Structural Fire Engineering Research Articles

A plastic-damage model for concrete in fire: Applications in structural fire engineering

The research aims at developing a new multiaxial constitutive model for concrete in the fire situation. In addition to validity at the material level, a crucial feature of a constitutive model is the applicability at the structural level; yet for concrete in fire there remains a serious lack of models combining reliability and robustness. The theoretical aspects and validation of the new model, which rely on a plastic-damage formulation, have been the subject of a former publication; they are briefly summarized here. This paper explores the capabilities of the concrete model for being used in a performance-based structural fire engineering framework. Several examples of numerical simulations by non-linear finite element method are discussed, with emphasis on practical applications that are demanding for the material model. In particular, it is shown that the simulations using the new concrete model succeed in capturing, at ambient temperature, the crack pattern in a plain concrete specimen and the influence of the loading path on reinforced concrete (RC) slabs. At high temperature, the presented applications include a RC slab subjected to furnace fire and a large-scale composite steel–concrete structure subjected to natural fire. In the numerical analyses, no parameter calibration was required on the particular concrete type, except for the uniaxial strengths and tensile crack energy which are to be defined case-by-case. The results illustrate the reliability and numerical robustness of the model. Also, they suggest that satisfactory prediction of structural behavior in fire can be obtained when no additional data is available on the specific properties of the particular concrete mix that is used in the project, as is often the case in practice, by using standard values of parameters.

Comparing simple and advanced tools for structural fire safety engineering

SummaryThe present paper gives an overview of the actual tools available for the estimation of the fire development and of the resulting thermal actions on structural members. A case study is developed on the basis of the Fire Safety Engineering methodology, respectively with two different approaches, one based on ‘advanced’ tools and another one based on ‘simplified’ tools. Indeed, three categories of fire models are used in this study, each of which corresponding to a different level of precision and complexity: hand calculations, zone models, and computational fluid dynamics (CFD) models. The case study is relative to the calculation of the heating of a portal frame in a gymnasium, under localised real fire conditions. It is shown through comparisons that, in this case, predictions of analytical methods are, to certain extent, in good agreement with predictions of the CFD model. In particular, it is demonstrated the relevance of using a simplified method of EN 1991‐1‐2 to predict thermal actions to vertical members. The obtained results also highlight the need to develop more relevant analytical methods in order to predict the temperature field during a fire in a large volume. Copyright © 2014 John Wiley & Sons, Ltd.

Mechanical response of a partially restrained column exposed to localised fires

Recent trends in structural fire engineering research have focussed on the response of buildings with large open plan spaces to so-called travelling fires. These fires travel horizontally across the floor plate of a building and result in time and spatially varying thermal exposure and response of the structure to the fire. What has received little attention, however, is the effect that non-uniform thermal exposure has on columns. Recent tests conducted at SP demonstrated the effect of a small non-uniformity of thermal exposure, resulting in a thermal gradient of around 1°C/mm, on a column exposed to a pool fire. The curvature resulting from a non-uniform thermal exposure where the column is pinned, or in cases where the column is partially restrained, will result in an eccentricity in the column’s loading and large second order effects.This paper describes the effect of thermal exposure varying in both the horizontal and vertical axes to columns by means of including this thermal boundary in a solution of classical Euler beam theory. The resulting solution allows for a variation in the stiffness of the rotational restraint at both ends of the column and a non-uniform temperature exposure through the column’s section and along its height. The resulting equations help to understand better the impact of the assumptions of ‘lumped capacitance’ on steel columns, suggesting a challenge to this assumption in some instances, as well as to enhance understanding of the impact of non-uniform fires on steel columns.

An application of the PEER performance based earthquake engineering framework to structures in fire

The Pacific Earthquake Engineering Research (PEER) Center’s Performance Based Earthquake Engineering (PBEE) framework is well documented. The framework is a linear methodology which is based upon obtaining in turn output from each of the following analyses: hazard analysis; structural analysis; loss analysis, and finally decision making based on variables of interest, such as downtime or cost to repair. The strength of the framework is in its linearity, its clear flexibility and in the consideration of uncertainty at every stage of the analysis. The framework has potential applications to other forms of extreme loading; however in order for this to be achieved the ‘mapping’ of the framework to the analysis of structures for other loading situations must be successful.This paper illustrates one such ‘mapping’ of the framework for Performance Based Fire Engineering (PBFE) of structures. Using a combination of simple analytical techniques and codified methods as well as random sampling techniques to develop a range of response records, the PEER framework is followed to illustrate its application to structural fire engineering. The end result is a successful application of the earthquake framework to fire which highlights both the assumptions which are inherent in the performance based design framework as well as subjects of future research which will allow more confidence in the design of structures for fire using performance based techniques.This article describes the PEER framework applied to structural earthquake design then follows the framework from start to completion applying suitable alternative tools to perform each stage of the analysis for structures in fire.

