Code-stipulated Design Spectra
NEHRP Spectra

The 1997 NEHRP Recommended Provisions for Seismic Regulations for New Buildings has developed a more detailed procedure for estimating site specific design response spectra. This spectrum, with minor changes, will be incorporated into the year 2000 International Building Code (IBC). This spectrum is based on a maximum considered earthquake (MCE) with a 2% probability of occurence in 50 years (2500 year recurrence interval).

Detailed maps, which are based on probabilistic estimates by USGS [1], provide spectral ordinates at periods of 0.2 and 1.0 seconds. These maps are for medium rock sites, but factors to account for soil conditions are included.

However, the IBC implementation of these provisions will be for a single design level event with a probability of 10% in 50 years. Since the code uses a single-level indirect method rather than performance-based engineering, only one level of event is specified. This event is taken to be 2/3 of the MCE event from the NEHRP provisions. For California, this relationship is about right, but for other areas (such as New Madrid) this results in unnecessarily large events. However, lower standards for design (e.g. ordinary moment frames) are permitted in these areas.


The basic form of the spectrum looks like a typical code or Newmark and Hall spectrum. The corner points are:

To = 0.2Sd1/Sds
Ts = Sd1/Sds

V = CsW
Cs = Sds/(R/I) < Sd1/(TR/I)