for certain city sites such as St. Louis, Chicago, and New York, where the changes lower the Seismic Design Category (SDC).
2) In the western United States, the changes from ASCE 7-05 to ASCE 7-10 cause a modest increase, or decrease, in design ground motions (plus or minus 10 percent).
3) For certain city sites, the changes from ASCE 7-05 to ASCE 7-10 substantially change design ground motions, due primarily to changes in underlying hazard functions. For instance, there have been sizable increases in the San Bernardino, Calif., area and significant decreases in the San Diego area.

Liquefaction potential evaluation
Section 11.8.3 (Additional Geotechnical Investigation Report Requirements for SDC D through F) has been modified to require that evaluations of liquefaction potential be made for MCE, rather than design earthquake, ground motions.

The provision also requires that liquefaction potential evaluations are conducted using mapped peak ground acceleration adjusted for site effects, rather than using the current approximation for peak ground acceleration equal to the short-period spectral acceleration divided by 2.5. The new maps provide substantially more accurate values for PGA, since they are based on PGA attenuation relationships. PGA is modified for site class effects using site coefficient FPGA obtained from Table 11.8-1 in the document. Values of FPGA in the table are identical to Fa in Table 11.4-1 but are a function of PGA, rather than SS.

The mapped peak ground accelerations in Figure 22-7 through 22-11 are geo-mean values and not risk-targeted values. These are designated as MCEG peak ground accelerations, unlike the spectral accelerations in Figures 22-1 through 22-6, which represent MCER ground motion.

Changes in seismic design requirements for building structures
Changes in ASCE 7-10 Chapter 11 that are not strictly related to earthquake ground motion and Chapter 12 changes are discussed in this section.

Structural integrity
Section 11.7 (Design Requirements for SDC A) is now greatly reduced in size. Much of the text in ASCE 7-05 Section 11.7 has been transferred in modified form to Section 1.4 General Structural Integrity, a more logical location.

Design coefficients and factors for seismic force-resisting systems
The following significant changes have been made in Table 12.2-1.
1) Seismic design factors and height restrictions for bearing wall systems consisting of ordinary reinforced and ordinary plain autoclaved aerated concrete (AAC) masonry shear walls have been added. The values and restrictions are consistent with those in 2009 IBC Section 1613.6.4.
2) A newly defined seismic force-resisting system titled —€œCold-formed Steel – Special Bolted Moment Frame—€ – or CFS-SBMF – has been introduced, subject to a height limit of 35 feet for all SDCs, based on practical use considerations. Design provisions for the new system are provided in AISI S110, Standard for Seismic Design of Cold-Formed Steel Structural Systems – Special Bolted Moment Frames, which has been adopted by ASCE 7-10.

Vertical combination of structural systems
When different lateral force-resisting systems are vertically stacked, the ASCE 7-05 rule concerning seismic design coefficients was that the R-value could not increase and that the values of O0 and Cd could not decrease as one went down a building. According to ASCE 7-10, the R-value still cannot increase as one goes down a building; however, O0 and Cd now always must correspond to the R-value.

Steel cantilever column systems
ASCE 7-05 contained provisions for steel ordinary, intermediate, as well as special cantilever column systems. Previous editions of AISC 341 did not explicitly address cantilever column systems. Modifications made to Section 12.2.5.2 and Table 12.2-1 have been coordinated with parallel changes in the 2010 edition of AISC 341. AISC 341-10 does not have separate requirements for intermediate cantilever column systems. Consequently, this system has been removed.

Height limit for special steel plate shear walls
Steel special plate shear wall systems were first introduced in the 2005 editions of ASCE 7 and AISC 341. At that time, the inclusion of this system in the permitted height increase of ASCE 7-05 Section 12.2.5.4 was overlooked. A modification now includes these systems in the permitted height increase of ASCE 7-10 Section 12.2.5.4.

Steel ordinary and intermediate moment frames
Steel ordinary moment frame construction has been used for many years for tall, single-story buildings of varied types. ASCE 7-05 prohibited the use of ordinary and intermediate moment frames in higher seismic design categories for many of these structures. New exceptions are added for SDC D and E ordinary and intermediate moment frames. However, the exceptions apply only if a number of stringent restrictions are met.

Flexible diaphragm condition
ASCE 7-05 Section 12.3.1.1 set forth conditions under which certain diaphragms may be considered flexible for lateral force distribution purposes. 2006 and 2009 IBC (Section 1613.6.1) modified this ASCE 7-05 section to add one set of other conditions, the satisfaction of which would qualify a diaphragm as flexible. A modified set of the conditions included in this IBC modification is now part of ASCE 7-10 Section 12.3.1.1.

Horizontal structural irregularities
In ASCE 7-05 Table 12.3-1, torsional as well as extreme torsional irregularity was defined in terms of computed maximum story drift, including accidental torsion. Clarification is now provided that it is accidental torsion with the torsional amplification factor Ax = 1, to avoid iteration.

The revised definition of nonparallel systems irregularity clearly indicates that it exists only where the vertical elements are not parallel to the major orthogonal axes. The ASCE 7-05 text of —€œparallel to or symmetric about—€ was sometimes misread to require that the system be both parallel to and symmetric about the major orthogonal axes. By that reading, Figure 1 has a nonparallel systems irregularity. By the revised ASCE 7-10 definition, it does not.

Figure 1: Asymmetric seismic force-resisting systems.

Vertical structural irregularities
The definition of vertical structural irregularity Type 4 has been revised in ASCE 7-10 to read: —€œin-plane discontinuity in vertical lateral force-resisting element is defined to exist where there is an in-plane offset of a vertical seismic force-resisting element resulting in overturning demands on a supporting beam, column, truss, or slab.—€ This provides needed rectification of a deficient ASCE 7-05 definition.

Conclusion
Changes in the seismic design provisions from ASCE 7-05 to ASCE 7-10 are numerous and often quite substantive. Arguably, changes in the seismic ground motion maps are the most significant. Changes that could not be accommodated in this Part 1 will be discussed in Part 2 of this paper in the September 2011 issue of Structural Engineer. For a fuller treatment of all seismic changes, reference may be made to Significant Changes to the Seismic Load Provisions of ASCE 7-10: An Illustrated Guide by S. K. Ghosh, Susan Dowty, and Prabuddha Dasgupta, published by ASCE.

S.K. Ghosh Associates Inc. is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S. K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com.


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