Negar Elhami Khorasani

Probabilistic performance-based evaluation of a tall steel moment resisting frame under post-earthquake fires

Purpose This paper aims to develop a framework to assess the reliability of structures subject to a fire following an earthquake (FFE) event. The proposed framework is implemented in one seamless programming environment and is used to analyze an example nine-story steel moment-resisting frame (MRF) under an FFE. The framework includes uncertainties in load and material properties at elevated temperatures and evaluates the MRF performance based on various limit states. Design/methodology/approach Specifically, this work models the uncertainties in fire load density, yield strength and modulus of elasticity of steel. The location of fire compartment is also varied to investigate the effect of story level (lower vs higher) and bay location (interior vs exterior) of the fire on the post-earthquake performance of the frame. The frame is modeled in OpenSees to perform non-linear dynamic, thermal and reliability analyses of the structure. Findings Results show that interior bays are more susceptible than exterior bays to connection failure because of the development of larger tension forces during the cooling phase of the fire. Also, upper floors in general are more probable to reach specified damage states than lower floors because of the smaller beam sizes. Overall, results suggest that modern MRFs with a design that is governed by inter-story drifts have enough residual strength after an earthquake so that a subsequent fire typically does not lead to results significantly different compared to those of an event where the fire occurs without previous seismic damage. However, the seismic damage could lead to larger fire spread, increased danger to the building as a whole and larger associated economic losses. Originality/value Although the paper focuses on FFE, the proposed framework is general and can be extended to other multi-hazard scenarios.

Overview of fire following earthquake: historical events and community responses

Purpose This paper aims to present a literature review on the problem of fire following earthquake (FFE) as a potential hazard to communities in seismically active regions. The paper is important to work toward resilient communities that are subject to extreme hazards. Design/methodology/approach The paper lists and reviews the historical FFE events (20 earthquakes from 7 countries), studies the available analytical tools to evaluate fire ignition and spread in communities after an earthquake, discusses the available studies on performance of individual buildings under post-earthquake fires and summarizes the current literature on mitigation techniques for post-earthquake fires. Findings FFE can be considered a potential hazard for urban communities that are especially not prepared for such conditions. The available analytical models are not yet fully up to the standards that can be used by city authorities for decision-making, and therefore, should be further validated. Limited structural analyses of individual buildings under FFE scenarios have been completed. Results show that the drift demand on the building frame increases during post-earthquake fires. Despite the mitigation actions, there are still urban cities that are not prepared for such an event, such as certain areas of California in the USA. Originality/value The paper is a complete and an exhaustive collection of literature on different aspects of FFE. Research in earthquake engineering is well advanced, while structural analyses under fire load and performance of communities under FFE can be further advanced.

Tools for Measuring a City’s Resilience in a Fire Following Earthquake Scenario

The paper provides a framework to evaluate the response of buildings in a community subject to fire following earthquake. First, a model is developed to determine the probability of ignition in buildings of a community due to an earthquake. Second, fragility functions are developed for buildings subject to fire, to quantify the structural damage and the expected losses. The ignition model, combined with the fragility functions, can be implemented in a GIS based risk management platform to evaluate economical losses in a region from fire following an earthquake.

Comparative fire analysis of steel-concrete composite buildings designed following performance-based and U.S. prescriptive approaches

Performance-based structural fire design provides a rational methodology for designing modern buildings with cost-effective solutions. However, in the United States, fire design still largely relies on design at the component level using prescriptive approaches. With performance-based approaches, there is an opportunity to benefit from increased flexibility and reduced cost in the design, but these advantages need to be explicitly described and disseminated to promote this shift in paradigm. In this paper, a comparative analysis is conducted on multi-story steel-concrete buildings designed following performance-based and U.S. prescriptive approaches. The steel-concrete composite structure allows taking advantage of tensile membrane action in the slab during fire, and therefore removing the fire protection on secondary beam elements. The nonlinear finite element software SAFIR is used to model the behavior of the buildings under the standard ASTM fire and a natural fire determined using the two-zone fire model CFAST. The numerical simulations show that performance-based design can be used to achieve the required level of safety currently enforced in the U.S. prescriptive guidelines, while providing an opportunity for cost reduction in fire protection material. the thickness of spray fire protection for the prescribed fire rating to be applied on the elements. Meanwhile, previous research shows that, when system-level performance is considered, fire protection on secondary beam elements is not necessary due to the development of a membrane action in the concrete slab during fire (Bailey and Moore, 2000; Gillie et al., 2002; Vassart et al., 2012). In the second part of this research, performance-based approach based on numerical simulations is adopted to design the fire protection in the building while taking into account the interaction of structural members at the system level. Different alternatives are studied considering different peripheral beam sections and amount of steel mesh in the slab. Two types of fire exposure are considered, namely the standard ASTM E119 fire and a natural fire. The prescriptive design philosophy is based on the use of standard fire curves such as the ASTM E119 fire. Consideration of the same fire exposure for the structure designed with the performance-based approach allows comparing the response of the two designs for a same thermal load, hence focusing on the structural behavior. The safety level can be discussed in terms of amount of time that the structure withstands the applied loads under this standard fire. Yet, performance-based approach entails the possibility to consider a natural fire exposure evaluated by considering the real characteristics of the compartment. Natural fires are by nature very different from standard fire and therefore can lead to distinct structural response. So, this study also investigates the response of the different alternative designs under a natural fire which is determined using a two-zone fire model implemented in the computer program CFAST. 2 PROTOTYPE BUILDING 2.1 Multi-story building The prototype building studied in this paper is a nine-story steel frame office building. The building is 45.72 m by 45.72 m in plan, consisting of five bays of 9.14 m in the two directions. The structure is composed of four moment resisting frames on the perimeter, as the lateral load resisting system, and four interior gravity frames, see Figure 1. The columns of the interior frames are continuous on the nine-story but the beams have pinned connections (statically determinate beams). The total height of the building is 37.18 m, divided between a 5.49 m high first floor and eight other floors each with a height of 3.96 m.

Probabilistic Evaluation Framework for Fire and Fire Following Earthquake

This work provides a probabilistic framework to evaluate the performance of a structure under fire and fire following earthquake, by studying response of the structure for several limit states and incorporating uncertainties in demand and capacity parameters. The multi-hazard framework is then applied to a steel moment resisting frame (MRF) to evaluate the structural performance of the MRF under post-earthquake fires. The study develops probabilistic models for key quantities with uncertainty including fire load, as well as yield strength and modulus of elasticity of steel at elevated temperatures. The MRF is analyzed under several fire scenarios and fire locations. Results show that the location of fire in the frame (e.g., lower vs. upper floors and interior vs. exterior bays) affects the element response. Compartments in the interior bays reach limit states faster than those on the perimeter of the frame, and upper floors reach limit states sooner than lower floors. The post-earthquake damage does not affect the structural response under fire for the considered limit states, but post-earthquake fire increases the drift demand on columns located at the perimeter of the structure.