The design response spectrum in the 2006 IBC is based on work developed by Newmark and Hall and consists of the three color-coded regions for short, long, and very long period ranges, shown in Figure 1 below.
In the 2003 IBC, only the first two curves were represented because the thinking was that relatively few structures had a period long enough to fall into the very long period range. Also, a minimum design base shear of 0.044ISDSW was imposed in all seismic design categories in ASCE 7-02 and the 2003 IBC. This minimum was deleted at the same time that a companion code change added the third curve to the design response spectrum. (Note that the “ramp” up to the “flat top” does not count as a branch).
The period at which the new third branch starts, referred to as the “Long-Period Transition Period, TL” is determined from new maps, which are similar to zone maps, for all 50 states. These maps are provided in Figures 22-15 through 22-20 of ASCE 7-05 and are shown in Figure 2 below.
One must locate the project site on these maps to determine TL, which ranges between 4 and 16 seconds, depending upon the location.
The procedure used to develop the TL maps is described in the Commentary to the National Earthquake Hazards 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. The minimum base shear equation in ASCE 7-02, which was a function of SDS, has been replaced by 1percent W in ASCE 7-05.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 requirement has changed.
A: The reason for the removal of the minimum base shear (0.044ISDSW) that was a function of SDS was that there is now the constant-displacement branch to the design spectrum, so that no arbitrary minimum should be necessary. The proponents of the removal did not even want the 1 percent minimum, but had to relent when it was pointed out that it was a structural integrity minimum, required even in SDC A (see ASCE 7-02 Section 11.7.2). However, you are correct that the seismic design force levels for tall buildings have been changed rather drastically. For buildings and other structures where S1 ≥ 0.60g, a minimum base shear of 0.5S1W/(R/I) is required (see ASCE 7-05 Equation 12.8-6). This equation produces results that exceed those associated with the former equation 0.044ISDS, which is consistent with near-source effects.
S.K. Ghosh Associates, Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President 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 email@example.com and firstname.lastname@example.org, respectively.