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ESTIMATING THE RESPONSE OF STRUCTURAL
SYSTEMS TO FIRE EXPOSURE: STATE-OF-THE-ART
REVIEW
Mehrafarid Ghoreishi1, Ashutosh Bagchi1& Mohamed A. Sultan2
1Dept of Building, Civil and Environmental Eng., Concordia University, Montreal, Canada 2Institute of Research in Construction, National Research Council, Ottawa, Canada
Abstract: Over the past century, fire protection design approaches have been traditionally based
on the prescription of fire resistance ratings achieved from standard tests. Standard furnace tests do not consider a wide range of typical structural conditions such as size; restrain conditions and loading that are encountered in real practice. On the other hand, fire resistance behavior of a single structural member is different from that of complete structure because of factors such as continuity, interaction between members and moment redistribution which are present in the whole structure. In the past few decades, there has been significant progress in structural fire safety analysis and design. However, such analysis relies on simplified methods and experimental data related to structural components, not of the complete systems. There has been very limited attention given on fire safety evaluation at the system level. This paper presents a review of the available experimental and numerical studies on structural systems under fire, current approaches and identifies the research needs in this area.
INTRODUCTION
Most new buildings behave better in fire because of the general reductions in fire load and combustible contents, better fire protection and faster fire fighting response times. On the other hand, slender and lightweight building structures incorporating plastics and other more flammable materials are more vulnerable to fire. The building industry has expended considerable effort on determining the inherent fire resistance of unprotected structures and designing appropriate fire protection systems. Conventional fire protections add the cost to the bare structure, and there are many forms of structure where such protection may be avoided, leading to considerable cost savingd.
A huge volume of researches into the behavior of structures exposed to a fire have been concerned with the structural performance of single structural elements, and the development of analytical techniques. In recent years, various studies have been undertaken on the overall behavior of structures when subjected to fire. The aim of this paper is to give an overview of the research, both experimental and analytical, on the complete steel and concrete structure and identify the gap in this area. Timber structures have special requirements for fire, and because of the restricted use of timber structures in low-rise buildings, they are not covered in the present review. The experimental timber fire test in Cardington and current researches in National Research Council of Canada (NRC) are valuable resources.
FIRE AND HEATING MODELS
Fire is a component which may be characterized by the development of temperature, pressure and burning products in time. There are three stages of fire as shown in Figure 1, which are: the growth phase, the burning phase which starts by a flashover, and the cooling and decay phase.
Figure 1. Modeling of gas temperature during fire1.
The temperature distribution inside the structure is usually calculated based on the gas temperature from nominal fire curves, parametric fire curves and the zone or fluid dynamics models, using heat transfer analysis. Simplified and advanced models of fire may be distinguished. The simplified models of fire are based on fundamental physical parameters, which allow temperature prediction, are the design density of fire load and the conditions of the ventilation.
Advanced models take into account properties of gas and the exchange of mass and energy. The zone models apply homogeneous time dependent temperature development in the fire compartment. The fluid dynamics models forecast the temperature and pressure growth in the finite elements of space in time. Figure 1 shows the Modeling of gas temperature in fire compartment during fire.
STRUCTURAL MODELS
One or a combination of the following three structural models in increasing complexity can be used for evaluating the fire resistance of a structure: simple structural elements, subassemblies, and the complete structural system. Isolated structural elements exposed to the fire are used for determining the element level fire resistance, while a subassembly or the whole structure model gives a realistic estimate of fire resistance. Models of an assembly or the whole structure can account for the connectivity between the structural elements, redistribution of stresses, joint rigidity, and continuity. Thus, these models yield higher level of fire resistance as compared to that calculated based on the element level fire resistance. Subframes or subassemblies are neither as simple as individual member nor complex as complete structure also used to predict the structural behavior in fire. Comparison between the behavior of the whole frame and subframe indicated slight variation 2and choice of subframe models has little influence on the results 3. In general, subframes may be used to replace the complete frame.
3D modeling is the most natural way to represent structures. However, 3D models are usually more complicated to generate and computationally more demanding than the equivalent 2D models. Performance of a 3D frame model versus a 2D plane frame show close agreement. However, the 2D model cannot capture the performance of the composite floor system and redistribution of load that occurs. In the 2D models, the slab should be considered in the thermal analysis of the girder, but it can be neglected in the structural analysis since it has a negligible effect4.
Two basic type of computer-based calculation, Finite-Element Method (FEM) and Moment-Curvature Method (MCM) can be used to analyze the internal forces in a member exposed to fire. The FEM stress-analysis programs such as NASTRAN, ANSYS, VULCAN, ABAQUS and SAFIR are most powerful and often used for analyzing large structures. MCM is based in the development of moment-curvature-thrust relationships for the heat-affected cross sections and most suitable for individual members.
