The community impact of post-earthquake safety decisions based on damage to tall buildings and elevated hazard due to aftershocks

A key component of community resilience is the ability to recover community functions after an earthquake. Resilience planning focuses on identifying critical functions, setting recovery time targets for various hazard levels, and assessing the gap between the desired and the anticipated performance. These performance assessments should consider all potential disruptions to community functions, such as damage that makes buildings unsafe, damage to equipment and architectural finishes that makes buildings non-functional, and loss of utility services or transportation access. A recovery-based assessment includes both the initial loss of functionality and the process for restoring it, including stabilizing unsafe buildings and securing the necessary resources to initiate building repairs. A heavily damaged building has an elevated risk of collapse during an aftershock, endangering the occupants in nearby buildings. Post-earthquake safety inspectors evaluate which damaged buildings to red tag and where to set up safety cordons, restricting access to potential fall zones. This dissertation focuses on these access restrictions associated with unsafe buildings. Post-earthquake decisions regarding which buildings are safe to re-enter are an important part of a community's recovery process. Unsafe buildings and neighborhoods should be marked as such, alleviating the public's uncertainty as they take stock of their surroundings. However, these decisions should be made carefully, recognizing their impact on the community. Displacing too many residents through overly conservative red tagging would increase the potential for permanent outmitgration. Businesses in an inaccessible downtown may fail or need to relocate--to the outlying areas or to another city. Therefore, while safety cordons are imperative, their presence should be minimized where feasible, either by building retrofits to proactively reduce the likelihood of earthquake damage or by facilitating building inspection and stabilization efforts to remove the cordons as soon as possible. This dissertation proposes tools for quantifying three aspects of access restrictions: the additional community downtime due to safety cordons, the reduced safety of a damaged building, and the elevated risk of building collapse in the presence of aftershocks. First, the dissertation proposes a community recovery framework that accounts for access restrictions due to both individual building damage and safety cordons. The framework extends Performance-Based Earthquake Engineering tools developed for individual building assessments (FEMA P-58 and REDi) to the community level, using geospatial information to consider both regional-scale ground shaking and neighborhood-scale access restrictions. The cordons are incorporated into the impeding factor model for the logistical delays that contribute to individual building downtimes. A case study applies the framework to downtown San Francisco after a $M_w7.2$ scenario earthquake on the San Andreas Fault, focusing on loss of functionality in the community's office buildings. Downtime is quantified as community days lost in the first year. (The units are the community's pre-earthquake daily capacity, meaning that 180 community days lost in the first year is equivalent to the all the office space being non-functional for six months.) The presence of safety cordons may increase the downtime by an additional 50\%, as compared to downtime due to building damage and impeding factors alone. The framework can aid in resilience planning by comparing various policy options for mitigating downtime, such as mandating retrofits to reduce the number of buildings that would require a safety cordon or establishing contingency plans to shorten the duration of impeding factors. Evaluating the probability of achieving community recovery targets shows that retrofits are effective for 4 month, short-term recovery targets, while contingency plans are only effective for longer time frame recovery targets (e.g. 1 year). Next, the proposed damage indicator evaluation methodology assesses various building damage metrics for their ability to classify a damaged building as safe or unsafe to occupy. A trilinear regression model for collapse performance as a function of building damage reveals damage thresholds beyond which the building safety rapidly deteriorates. The methodology prioritizes damage indicators that are applicable to a broad range of buildings that share a specific structural system, considering the sensitivity to building characteristics such as the number of stories and uncertainty in analytical modeling parameters. A case study for reinforced concrete moment frames offers three new damage indicators, conceptually based on FEMA 352's floor damage indicator for welded steel moment frames, and their associated damage thresholds for identifying unsafe buildings. The recommended damage indicator focuses on damage to floors that are adjacent to the story with the largest drift ratios, thereby reducing the number of floors that need detailed inspections. Finally, the elevated collapse risk assessment methodology incorporates both the increased hazard due to aftershocks and the reduced collapse safety due to building damage. The elevated hazard curve includes both the steady state hazard and the aftershock hazard, which decays over time. A case study for a downtown San Francisco site considers three potential mainshock magnitudes on the San Andreas Fault and the subsequent aftershocks that may occur within the mainshock's rupture surface. The aftershock hazard contribution to the elevated risk of building collapse depends on the mainshock magnitude and decreases over time with the decaying rate of aftershocks. While the building damage contribution to the elevated risk may be low as compared to the initial aftershock hazard, the additional risk will persist indefinitely, unless the damage is repaired. These two dimensions of the collapse risk can be addressed through complementary risk- and damage-based tagging criteria. The case study compares the elevated collapse risk to the implied risk limit in building codes, applying the methodology to an individual building and to a parametric study representing a broader building inventory. The tools developed in this study are intended to support multi-disciplinary efforts to consider the societal implications of restricting access to a community's buildings. The broader discussion should address the appropriate balance of safety-based versus socio-economic priorities, including the community's perception and tolerance of risk

Advisor: Deierlein Gregory G. (Gregory Gerard)
Baker Jack W. 
Molina Hutt Carlos