Optimum Design of Seismic Joints in Bridges

A key aspect of seismic design of bridges is the appropriate selection of joint gaps, an issue that has hardly received proper attention so far, while pertinent code provisions are far from comprehensive. End gaps define the boundary conditions of the bridge and affect its dynamic response; their proper design can lead to an improved structural performance under dynamic actions. This contribution poses the problem, assesses the effect of gap size on the seismic response of bridges and puts forward a methodology for optimising this size, using a number of criteria such as maintaining the functionality of the bridge for moderate earthquakes, ensuring the safety of the bridge under earthquakes stronger than that used for code design, and, last but not least, optimising the cost of the bridge (as affected by joint gap size) using a life-cycle cost approach. The idea of the ‘Dynamic Intelligent Bridge’ recently coined by the first author is revisited here, wherein current bridge joints that have a fixed width are substituted by variable-width joints and, under seismic loading, the joint gap is optimised either with a one-off adjustment, or continuously (in real time) through semi-active control. In all cases a novel device is used that permits this improved behaviour of the joints, the moveable shear key (MSK), a device for blocking the movement of the bridge deck, which has the possibility to slide, hence varying the size of the existing joint gap.