Seismic Vulnerability Assessment of Steel Moment Resisting Frames with Consideration of Mainshock Aftershock Seismic Sequence

This study assessment of the collapse capacity and mechanism of special steel moment frames (SSMFs) with considering earthquake shock sequences for structures height from 4 to 20 stories. The collapse capacity is expressed in terms of the Sa(T1) value at which the system reaches the collapse state. Collapse identification relies on the capability of the model to accurately simulate damage of members and the resulting behavior deterioration. The softening in the lateral response expressed in terms of maximum interstory drift (MID) when it is expressed against the Sa(T1) values is thus used to identify collapse occurrence. The mainshock-aftershock (MS-AS) effect and the uncertainties associated with it are simulated by utilizing a set of 32 natural record pairs. The level of damage induced by MS is represented by two parameters including MID and peak residual story drift (PRSD). Using these parameters, the MS is applied so as to various pre-selected damage levels are imposed. For each MS damage level, an incremental dynamic analysis (IDA) is performed using the AS record that follows MS and a free vibration phase. The AS intensity causing the collapse state is finally identified and respected for evaluating the effect of MS damage level. The correlation between the median collapse Sa’s, called aftershock median collapse capacity, and the MS damage indicated by MID and PRSD parameters is quantified. The collapse mechanism under the AS is also investigated by studying the distribution of maximum ductility demand (MDD) and contribution of members in the total hysteretic energy absorption, called energy absorption contribution (EAC). This investigation revealed that the EAC parameter provides better metrics for studying the collapse mechanism which is remarkably affected by the MS-AS sequence. Accordingly, the role of elements experiencing damage under the MS becomes less prominent in absorbing the AS energy and is substituted by contribution of elements with less deterioration under the MS. The members’ EAC is also shown to be affected by the percentage of the energy imposed by each individual shock as a fraction of the total energy imposed by the MS-AS sequence.