ASCE 7-05 Figure 11.4-1 Design Response Spectrum cenews.com /article/4842/asce_7_05_figure_11.4_1_design_response_spectrum

December 2006 » Columns » CODE SIMPLE

Code Simple: ASCE 7-05 Figure 11.4-1 Design Response Spectrum

S. K. Ghosh, Ph.D., and Susan Dowty, S.E.

The design response spectrum in the 2006 IBC is based on work developed by Newmark and Hall andconsists of the three color-coded regions for short, long, and very long period ranges, shown in Figure 1below.

In the 2003 IBC, only thefirst two curves wererepresented because thethinking was that relativelyfew structures had a periodlong enough to fall into thevery long period range.Also, a minimum designbase shear of 0.044ISDSWwas imposed in all seismicdesign categories in ASCE7-02 and the 2003 IBC.This minimum was deletedat the same time that acompanion code changeadded the third curve to thedesign response spectrum.(Note that the "ramp" up tothe "flat top" does not countas a branch).

The period at which the new third branch starts, referred to as the "Long-Period Transition Period, TL" isdetermined from new maps, which are similar to zone maps, for all 50 states. These maps are provided inFigures 22-15 through 22-20 of ASCE 7-05 and are shown in Figure 2 below.

One must locate theproject site on these mapsto determine TL, whichranges between 4 and 16seconds, depending uponthe location.

The procedure used to develop the TL maps is described in the Commentary to the National EarthquakeHazards Reduction Program’s (NEHRP) Recommended Provisions for Seismic Regulations for New

Buildings and Other Structures.

Answers to FAQ’s:

Q: I have a question regarding changes in seismic provisions in ASCE 7-05 compared to ASCE 7-02. Theminimum base shear equation in ASCE 7-02, which was a function of SDS, has been replaced by1percent W in ASCE 7-05. In some cases this change can reduce the minimum base shear by 50 percent.I’d greatly appreciate it if you can give an explanation as to the basis for which this minimum requirementhas changed.

A: The reason for the removal of the minimum base shear (0.044 ISDSW) that was a function of SDS wasthat there is now the constant-displacement branch to the design spectrum, so that no arbitrary minimumshould be necessary. The proponents of the removal did not even want the 1 percent minimum, but had torelent when it was pointed out that it was a structural integrity minimum, required even in SDC A (seeASCE 7-02 Section 11.7.2). However, you are correct that the seismic design force levels for tall buildingshave been changed rather drastically. For buildings and other structures where S1 ≥ 0.60g, a minimumbase shear of 0.5S1W/(R/I) is required (see ASCE 7-05 Equation 12.8-6). This equation produces resultsthat exceed those associated with the former equation 0.044ISDS, which is consistent with near-sourceeffects.

S.K. Ghosh Associates, Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., andLaguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the developmentand interpretation of national structural code provisions. They can be contacted at skghosh@aol.com anddowtyskga@cox.net, respectively.