This paper presents a numerical method for the analysis of infilled frames subjected to earthquake excitation. The method is based on the concept of equivalent strut used to idealize the infill panels. Properties of the mathematical model are determined using the stiffness matrix method. Dynamic equations of motion are solved numerically in the time domain using the modified step-by-step linear acceleration method and known as the Wilson-theta method.

The above procedure is implemented into a computer program SAPF (Seismic Analysis of Plane Frames) to be run on an ICL computer. The program has many capabilities such as mesh automatic generation, static, dynamic and spectrum analysis. This program has been used to solve thirteen cases of bare and infilled frames in order to assess the effect of different factors such as the presence and continuity of infill panels, the height of the structure, infill material, panel rectangularity ratio and width of the equivalent strut. Results show that infill panels have a great effect on the behavior of such structures. A comparison of this method with existing seismic code formulae showed that the latter overestimate the shear forces in the building due to the fact that they do not consider the effect of infill panels.

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