Structural Fire Design Research Articles

A comparative study between ultra-high-performance concrete structures and normal strength concrete structures exposed to fire

The use of ultra-high-performance concrete (UHPC) in civil infrastructure is increasing due to its numerous advantages compared to normal strength concrete (NSC). In this research, thermal analysis of NSC and UHPC beams and columns was developed to understand and compare the fire performance of these structures. A numerical analysis was performed using finite element (FE) modeling with ABAQUS software. Compared to NSC, UHPC structures in some cases have lower fire resistance due to their slender cross-sections and unfavorable thermal properties and mechanical capacity degradation due to temperature effect. The results show that UHPC structures experience significantly higher cross-sectional temperatures when exposed to fire compared to NSC. The results are particularly relevant for high-rise buildings, where fire safety requirements are more stringent and the use of UHPC is attractive, leading to an unprecedented contradiction. Alternative strategies for the application of UHPC, such as the contradictory increases in cross-section or the use of fire protection coatings, must be considered. In some cases, NSC may be more attractive as a structural material than UHPC if structural fire design requirements are taken into account.

The role of large language models (AI chatbots) in fire engineering: An examination of technical questions against domain knowledge

This communication presents a short review of chatbot technology and preliminary findings from comparing two recent chatbots, OpenAI’s ChatGPT and Google’s Bard, in the context of fire engineering by evaluating their responses in handling fire safety-related queries. A diverse range of fire engineering questions and scenarios were created and examined, including structural fire design, fire prevention strategies, evacuation, building code compliance, and fire suppression systems. The results reveal some key differences in the performance of the chatbots, with ChatGPT demonstrating a relatively superior performance of 88% (vs. 80% for Bard). Then, this communication highlights the potential for chatbot technology to revolutionize fire engineering practices by providing instant access to critical information while outlining areas for further improvement and research. Evidently, and when it matures, this technology will likely be elemental to our engineers’ practice and education.

Numerical analysis of full-scale structural fire tests on composite floor systems

Recent experiments conducted at the NIST examined steel-concrete composite floor systems designed per U.S. practice under standard fire. The first experiment designed per prescriptive provisions for a 2-h fire rating with 60 mm2/m of reinforcement, developed a central breach integrity failure after approximately 1 h of exposure. The second and third experiments, designed with 230 mm2/m of reinforcement, with the third one omitting the fire protection on the central steel beam, showed no failure within 2 h. This paper describes a numerical investigation to gain further insights into the fire behavior of the composite systems tested in these experiments. Nonlinear finite element models were validated against the tests. Simulation of the first test shows concrete damage and rebar fracture in the hogging moment area corresponding with the cracks observed experimentally. Simulation of the third test captures the development of tensile membrane action, confirming the redistribution from the unprotected secondary steel member to the floor reinforcing steel. A sensitivity analysis allows identifying the minimum reinforcement steel for protected and unprotected central beams configurations. The results can support improvements of the fire requirements in the U.S. codes as well as application of performance-based structural fire design for composite structures.

Probabilistic Performance-based Fire Design of Structures: A Hazard-Centred and Consequence-Oriented Perspective

Risk-based design and assessment methods are gaining popularity in performance-based structural fire engineering. These methods usually start by defining a set of hazard scenarios to use as analysis inputs. This approach, proven highly effective for other hazard types such as earthquakes, may not be optimal for fire safety design. Indeed, the strong coupling between the fire phenomenon and structural features enables an ad-hoc design variable selection (and/or optimisation) to reduce fire intensity, making fire scenarios additional design outputs. In addition, such a coupling effect implies that fire scenarios maximising consequences are structure specific. Building on these considerations, this paper discusses the limitations that arise at different analysis steps (i.e., fire-scenario and intensity treatment, identifying fire intensity measures, probabilistic fire hazard analysis, developing fire fragility models, and risk calculation) when using conventional risk-based approaches for design purposes. Furthermore, it compares such approaches with a fire safety design methodology (the Consequence-oriented Fire intensity Optimisation, CFO, approach) that addresses the identified limitations. The potential benefits of integrating the two approaches are also discussed. Finally, the fire design of a simplified steel-girder bridge is introduced as an illustrative example, comparing the consequence metrics and design updating strategies resulting from the two approaches.

Cost-optimization based target reliabilities for design of structures exposed to fire

Adequacy of structural fire design in uncommon structures is conceptually ensured through cost-benefit analysis where the future costs are balanced against the benefits of safety investment. Cost-benefit analyses, however, are complicated and computationally challenging, and hence impractical for application to individual projects. To address this issue, design guidance proposes target reliability indices for normal design conditions, but no target reliability indices are defined for structural fire design. We revisit the background of the cost-optimization based approach underlying normal design target reliability indices then we extend this approach for the case of fire design of structures. We also propose a modified objective function for cost-optimization which simplifies the evaluation of target reliability indices and reduces the number of assumptions. The optimum safety level is expressed as a function of a new dimensionless variable named “Damage-to-investment indicator” (DII). The cost optimization approach is validated for the target reliability indices for normal design condition. The method is then applied for evaluating DII and the associated optimum reliability indices for fire-exposed structures. Two case studies are presented: (i) a one-way loaded reinforced concrete slab and (ii) a steel column under axial loading. This study thus provides a framework for deriving optimum (target) reliability index for structural fire design which can support the development of rational provisions in codes and standards.

