Experimental Studies of Dynamic Response of Foundations

Centrifuge testing;dynamics of foundations;earthquake loading;foundation embedment effect

Hushmand, Behnam ; Scott, Ronald F.

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: Centrifuge testing;    dynamics of foundations;    earthquake loading;    foundation embedment effect;   

Others  :  https://thesis.library.caltech.edu/3131/1/Hushmand_b_1984.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

An investigation was made into the behavior of rigid foundations and structures resting on the surface or embedded in a cohesionless soil and subjected to transient active or passive excitations and forced vibrations using the centrifuge modeling technique. The investigation was aimed at studying both low and high amplitude vibrations of foundations under machine type loadings, earthquake or wave induced vibrations, and other sources of dynamic loads. Rigid "prototype" foundations of mass and size comparable to foundations of a low rise building were simulated in the centrifuge at a centrifugal acceleration of 50g. Rigid model structures (aluminum towers) attached to foundations of different shapes, sizes, masses, and moments of inertia were tested. The effect of soil depth, boundary conditions, and depth of foundation embedment were investigated. Mainly rocking and horizontal modes of vibration were studied. The impulse rocking-horizontal excitation of the models was provided by actively perturbing the model structures using explosive energy or by passively exciting them by shaking the whole soil bucket using a hydraulic shaking system. The forced vibration was produced by a miniature air-driven counterrotating eccentric mass shaker mounted on the model structures. During the tests detailed measurements of the static and dynamic contact pressure distributions, displacement components of the model, and acceleration amplitudes at different elevations of the model structure were obtained. The acceleration ratios were used to determine the modes of vibration of the foundation systems. Natural frequencies and damping coefficients of the modes were calculated by fitting the amplitude-frequency response of a single degree of freedom mass-spring-dashpot oscillator to the experimental response curves derived from the test data. Experimental results provided information regarding the influence of different geometrical, inertial, and loading conditions on the vibrational characteristics of the soil-structure system. In particular the effect of foundation embedment was to increase the model resonant frequencies and to cause an appreciable change in contact pressure distribution underneath the footing. However, the resonant frequencies predicted by the lumped parameter analysis for a simple two-degree-of-freedom (rocking and translation) model were about 20 to 55 per cent higher than those measured experimentally. These results were consistent with the comparisons made in similar theoretical and experimental studies such as those performed by Morris in the Cambridge centrifuge and those performed on full-scale footings by Stokoe and Richart. Damping ratios of the rocking-sliding vibration did not change considerably when footing size or depth of embedment changed. The existence of rigid boundaries around the soil mass in the bucket, and an inefficient contact between soil and the foundation side walls and lower surface could account for these observations. Uplift and nonlinear large amplitude vibrations were consistently observed during the steady-state vibration tests. Uplift led to a softer vibrating system which behaved non-linearly. As a result the frequency of vibration decreased with the amount of lift-off. In transient vibration uplift reduced the intensity of higher frequency vibration. Soil around the foundation edge yielded and plastic deformations and subsequent softening of the contact soil increased the material damping while it decreased the resonant frequency of the system. It was concluded that elastic half-space theory does not satisfy the needs for analysis of foundation behavior under high amplitude vibrations and more sophisticated methods of analysis are required.