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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Seismic Performance of Slab-on-Girder Bridges Using New Ductile End-Diaphragm Retrofitting Systems
M. Lotfollahi and M. Mofid
Department of Civil Engineering, Sharif University of Technology, Tehran, Iran
M. Lotfollahi, M. Mofid, "Seismic Performance of Slab-on-Girder Bridges Using New Ductile End-Diaphragm Retrofitting Systems", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 251, 2006. doi:10.4203/ccp.83.251
Keywords: bridge, slab-on-girder, retrofit, nonlinear analysis, eccentrically braced frames, triangular-plate added damping.
In this paper the validity of several effective ductile end-diaphragm systems for seismic retrofit of slab-on-girder steel bridges are investigated. 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 investigation discusses the adequacy of several systems to present a seismic retrofit strategy that relies instead on ductile end-diaphragms inserted in the steel superstructures. In the other words, the substructure is protected by replacing the steel diaphragms on superstructures with specially designed ductile diaphragms calibrated to yield before the substructure reaches the initial yield point.
Although damage to superstructures components of these bridges is also possible, mostly in the form of buckling and, or connection fracture of diaphragm braces, damage to substructure components such as, abutments, piers, and bearings, have proven to be of far greater consequence, often leading to span collapses. Therefore, when the existing bridges are targeted for seismic rehabilitation, much attention is paid to these substructure elements. Generally, the current retrofitting process is to either strengthen or replace the existing nonductile members, increase their ductility capacity, or reduce the force demand on the vulnerable substructure elements using base isolation techniques or other structural modifications. Whereas all these approaches are proven effective solutions, only the base isolation concept currently recognizes that seismic deficiency attributable to substructure weaknesses may be resolved by operating elsewhere other than on the substructure itself. Moreover, all approaches can be costly, even base isolation in those instances when significant abutment modifications and other structural changes are needed to permit large displacements at the isolation bearings and lateral load redistribution among the piers.
A seismic retrofit strategy that relies instead on ductile end-diaphragms inserted in the steel superstructure, if effective, could provide an interesting alternative. In some cases, by replacing the steel diaphragms over abutments and piers with specially designed ductile diaphragms adjusted to yield before the strength of the substructure is reached, damage can be prevented from developing in the nonductile substructural elements, foundation, and bearings. Many types of systems capable of stable passive seismic energy dissipation could be used for this purpose. Among those, eccentrically braced frames (EBF) , steel triangular-plate added damping and stiffness devices (TADAS) , and chevron knee bracing (CKB)  have received particular attention in building applications.
Linear static and dynamic analyses were conducted on the two and three-dimensional models of the ductile end-diaphragm systems and the stiffness, and the other specifications of the proposed models are obtained and the results are compared. To provide computational efficiency and to allow a creative idea for the design procedure based on the graph and formulation, a simplified two-dimensional model capturing the essence of the three-dimensional behaviour of the slab-on-girder bridges was developed. Nonlinear inelastic static pushover analyses were conducted to illustrate the trilinear behaviour of the resulting ductile diaphragms and the expected extent of yielding at the design level. These analyses were conducted on the two-dimensional model of the end-diaphragms and they revealed that the yielding of the device, shear yielding of EBF and CKB systems, and flexural yielding of TADAS, were obtained at device yield strength of the ductile devices and all other members remained elastic while the energy dissipating devices yielded. To validate the ductile diaphragm seismic retrofit concept, the two-dimensional structural model was also analyzed using nonlinear inelastic time history analyses and considering three different earthquake excitations scaled to 0.4g and applied transversely to the span direction. The resulting inelastic time history analyses, deck displacement and link deformation, for the example bridge models with three end-diaphragms (EBF, TADAS, and CKB) subjected to different earthquakes are compared in the paper. As shown from these analyses for the average of all earthquakes considered, maximum ductilities and drift results remain within the expected range for bridges on stiff pier bents, when excessive ductility demands were obtained for the bridges on more flexible piers. Finally, the results of this paper reveal the capability of the three new end-diaphragms for the slab-on-girder steel bridges, and demonstrate a simple design technique for retrofitting purposes by use of the three mentioned end-diaphragms.
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