STR-866: NON LINEAR FINITE ELEMENT MODEL FOR POST-EARTHQUAKE FIRE PERFORMANCE EVALUATION OF STEEL PORTAL FRAMES

Post-earthquake fires (PEF) especially in densely populated urban areas have been catastrophic in recent seismic events. It appears to be an important design load which has not been considered critical by most design standards. Moreover, current performance-based seismic design philosophy permits certain level of damage to a structure based on the assumed design seismic hazard. These damaged structures are extremely vulnerable to post-earthquake fires. Even after the outbreak of fire, the structural integrity of the damaged structure must be intact for sufficient duration enabling the firefighters to evacuate and extinguish the fire in the affected building. The recent performance-based design, necessitates evaluation of the fire resistance level of earthquake damaged building with or without the outbreak of post-earthquake fire. In this study an integrated seismic and thermal analysis model was developed using the sequential thermal–structural analysis scheme using the finite element program, ABAQUS. A simple portal frame was considered to investigate the global behaviour of the frame and determine post-earthquake fire resistance. A 2D transient heat transfer analysis was conducted and the transient nodal temperatures across the structural elements cross sections were stored for subsequent thermal structural analyses. The state of earthquake inflicted damage, corresponding to desired performance level was realized using pushover analysis. The results of the simplified 2D model matched reasonably well with that of 3D finite element model considered for validation study. The developed model is being used for subsequent study to investigate the multi-story moment resisting frames with fire scenarios resulting in asymmetric heating of the frame

Performance of steel structures subjected to fire following earthquake

Includes bibliographical references.2016 Summer.Fires following earthquakes are considered sequential hazards that may occur in metropolitans with moderate-to-highly seismicity. The potential for fire ignition is elevated by various factors including damage to active and passive fire protections following a strong ground motion. In addition, damage imposed by an earthquake to transportation networks, water supply, and communication systems, could hinder the response of fire departments to the post-earthquake fire events. In addition, the simultaneous ignitions – caused by strong earthquake – might turn to mass conflagrations in the shaken area, which could lead to catastrophic scenarios including structural collapse, hazardous materials release, loss of life, and the inability to provide the emergency medical need. This has been demonstrated through various historical events including the fires following the 1906 San Francisco earthquake and the 1995 Kobe earthquakes, among others, making fire following earthquake the most dominant contributor to earthquake-induced losses in properties and lives in the United States and Japan in the last century. From a design perspective, current performance-based earthquake design philosophy allows certain degrees of damage in the structural and non-structural members of steel-framed buildings during the earthquake. The cumulative structural damages, caused by the earthquake, can reduce the load-bearing capacity of structural members in a typical steel building. In addition, potential damage to active and passive fire protections following an earthquake leaves the steel material exposed to elevated temperatures in the case of post-earthquake fire events. The combined damage to steel members and components following an earthquake combined with damage to fire protection systems can increase the vulnerability of steel buildings to withstand fire following seismic events. Therefore, there is a pressing need to quantify the performance of steel structures under fire following earthquake in moderate-to-high seismic regions. The aim of the study is to assess the performance of steel structural members and systems under the cascading hazards of earthquake and fire. The research commences with evaluation of the stability of hot-rolled W-shape steel columns subjected to the earthquake-induced lateral deformations followed by fire loads. Based on the stability analyses, equations are proposed to predict the elastic and inelastic buckling stresses in steel columns exposed to the fire following earthquake, considering a wide variety of variables. The performance of three steel moment-resisting frames – with 3, 9, and 20 stories – with reduced beam section connections is assessed under multi-story fires following a suite of earthquake records. The response of structural components – beams, columns, and critical connection details – is investigated to evaluate the demand and system-level instability under fire following earthquake. Next, a performance-based framework is established for probabilistic assessment of steel structural members and systems under the combined events of earthquake and fire. A stochastic model of the effective random variables is utilized for conducting the probabilistic performance-based analysis. This framework allows structural engineers to generate fragility of steel columns and frames under multiple-hazard of earthquake and fire. The results demonstrate that instability can be a major concern in steel structures, both on the member and system levels, under the sequential events and highlights the need to develop provisions for the design of steel structures subjected to fire following earthquake. Furthermore, a suite of recommendations is proposed for future studies based on findings in this dissertation

