Dynamic Analysis of RCC Retaining Walls
Professor, Department of Civil Engineering,
Chief Discipline Supervisor
Present state-of-the-art for the analysis and design of retaining walls under earthquake loading is based on the method proposed by Mononobe and Matsuo (1926) and Okabe (1926) (M-O analysis). Dynamic pressure induced in the soil both due to active and passive conditions are computed using a pseudo-static force acting within the failed soil wedge and equivalent dynamic coefficients of active and passive earth pressures are obtained. The pressure distribution is obtained considering the wall, as a gravity-type having infinite stiffness while the ground acceleration is considered maximum. However, a typical RCC retaining wall is much more flexible than the gravity wall and such analysis could either be too conservative or unsafe.
In the present paper, a method is proposed to obtain the time period of a retaining wall quite accurately and carry out a dynamic analysis of such wall based on modal response technique. The results based on proposed method are then compared with the existing methods to estimate the variations, if any. The present procedure establishes both the general and particular cases of dynamic response of RCC retaining wall based on improved Rayleigh-Ritz method.
The present dynamic analysis agrees closely with M-O analysis when the basis of ah is same for the both. However for M-O method based on seismic zone method, significant differences in both shear and moment are observed for active earth pressure case. The reason is, in the seismic coefficient method, a constant value of ah = 0.03 is assumed both for active and passive cases. However, for dynamic analysis, as ah is time dependent, one can arrive at a value of ah = 0.054 and 0.03 for active and passive cases respectively. That the results match in passive case is coincidental.
Keywords: Analysis, computation, earthquakes, earth pressure, dynamic loads, moments, retaining walls, shear stress, stiffness, vibration.
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