| Abstract: |
6.1 In this research, a nonlinear finite element computer program is developed to solve soil-structure interaction problems such as coupled shear walls-soil interaction, frame-soil interaction…etc, due to static or different dynamic loads. The introduced program constructed by (Shallan 1994) to solve a plane structural system under static or dynamic loads is developed to study the nonlinear behavior of reinforced concrete building-soil interaction by using a nonlinear finite element models. The material model treats concrete element as a nonlinear isoparametric quadratic element with elasto-plastic behavior in compression and also, tension cracks are taken into account through the development of one or two perpendicular cracks. Also, the nonlinear finite element model is modified to represent the nonlinear behavior of the soil. Steel reinforcement is represented as a bar element with bilinear stress-strain relationship possessing strain hardening.The soil structure interaction problem is solved by introducing a modified model to challenge the problem introduced from tension in soil. The new model based on a combination between Duncan model and Mohr- Coulomb model. This proposed procedure takes the relations of Duncan model when the soil in compression, which means the stiffness, is varied according to the state of stresses. But when the soil satisfies the failure criteria, the model makes redistribution of the stresses according to the Mohr-Coulomb model. Also, in this program the infinite elements are used to represent the infinite domain of soil. The infinite elements are used to minimize the mesh size and to prevent the effect of wave reflection.The developed computer program is used to investigate the seismic behavior of the soil-structure interaction problem. At first, three earthquakes are used to show the behavior of the reference model under different earthquakes excitation. The developed computer program is verified by comparing its results with previously published experimental results. Good agreement is observed during such comparison.The effects of several parameters on the nonlinear behavior of soil-structure interaction are investigated. Important parameters are considered into the problem including the effect of soil type, depth of connecting beam, ratio of edge steel reinforcement and the depth of the raft foundation. Finally, this research studies the behavior of wall-soil system for unequal coupled shear walls. The effects of these parameters are investigated through performing full nonlinear analysis of coupled shear walls-foundation-soil model. Comparisons among the different parameter results are demonstrated and the recommended parameters are introduced for this study.6.2 CONCLUSIONS:from the previous analysis the most important conclusions can be summarized as the follow:1. The computer program is considered as a very helpful tool for solving a plane stress nonlinear problem for both soil and structure interaction under static or earthquake excitation.2. The results demonstrate that, in case of small beam depth (Db/L=1/3), the distribution of the shear force at the connecting beams along the height of the shear wall is small. Also, the cracks propagated at all the ends of the connecting beams at the same time. In this case, the coupled shear walls become separated shear walls due to the decrease of the stiffness of the connecting beams.3. The results illustrate that, as the depth of the connecting beams increases from (Db/L=1/2 to 2/3), the shear force in the connecting beams especially in the first floor to the third floor of the wall increases by (80%).4. When the depth of the connecting beams increases to large beam depth (Db/L=5/6), the wall behaves as a single shear wall which the maximum shear force occurs at the base of the wall and the value of shear force decreases along the height as in the cantilever shear wall. Also, the cracks in the connecting beams occur after the propagation of the cracks in the wall.5. The lateral top displacement in the wall increases by decreasing the stiffness of the soil. Also, the stresses at the base of the wall decrease with decreasing the stiffness of the soil. The decrease in the overall displacement is about (65%) and the increase in the vertical normal stresses at the base are about (9%) when changing the type of soil from sand soil to silty sand soil.6. By increasing the edge steel reinforcement in the wall, the response of the overall displacement in the wall improves due to increasing the stiffness of the wall. More damage is indicated by increasing the edge steel reinforcement near the upper limits of Egyptian Code of practice. This response is due to increasing the pier stiffness relative to the beam stiffness, so several cracks are propagated at the connecting beams which make the coupling shear walls mostly behave as a cantilever shear wall. Also, by increasing the ratio of edge steel reinforcement in the wall piers, stresses in the steel reinforcement can be transmitted to concrete through bounded between reinforcement steel and concrete and make several cracks in the system.7. The results show that, the increase of the foundation rigidity has beneficial effect on the displacement response which leads to the increase of the overall stiffness of the structure and better distribution of loads on soil. The percentage decreases of the overall displacement from (Dr/L =0.1 to 0.13) is equal to (75%). When the rigidity of the foundation is extremely increased (Dr/L=0.2), the stresses in soil are concentrated near the edges of the foundation and leading to earlier failure in soil. But in case of medium rigidity of the foundation (Dr/L =0.13) the stresses is distributed under the foundation as uniform shape.8. The results demonstrate that, by changing the position of the opening between two wall piers, in case of linear state, no significant changes in the maximum lateral top displacement in the wall are observed. But when the behavior of the system reached the nonlinear state, the stiffer unequal shear wall (B1/B2=1.4) cracks early due to the sudden failure is observed. Also, as the ratio between two wall breadths increases, the behavior of the shear wall converts to shear wall-frame system.9. The behavior inside the connecting beams changes by changing the ratio of wall pier breadth which the distribution of the shear force increases in the connecting beams by increasing the ratio between two wall pier breadths. Also, the position of the maximum shear force in the connecting beams is moving upward. This response occurs due to increasing the slope angle of the connecting beam by increasing the ratio between two wall pier breadths.10. The results show the importance of including the soil on the response of the structure.11. The fundamental time period of any buildings increasing by decreasing the stiffness of the soil. The increase of the fundamental time period moves the system far from the region of maximum response of the earthquake spectrum.12. Using the infinite element in static and dynamic analysis decreases mesh size which leads to minimum labor, time and computer memory.6.3 RECOMMENDATIONS FOR THE FUTURE WORK:The following recommendations are proposed for the future work:1. Using the developed program for studying the effect of different types of the structure-soil interaction, change the upper and lower steel reinforcement in the connecting beams with changing the edge steel reinforcement in the wall piers.2. This work may be extended to study the effect of the vertical ground motion.3. This work may be extended to study the behavior of stepped shear walls.4. This work may be extended to study three dimension shear wall-soil interaction also, to study many piers of shear wall connected at the same level by connecting beams.5. This work may be extended to study the geometric nonlinearity.
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