Developing OpenSees software framework for modelling structures in fire

Fire following an earthquake (FFE) is a hazard that is not usually accounted for in either earthquake or fire resistant design of structures. There have however been many instances in the past of FFE events causing even greater damage and even loss of life than the original earthquake. The potential damage associate with this hazard is increasing considerably with increasing urbanisation in seismically vulnerable regions. It is reasonable for users to expect that structures should maintain their integrity for a long enough period in an FFE event allowing emergency crews to assist the most vulnerable occupants to evacuate the building safely. Because of the lack of regulatory requirements there is naturally very little research on the response of structural frames under FFE events so far, but given the reasons discussed earlier, it is clearly a matter of increasing importance that engineers should develop a better understanding of the behaviour of seismically damaged structural frames in fire. This thesis project was fortunate to have occurred at a time when a set of full-scale fire tests were taking place at IIT Roorkee in India, in collaboration with the University of Edinburgh to address exactly this topic. This thesis research was undertaken to model these experiments (to determine the fire resistance of a reinforced concrete frame first subjected to simulated seismic damage). The open source software framework OpenSees was chosen for the modelling work as it was considered to be the best software tool for modelling structures under earthquake loading. The first part of this thesis reports the development work done on OpenSees for adding thermomechanical analysis modules to enable the modelling of FFE events using this software framework. The code developed for OpenSees has been allowed the introduction of features not available in commercial software such as ABAQUS. Many new classes were developed, such as ThermalAction, ThermalElement, ThermalSeciton, TheramalMaterial, etc. The newly developed code was tested using a number of benchmark problems and modelling of real fire experiments on steel and composite framed structures. The results from these tests showed that the new developments were successful. The second part of the thesis describes the modelling of the reinforced concrete (RC) frame tested at IIT Roorkee, which was first subjected to cyclic displacement loading (to introduce damage in the frame similar to that of a seismic event) and then to a one hour kerosene fire. The modelling was first used to provide predictions of the performance of the test frame under the proposed loading, to fine tune the design of the experiment. The modelling subsequent to the tests was gradually improved to achieve better comparisons with the test results and to develop a detailed understanding of the behaviour of seismically damaged RC frames in fire, which was also compared to the behaviour in fire of undamaged frames

Effect of fire and fire following an earthquake on steel reduced beam section moment connections

2013 Summer.Includes bibliographical references.The main objective of this research is to investigate the behavior of steel frames with reduced beam sections (RBS) during a fire as well as during the combined events of fire following an earthquake (FFE). Historical events and recent disasters have clearly demonstrated that the occurrence of these two events (fire and FFE) within steel framed buildings represents a probable scenario that warrants further investigation. Accurate analytical evaluation of the structural behavior of steel buildings under fire, and to a lesser extent an earthquake, is difficult due to the many complex and uncertain phenomena involved. Detailed numerical modeling of the overall structural system has been shown to provide the most reliable simulation results under current research development. However, detailed analysis is generally computationally expensive and as such not practically applicable. In addition, the nonlinear behavior of entire structures is complex and not fully understood. Therefore, detailed numerical models of the overall structural system often have difficulty capturing local failure modes. This research provides a practical analytical approach to perform accurate numerical evaluation of steel structures under fire and FFE and to closely investigate its characteristic behavior. The approach utilized is to limit the focus on localized compartment fires and investigate the behavior of a single beam-column subassembly within the chosen compartment. By limiting the focus of the study the numerical models can be simplified by utilizing specifically appropriate subassembly models for the analysis. Using the finite element program ABAQUS, two different beam-column subassemblies with RBS were created and analyzed. The subassemblies are representative of actual connections in two steel special moment resisting frames that were designed for the highly seismic Los Angeles region. The frames selected for analysis are an 8-story 4-bay frame and a 16-story 4-bay frame and the selected subassemblies are located at the exterior of the frames at the mid and lower levels, respectively. Both subassemblies were analyzed under fire alone to determine their structural behavior during the event as well as allow for a better understanding of the influence the seismic demand has on the behavior of the connection when exposed to FFE. For the FFE simulations both models were analyzed under a suite of earthquake ground motions followed by a fire simulation. For the fire analysis portion of both simulations (fire alone and FFE) a sequentially coupled thermo-mechanical modeling technique, which includes representative constraint elements to simulate the restraint imposed by the frame is employed. The results of the study highlight the significance of including realistic boundary conditions during fire simulations and points towards the possibility for the occurrence of substantial damage in unprotected steel frames during fire as well as protected steel frames during fire following an earthquake