FIRE SAFETY APPROACH
Figure 2 shows the current approaches to the evaluation of the fire performance of structures using analytical and experimental methods. Standard furnace tests (standard fire tests) do not consider a broad range of typical structural conditions, such as size, restrain conditions and loading, that are encountered in real practice. The fire resistance of the whole structure related to the actual fire characteristics is difficult to determine experimentally. The temperature-time relationship generated in the standard fire cannot reflect all aspects of actual fire such as cooling phase that was demonstrated in the Cardington test5.
Analytical Study Experimental Study Verify with Experimental Data if it is possible. 2D 3D
Full Scale Prototype
Real Fire / Real Fire Model
Furnace / Standard Fire Model
Sub Frame Isolated Element Complete Frame Experimental Analytical Analytical / Expeimental? Access Structure Save Valuable Data
Assess Structure
Figure 2. A schematic diagram for evaluation of the fire performance of structures.
The real fire models and complete frame analysis as an advanced and complex approach may not be a reasonable level of study for all forms of structures. For each building or structure, the level of analysis should vary depending on the desired performance level. For example, a 1-2 storey single family residential building will have a lower building importance level, fire risk, and life-safety risk to that of a 100-storey steel office building.
In any event, it is clear that fire should be treated as a credible design load for structures. The method of assessment, however, should be based on the most practical yet appropriate method to meet the desired performance deemed acceptable for the level of risk. For many high-rise structures, a performance-based structural fire assessment may be the most appropriate and reliable approach. The introduction of the performance–based code in a few countries such as New Zealand and Australia has created significant interest around the globe in assessing the realistic fire resistance of a complete structure.
Fire following an earthquake is also an important event as frame subjected to fire subsequent to a major earthquake may cause extensive damage to buildings and life-line structures. Mousavi et al. (2008)6 reviewed the current state of the art of the effects of earthquake-induced fires on building structures, including the corresponding hazard analysis, structural fire safety design, mitigation methods, and techniques and tools for performance evaluation of structures.
EXPERIMENTAL STUDIES
A limited volume of experimental studies are available because of the difficulties and high cost associated with test setup and experimental facilities. Objectives of the experimental studies included: (i) characterizing the fire performance of structural and non-structural elements, (ii) assessing the overall behavior of the structure under real fires, and (iii) acquiring data for a range of fire scenarios so that the analytical models can be calibrated. One of the earliest fire tests in Europe on a structural assembly is reported by Cooke and Latham (1987) 7. They tested a two-dimensional unprotected steel frame subjected to fire exposure, and found that the performance of the frame is better than the individual members. In the Australia a fire test on a 41-storey prototype structure was conducted to investigate the probable effect of fire in the building. The experimental study was also intended to provide data for a risk assessment on fire safety in the prototype buildings. In Japan, fire tests on the behavior of multi-storey steel frame buildings were conducted to investigate the impact of using fire-resistance steel for structural elements for buildings8. One of the vast and complete studies has been conducted at the British Research Establishment (BRE) in Cardington, United Kingdom on a full scale eight-storey steel-framed composite building 9. The majority of experimental fire test is in steel or composite structures.
CARDINGTON FIRE TEST AND FOLLOWING APPROACHES
Following four tests were designed in Cardington to investigate various aspects of structural behavior: restrained beam, plane frame, corner, and office fire (demonstration). The observations made from these tests are briefly described below.
(a) The influence of the continuity and composite action with the slab on the overall stability of the exposed beam when forming part of a complete frame was demonstrated in the first test10. (b) The plane frame test demonstrated the structural stability of the whole frame which developed
alternative load paths, although the extent of deformation in a column or some columns was well beyond the normal acceptable limits of a standard fire test on a single column. This test demonstrated the importance of providing protection to critical members such as columns.
(c) The corner test (third test) was designed to show that the membrane action between the beams and floor slabs improved the performance of the overall behavior of the frame under fire conditions. The analytical result shows that a much higher fire load can be resisted by the floor slab than that used for experimental investigation, and the load on the floor slab above the beam was resisted by the tensile membrane action of the concrete slab 11.
(d) The results obtained from the three tests were applied in the fourth test by creating a real fire in an office situation. The objectives of the fourth test, demonstration test, were to reveal some of the important observations in the first three tests in a more realistic fire scenario and to assess
other aspects of structural behavior which were not addressed in the previous three tests. In this test, a compartment was built on the first floor of the structure to represent an open office and was fitted out with a series of work stations, furniture, computers, and filing systems12.
