ASCE 7-10 – Section 20.3.1 requires performing site response analysis when site class is F and fundamental vibration period of structure is greater than 0.5 seconds. In this article we want to achieve a clear understanding of this important requirement of the Code.

Building Codes of almost all countries, including ASCE 7-10, provide site amplification coefficients for typical ground conditions. These factors, namely F_a and F_v, are used to adjust mapped spectral response accelerations S_s and S_1 for site conditions. ASCE 7-10 recommends typical values for F_a and F_v for different site classification which is determined based on insitu soil properties such as shear wave velocity, standard penetration, and/or undrained shear strength averaged over a 100 feet depth.

In the case that soil under a building is liquefiable (site class F) the soil properties (V_s, N_{60}, and shear strength) will drastically decrease during earthquake shaking due to liquefaction. Because soil parameters varies overtime during earthquake event, the code provisions for determination of F_a and F_v would not be applicable anymore. To this end a site-specific site response analysis is required to account for soil softening behavior in determination of MCE_R spectral response acceleration. Estimation of soil response during liquefaction requires an advanced numerical modeling.

Because of low stiffness, liquefied soil would mainly transfer motion pulses with long periods. Therefore, structures with short fundamental periods would NOT be excited considerably under the earthquake shakings. This is why ASCE 7-10 exempts site response analysis if the fundamental period of structure is less than 0.5 sec.