Structural System Costs and IBC 2000

Under the currently-adopted International Building Code (2000), seismic design requirements for Georgia have become more stringent, and consequently, in most cases, structures will be more expensive. The change is due to the adoption of Maximum Considered Earthquake (MCE) maps having a longer return period. The previous code (Standard Building Code 1994) used MCE maps having a return period of about 475 years (10% probability of exceedance in 50 years), while IBC 2000 MCE maps use a return period of about 2,500 years (2% probability of exceedance in 50 years). The MCE in IBC 2000 will be of greater magnitude than in the old SBC. In short, the design forces (base shear) to be resisted by structures will be greater under IBC 2000. In most cases, projects hardest hit by IBC 2000 seismic provisions will be 3- to 6-story buildings.

Greatly affecting the magnitude of the seismic forces to be used in design is the Site Class. Conceptually, the Site Class is the approach by which the code considers the composition of the soil above the rock surface and provides for attenuation or amplification of ground motions. Using standard penetration testing (ASTM D 1586), which is the most common soil sampling technique used in Georgia and the U.S., will often yield a Site Class D. Although it could be worse, structural engineers will not be happy with a "D". Owners and architects will increasingly hear groans and moans from structural engineers once they get the news of having to design using a Site Class "D". And the increased cost of the structural system will make owners and architects cringe.

Although a Site Class D is not avoidable on many sites, there are some options. The Site Class determination can be refined by obtaining the actual shear wave velocity (Vs) of the materials in the top 100 feet of soil/rock, instead of using the generally more conservative correlations provided in the code to allow the use of standard penetration testing data (blows per foot).

Based on our recent experience, even if a more refined Site Class determination still results in a Site Class D , the parameters used to derive the base shear, and therefore the magnitude of the base shear, can generally be reduced by performing a site-specific Probabilistic Seismic Hazard Analysis (PSHA). From section 1615.2 of IBC 2000, a PSHA "accounts for regional seismicity and geology; the expected recurrence rates and maximum magnitudes of events on known faults and sources; the location of the site with respect to these; near source effects if any; and the characteristics of subsurface site conditions."

Geo-Hydro can perform a PSHA and determine the Vs of overburden materials and underlying rock for your site. We can advise you early in the design process whether either approach, or both, may be beneficial to the project. Although not a magic bullet (nothing is), putting a little more effort up front by performing a PSHA and determining V s can provide significant savings for most projects.

Luis E. Babler, P.E.

February 2003