A numerical approach for modeling the fire induced restraint effects in reinforced concrete beams

In this paper, a model to predict the influence of fire induced restraints on the fire resistance of reinforced concrete (RC) beams is presented. The three stages, associated with the fire growth, thermal and structural analysis, for the calculation of fire resistance of the RC beams are explained. A simplified approach to account for spalling under fire conditions is incorporated into the model. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. The program is used to conduct two case studies to investigate the influence of both the rotational and the axial restraint on the fire response of the RC beams. Through these case studies, it is shown that the restraint, both rotational and axial, has significant influence on the fire resistance of the RC beams.     

Macroscopic FE model for tracing the fire response of reinforced concrete structures

A macroscopic finite element model for tracing the fire response of reinforced concrete (RC) structural members is presented. The model accounts for critical factors that are to be considered for performance-based fire resistance assessment of RC structural members. Fire induced spalling, various strain components, high temperature material properties, restraint effects, different fire scenarios and failure criteria are incorporated in the model. The validity of the numerical model is established by comparing the predictions from the computer program with results from full-scale fire resistance tests. Case studies are conducted to demonstrate the use of the computer program for tracing the response of RC members under standard and design fire exposures. Through the results of the case studies, it is shown that the fire scenario has a significant effect on the fire resistance of RC columns and beams. It is also shown that macroscopic finite element models are capable of predicting the fire response of RC structural members with an adequate accuracy for practical applications.     

An approach for evaluating the residual strength of fire exposed bridge girders

This paper presents an approach for evaluating residual strength of fire exposed steel bridge girders. The approach involves three stages of analysis that is to be carried out at ambient conditions, during exposure to fire, and after cooling of the fire exposed bridge girder. In the first stage, load carrying capacity of the girder is evaluated at room temperature. In the second stage of analysis, thermal and structural response of the bridge girder is traced under specified fire exposure and loading conditions. In the third stage (after the bridge girder has cooled down), residual capacity of the girder is evaluated by incrementing load on the girder till failure occurs. The proposed approach is applied to carry out a set of numerical studies on a typical steel girder using finite element computer program ANSYS. Results from numerical studies indicate that the maximum fire temperature (and associated temperature in steel) is the most critical factor that influences residual strength of a fire exposed bridge girder. A girder exposed to typical “external” fire conditions, with maximum fire temperatures reaching to 600–700 °C, retains about 70 to 80% of its strength on cooling. On the other hand, a steel bridge girder exposed to hydrocarbon fire, with a maximum temperature of about 1100 °C, looses most of its strength during heating phase of the fire and experiences failure.     

压型钢板-陶粒混凝土组合楼板火灾响应及火灾后受力性能试验研究

为了分析压型钢板-陶粒混凝土组合楼板的火灾响应,首先对受荷条件下的2块简支陶粒混凝土组合楼板进行了火灾行为试验,得到了其火灾温度响应及位移响应。在此基础上,进一步对2块受火后的压型钢板-陶粒混凝土组合楼板进行了火灾后受力性能试验研究,并对1块具有相同参数的组合楼板开展了常温下静载试验。结果表明：在楼板内部设置焊接栓钉将减弱组合楼板的抗火性能、降低组合楼板火灾后的承载力;与未受火作用的同样规格的组合楼板相比,火灾后压型钢板-陶粒混凝土组合楼板的破坏形式发生较大变化,但仍具有较高的承载力和良好的受力性能;压型钢板-陶粒混凝土组合楼板受火后为弯曲破坏,而未受火楼板则为剪切滑移破坏。     

