Study of Response Modification Factor in Concrete Bridges with Seismic Isolation System

One of the most important factors in seismic design of structures is ductility and energy dissipation that appear with taking the structure to inelastic region. By applying the response modification factor, the seismic forces are reduced in elastic design procedure. This reduction is due to the formation of plastic hinges and energy dissipation of the structure in inelastic range. A problems in bridge design is the uncertainty in codes for defining the role of isolators in terms of R value; for example, the code No.463 has not specified the effect of isolators in R value. Besides, AASHTO guide specification for seismic isolation design, expresses that R-factor for all substructureal elements should be half of those expressed in codes for usual cases but not less than 1.50. Small value for R is due to remaining the structure in elastic range. In this study, for investigation of the R-factor, five models with an isolator connection between substructure and superstructure are developed. LRB isolators are designed according to “AASHTO guide specification for seismic isolation design”. Research results indicate that R-factors are estimated about half of the corresponding values for ordinary cases, while justifying AASHTO guide specification, it shows that the substructure of isolated bridges usually remains in elastic region.