A number of analytical studies have been performed based on the results from the Cardington test. Some of these analytical simulations confirm the experimental results, some of them were calibrated for future usage or data loses in other experimental tests and the rest were modeled to be validated with experimental results. Ramli-Sulong et al. (2007)13 validated their proposed connection model by comparing against a range of available experimental results from the Cardington fire test. Computer simulations as well as experimental results show that the continuity of slab in the composite steel frame has significant effect on fire performance14. Significant moments are generated in the heavily fire protected columns due to thermal expansion of the connected heated beams displacing the columns at floor level as observed in Cardington test. This lateral displacement is sensitive to the stiffness and location of the braced core areas. The four Cardington fire tests have provided a wealth of information about the temperatures in the fire environment and the protected and unprotected steel. But, there are only a few points where temperatures data could be obtained across the depth of concrete slab. The numerical model that was calibrated with these results predicts the slab temperatures in other points15 as well in order to determine the temperature profile across the slab depth. Although the model including the metal deck predicts the concrete temperatures very well, it over-predicts the steel temperatures.
ANALYTICAL/NUMERICAL STUDIES
At present, considerable attention has been given to simulating the behavior of structures in fire by analytical and numerical approaches. During the last decade numerical models have been used more often to analyze the behavior of structures under fire conditions. Some of these studies concentrated on the development of models, which were generally based on the finite element method (FEM) to determinate the temperature distribution across the cross-section of a structural element (thermal analysis). In the U.K., at university of Sheffield, University of Nottingham, Loughborough University of Technology and British Research Establishment (BRE), considerable research on the development of numerical models to study the overall behavior of steel structures in fire is being carried out. Similar studies have been conducted in Japan, Belgium, Netherlands, Canada, the United States, and all over the world. Some of the existing numerical studies are discussed in the following sections.
COMPOSITE BEHAVIOR
Some of the models that deal with the concept of composite action have included the effect of the floor slab by representing the beams as composite elements based on the effective width concept. The experimental and analytical studies investigate the influence of membrane action of a concrete floor slab on the behavior of a steel framed composite building exposed to fire. The extension of the membrane theory as proposed by Kemp (1967)16, was applied to simulate them. As general results, the stability of the floor slab was found to be insensitive to the fire load, and This implies that after the failure of some structural members in a building due to fire, the stability of the building can still be preserved as a result of the membrane action of the floor slab, which may possibly develop an alternative load path to bridge over these failed structural members. Lawson (2001)17 reviewed the earlier works in composite buildings exposed to fire. In his paper, elemental performance in standard fires and full-scale behavior of floors and complete structures in natural fires were discussed as well.
Simplified approaches of membrane action and composite beams in fire showed a very good agreement with the experimental test results. Fourier series expansion methods can be used for simplifying the
composite beam18, while yield-line theory is used for modeling the membrane action19. Such simplified models are easier to apply as compared to the finite element (FE) models which can deal with higher complexity and require longer solution time. However, the FE models provide comprehensive information that may be more reliable20.
COMPLETE STRUCTURE UNDER HEATING MODELS a) Concrete Structure
The fire resistance of a concrete structure is affected by many factors including concrete cover, the cross section of the structural elements which is not essentially constant during fire exposure, and the material properties concrete and reinforcing steel. Concrete has excellent fire resistance properties. However, the strength of concrete also reduces as temperature rises, and high strength concrete is susceptible to explosive spalling at high temperature. Although there are some studies on the element levels fire resistance of reinforced concrete (RC) components, high strength concrete, concrete columns and beams strengthened with fiber reinforced polymer, there is a lack of analytical and experimental studies on assemblies or whole structural systems. Available research shows that structural collapse in concrete structures is always due to column failure and the fire resistance of columns is vitally important for reinforced concrete buildings21. Fire scenarios are found to have significant influence on fire resistance RC beams22. Continuity of slab is an important factor in fire resistance of RC structures and membrane action in reinforced concrete frames as well as composite structures were reported to have beneficial effect. As a result, slabs and beams have much better fire resistance if they are restrained rotationally; and the behavior of one-way slabs is very sensitive to the end support conditions and the axial restraint23.
b) Steel Structure
A number of analytical studies have been conducted on the overall behavior of structures in fire. The main objectives of these studies are to demonstrate the inherent fire resistance of structures or to show that the amount of fire protection required can be reduced by investigating the overall response of a complete frame exposed to fire. These models have been used to study the effects of parameters such as continuity, end restraint and continuous floor slabs, which influence the behavior of a heated frame. As a result of frame analysis, the performance of the frame members in fire is better than that of isolated members under the same fire and load conditions.
