This article is Part II of an article published in the July 2011 issue of Structural Engineer and briefly discusses changes in ASCE 7-10 seismic design provisions, with scope limited to those changes found in Chapter 12.
Redundancy: Clarificationof height-to-length ratio (Figure 12.3-2)
The definition of height-to-length ratio of shear walls and wall piers has been clarified for the purpose of determining the redundancy coefficient ρ. Wall height is from the top of a floor to the underside of the horizontal framing for the floor above, rather than to the top of the floor above. Four-feet-long shear walls are thus sufficient to produce a redundancy factor of one for top-of-floor to top-of-floor height exceeding 8 feet, provided the wall height does not exceed 8 feet.
Increase in forces due to irregularities for SDC D-F (Section 126.96.36.199)
The ASCE 7-05 requirement concerning increase in forces in diaphragm elements due to irregularities for SDC D-F has been simplified by revising language and creating an exception. The change also corrects the incorrect reference to the equivalent lateral force base shear in ASCE 7-05 Section 12.8.1 (and, by implication, the corresponding vertical distribution) and refers to the diaphragm design force in ASCE 7-10 Section 188.8.131.52 instead.
Conditions where value of ρ is 1.0 (Section 184.108.40.206)
The redundancy factor can now be taken equal to one in the design of structural walls for out-of-plane forces, including their anchorage. The intent never was to penalize out-of-plane wall designs for non-redundant seismic force-resisting systems.
Allowable stress increase for load combinations with overstrength (Section 220.127.116.11)
Where allowable stress design methodologies are used in conjunction with load combinations with overstrength, allowable stresses are permitted to be determined using an allowable stress increase of 1.2. The accompanying text has been changed in ASCE 7-10 to: “This increase shall not be combined with increases in allowable stresses or load combination reductions otherwise permitted by this standard or the material reference document except for increases due to adjustment factors in accordance with AF&PA NDS.”
The reference in ASCE 7-05 was to duration of load increases. Several adjustment factors for the design of wood construction can result in increases to the reference design values of the AF&PA NDS; some examples are the flat use factor, repetitive member factor, buckling stiffness factor, and bearing area factor. These factors are material-dependent in much the same manner as load duration factor.
Permitted analytical procedures (Table 12.6-1)
Significant beneficial changes have resulted in revised provisions for when dynamic analysis is required, as summarized in Figure 1. (Note: ELF is equivalent lateral force procedure)
Effective seismic weight (Sections 12.7.2 and 18.104.22.168)
What is required to be included in the effective seismic weight of a building is better defined. Weight of landscaping and other materials at roof gardens and similar areas is specifically required to be included. Floor live load in public garages and open parking structures need not be included.
Minimum design base shear (Section 22.214.171.124)
The minimum design base shear of 0.044 SDSIe W, applicable for SDC B-F, was part of ASCE 7-02 and the 2000 and 2003 IBC. However, when the third (constant-displacement) branch, starting at TL, was added to ASCE 7-05, this minimum base shear was deleted in favor of just 1 percent of weight, which is a structural integrity minimum, applicable irrespective of SDC.
ASCE subsequently processed a Supplement No. 2 to ASCE 7-05, reinstating this minimum design base shear. Supplement No. 2 was adopted by the 2009 IBC. ASCE 7-10 has now incorporated Supplement No. 2 in its body.
Approximate fundamental period (Table 12.8-2)
The approximate period formula is now Ta = 0.03hn0.75 for steel eccentrically braced frames in accordance with Table 12.2-1 lines B1 or D1 and steel buckling restrained braced frames.
Amplification of accidental torsional moment (Section 126.96.36.199)
The ASCE 7-05 exception, reading: “The accidental torsional moment need not be amplified for structures of light-frame construction” has been deleted.
Story drift determination (Section 12.8.6)
Many computer programs can explicitly provide drift ratios; however, such computer programs often do not use the same vertically aligned points to compute these ratios, thus yielding inaccurate measures of drift. A sentence has been added in Section 12.8.6 to permit vertical projections of points when centers of mass do not align vertically. Other clarifications have been made in drift computation provisions.