An Overview Of Optimising Passive Fire Protection Using A Limiting Temperature By Structural Assessment

The British Standards and the Eurocodes both cover the design of steel structures in fire. These provide designers with the opportunity to exploit the properties of structural steel to its maximum capacity in the fire limit state, termed Structural Fire Engineering. This allows fire protection measures to be integrated into structural design as part of the passive fire protection specification. If used effectively in the specification process it can bring significant benefits to the project, including robust and safe designs, quantified structural performance and cost savings.The suitability of a member in a structural design is generally governed by serviceability limitations such as deflection. Generally this approach provides a conservative working stress for the steel sections of approximately 50% of their overall capacity. Within the UK, this simplified approach led to the development of prescribed limiting steel temperatures of 550°C for columns and 620°C for beams.Best-practice industry guidance published in the UK encourages designers to specify a limiting steel temperature together with the fire resistance period as part of the overall steelwork specification. Passive fire protection is generally specified post design-stage at a contractor or applicator level and sometimes very little is known about the limiting steel temperature.This paper provides an overview of the fire protection specification and provides advice on structural fire engineering to determine a limiting steel temperature and the benefits that can be gained from doing so. The paper highlights how supplier expertise in fire engineering approaches and product performance knowledge can bring value to a design and play an important role in providing safe, cost effective and compliant solutions.

Modelling of Steel-Concrete Composite Structures in Fire Using OpenSees

This paper presents the extension of the structural analysis software framework OpenSees for modeling steel framed composite structures subjected to fire including the development of a geometrically nonlinear shell element. The new shell element is formed by a combination of membrane elements and Mindlin plate bending elements using a general total Lagrangian formulation. The MITC technique (Mixed Interpolation of Tensorial Components) is applied to alleviate shear locking problems and the addition of drilling degrees of freedom is included. A new thermal load class was created to define the temperature distribution through the thickness of the shell section. The two-dimensional OpenSees material, DruckerPrager, was modified to model the concrete in the composite deck slab at elevated temperature with temperature-dependent material properties according to the Eurocode 2. A three-dimensional finite element model of a composite structure was built in OpenSees, consisting of a flat reinforced concrete slab modeled by the developed shell element as well as concrete ribs and beams/columns modeled by three-dimensional beam elements. These components were connected by rigid link elements to model composite action. The performance of the developed model is verified and validated by a series of analytical solutions and experimental results respectively. Among these are: one-way bending of steel plates; fire tests on simply supported composite beams; and reinforced concrete slabs where membrane actions are investigated. Cardington restrained beam test and British Steel Corner test are also modeled. The reasonable agreement achieved between OpenSees predictions and experimental measurements shows the validity of the developed OpenSees extension to model composite structures in fire. The horizontal displacement of the column at floor level was modeled for the first time with reasonable agreement. This work is part of a wider project which, upon completion, will provide a user-friendly open-source computational platform for structural fire engineering analyses from fire dynamic simulation through to heat transfer analysis and mechanical analysis.

Structural Fire Engineering: Overview and Application Examples of Appendix 4 of the AISC Specification

This paper presents an overview of current conventional practices for providing passive fire protection of building structures and describes alternative engineering approaches covered in Appendix 4 of the 2010 AISC Specification, ANSI/AISC 360-10. The concept of structural fire engineering is discussed, along with guidance and design references that are available to support performance-based structural fire engineering analyses. The roles and responsibilities typically assumed by design team members and other stakeholders in a structural fire engineering project are presented, as are considerations associated with peer reviews and approval by authorities having jurisdiction. The paper concludes with a series of four design examples that demonstrate a range of structural fire engineering applications for steel buildings.

