A rationale for determining the natural period of RC building frames having infill

Building design codes generally impose some upper limit on the magnitude of the natural period determined from a rational numerical analysis if the period is longer than that predicted by empirical code equations since the code equations are derived on the basis of measuring the period of real buildings during an earthquake. In this study, the fundamental periods of vibration of a series of regular RC framed buildings are studied using 3D FE modeling and modal eigenvalue analysis including the effects of infill. It has been found that when the models do not include infill, as is done in conventional analysis, the period given by the analysis is significantly longer than the period predicted by the code equations justifying the imposition of upper limit on the period by the code. However, when the effect of infill is included in the models, the time periods determined from eigenvalue analysis were remarkably close to those predicted by the code formulas. It is also observed that the randomness in the distribution of infill does not cause much variation of the period if the total amount of infilled panels is the same for all models. It is also observed that varying amount of infilled panels causes some changes in the determined period. Based on the findings of the study, some guidelines are suggested for determining the period. The findings of the study have shown us a practical way to determine the fundamental period of RC frames using rational approaches like modal analysis, and eliminate the necessity of imposing code limits.