An accurate estimation of seismic demand is essential for the safe and economical design of buildings on soft soils. The ASCE/SEI 7-16 Section 11.4.8 exceptions were introduced to simplify seismic design by allowing engineers to omit site-specific ground motion analyses for Site Classes D and E under prescribed conditions. However, these simplifications can significantly influence the calculated base shear and drift responses, particularly for long-period structures. This study investigates the impact of these exceptions on the seismic response of 4-, 8-, and 12-storey reinforced-concrete moment-resisting frames. The analyses were conducted in ETABS using both the Equivalent Lateral Force (ELF) and Modal Response Spectrum (MRS) procedures in accordance with ASCE/SEI 7-16. Sensitivity studies were conducted to assess the impact of different long-period spectral accelerations (S1). When using the ELF procedure, the results demonstrate that the base shear estimates are conservative for Site Class D, especially for the middle- and long-period structures, where the 1.5 amplification factor increases the design forces significantly. The longer S1 values clearly indicate that this conservatism is most pronounced. However, the MRS analysis for Site Class E indicates that the base shear demands were disproportionately high for taller structures, which is a reasonable assumption for longer-period soil amplification. The overall conclusion from this study is that while the exceptions maintain a high level of design safety, especially for low-rise simple structures, the generalised application of these exceptions is likely to yield overly conservative seismic demands for structures with greater flexibility.
Suwondo et al. (2026) studied this question.