The dynamic foundation interaction of multistorey frames

This paper discusses the extent to which foundation properties influence the response of some framed structures to earthquakes. Three stages can be isolated, the first of which checks the validity of a theoretical method by means of model studies. Five models were made, a five-storey plane frame rigidly fixed at its feet, two models of the same frame but now in association with a linearly elastic isotropic foundation, Adiprene L-100 and L-167, which had E-values of 4.4 and 39.7 N/mm², respectively. A five-storey, asymmetric space frame was also made, with fixed feet and with a foundation of modulus 4.4 N/mm². In this first stage, resonance tests were made, using the MAMA,² to obtain frequencies, mode shapes and damping values. Corresponding frequencies and mode shapes were calculated using a finite element idealization. The second stage uses a proven computer program for a parametric study to obtain a large number of results, which give a good indication of the effect of different foundation conditions on this type of structure. Some calculations are also made in which the foundation is represented by equivalent springs⁸ instead of finite elements. In the third stage dynamic response was considered. A shaking table was built on which the models were subjected to a number of actual earthquake records, which had been suitably scaled and recorded on magnetic tape. The structural response was measured, and compared with calculated values obtained by step-by-step methods and, alternatively, the modal addition approach. For the asymmetric space-frame, particular attention was paid to the torsional effect.