Findings of the International Road Tunnel Fire Detection Research Project

Fire detection systems play a crucial role in ensuring safe evacuation and firefighting operations in road tunnels, but information on the performance of these systems in tunnels has been limited and guidelines for their application in tunnel environments are not fully developed. Recently, the National Research Council of Canada (NRC) and the Fire Protection Research Foundation completed a 2-year international research project, with the support of private- and public-sector organizations, to determine some of the strengths and weaknesses of the various types of fire detection systems and the factors that can affect their performance in tunnel environments. The project included both laboratory and field fire tests combined with computer modeling studies. Although this research was conducted on road tunnels, the findings should apply to other tunnels, such as those used in subway systems. As part of the project, the NRC conducted two series of tests in the Carleton University-NRC tunnel facility to investigate the performance of detection systems under minimal and longitudinal airflow conditions. In addition, NRC conducted tests in the Carré-Viger Tunnel in Montréal, as well as a computer modeling study. The project studied nine fire detection systems that covered five types of currently available technologies. The performance of the detection systems, including response times and ability to locate and monitor a fire in the tunnel and the effect of the tunnel environment, were evaluated under the same conditions. This article provides an overview of the findings of the project. Fire detectors, fire scenarios and test protocols used in the test program are described. A summary of the research results of the full-scale fire tests conducted in a laboratory tunnel facility and in an operating road tunnel as well as of the computer modeling activities is reported.     

The behaviour of asymmetric slim floor steel beams in fire

Computer software has been developed to predict the structural response of asymmetric slim floor steel beams, used with composite concrete floor slabs consisting of deep profiled steel decking. Comparisons between predicted behaviour and that recorded in standard fire tests, showed that the software is very accurate. By including the rotational stiffness of the beam-to-column connections, the fire resistance of the beam is significantly enhanced. This is mainly due to the connections retaining most of their strength during a fire, since they are fully encased in concrete as a consequence of this type of construction. The analyses presented in this paper indicate that it may be possible to increase the fire resistance of the steel beams from 60 to 90 minutes, by including the connection behaviour. The software has also been used to aid the design of a future large-scale fire test on the asymmetric slim floor system. Predictions of the structural response have been presented. These will enable the fire load and ventilation conditions in the test to be designed. In addition the software has been used to identify the minimum amount of fire protection that is required for the supporting columns.     

Effect of Location of Restraint on Fire Response of Steel Beams

Restrained steel beams, when exposed to fire, develop significant restraint forces and often behave as beam-columns. The response of such restrained steel beams under fire depends on many factors including: fire scenario, beam slenderness ratio, location of axial restraint at the supports, and high-temperature properties of steel. A set of numerical studies, using finite element computer program ANSYS, is carried out to study the fire response of steel beam-columns under realistic fire and restraint scenarios. Results from the parametric studies indicate that fire scenario, beam slenderness, location of axial restraint and high-temperature creep have significant influence on the behavior of restrained beams under fire conditions. Severe fires produce high axial forces at early stages of fire exposure; whereas in moderate fires, significant axial force develops only at later stages of fire exposure. Axial restraint enhances the fire resistance due to the development of tensile catenary action in restrained beams. Furthermore, restrained beams with low slenderness ratio exhibit better fire performance when the axial restraint at the support is located at the bottom flange.     

火灾下大空间网架结构的抗倒塌分析方法

基于性能化防火设计方法和结构连续性倒塌设计方法,提出性能化抗倒塌分析方法,并通过开发相应的有限元程序模块(FireStru)连接具体的火场模拟结果,实现结构火灾反应计算过程。其中采用的非线性静力分析方法可以进行结构在局部构件破坏后的抗倒塌分析,并判断结构是否倒塌、是否局部倒塌或是否整体倒塌。最后,通过一具体的体育场馆结构进行验证,从而为其在火灾下的结构安全提供科学依据。     

Modeling post-fire behavior of aluminum structural components using a maximum temperature approach

Aluminum alloys exhibit permanent degradation of material strength subsequent to reaching temperatures above 200 °C. While modeling of aluminum structural components during fire is becoming more heavily researched, modeling response of aluminum structures following fire events has still not been explored. Previous research suggests that the post-fire strength of AA6061 is heavily correlated to the maximum temperature reached during the fire. This research uses the maximum exposure temperature as an input to finite element simulations of a small scale aluminum beam to predict the post-fire behavior of the beam. Results of the simulations were compared to a set of post-fire thermo-mechanical experiments. Samples were heated using a radiative source, water quenched, and then loaded to failure in a four point bending configuration. The simulated thermal response of the aluminum was within 10% of measured values across the entire side of the beam. Prediction of the unexposed mechanical response was within 5% of experimentally measured response. The post-fire peak bending load from simulations generally over-predicted the experimentally measured value while predictions of the flexural stiffness were within 15% of experimental results.     

