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UBC Theses and Dissertations

Modulus reduction dynamic analysis Purssell, Tanis Jane


A semi-analytical method of dynamic analysis, capable of predicting both the magnitude and pattern of earthquake induced deformations, is presented. The analysis is based on a modulus reduction approach which uses a reduced modulus to simulate the softening induced in soils during cyclic loading. The effects of the inertia forces developed during dynamic loading on the induced deformations are also included through an appropriate selection of the reduced modulus. The reduced modulus is utilized in a static stress-strain analysis to predict the magnitude and pattern of the deformations induced during earthquake loading. The appropriate modulus reduction is determined from laboratory tests on undisturbed soil samples. Three methods of computing a suitable post-cyclic modulus were investigated but only the cyclic strain approach, in which the modulus is determined from cyclic loading tests that duplicate the field stress conditions, yields reductions of sufficient magnitude to provide realistic estimates of earthquake induced deformations. The modulus reduction analysis was used to predict the deformations occurring during dynamic loading of a model tailings slope in a laboratory shaking table test and of the Upper San Fernando Dam during the earthquake of February, 1971. These studies showed that the modulus reduction analysis is capable of reproducing the dynamically induced deformations and that reductions in the modulus of up to 1000 times may be required. Unfortunately, limitations of the testing equipment and inadequacies in the available data required that the appropriate modulus reductions could not be determined entirely through laboratory and field investigations. Some assumptions were necessary in selecting the reduced modulus values used in the analyses. Although these case studies were, hence, unable to provide full verification of the proposed method, they do demonstrate the reliability and simplicity of the analysis as a method of assessing the performance of soil structures during earthquake loading.

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