In recent years, masonry infills have been gradually replaced by concrete sandwich infills in some countries. Therefore, the effect of these infill walls on the seismic behavior of building frames still needs more research. This study aims to evaluate the parameters affecting the fundamental period of steel moment-resisting frames with concrete sandwich infill walls and then establish a relationship to predict that period. For this purpose, first, many steel frames are designed to form a sample space for statistical analysis considering the effective factors including the number of stories, number of spans, span length, thickness of the sandwich panel and some mechanical properties of the infills. Concrete sandwich infill walls are modeled as compression struts in the frames. The effects of different parameters on the fundamental period are identified by the modal analysis. Using nonlinear regression analysis, an empirical relationship is proposed to estimate the fundamental period of the steel frames with concrete sandwich infill walls for the first time. This empirical relationship is also extended to estimate the fundamental period of bare frames. The results demonstrate that the proposed relationship is more accurate in predicting the fundamental period than several relationships presented in the design codes.
