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.
