Comparison of seismic fragility of special moment frames in recent editions of ASCE 7 and ACI 318 regulations

The seismic safety levels provided by the three most recent editions of ACI and ASCE regulations for new moment frame structures are determined. Five special RC moment frames having 3, 5, 10, 15, and 20 stories are designed separately based on ACI 318-99, ACI 318-05, and ACI 318-11, and the associated seismic regulations of UBC 97, ASCE 7-05, and ASCE 7-10. A suit of 10 consistent earthquake records is selected for non-linear dynamic analysis of buildings. Incremental dynamic analysis is conducted, and the corresponding fragility curves are calculated. The comparison of results shows that seismic safety of special moment frame buildings has considerably improved from ACI 318-99 to ACI 318-11, and from UBC 97 to ASCE 7-10, owing its larger part to the improvements made in the two latter versions of the mentioned building codes. However, the safety enhancement is not uniform and is much less for buildings with larger fundamental periods, especially for periods larger than 1.0 sec. For structures with fundamental periods smaller than 1 sec, the collapse ratio has increased up to more than 70%. For larger fundamental periods, the increase is less than 20%. Changes in ASCE7 with regard to the R-factor and coefficients of the load combination equations and its introduction of stricter requirements for the allowable story drift are pinpointed to be the most important factors. For the ACI codes, stricter requirements regarding the confinement of the plastic hinges and configuration of the longitudinal reinforcement are recognized to be the most influential changes.