Simplified Analysis of Vertically Irregular Building Structures
Date Issued
2014
Date
2014
Author(s)
Tsaur, Chih-Chia
Abstract
Most seismic building design codes describe the features in the vertically irregular structures and prescribe that the dynamic analysis procedures be adopted in the design of these irregular structures. In recent researches, it has been shown that seismic responses for vertically irregular structures can be estimated reasonably well by simplified analysis procedures such as modal pushover analysis (MPA) or uncoupled modal response history analysis (UMRHA), as long as the building does not have a strong lower-half or a strong first-story feature. For either one of these two vertical irregularities, it is suggested that a rigorous finite element model (FEM) response history analysis procedures be considered in order to avoid the unsatisfactory results computed from the simplified procedures. These simplified methods consider each mode as one single-degree-of-freedom. Thus, it is identified as the single DOF method (SDM) in this research.
In this study, a vertically regular reference structure is modified into a vertically irregular two-part structure, consisting of a super-structure and a sub-structure. During the modifications, one of the two parts is adjusted by tuning its story masses, strengths or stiffness, making the modified structure vertically irregular. Responses of the two parts in a given mode are no longer considered as mode-shape-proportional at the inelastic state. That is, the top and bottom part’s responses are not determined by using only one degree of freedom Dn. In this study, a two-degree-of-freedom method (2DM) is developed to describe the non-proportional responses of the super-structure and the sub-structure in a specific mode. A two-degree-of-freedom (2DOF) modal stick model is constructed accordingly in order to incorporate the two response variables Dpn and Dbn into the 2DOF equation of motion.
The LA-9, 9-story steel 2D frame model with a fundamental period T1=2.78 sec. from SAC project is modified into four irregular building structure models: each having 1. a strong first story 2. a strong-and-stiff first story 3. strong lower half 4. strong-and-stiff lower half. Using 60 (20 each for OBE, DBE and MCE) ground motion records, results computed from 2DM, SDM and the FEM (considered as the exact responses) are compared. Analytical results show that the 2DM has an overall more accurate evaluation for inter story drift ratios under DBEs and MCEs, reducing the overestimation of SDM for lower stories from 80-140% to at most 30%, and the underestimation of SDM at upper stories from 70% to at most 40%. The 2DM also have consistent results with SDM in OBEs. It appears that the 2DM improves the precision of modal superposition and reliability of the MPA or UMRHA. It expands the applications of the simplified analysis procedures.
Subjects
立面不規則
簡化分析
振態分析
動力歷時分析
非線性結構分析
Type
thesis
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