Performance of Flexure-Controlled Reinforced Concrete Structural Walls Under Sequential Fire-Earthquake Loads

The performance of reinforced concrete (RC) structural walls under individual hazards has been well studied. However, little is known regarding the behavior of RC structural walls under sequential hazards. The research presented here seeks to address the performance of RC structural walls under sequential fire-earthquake loads (both post-earthquake fire and post-fire earthquake). Longer burn times of post-earthquake fire and initial seismic damage can have significant structural impacts on RC structures which are usually considered to have superior performance in a fire. An 8-inch wall with characteristics representative of typical construction in seismic regions was utilized as the basis of the simulations. The wall with non-uniform layout of reinforcement provides a complex deformed shape under fire. Individual typical earthquake damage states were introduced to the wall to assess impact on fire resistance. The fire resistance of a wall was discussed according to thermal-insulation and load-bearing criteria in codes. The results show that crack does not impact the fundamental response of a wall under fire while cover loss decreases its load-bearing capacity significantly. Moreover, the location of cover loss has remarkable impact on the deformed shape of a wall and its load-bearing fire resistance. While the thermal-insulation capacity decreases below code requirements, the load-bearing fire resistance of earthquake-damaged walls is still acceptable. Another potential but infrequently studied hazard is the post-fire earthquake scenario. The impact of fire damage on the earthquake behavior of RC walls is not well understood, which leads to some safety concerns in earthquake after fire or aftershocks after post-earthquake fire. A simulation procedure combining SAFIR and OpenSees is proposed and validated for the PFE analysis of RC structural walls. Based on the validated the simulation procedure, a parametric study on the PFE performance of RC walls was conducted. Results indicate that fire damage decreases the load-bearing capacity and stiffness of RC walls under reversed-cyclic loads while fire damage decreases the deformation capacity in most cases. Severe fire exposure may shift damage from the boundary element to the web. Wall characteristics which significantly impact the residual wall response quantities are wall thickness, boundary element length, and axial load ratio. In addition, a framework for simplified nonlinear modeling was proposed for the PFE performance of RC walls. The models are defined by modification factors that account for the change in wall response relative to that of a wall without fire damage. Modification factors, established from the results of the parametric study, are a function of fire damage indices that account for the effect of fire on the material properties of steel and concrete. Results indicate that the model is generally able to predict the response of a fire-damaged wall

Ductile moment-resisting timber connections: a review

In the last two decades, high-rise timber buildings have been built using the glulam truss system, even with limited openings. Moment-resisting timber frames (MRTF) with semi-rigid beam-to-column connections can be an architecture-friendly way to provide a load-carrying system to vertical and horizontal loads for timber buildings. In these structures, connections of adequate ductility are crucial to ensure robustness and energy dissipation. This paper presents a review of the main types of timber beam–column moment connections with improved ductility and proposes to carry out a ductility assessment of these connections based on the most relevant ductility factors. Joints have a significant influence on the global performance of MRTF, and the application of ductile connections have improved the mechanical parameters of the timber frame. The reinforced bolted slotted-in steel plate and glued-in rods connections have similar mechanical performance, with high rotation capacity and good ultimate moment, but exhibited different failure modes under cyclic loading. The connections were classified within ductility classes. In general, the glued-in steel rods presented better results because of the high influence of steel profiles in the connection yielding. Despite the excellent mechanical behavior, the reinforced bolted slotted-in steel plate connections presented medium ductility values.This research was funded by Fundação para a Ciência e a Tecnologia (FCT) grant number BD/06301/2022