Bridges are strategic structures and any failure of them due to earthquakes can affect the transportation network causing substantial economic losses and life safety threats. Considering the likelihood of occurrence of aftershocks following a major earthquake, there is a need to specify the operational state of a mainshock-damaged bridge in order to guarantee that it has the ability to resist aftershocks against collapse.
In this paper, component and system-wide aftershock fragility functions of the multi-span simply supported concrete girder bridges are determined. For this purpose, two different probabilistic aftershock demand models proposed by Mackie & Stojadinović (2004) and Jeon et al. (2015) have been utilized and the resulting fragility functions for different post-mainshock damage states of the mainshock-damaged bridge are produced by performing back-to-back nonlinear dynamic analyses on the 3D finite-element analytical bridge models. The generated bridge system median fragilities in slight, moderate, and extensive post-mainshock states have shown a small difference, although, the aftershock demand model presented by Jeon et al. (2015) resulted in more conservative median values. Since, the correlation between the mainshock and aftershock, as well as the aftershock intensity measure are taken into account in the demand model proposed by Mackie & Stojadinović (2004), and considering the negligible difference between the resulting fragilities, the fragility functions generated using the model presented by Mackie & Stojadinović (2004) led to more precise results.
It was found that bearings are the most vulnerable components when the damaged bridge is in the slight damage state after the mainshock with due consideration of the potential aftershocks. Moreover, in the moderate and extensive post-mainshock damage states, columns have been proved to be the most fragile components. These findings can be useful for critical decision making regarding the retrofit, repair, and inspection prioritization in the aftermath of an earthquake.