Heat transfer analysis of the composite slab in the Cardington frame fire tests

The structural modelling of the Cardington Frame fire tests as part of the Department of Environment, Transport and Regions funded Partners in Technology project has highlighted the importance of the temperature evolution both temporally and spatially in determining the structural response. Restrained thermal expansion/contraction and thermal bowing are the main driving force behind almost all the structural phenomena witnessed in the tests.

The four British Steel fire tests carried out on the 8-storey composite steel and concrete building at Cardington have provided a wealth of information about the temperatures in the fire atmosphere and the protected and unprotected steel. Unfortunately, there is considerably less information on the temperatures attained in the concrete slab. In Tests 1–3, the temperatures through the depth of the slab have been recorded only at a few points and in terms of the structural modelling this has been just about adequate. There were no temperatures recorded in the slab in Test 4 (Office demonstration test).

The finite element, adaptive heat transfer program HADAPT has been used to model the heat transfer to the composite steel and concrete slab. HADAPT is a 2D adaptive heat transfer code capable of carrying out a nonlinear, transient, thermal analysis. The code models moisture evaporation from the pores of the concrete by assuming a phase change in the region of 100°C. The measured concrete temperatures in Tests 1–3 have been used to calibrate the model which has then been used to predict the slab temperatures in Test 4.     

A method for determining fire accidental loads and its application to thermal response analysis for optimal design of offshore thin-walled structures

More than 70% of accidents that occur on offshore installations stem from hydrocarbon fire and explosion, and as they involve heat and blast effects, they are extremely hazardous with serious consequences in terms of human health, structural safety and the surrounding environment. To prevent further accidents, substantial effort has been directed towards the management of fire and explosion in the safety design of offshore installations. The aim of this paper is to present a risk-based methodology procedure to help determine the fire accidental design load of an offshore installation (AL Living Quarter) in association with the thermal response characteristics for structural optimisation. A probabilistic sampling approach with numerical fire simulations was taken to determine the fire accidental load. To determine the optimisation of the thin-walled structures of the living quarter, an A60 based on the results of thermal response analyses was conducted and the temperature distribution calculated. The analysis results suggest incorporating both the design and safety planning aspects of offshore Living Quarter.     

Transient high-temperature stress relaxation of prestressing tendons in unbonded construction

Unbonded post-tensioned (UPT) flat plate concrete slabs are popular for modern continuous multiple bay floor assemblies due to economic and sustainability benefits (reductions in slab thickness and building self-weight) and structural advantages (decreased deflections over larger spans). Only limited research has been conducted on the performance of UPT flat plate slabs under fire conditions, yet the inherent fire endurance of these systems is sometimes quoted as a benefit of this type of construction. One concern for these structures in fire is that high-temperature stress relaxation of the unbonded prestressed reinforcement may cause considerable and irrecoverable prestress loss, with subsequent structural consequences. This paper uses a computational model which has been developed to predict the transient high-temperature stress relaxation (i.e., prestress loss) for typical UPT multiple span flat plate slabs in fire, to study the potential prestress relaxation behaviour under various plausible temperature conditions as might occur during exposure to a standard fire. The model is validated using experimental data from relaxation tests performed on locally heated unbonded seven-wire prestressing stand. The initial prestress level, concrete cover to the prestressed reinforcement, and ratio of heated length to overall tendon length are varied to investigate the potential implications for prestress loss, and subsequently for flexural and punching shear capacity. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate concrete cover for structural fire safety.     

Analyses of the effects of cooling and fire spread on steel-framed buildings

An existing computer model capable of predicting the response of three-dimensional semi-rigid steel framed buildings, including continuous floor systems, at elevated temperatures has been extended to include the possibility of extensive strain reversal within the material constitutive relationship. This allows the behaviour in cooling and the effect o f fire spread on steel-framed buildings to be investigated. A number of examples are presented to give an indication of the likely effects of cooling behaviour within the analysis. An initial study has also been made into the structural effects of fire spread from a single ignition point into adjacent bays of a two-dimensional steel building frame. It is shown that some of the distortions caused by the fire are increased by progressive fire spread, as compared with simultaneous burning across the same range of compartments.     

