Behzad Haseli - M.Sc. student of earthquake engineering, civil engineering faculty, Kharazmi University, Tehran, Iran
Dr.gholamreza nouri - Faculty of civil engineering, Department of engineering, Kharazmi University, Tehran, Iran

Bridges are a vital part of transportation system of every country. A bridge damage or collapse during strong earthquakes can lead to irreparable economic damages. Thus study and assessment of effective factors in seismic response of bridges is obvious. One of these important factors is abutment and how it assumed in creating model. Many researches done on the response of highway bridges subjected to seismic loads after San Fernando 1971. Most of studies on seismic response of bridges have focused on the dynamic response of superstructure and considering the abutment behavior and its effect on response is not studied much. Abutments behavior considering the soil-structure interaction have a salient effect on seismic behavior of whole bridge structure system subjected to moderate to strong earthquakes. Megally and Zhang did important researches to consider the effect of different parts of bridges on strength of bridges against earthquakes. According to Megally et al. 2001 experimental researches on abutments shear keys, if the effect of soil stiffness considered in a bridge model, seismic responses of different parts of bridge will change with comparison to the case that abutment assumed as simple or rolled support. On assessment of the behavior of abutment, important researches done by Wilson and Shamsabadi. Abutments are proper members to transmit inertial forces during earthquake and are designed with principles of retaining walls and active and passive pressure assumption. As earthquake occurs the inertial forces exceed the assumed condition of soil active pressure, therefor application of simplified assumption in designing are not proper. Consideration of effect of soil stiffness on designing of abutments according to the type of backfill is an important matter. In this paper abutments modeled as walls and the equivalent stiffness of soil allocated to abutment by subjecting static and dynamic pressure to soil behind the abutment wall. Equivalent stiffness of abutment calculated and the results under 5 different earthquake records compared with suggested equations of CALTRANS code. The results for all records demonstrates that assuming abutment as a wall (sixth case) have conformity to CALTRANS code design procedure for sandy soil (second case). Hence the assumption of this research for design can apply for next studies and design procedures.

bridge seismic design, abutment model, soil-abutment interaction, abutment stiffness, wallpier