الفهرس 
INTRODUCTION
Effects of SSI on the Structure ResponseOnly 14 pages are availabe for public view
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Abstract
In this thesis, a detailed and enhanced soil-structure model is developed in finite element software Plaxis 3D to investigate the influence of SSI on high-rise reinforced concrete building over piled-raft foundations. The soil column of the 3D finite element mesh is simulated by the 3D 10-node tetrahedral elements and the Mohr-Coulomb model is adopted. Firstly, the accuracy of the numerical model is verified according to the shaking table test’s results attained by Zhang and Far. After that, a novel attempt is made to quantify the effect of the structure existence on the surface response by numerically incorporating the SSI. Moreover, numerical simulations are conducted on the 25-story building to achieve the direct approach, predict the effects of base flexibility, and perform the seismic analyses by applying two far-field and two near-field earthquakes to rigid base and flexible-base models, respectively. Additionally, to reduce the detrimental effects on high-rise buildings due to earthquakes, piles-raft foundations of various piles lengths are adopted. According to the results of numerical simulations, this study can draw the subsequent conclusions.
5.2 Conclusions
After performing a series of numerical SSI simulations on the high-rise building with 25-stories, using far-field and near-field time earthquakes data, the following conclusions are made:
1. The near-field nature of the input motion could excite the soil nonlinearity leading to high energy dissipation and damping ratio, and consequently, substantial suppression of the surface acceleration and displacement.
2. The building existence almost increases the ground surface acceleration amplitude and displacement to different extents depending on the input motion characteristics.
3. Considering SSI for a building supported over piled-raft, lengthens the building fundamental period by 12.2%, and results in increased maximum lateral displacements and inter-story drifts as well as reduced base shear. The story shear forces of the building decrease to some extent after taking the SSI into account, except for the topmost stories (24th and 25th floors), possible reason is the shear wall stiffness that considered same along building height in the numerical model.
4. In designing high-rise buildings, it is not reasonable to use the same shear force reduction factor for all stories in a structure.
5. The increase of the maximum lateral displacements of the structure considering SSI under the two far-field seismic records (average ratio of 97.4%) is much more obvious than that of the structure under the action of two near-field seismic motions (average ratio of 34.7%).
6. For the two far-field earthquakes, the average base shear force of flexible-base structure is about 30.11% of that of fixed-base structure, while the average ratio of the two near-filed earthquakes is 37.28%.
7. The assumption of fixed-base is neither always accurate nor conservative to represent subsoil conditions for distribution of story shear forces and lateral deflections.
8. Under the influence of El-Centro far-field earthquake, increasing piles embedment depths by 20% up to 80%, leads to reduction in maximum accelerations of the building ranging from 15.0% to 22.8% corresponding to depression in maximum horizontal displacements by 17.5% to 32.2%. Same response is resulting under Hachinohe far-field excitation as the maximum accelerations are decreased by 12.6% to 14.3%, and maximum lateral deflections are depressed by 15.8% to 26.7%.
9. In cases of Northridge and Kobe near-field earthquakes, utilizing piles lengths with 1.2L to 1.8L, results in reducing the maximum accelerations of the structure by 4.1% to 6.6%, and by 6.7% to 10.3%, respectively. The maximum horizontal displacements also display a decrease ranging from 15.9% to 18.9% and from 7.6% to 12.3% when subjected to Northridge and Kobe input motions, respectively.
10. Increasing the piles embedding depths causes inter-story drifts to be decreased in all cases of seismic excitations, representing more stiffer building seismic performance. It is worth mentioning that the base shear and story shear forces increase by increasing of piles embedment depths. Nevertheless, maximum shear forces of base, using 1.8L, are only 38.98%, 33.24%, 50.18%, and 29.46% of that of fixed-base structure under influence of El-Centro, Hachinohe, Northridge, and Kobe earthquakes, respectively.
11. The prevailing belief that the deeper the piles are, the closer they are to the fixed-case is not accurate. It’s obvious that piles with 1.4L, 1.6L, and 1.8L lengths are providing nearly identical response. Consequently, increasing embedment depth more than 40% doesn’t affect significantly on the structure response. Therefore, 40% could be considered as an optimum piles’ lengths increase for enhancing the building seismic performance. This ratio depends mainly on the building height and the subsurface lithology conditions.