Continuity of a column has a major effect in fire performance11 but the column location, internal, edge or corner situation, only slightly affects the predicted column failure load 24. Restraint of structures against lateral movement is another important issue when considering the survival of that structure in fire conditions. Therefore, the enhancement of the column fire resistance could only be achieved when it is in a sway prevented frame, which is the case in real practice. The performance of the column in the fire is much more enhanced for the nonsway situation than for the unbraced case. End restraints, rotational or axial, generally increase fire resistance of a steel beam in a fire8. High-temperature creep as well as fire scenario, load level, degree of end-restraint has significant influence on the behavior of restrained beams under fire conditions. The creep effects have to be properly accounted for in the analysis to obtain realistic response of beams under fire conditions25. As a clear demonstration of the influence of frame continuity, Franssen et al. (1995)26 showed that the fire resistance of a frame calculated based on the whole structure is 24% higher than that calculated by summing the resistance of individual members.
After the collapse of World Trade Center (WTC) towers, some analytical studies have been conducted (e.g., Flint et al. 2007) 27 which indicated that large displacements may take place in long span structural floor systems without failure. However, the interaction of the highly deflected floors with the exterior or perimeter columns can lead to structural collapse. Flint et al. (2007)27 modeled long span 2D truss floor systems in tall buildings that represent the type of construction used in the WTC Towers. In high rise buildings, a corner compartment fire will lead to the inelastic buckling of the interior gravity columns.
However, the loads carried by the interior column can be redistributed to the remaining columns through catenary action in the floor system, and the structure may remain stable in spite of the interior column failure. When the complete storey is exposed to fire, then failure initiates by inelastic buckling of an interior gravity column. This failure propagates further by inelastic buckling of additional interior gravity columns. The overall structural load capacity reduces by about 15% with the failure of each gravity column, and complete story collapse mechanism develops if the loads are maintained constant at the level corresponding to the failure of the first column28. A simple and straightforward method for assessing tall steel buildings for collapse of exterior columns due to fire was introduced by Usmani (2008)29, which is cost effective and quick. The method can be used for assessing regular frames where a number of floors have high deflection and reduced stiffness such that the main load carrying mechanism consists of a catenary action leading to destabilizing pull-in forces to be exerted on exterior columns.
RESEARCH NEEDS
Historically, iron, as the predecessor of steel, was developed as a ‘fire-proof’ material in the early 19th century after dreadful fires in large timber mills in the north of England, and nowadays the challenge is how steel structure can be protected against fire loads. The present review indicates that the system level performance of a structure is quite different from that estimated from the components level performance. While standard tests of components provide important information about the fire resistance of a particular material and structural element, it is difficult to estimate the system level fire performance of a structure from such standard tests. While conducting an experiment with fire exposure on a full scale building is expensive and difficult, numerical modeling lends itself to conduct detailed investigation on the system level performance, once such model is calibrated against available test data (e.g. Cardington tests). More of such analytical studies for various systems and fire scenarios are needed.
Performance-based structural fire engineering is emerging as a preferable design approach where the design of the fire protection policy and structural detailing of a building is based on the evaluation of the structure’s behavior under realistic fire scenarios. While current practice in the world is principally prescriptive, performance-based fire engineering is beginning to have an impact on building design particularly as architects conceive more complex designs and engineers have an increased understanding of structural fire response from the WTC collapse and Windsor tower fire in Madrid30.
In the performance-based design of structures exposed to fire, the earthquake-induced damage scenario should also be considered for the buildings located in seismic areas. Since post-earthquake fires are not uncommon, fire codes need to distinguish between structures located in seismic and non-seismic areas, by requiring more stringent fire resistance provisions for those buildings potentially subjected to seismic actions 6.
SUMMARY AND CONCLUSIONS
From present review, it can be noted that under the same fire conditions the fire resistance of a complete structure is different from that calculated based on individual structural members. The fire resistance of a member is improved when it is considered as an integral part of a complete structure. Factors such as connections, continuity, load intensity, interaction between members, tensile membrane action of floor slabs, and fire loading type have a great influence on the local and overall collapse of a structure in fire. In a conventional prescriptive design approach, the above aspects would not have been addressed or realized for whole concrete and steel buildings and optimization of fire protection of structure will not be possible. As it is mentioned in most researches, fire scenario has a large influence in fire resistance and
the use of the standard fire curve is conservative. Performance-based design is suitable in this case and evaluation of the structural performance is essential. As such, an advanced analysis of the whole structure may not be necessary or practical in every case. On the other hand, for many high-rise or complex structures, performing an advanced performance-based fire safety design may be the most reliable approach. However, reliable simplified methods need to be developed for their use in the design process.
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