A multiaxial constitutive model for concrete in the fire situation: Theoretical formulation

This paper aims to develop a multiaxial concrete model for implementation in finite element software dedicated to the analysis of structures in fire. The need for proper concrete model remains a challenging task in structural fire engineering because of the complexity of the concrete mechanical behavior characterization and the severe requirements for the material models raised by the development of performance-based design. A fully three-dimensional model is developed based on the combination of elastoplasticity and damage theories. The state of damage in concrete, assumed isotropic, is modeled by means of a fourth order damage tensor to capture the unilateral effect. The concrete model comprises a limited number of parameters that can be identified by three simple tests at ambient temperature. At high temperatures, a generic transient creep model is included to take into account explicitly the effect of transient creep strain. The numerical implementation of the concrete model in a finite element software is presented and a series of numerical simulations are conducted for validation. The concrete behavior is accurately captured in a large range of temperature and stress states. A limitation appears when modeling the concrete post-peak behavior in highly confined stress states, due to the coupling assumption between damage and plasticity, but the considered levels of triaxial confinement are unusual stress states in structural concrete.

A Note From the Guest Editor

The Action COST TU0904 enables researchers in different specialisms and countries to understand better the recent advances in research in parallel fields, as well as their limitations, so that they see their own research in context and identify opportunities. Practitioners, including fire engineers and building/fire control authorities, benefit from exposure to research findings, discussion with the research community and the sharing of best practice. Their input makes researchers aware of real-world constraints, and requirements for new research. The Action divides its membership loosely into three themed Working Groups, although clearly its overall mission of promoting integration means that these groups must interact on many of the key activities. The Working Groups are: WG1 Fire Behaviour and Life Safety focuses on the behaviour and effects of fire in buildings, combining this research-based knowledge with the most effective means of protecting human life against the occurrence of fire in the built environment. This includes active measures in fire-fighting with the effects of building form on the inherent risk to inhabitants. WG2 Structural safety covers the response of different building types to fires and the rapidly developing research field of structural fire engineering, including new materials and technologies and passive protection measures.Crucial problems of structural fire engineering concern change of use of buildings and the current imperatives of sustainability, energy saving and protection of the environment after fire. WG3 Integrated Design brings together design, practice and research across the disciplines of fire in the built environment. In structural design this includes integration of fire resistance with all the other functional requirements of a building, from concept onwards, rather than simply adding fire protection after all other processes are complete. Active input from practitioners, regulators and fire-fighters through this group is vital to the success of the Action. The Action started in March 2010, and now has 22 nations of the EU and New Zealand participants. Its first deliverable, State of the Art Report attempts to bring together the current state of research, mainly in the participating countries but set into the context of knowledge world-wide. The deliverable, Case Studies presents current practice and accumulated knowledge in fire engineering.The authors, experts of the Action, are trying to include on the selected fire engineering applications clear explanations of the decision processes, the scientific assumptions and the practical constraints, as well as how different aspects of fire engineering are integrated. The deliverable, Fire Brigade Reports and Investigations consists of contribution to organisation of national fire and rescue arrangements in different EU, of contribution to comparability of national statistics and recommendations for questions to be included in standardised national fire fighters reports, and of lessons learned from the particular disasters. The current activity of Actionis focussed to Benchmark studies, contribution to Eurocodes backgrounds and Educational dimension in area of structural fire design. The papers in this issue allowed experts in Action to inform about its research findings in Proceedings of Action Conference 29 April 2011 outside the Action as well.

Deformation-reversal in component-based connection elements for analysis of steel frames in fire

A previous paper covered the principles of practical modelling of common steel beam-to-column connections in frameworks at elevated temperatures, using a component-based approach to represent the main structural actions generally grouped at bolt rows. This is needed for the structural modelling which is a necessary part of performance-based structural fire engineering design based on analysis of a range of natural fire scenarios. This paper extends the treatment of components to the effects of change of temperature during an analysis. This includes the treatment of reversal of displacement, which is essential in order to allow components within the whole model to change their temperatures, even when temperatures are increasing monotonically. Reversal curves, and their Intersection Points with the zero-force axis, are used to link the states of deformation of any component on adjacent force–displacement curves at different temperatures. A desirable feature of this treatment is that it encompasses the effects of cooling after heating, which has been observed in full-scale tests to cause failure of connections as beams contract. A simple sub-frame model including beams, columns and endplate connections, has been analysed using the component-based model, in order to illustrate how the connection and its components behave, and how failure develops when the members and connections are subjected to a heating–cooling sequence.

