Earthquake papers Seismic Response of Structures with Underground Storeys M. Hesham El Naggar, Ph. D, P. Eng.
ICLR Research Paper Series – No. 26
The objective of the research is to investigate the effect of the interaction between basements and the surrounding soil medium on the estimated earthquake forces and deformations, for a variety of building types. To achieve this objective, the methodology involves the computer modeling of different buildings supported by different foundation systems. The computer model involves three-dimensional dynamic analysis of the combined superstructure and its foundation.
The first part of the study involved the analysis of the seismic response of tall slender R/C towers supported on flexible foundations. A new hybrid analysis was developed to model the nonlinear behaviour of the soil underneath the structure. A three dimensional finite element model was developed to represent the superstructure. The effect of the foundation flexibility on the seismic response of the tower was evaluated in terms of total displacements at the tip of the tower and the base forces (base bending moment and base shear). It was found that the foundation flexibility has a significant effect on the tower response to earthquake loading. The results showed that the soil-structure interaction could have a detrimental effect on tall structures contrary to what is postulated in design codes.
The foundation rocking behaviour could greatly contribute to the response of the supported structure to seismic loading, and in some cases it may become the governing factor when choosing a retrofitting scheme. In the second part of the study, analytical equations for the moment-rotation response of a rigid foundation are presented. An equation is derived for the uplift-yield condition and is combined with equations for uplift-only and yield-only conditions to enable the definition of the entire static moment-rotation response. The results obtained from the developed model show that the inverse of the factor of safety, c, has a significant effect on the moment-rotation curve. The value of c = 0.5 not only determines whether uplift or yield occurs first but also defines the condition of the maximum moment-rotation response of the footing. The computed moment-rotation response agreed well with experimental results found in the literature, as well as the recommended NEHRP guidelines based on the FEMA 273/274 for the foundation moment-rotation. Three-dimensional finite element models are being developed for the seismic response analyses of multistory buildings. The soil model will be incorporated in the structural model and the global model will be used in the analysis.