Minimum base shear for computing drift (Section 188.8.131.52)
The minimum design base shear of 0.044 SDS Ie W, applicable to all structures irrespective of SDC, need no longer be considered for computing drift.
P-Delta effects (Section 12.8.7)
The importance factor Ie has now been included in the denominator of the expression for the stability coefficient (q), Equation (12.8-16).
Scaling of drifts in modal response spectrum analysis (Section 184.108.40.206)
Provision has been added for scaling of drifts where the near-fault minimum base shear equation (Eq. 12.8-6) governs. Where the combined response for the modal base shear (Vt) is less than 0.85 Cs W, where Cs is determined in accordance with Eq. 12.8-6, drifts are required to be multiplied by 0.85 Cs W/Vt.
Diaphragm and collector design forces (Section 220.127.116.11)
It has been clarified that diaphragm design forces are earthquake load effects QE as used in the load combinations of Section 12.4.
Important clarification has been provided concerning design forces for collector elements and their connections in structures assigned to SDC C-F, as summarized below:
|Table 1: Governing collector design force: SDC C, D, E, and F|
Design for out-of-plane forces (Section 12.11.1)
In ASCE 7-05 there was no logical path for out-of-plane structural wall forces to be included in seismic load combinations because they were not specifically defined as either V or Fp; QE was identified as derived only from V or Fp. This is resolved by stating Fp = 0.4SDS Ie times the weight, with a minimum of 10 percent of the weight, of the structural wall.
Structural separation and property line setback (Sections 12.12.3)
Structural separation and setback provisions were included in the 2000 and the 2003 IBC. However, when much of the structural provisions were deleted from the 2006 IBC and incorporated only through reference to ASCE 7-05, the building separation provisions got deleted, overlooking the fact that ASCE 7-05 did not include any such requirements. This error was rectified by having the building separation provisions included in the 2009 IBC by way of a modification to ASCE 7-05. This modification has now been incorporated in ASCE 7-10.
Anchorage of structural walls and transfer into diaphragms (Sections 12.11.2 and 18.104.22.168)
There are several substantive changes to the anchorage provisions. First, there is no longer any distinction between concrete and masonry walls and all walls. Second, the lower-bound anchorage force of 0.10Wp (280 plf in the case of concrete and masonry walls) is replaced by a minimum force of 0.2 ka Ie Wp. The multiplier ka increases from 1.0 to 2.0 as the span of a flexible diaphragm increases from zero to 100 feet or more. The span is considered to be zero for a rigid diaphragm, yielding a ka of 1.0. Third, the anchorage design force for walls supported by flexible diaphragms used to be twice that for walls supported by rigid diaphragms. ASCE 7-10 provides a gradual increase in anchorage design force through the multiplier ka. In another important change, where the anchorage is not located at the roof and all diaphragms are not flexible, the anchorage design force given by Eq. (12.11-1) may be reduced through multiplication by (1 + 2 z/h /3, where z is the height of the anchor above the base of the structure and h is the height of the roof above the base. This is consistent with the variation in seismic design force for nonstructural components attached to a building, along the height of the building, as given in Section 13.3.1.
Members spanning between structures (Section 12.12.4)
Large relative movements of seismically separate building portions may lead to loss of gravity support for members that bridge between two portions, unless supports are designed to accommodate such displacements. Five requirements are given for the first time for estimating these movements.
It should be obvious from the above that changes in the seismic design provisions from the 2005 to the 2010 edition of ASCE 7 are numerous and often quite substantive. Arguably, the biggest changes are those in the seismic ground motion maps discussed in Part I of this article. For a fuller treatment of the changes, including changes to the nonstructural components and nonbuilding structures provisions, 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 in 2010 by ASCE and available at www.skghoshassociates.com
S.K. Ghosh Associates Inc. is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. President S. K. Ghosh, Ph.D., and Vice President Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at firstname.lastname@example.org and email@example.com, respectively, or at www.skghoshassociates.com