An improved displacement-based seismic design methodology for bridges accounting for higher mode effects

Weaknesses of existing methods for direct displacement-based design (DDBD) of bridges are pointed out and an improvement is suggested to account for higher mode effects, the key idea being not only the proper prediction of a target-displacement profile through the effective mode shape (EMS) method (wherein all significant modes are considered), but also the proper definition of the corresponding peak structural response. The proposed methodology is then verified by applying it to an actual bridge wherein the different pier heights and the unrestrained transverse displacement at the abutments result in an increased contribution of the second mode. A comparison between the extended and the 'standard' DDBD is conducted, while further issues such as additional design criteria leading to optimum design, the proper consideration of the degree of fixity at the pier's top, and the effect of the deck's torsional stiffness are also investigated. The resulting designs are evaluated using nonlinear response- history analysis (NLRHA) for a number of spectrum-compatible motions. Unlike the 'standard' DDBD, the extended procedure adequately reproduced the target-displacement profile providing at the same time a good estimate of results regarding additional design quantities such as yield displacements, displacement ductilities etc., closely matching the results of the more rigorous NLRHA. However, the need for additional iterations clearly indicates that practical application of the proposed procedure is feasible if it is fully 'automated', i.e. implemented in a software package.