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Investigating the Effect of Bi-Directional Input for Near Field Earthquakes Considering the Soil Structure Interaction

Heydari Asl, Saeedeh | 2008

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39202 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Rahimzadeh Rofooie, Fayyaz
  7. Abstract:
  8. One of the main issues concerning the irregular building structures is the determination of the critical directions of earthquake input to them that leads to maximum structural response parameters. In applying the seismic excitations to the structures, there is always a critical direction in which the application of seismic loads in those directions, would cause the structural system to experience its maximum responses. Therefore, considering the probability of positioning of the structure’s principal axes in that critical direction, the structure needs to be designed based on the critical seismic induced demands. In this project, the CQC3 method has been used to determine the critical direction of the earthquake input on the structure for computing its maximum response under the application of bi-directional horizontal earthquake components. Then, the calculated results have been compared with those obtained by combining the maximum response parameters of the structure under the application of each earthquake component alone, using different combinational rules such as 30%, 40%, and SRSS. On the other hand, decreasing the soil rigidity sometimes leads to significant changes in the dynamic characteristics and damping of the structural system. In order to investigate this issue, according to sub-structural method, first the soil-structure system was divided to two parts including the structure and its foundation and the under-laying soil. Then, the force-displacement characteristic of the soil-foundation interface nodes is modeled using a number of springs and dampers with frequency dependent coefficients. Also, two 3 and 8 story steel structural models with 0 and 10% eccentricity were considered for parametric study. The prepared structural models were analyzed using 5 pairs of near field records and 2 pairs of far field records for comparison purposes. The results of the analyses indicates that under far field earthquakes and for stiff soil, the 40% method leads to conservative results for both structural models. However, for the 8 story model subjected to near field earthquakes, 40% method underestimates the dynamic responses of the system. Using flexible soil (with shear wave velocity 350 m/sec) for investigating the soil-structure interaction, almost all the 30%, 40% and SRSS approaches underestimate the real response of the structural models and as the under-laying soil becomes softer(shear wave velocity 150 m/sec), the 40% method loses its precision for both 3 and 8 story models as well. Thus, common combinational methods are not suitable in estimating the structural response under near field earthquakes more investigations are necessary in this regard
  9. Keywords:
  10. Soil-Structure Interaction ; Near-Field Earthquake ; Irregular Structures ; Bi-Directional Input

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