A Structural Fire Engineering Prediction for the Veselí Fire Tests, 2011

Fire hazards and full-scale structural tests have provided evidence that the beam-column connections of building frames are the weakest structural elements, which are vulnerable to fracture in fire. Connection fractures may lead to extensive damage or even progressive collapse. However, current design methods for connections are solely based on ambient-temperature behaviour, the additional forces and rotations generated in fire are not taken into account. The Structural Fire Engineering Research Group of the University of Sheffield is involved in a European-collaborative project which concerns the behaviour and robustness in fire of practical connections to composite columns. This includes two natural fire tests in a full-scale composite structure in Veselí, the Czech Republic. The Sheffield team was responsible for predicting the structural behaviour in the tests before they were conducted. This assessment was conducted using the specialist structural fire engineering FEA program Vulcan. This paper reports the results of this predictive analysis.

Tensile Membrane Action of Thin Slabs Exposed to Thermal Gradients

A number of simplified design methods have been developed to predict composite slab capacities in fire. Most of these extend ambient-temperature large-deflection slab behavior to the elevated-temperature phase by reducing the strengths of fire-exposed concrete and reinforcement while neglecting the effects of thermal expansion and thermal bowing of the slab. Experiments have shown that there are significant differences between the predictions from these methods and the actual behavior and failure modes of ambient- and elevated-temperature concrete slabs in tensile membrane action. Therefore, this paper describes the development of a new analytical method that incorporates both thermal and mechanical effects into the prediction of slab behavior in fire conditions. It uses the variational Rayleigh-Ritz approach to classical large-deflection plate theory. The method is found to produce accurate predictions of deflections and membrane tractions; however, it requires further refinement for accuracy of stresses. The results are compared with numerical modeling using VULCAN, a specialist finite-element (FE) program for structural fire engineering.

A contemporary review of large-scale non-standard structural fire testing

In recent years, large-scale structural fire testing has experienced something of a renaissance. After about a century with the standard fire resistance test being the predominant means to characterize the response of structural elements in fires, both research and regulatory communities are confronting the many inherent problems associated with using simplified single element tests, on isolated structural members subjected to unrealistic temperature-time curves, to demonstrate adequate structural performance in fires. As a consequence, a shift in testing philosophy to large-scale non-standard fire testing, using real rather than standard fires, is growing in momentum. A number of custom made, non-standard testing facilities have recently been constructed or are nearing completion. Non-standard fire tests performed around the world during the past three decades have identified numerous shortcomings in our understanding of real building behavior during real fires; in most cases these shortcomings could not have been observed through standard furnace tests. Supported by a grant from the Fire Protection Research Foundation, this paper presents a review of relevant non-standard structural fire engineering research done at the large-scale around the world during the past few decades. It identifies gaps and research needs based both on the conclusions of previous researchers and also on the authors’ own assessment of the information presented. A review of similar research needs assessments carried out or presented during the past ten years is included. The overarching objective is to highlight gaps in knowledge and to help steer future research in structural fire engineering, particularly experimental research at the large-scale.

On the use of fire brigade statistics for structural fire safety engineering

In this paper the fire brigade intervention is considered for the assessment of structural fire safety through the concept of a maximal controllable fire area. Based on a literature survey probabilistic models are developed to consider the uncertainties associated with the fire development and the fire brigade intervention. A sensitivity analysis identifies the most important parameters and suggestions for future data collection are made to improve the probabilistic models.

Modeling fire-induced radiative heat transfer in smoke-filled structural cavities

Accurate heat transfer prediction is important to structural fire engineering in order to analyze thermal–mechanical behavior of structures in fire. However, the presence of cavities and participating media complicates radiative calculations. A numerical approach with finite element and discrete ordinates method is presented in this paper to predict fire imposed radiative heat fluxes to these type of structural members. This approach is used to simulate heat transfer to unprotected steel I-sections with symmetrical cavities exposed to post-flashover fires. Results show that the cavity geometry could strongly attenuate the radiative energy, while the presence of hot smoke enhances radiative transfer by emission. Average radiative fluxes for inner surfaces of the I-sections are seen to increase with smoke opacity. In addition, the radiative fluxes are observed to decrease faster for I-sections with higher section factors. This work also shows that the self-radiating mechanism of I-sections is important in the optically thin region, and existing methodologies neglecting these physics could significantly underpredict steel temperatures.