火灾下钢结构和组合结构的非线性模拟

近年来Sheffield大学开发了一种计算机程序Vulcan，用来对火灾下组合结构的性能进行三维模拟。本简要叙述了程序的理论基础。1995—1996年在Cardington进行了三个组合框架的抗火试验，各试验中相邻结构所提供的平面内约束程度不同，通过模拟这三个试验来显示受热区域内约束对结构性能的影响。为了阐明薄膜作用的影响及其和边界约束的关系，所有的试验都分别用几何线性以及非线性的板单元进行了分析。为此，首先对引起这种作用的钢筋的性能做了一系列参数研究。很明显，板内的薄膜作用对隔间最终的完整性很重要，并且在模拟火灾下这种类型的结构时应加以考虑。     

Analysis of the Response of Smoke Detectors to Smoldering Fires and Nuisance Sources

Since the initial smoke detector was developed, research has been ongoing to reduce the response times to fire sources and improve the ability of detectors to ignore nuisance sources. Research has been conducted to analyze the signatures from cooking activities, ranging from normal cooking to the flaming ignition of food products with the intent of identifying the precursors to flaming ignition. In particular, the goal of the research is to provide an alarm sufficiently prior to flaming ignition to allow homeowners to take corrective actions to prevent a fire. For the 11 experiments analyzed, the optical density measured in the range hood was the most accurate precursor signal, having the greatest ability to predict the imminent transition to a flaming fire with a minimum of false positives. Though plagued by false positives, the temperature of the heating element and response of an ionization detector provided a faster response than the optical density measure. Given that this research only included 11 experiments, further research should be conducted on a broader range of cooking styles and items being cooked.     

Analysis of the effect of an external fire on the safety operation of a power plant

The basic aim of this paper is to examine the relevant features of an outdoor fire event and its influence on the surrounding area. The work is based on the analytical study of fire origin, its development and spread. The computer program fire dynamic simulator (FDS) is used to simulate fire behaviour. This program uses large Eddie simulation (LES) to calculate fire development and the spread of combustion products in the environment. The fire source is located in the vicinity of a hazardous plant; e.g., power, chemical, etc. The article presents the brief background of the FDS computer program and the initial and boundary conditions used in the mathematical model. In this analysis, it is often necessary to carry out many repeat simulations of a hazardous event and the computer run time becomes an important factor. Ideally, results should be obtained with a dense computational grid but hardware resources and excessive computational time can preclude this option. The response is to reduce a grid density but to ensure that results are only acceptable if comparable to those obtained with a dense grid. The work presents some corrections of the physical model used and its validation by experimental data, which influence the quality of results.     

无粘结预应力混凝土框架火灾下结构反应分析

对五榀单层单跨无粘结预应力混凝土框架模型分别进行了常温加载破坏试验,以及不同恒载、不同升温曲线、不同预应力度下的火灾试验,对火灾中结构反应进行了分析研究。编制了火灾过程中的结构反应分析程序,进行温度场计算机模拟,对火灾进程中结构的变形进行计算,并由试验验证。     

Effects of heating and loading histories on post-fire cooling behaviour of concrete-filled steel tubular columns

A finite element method (FEM) program is developed and used in this paper to analyse the behaviour of concrete-filled steel tubular (CFST) columns during the entire stage of fire exposure, including: loading at ambient temperature, heating, cooling to the ambient temperature and post-fire loading to failure. The emphasis of this paper is on CFST column behaviours during the cooling and post-fire stages because these behaviours are affected by the loading and heating histories, but they have not previously been studied. This paper will present the mechanical property models for these different loading and heating stages. To validate the FEM program, some experimental data, including fire resistance, axial deformation and ultimate strength of CFST columns are compared and it is found that the FEM program can predict the test results with good accuracy. Using the FEM program, a parametric study is then conducted to investigate the influences of ambient temperature loading and heating history on the cooling and post-fire behaviours of CFST columns. It is concluded that various parameters (such as load ratio and elevating temperature time ratio etc.) affect the residual strength of CFST columns severely. Finally, this paper proposes a set of formulas which can be used to predict the residual strength of CFST columns after going through the whole fire exposure process.