Journal of Civil Engineering and Environmental Technology Print ISSN: 2349-8404; Online ISSN: 2349-879X; Volume 2, Number 4; April-June, 2015 pp. 314-318 © Krishi Sanskriti Publications http://www.krishisanskriti.org/jceet.html Translational and Rotational Effect of Earthquake Ground Motion on a Bridge Substructure Tauhidur Rahman 1 and Gitartha Kalita 2 1,2 National Institute of Technology Silchar Cachar, Assam E-mail: 1 tauhid_srm@yahoo. com, 2 gitartha7@gmail. com Abstract—In the present paper, the rotational and translational effect of earthquake ground motion for a four span box box girder bridge has been thoroughly investigated. This study is motivated by the fact that in many countries the translational and rotational components of earthquake ground motion is not adequately considered in analyzing the overall response of the structures subjected to earthquake ground excitations. Much consideration is given to only the horizontal components of the earthquake ground motion during the response analysis of structures. In the present research work, P waves, SV waves and Rayleigh wave excitations are considered for different angle of incidence. In the present paper, the four span box girder bridge is modeled considering the effects of vertical and rocking components of P, SV and Rayleigh wave excitations. Ground responses namely displacement, velocity and acceleration of the substructures of the bridge have been considered for rotational and translational effects in addition to the horizontal ground motion due to earthquake and wind. Keywords: Ground motion, Response, Rotational effects, Translational effects. 1. 1NTRODUCTION The lack of adequate information about the effects of the rotational components of earthquake ground motion, specially rocking, in the overall response of structures and of bridges to earthquake ground excitations has motivated this study. Since only the horizontal components of earthquake ground motions is given importance during analysis of structures subjected to earthquake excitation. The objective of this work is to investigate these effects on a box girder bridge model. The model is converted to a lumped mass model consisting of a massless column supporting a concentrated mass. 2. METHODOLOGY The response of a four span box girder bridge subjected to earthquake excitations is studied. Each span is considered to be of equal length of 50m. The box girder depth is considered to be 2. 5m. The bridge is supported on both sides by abutments of depth 4m and width 2m along the length. There are three piers in between the two abutments which has a depth of 15m. The girder is spported on hinge over the left abutment. The spans are considered to be discontinuos and they are supported on both hinge and roller supports over the piers. The girder is supported on roller supports over the right abutment. The the piers caps above the piers are considered to be of 4m depth. The piers and the abutments are analyzed for incident P, SV and Rayleigh wave excitations. For SV wave excitations the angles of incidence below the critical angle of incidence are analysed. A program is prepared to obtain the response for each wave excitation in the form of equation of motion which will be second order differential equations. The solution of the differential equations will provide the response of the structure. The input parameters of the earthquake effects will then be modified in the program. The input parameters will be the frequency of excitation and the angle of incidence of the earthquake waves. In case of Rayleigh waves only the frequency parameter will be given importance. The change in the response of the structure by modifying the input parameters of the waves will be studied. Table 1: Terms used in the equations Symbol Quantity SI UNIT L  Rocking of the left abutment Radian M  Rocking of the middle pier Radian R  Rocking of the right abutment Radian L n  Natural frequecy of the left abutment Hertz M n  Natural frequency of the middle pier Hertz R n  Natural frequency of the right abutment Hertz 1 m Mass on the left abutment Kilogram 2 m   Mass on the middle piers between the abutments Longitudinal wave number Kilogram Radians/ meter