Design of pre-cast two-column bents using strut-and-tie models and nonlinear finite element analysis

Abstract

The collapse of the Cypress Street Viaduct during the 1989 Loma Prieta earthquake raised awareness to the seismic design and detailing of the connections between cap beams and columns. Current standards require a considerable amount of transverse reinforcement inside the joints, leading to reinforcement congestion and constructability problems. Moreover, some design rules are rather unclear and the link to the flow of forces in the joint is sometimes lost. However, a clear understanding of the forces flow is essential for the design of new structural systems. This work belongs to a broader research project in which pre-cast solutions of railway viaducts are being developed. Besides the specific requirements of railway bridges, the viaducts shall be able to withstand the seismic excitation of southern Europe. In the solution being considered, the superstructure is bearing-supported and rests on a two-column bent. In order to make it feasible for pre-fabrication and simple assembly on-site, the reinforcement layout at the joint must be optimized without compromising the transmission of forces generated inside the joint during a seismic event. The beam-column joints were designed resorting to strut-and-tie models that enabled the understanding of the force transfer mechanisms within the joints when subjected to the forces from the seismic load combination. The optimized solution was achieved through a superposition of models, which leads to a statically undetermined problem. Detailed 3D nonlinear numerical analyses in the DIANA FEA software are developed to confirm the adequacy of the reinforcement layout. An experimental campaign is being planned to assess the structural performance of the designed solution under cyclic loading.