Ductile end-diaphragms for seismic retrofit of slab-on-girder steel bridges

Steel bridges are frequently supported by seismically vulnerable substructures, as clearly demonstrated by recent earthquakes. The seismic retrofit of these nonductile substructures can be, in many cases, a rather costly operation. This paper investigates the adequacy of a seismic retrofit strategy that relies instead on ductile end-diaphragms inserted in the steel superstructure: the objective is to protect the substructure by replacing the steel diaphragms over abutments and piers with specially designed ductile diaphragms calibrated to yield before the strength of the substructure is reached. For a type of steel slab-on-girder bridge widely found in North America, this paper presents simplified analytical models as well as a step-by-step design procedure developed for three types of ductile diaphragm systems (such as shear panels, eccentrically braced frames, and triangular-plate added damping and stiffness devices), followed by results from nonlinear inelastic analyses conducted to investigate the seismic behavior of these retrofitted bridges. At this time, only bridges on stiff substructure are considered, although a few examples are presented to illustrate the potential inadequacy of this retrofit approach for bridges on flexible substructures.