The Effect of Mass, Depth, and Properties of the Soil Below the Raft Foundation on the Seismic Performance of R.C. Plane Frames

J. A. Alomari

Abstract


Soil structure interaction has been the subject of numerous studies. The foundation soil has a definite effect on the performance of structures during seismic excitation. Recent studies show that the effect of soil-structure interaction SSI may be detrimental to the structure during seismic excitation. In this study, the effect of consideration of the soil below foundation and its depth, and the soil modulus of elasticity on the response of structures is investigated. The number of mode shapes considered has an effect on the accuracy of the values of structure response. A structural model consisting of an 8-story reinforced concrete frame resting on raft foundation, and including the soil below the raft is analyzed. The frame is analyzed using SAP2000 software, and time history and modal analysis are carried out with varying values of both soil depth and soil modulus of elasticity. The soil below the foundation is connected to the raft elements by gap links. Gap element links are compression–only members with appropriate stiffness, which are active only in compression. Modal analysis results show that the periods of vibration decrease as the modulus of elasticity of the soil increases. Periods of vibration of the frame without the soil mass consideration are less than those when the soil mass below the raft is considered, and they increase with increased depth of foundation to a certain limit. The structures response in the form of columns shear forces and story displacements are also evaluated under the variable parameters considered.


Keywords


SSI; soil mass depth; number of mode shapes; seismic; period of vibration; soil modulus of elasticity; column shear force; story displacement; gap link

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References


B. Fatahi, S. H. R. Tabatabaiefar, B. Samali, “Soil-structure interaction vs site effect for seismic design of tall buildings on soft soil”, Geomechanics and Engineering, Vol. 6, No. 3, pp. 293-320, 2014

S. E. Abdel Raheem, M. M. Ahmed, T. M. A. Alazrak, “Evaluation of soil–foundation–structure interaction effects on seismic response demands of multi-story MRF buildings on raft foundations”, International Journal of Advanced Structural Engineering, Vol. 7, No. 1, pp. 11-30, 2015

B. Moczar, Z. Polgar, A. Mahler, “A comparative study of soil-structure interaction in the case of frame structures with raft foundation”, Materials and Geoenvironment, Vol. 63, No. 1, pp. 1-8, 2016

B. Fatahi, S. H. Reza Tabatabaifar, A. S. Hokmabadi, B. Samali, “Significance of Bedrock Depth in Dynamic Soil-Structure Interaction Analysis for Moment Resisting Frames”, 2nd International Conference On Performance-Based Design In Earthquake Geotechnical Engineering, Taormina, Italy, May 28-30, 2012

O. A. Lopez, M. Cruz, “Number of modes for the seismic design of buildings”, Earthquake Engineering and Structural Dynamics, Vol. 25, No. 8, pp. 837-855, 1996

M. Papadopoulos, R. Van Beeumen, S. Francois, G. Degrande, G. Lombaert, “Computing the modal characteristics of structures considering soil-structure interaction effects”, Procedia Engineering, Vol. 199, pp. 2414–2419, 2017

R. Obrzud, A. Truty, The Hardening Soil Model –A Practical Guidebook, Zace Services Ltd, 2012

S. Lamar, C. Fortoul, “Brick Masonry Effect in Vibrations of Frames”, in: Proceedings of the 4th World Conference on Earthquake Engineering, Vol. 2, pp. 91-98, 1969

A. K., Chopra, Dynamics of Structures Theory and Applications to Earthquake Engineering, Prentice Hall, 2017

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