Influence of structure parameters on period of frame structures for earthquake resistant design

Abstract

Codes of practice suggest equivalent static force method for earthquake resistant design of frame structures of moderate height. For regular frames, the codes propose approximate formulae for fundamcntal pcriod required for the evaluation of base shear. These formulae describe period cither as a function of height of the structure or number of stories. Lack of reflection of other structure parameters makes them grossly approximate. An invcstigation is conducted in this work to assess the influence of different structure parameters on the period of regular framc structures. Model frames, divided into six major groups are subjected to modal analysis to evaluate the fundamental frequency and corresponding period. General purpose finite clement package ANSYS, Revision 5.2, has been employed to conduct the analyses. The analysis tool assumes constant stiffuess and mass effects and neglects damping. Appropriate elcments arc selected from ANSYS element library to model the frames. ANSYS option of lumped mass approach has been used for mesh elements. Masses of secondary structures arc added as point mass elements at nodal points. Modal analysis has been used as analysis typc and reduced method as analysis option. The extracted cigenvaleus and the corresponding eigenvectors represent the frequencies and mode shapcs respectively. The fundamental period is obtained from the lowest frequency. An extensive parametric study is conducted to idcntify influence of parameters like numbers and width of bays, numbers and height of stories, stiffuess of columns and beams and strength and density of concretc on fundamental period. Effect of inclusion of floor slabs and masses of secondary structural elements in the analysis is also studied. The period has been found to increase with decreasing number of bays along the direction of motion or increasing number of bays transverse to the direction of motion. An increase of bay width, story height or number of stories leads to an increase of period. Approximate code formulae arc found to become increasingly conservative with increasing structure height in evaluation of earthquake forces. Usc of stiffcr columns and beams or concrete of higher strength has a reducing effect on period. It has been revealed that application of approximate code formulae in cases of low rise structurcs with stocky columns and beams or with concrete of higher strength may lead to an underestimation of earthquake forces. A reduction in beam height or column dimension in the direction of earthquake forces or choice of low strength concrete are found to reduce design earthquake forces. In most practical eases, the approximate code formutae lead to a conservative estimation of earthquake forces.