The effect of joint gap size on the seismic performance of railway bridges

Boundary conditions play an important role in the response of bridges to different actions, particularly dynamic loads. Gap closure at end joints, resulting in activation of the abutment-backfill-embankment system, considerably affects the dynamic characteristics and, hence, the seismic response of the bridge. In particular, consideration of abutment-backfill flexibility may often result in substantial reduction of the contribution of piers to the bridge response under earthquake loading. Moreover, gap size and closure are related to various damage stages including backfill deformation/damage and deck unseating, while the varying boundary conditions during an earthquake affect the inelastic behaviour of critical bridge components (i.e. piers, piles), as well as the damage sequence. The use of gap sizes larger than the ones resulting from Eurocode 8 provisions is a common practice in seismic design of bridges in Greece; this is apparently considered to be on the safe side and conveniently relieves the designer from the difficulty of accounting for gap closure effects in the analysis. However, this means over-dimensioning of bridge joints, with subsequent increase in both initial and maintenance costs, while it is not certain that preventing activation of the abutment-backfill system under seismic actions exceeding the design ones increases the safety of the bridge. In this context, the aim of this paper is to study the effect of gap size on the seismic performance of concrete bridges, providing insight into the distribution of seismic action effects among the key components of the bridge i.e. piers and abutments, accounting for varying gap sizes and hence boundary conditions. The behaviour of an existing railway bridge with a passive ssytem is evaluated using inelastic response history analysis for a set of design spectrum compatible artificial accelerograms, considering different gap sizes for the end joints, i.e. those resulting from Eurocode 8, as well as higher and lower values. Analyses are carried out for different levels of seismic action, up to twice the design one.