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Investigating the Effect of Vertical Irregularity (Setback) on the Load Pattern in Nonlinear Static Analysis of Steel Structural Models

Jasour, Omid | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: English
  3. Document No: 50207 (53)
  4. University: Sharif University of Technology, International Campus, Kish Island
  5. Department: Science and Engineering
  6. Advisor(s): Rahimzadeh Rofouei, Fayaz
  7. Abstract:
  8. Recent studies regarding the damage assessment of structures under severe earthquakes have shown that vertical irregularity of structures have considerable effects on the severity of the induced damages. In these cases, structure enters into its nonlinear behavior, and both structural and nonstructural elements sustain large forces and deformations. In general, nonlinear dynamic procedure (NDP) can provide more realistic results than nonlinear static procedures (NSPs). This is due to consideration of higher modes effects and the shifts in inertial load patterns as the structural system experiences nonlinear behavior. Nowadays, due to the complication of nonlinear dynamic analyses in estimating these forces and deformations on case of seismic ground motion, the use of nonlinear static analyses methods has increased significantly. The NSPs proposed by design codes do not lead to reliable results especially for tall buildings with vertical geometric irregularity (setback). They generally provide inconsistent estimates of inelastic seismic demands, especially for the top floors due to their inabilities in considering the higher modes effects. In recent years, researchers have made various efforts to modify and enhance the accuracy of NSPs on variety of structural models due to increasingly demand of engineers to use and develop and use performance based design method. In this study, the basis of the nonlinear static and dynamic analysis is discussed. Also, a number of existing enhanced pushover procedures are elaborated in details. Next, the code recommended load patterns and the aforementioned enhanced procedures are compared using nonlinear time history analyses. Two 3-D structural models with special moment resisting frames with 10 and 14 stories are considered for the parametric study. The story displacement and the story drift are compared with the results obtained through the nonlinear dynamic time history analyses (NDTHA) and other introduced NSPs. The results indicate that pushover methods utilizing the lateral force distributions which are based on a single mode, are not able to predict inelastic demands properly. Moreover, enhanced procedures are more accurate. In the last part, appropriate lateral load patterns for pushover analysis are proposed. To investigate the accuracy of these proposed methods, the resulting story displacements and the story drifts are compared with those obtained from NDTHA and some other available enhanced NSPs, it can be concluded that the proposed methods had a good agreement with NDTHA and can properly predict the nonlinear response of vertical irregularity (setback) structures. Also, the Sap2000 software are used for modeling and analyses of the considered building structures
  9. Keywords:
  10. Nonlinear Static Analysis ; Pushover Analysis ; Vertical Irregularity ; Lateral Load Pattern ; Nonlinear Dynamic Analysis ; Setback Structure

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