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Optimal Calibration of the Seismic Design Base Shear of Steel Structures by Minimizing the Lifecycle Cost and Its Uncertainty

Bagheri, Atiyeh | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58272 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Mahsuli, Mojtaba; Hosseini Varzandeh, Saeed
  7. Abstract:
  8. This research focuses on determining the optimal base shear coefficient for the design of conventional steel buildings based on the minimization of lifecycle cost (LCC) and associated uncertainties throughout the service life of the building. Current seismic design codes are calibrated based on engineering judgment, acceptable societal risk, or past earthquake experiences. However, technical literature has shown that such calibration does not lead to a design that optimally balances construction costs and seismic damage. As a solution, lifecycle cost-based calibration has been proposed. In this study, the optimal range of the base shear coefficient is determined by minimizing the expected LCC, ensuring that small changes in the coefficient result in negligible variations in the mean cost. Within this range, the coefficient that minimizes the variance of LCC is selected as the optimal and robust base shear coefficient. This approach not only adheres to the minimum expected cost theory but also minimizes uncertainties, reducing the likelihood of extreme costs and enhancing life safety. To evaluate the effect of building use on design, the optimal importance factor is defined as a function of the ratio of total seismic damage for a building with a given importance level to that for a residential building. Construction costs are estimated using local cost indices, and seismic damage is calculated based on FEMA P-58 guidelines through four stages: hazard analysis, response analysis, damage analysis, and loss analysis. Hazard analysis employs reliability-based methods for risk assessment. Nonlinear structural models are developed, and incremental dynamic analyses are conducted to estimate the probabilistic response of structures conditional on earthquake intensity and to derive collapse fragility curves. Damage analysis uses fragility curves for structural and non-structural components, and loss analysis is performed using consequence functions that are localized for Iran within the scope of this study. Seismic losses considered in the risk analysis include direct economic losses due to structural, nonstructural, and content damage; indirect economic losses due to building downtime; direct social losses from injuries and fatalities; indirect social losses due to reduced quality of life; and environmental losses due to greenhouse gas emissions and energy waste. The proposed framework is applied to 5706 sites across 14 countries in the West Asia region. For this purpose, 28 steel moment frame (MRFs) structures with base shear coefficients ranging from 0 to 0.35 are designed for each site. LCC, including construction costs and seismic losses, are calculated, and the optimal and robust base shear coefficient is determined for each site, considering site class and building importance level. Comparison with Standard 2800 and ASCE 7-22 reveals that these codes underestimate the design base shear coefficient for residential buildings in seismically active areas and for highly important buildings, resulting in high-risk designs. Conversely, in low-seismicity regions, the codes produce overly conservative designs. The proposed optimal and robust base shear coefficient for intermediate steel MRFs with residential use is approximately 1.1 times the ASCE 7 values on average, and 2.7 times for highly important buildings. For residential buildings in high and very high seismicity regions, the coefficient is about 1.07 times, and for highly important buildings, about 1.8 times the values in Standard 2800.This increase leads to an average rise in construction costs of only 0.7% for residential buildings and 5.4% for highly important buildings. However, it reduces the mean and variance of lifecycle costs by 1.7% and 1.0%, respectively, for residential buildings, and by 10.1% and 50.0%, respectively, for highly important buildings. The proposed framework also identifies the limitations or preferences for using specific structural systems under various conditions. For example, steel moment frame systems are unsuitable for highly important buildings on soil types D and E. While the collapse probabilities of intermediate and special moment frames with the same base shear coefficient are similar, the optimal coefficient for special moment frames is about 20% lower due to their higher stiffness. Given the higher construction costs of steel structures compared to reinforced concrete frames and the negligible seismic damage relative to construction costs, the optimal base shear coefficient for concrete structures is higher. On average, seismic losses in reinforced concrete structures are about 20% lower, and their lifecycle costs are 13% lower than those of steel structures
  9. Keywords:
  10. Life Cycle Cost (LCC) ; Risk ; Seismic Design ; Code Calibration ; Minimization ; Robustness ; Risk Aversion Programming ; Design Base Shear Coefficient

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