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Proposing The Appropriate Lateral Load Pattern for Pushover Analysis

Mirjalili, Mohammad Reza | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 41597 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Rahimzadeh Rofooei, Fayaz
  7. Abstract:
  8. Nonlinear dynamic procedure (NDP) can in general provide more realistic results than nonlinear static procedures (NSPs). That is due to consideration of higher modes effects and the shifts in inertia load patterns as the structural system experiences nonlinear behavior. Therefore, the simplified NSPs based on constant load patterns are inadequate to determine the inelastic seismic demands especially for top stories of tall buildings. The conventional procedures that have been implemented in design codes and guidelines such as ATC-40, FEMA356, FEMA440 and ASCE41-06, apply invariant load patterns such as inverse triangular, equal lateral force (ELF), first mode shape, uniform and SRSS modal response combination load pattern in pushover analysis. In recent years, researchers have made various efforts to modify and enhance the accuracy of nonlinear static procedures (NSPs) on variety of structural models due to increasingly demand of engineers to use and develop performance based design method. These attempts can be classified in three categories: a) Utilizing a single load vector obtained by combining the fundamental mode shape and factored higher mode shapes, such as MMC and MMP methods. The difference between these procedures is the coefficients used for combining defferent modes. b) The methods in which nonlinear pushover analysis is performed for a number of modes of the structure, and the results are combined using SRSS or CQC combination methods. MPA, MMPA and IMPA use this procedure. c) Adaptive pushover procedures that attempt to update the load vectors progressively to take into account the effect of higher modes and change in system modal properties during inelastic phase. Despite their conceptual superiority features, there is no consensus on the merits of adaptive load patterns. In this study, the theory of the nonlinear static and dynamic analysis is discussed. Besides, a number of existing enhanced pushover procedures are elaborated in details. Next, the code recommended load patterns and the aformationed enhanced procedures are investigated using nonlinear time history analyses. Three 2-D moment resisting steel models are considering for this parametric study. The story drift ratios and the plastic hinge moment and curvature predictions are compared with the results obtained by the nonlinear time history analyses (NL-THA) and other introduced NSPs procedures. 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, new lateral load patterns for pushover analysis are proposed. In these load patterns, a number of mode shapes are combined with appropriate coefficients which depend on mode participation factor, mode frequency and design spectrum. To investigate the accuracy of this procedure, the inter-story drifts as displacement response and the member plastic moment as force response are compared to those obtained from nonlinear time-history analysis and some other available enhanced NSPs. It is shown that the proposed method had a good agreement with NL-THA analysis and can properly predict the nonlinear response of structure.

  9. Keywords:
  10. Nonlinear Static Analysis ; Pushover Analysis ; Nonlinear Dynamic Analysis ; Modal Pushover ; Lateral Loading

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