NEHRP site amplification factors and the NGA relationships

The Next Generation Attenuation (NGA) relationships were developed using a large database of strong motion recordings of shallow crustal earthquakes. The functional form of the relationships provides an opportunity to assess and update if necessary the site class coefficients provided in the NEHRP Provisions and ASCE-7. Site amplification factors estimated using the Boore and Atkinson, Campbell and Bozorgnia, and Chiou and Youngs NGA relationships are computed and averaged for a combination of magnitude, distance, faulting type and period and for NEHRP/ASCE-7 Site Classes B, C, D and E and boundaries A/B, B/C, C/D and D/E. The average NGA site amplification factors show a clear dependency on period for V S30 smaller than 270 m/s and are substantially greater than the current NEHRP site class coefficients in some cases. The use of the NEHRP/ASCE-7 site class coefficients and spectral demands for the reference site condition computed using the NGA relationships may produce erroneous results. A family of site class coefficients is proposed to address these issues in a format suitable for inclusion in the NEHRP Provisions and ASCE-7.