Behaviour of restrained structural subassemblies of steel beam to CFT column in fire during cooling stage

During the well publicised Cardington structural fire research programme, parts of some connections suffered fracture during cooling. This led some to believe that fire spread and structural collapse may occur during the cooling stage of a fire and therefore this issue should be considered in structural fire engineering design. This paper focuses on steel framed structures using concrete filled tubular (CFT) columns and the objective of this paper is to find means of reducing the risk of structural failure during cooling. It reports the results of a study using the general finite element software ABAQUS to numerically model the behaviour of restrained structural subassemblies of steel beam to CFT columns and their joints both in fire, emphasising on the cooling stage. Validation of the finite element model was achieved by comparing the simulation and test results for the two fire tests investigating cooling behaviour, recently conducted at the University of Manchester on similar structures. In these two tests, the test assembly was heated to temperatures close to the limiting temperature of the steel beam and then cooled down while still maintaining the applied loads on the beam. One of the tests used reverse channel connection and the other test used fin plate connection. The finite element models give very good agreement with the experimental results and observations. Remarkable differences in tensile forces in the connected beams were observed during the tests depending on the beam temperature at which cooling started. This leads to the suggestion that in order to avoid connection fracture during cooling, it may be possible to reduce the limiting temperature of the connected beam by a small value (<50°C) from the limiting temperature calculated without considering any axial restraints in the beam.

Recent Lessons Learned in Structural Fire Engineering for Composite Steel Structures

The knowledge in the field of structural fire engineering has been greatly advanced through assessment of a number of real fires (WTC, Torre Windsor, Broadgate, etc.) and, especially, by the Cardington series of full scale structural fire tests. This knowledge has been used to validate and verify the use of computational finite element models that have expanded the range of structures that can be investigated under severe fire exposure. This paper presents a selection of key lessons learned by the authors through the assessment of structures in fire for real commercial building projects. The key areas of sensitivity that have been encountered are described and a discussion of each point presented. The paper is aimed at describing potential weaknesses that have been observed in the commercial work of the authors, often driven by the requirements for efficient ambient structural design. The paper concludes with some suggested advice for structural engineers aimed at increasing the general robustness of building structures. This is based on designing out as far as possible in the ambient design of a structure the potential weaknesses identified in past project work.

Effect of transient strain on strength of concrete and CFT columns in fire – Part 2: Simplified and numerical modelling

This paper presents finite element analysis on columns of concrete and concrete-filled tubular section. This study has been conducted with Vulcan, a specialist structural fire engineering FE program, which has been further developed to incorporate the transient strain of concrete. The implementation of transient strain in Vulcan has been validated against a Shanley-like simplified model. A further extension of the simplified model has been carried out in order to provide direct comparability with the FE modelling. The effects of transient strain, considering thermal gradients through the column cross-section, have been evaluated with both the simplified and FE models. Finally, parametric studies on concrete-filled tubular columns, considering the effects of slenderness ratio, reinforcement, the steel casing and thermal gradient within the cross-section, have been conducted using Vulcan.

Book review: Structural Fire Engineering

Book review: Structural Fire Engineering

A connection element for modelling end-plate connections in fire

In this paper a robust 2-noded connection element has been developed for modelling the bolted end-plate connection between a steel beam and column at elevated temperatures. The connection element allows the element nodes to be placed at the reference plane with offset and the non-uniform temperature distributions within the connection. In this model the connection failure due to bending, axial tension, compression and vertical shear are considered. The influence of the axial tensile force of the connected beam on the connection is also taken into account. This model has the advantages of both the previous simple and component-based models. A total of 23 fire tests were used to extensively validate the model. It can be seen that the current model is robust and has a capability to predict the behaviour of a bolted end-plate connection under fire attack with reasonable accuracy. Compared to the tested results the predictions of the current model were mainly on the conservative side. Hence, the model can be used for structural fire engineering design on steel-framed composite buildings. The idea described in this paper can also easily be applied to develop other kinds of connections, such as simple connections, column based connections or hollow section connections, and so on.