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Optimal Target Reliability for Seismic Design of Mid-rise Steel Buildings Based on Detailed Risk Analysis

Habibi, Kasra | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 53768 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Mahsuli, Mojtaba
  7. Abstract:
  8. Most of the seismic regulations in the Iranian design codes are adopted from the U.S. codes, while the severe earthquake rates with regards to the gross area and total population of Iran is 8.4 and 5.5 times the rates for U.S. respectively. In fact, Iranian rates are between the ones for U.S. and Japan which have up to 200% difference in prescribed strength level in their seismic codes. Poor construction quality regarding defected control in Iran adds to the problem. Therefore, the prescribed strength level of U.S. building codes and thus their target reliability index seems insufficient for use in Iran. Also, previous research has shown that the load and resistance factors in Iranian seismic design codes do not even fulfill the target reliability index of the reference code. All of these reasons have given rise to the catastrophic social and economic consequences of earthquakes in Iran. This study aims to quantify the target reliability index for Iranian seismic design code based on detailed risk analysis with an emphasis on steel buildings. To do so, in the first step, three mid-rise steel building archetypes with different lateral load bearing systems including intermediate moment-resisting frame and special concentrically braced frame with X configuration are designed. The design is repeated in different levels of base shear coefficient, each of which corresponds to a level of reliability. Country’s regulations are followed for the designs and initial construction cost is determined by local specifications. Then, non-linear models of these structures are generated and are rigorously verified. The response distributions of the structures conditioned on seismic intensity and their collapse fragilities are then assessed with IDA. In the next step, detailed risk analysis of the archetypes is conducted based on FEMA P-58 methodology, and seismic consequences are quantified in terms of direct economic losses due to repair and replacement, indirect economic losses due to buildings’ downtime, social losses due to fatalities and injuries, and also environmental losses due to carbon emission and embodied energy. Transforming all of the consequences to their equivalent monetary value, life-cycle cost of the building archetypes which is the sum of their initial construction cost and the costs associated with the seismic consequences are calculated. Based on the findings, the effect of increasing the earthquake design load on initial construction cost is 3% and 10% for braced frames and moment frames respectively, and thus is insignificant. That is because of the high contribution of non-structural components in the initial construction cost. Due to the high cost put upon the society by each person’s death based on the value of a statistical life, most of the losses are attributed to social consequences followed by direct and indirect economic losses. Also, the losses’ trends are different from each other such that increasing the earthquake design load can decrease one while increasing another, so considering each one of them alone to determine the target reliability level is not a prude manner. Based on the results obtained in this study, a 40% increase in the design base shear can decrease life-cycle cost of moment frames up to 10%, but for braced frames, status quo seems to be near the optimal one. Consequently, the target reliability index for moment frames with regards to collapse limit state is assessed as 1.96 which is higher than the 1.75 index suggested in AISC. This target index is calculated as 2.1 for braced frames. In addition, collapse probability given MCE is 26% for moment frames in the current state of the code, and it is 16% in the optimal one for both moment frames and braced frames. Moreover, the findings suggest that the distribution of lateral strength and stiffness in the height of a building has a major role in its seismic performance, such that merely increasing them without considering their distribution in height, not only does not decrease collapse probability and life-cycle cost, but also may increase them by concentrating plastic deformations in relatively weaker and softer stories. The issue is more pronounced for braced frames which are highly susceptible to the buckling of each story’s braces. Since the current regulations cannot guarantee a uniform distribution of lateral strength and stiffness in the height, it is suggested that a modification be carried out in the code to fulfill that goal
  9. Keywords:
  10. Code Calibration ; Incremental Dynamic Analysis ; Concentrically Braced Frame ; Life Cycle Cost (LCC) ; Probability Model ; Intermediate Concrete Moment Frame ; Collapse Analysis ; Optimal Reliability ; Detailed Risk Analysis

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