Multi-objective optimization of structural fire design

Probabilistic risk assessment can be used for determining a design with a residual risk that is As Low As Reasonably Practicable (ALARP). Existing risk-based design approaches however predominantly focus on single objectives, which can be impractical considering the need for structures to meet diverse performance criteria across various dimensions, including monetary costs, environmental costs, and structural performance measures. To address these challenges, this study proposes the use of a multi-objective optimization (MOO) approach within a risk-based framework. Within the MOO framework, design goals for sustainability and resilience are incorporated together with safety objectives. The MOO approach is developed here for the risk-based design of structures exposed to fire. A multi-objective problem is formulated for a reinforced concrete slab in a multi-family dwelling by identifying design parameters, objectives, and constraints. Taking environmental costs into account has little effect on the optimum design obtained through MOO for the case study, except in cases where CO2 emissions are highly valued. A limited increase in investment is observed to render the structure repairable after a fire incident, highlighting the importance of factoring in post-fire reparability in structural fire design.

Tensile Tests for Material Characterisation of High- and Ultra-High-Strength Steels S690QL and S960QL under Natural Fire Conditions

The mechanical material behavior of mild steels is reversible in the cooling phase of natural fires, which is proven by experimental evidence. For the material behavior of high-strength steels during cooling, no results are yet available. The paper provides the first comprehensive test program on the constitutive material behavior of high-strength steels S690QL and S960QL as well as mild steel S355 J2 + N in the case of natural fires. It is elaborated that the mechanical material behavior of high-strength steels in the cooling phase differs from the behavior in the heating phase and is not reversible due to phase changes of the microstructure. A constitutive material model for structural fire design purposes is developed on the basis of experimental data and the soundness and reliability of the model are proven by a statistical study that systematically evaluates the deviation of the model prediction from the test data.

Behavior of Zn5Al hot-dip galvanized steel members under fire exposure

PurposeBased on the known positive effects of conventional hot-dip galvanizing under fire exposure and indicative results on zinc–aluminum coatings from smallscale tests, a series of tests were conducted on zinc-5% aluminum galvanized test specimens under fire loads to verify the previous positive findings under largescale boundary conditions.Design/methodology/approachThe emissivity of zinc-5% aluminum galvanized surfaces applied to steel specimens was determined experimentally under real fire loads and laboratory thermal loads in accordance with the normative specifications of the standard fire curve. Both large and smallscale specimens were used in this study. The steel grade and surface conditions of the specimens were varied for both test scenarios.FindingsLargescale tests on specimens with typical steel construction dimensions under fire loads showed that the surface emissivity of zinc-5% aluminum galvanized steel was significantly lower than that of the conventionally galvanized steel. Only minor influences from the weathering of the specimens and steel chemistry were observed. These results agree well with those obtained from smallscale tests. The design values of zinc-5% aluminum melt (Zn5Al) required for the structural fire design were proposed based on the obtained results.Originality/valueThe novel tests presented in this study are the first ones to study the behavior of zinc-5% aluminum galvanized largescale steel construction components under the influence of real fire exposure and their positive effect on the emissivity of steel components galvanized by this method. The results provide valuable insights and information on the behavior in the case of fire and the associated savings potential for steel construction.

Holistic Performance-Based Fire Design of Steel Structures—Case Study: Sports Hall

Holistic Performance-Based Fire Design of Steel Structures—Case Study: Sports Hall

Constitutive model of austenitic high-strength A4L-80 bolts at elevated temperatures

In recent years, the use of austenitic bolts in building structures increased rapidly. It is important to conduct the fire-resistant design of austenitic bolts because their mechanical properties may deteriorate faster than normal high-strength steel bolts in a fire. Therefore, the mechanical properties of the austenitic bolts at elevated temperatures should be clarified to provide a basis for structural fire design. In this paper, an experimental study was conducted to investigate the mechanical properties of A4L-80 stainless steel bolts at elevated temperatures ranging from 20 to 900 °C. Stress-strain curves and mechanical property index were obtained by the tensile tests. The test results were compared with other types of austenitic bolts and base materials at elevated temperatures, and a constitutive model of A4L-80 stainless steel bolts was proposed. It was seen that the proposed reduction factors can effectively predict the mechanical property of A4L-80 at elevated temperatures; the constitutive model was consistent with stress-strain curves obtained from tests.

GMNIA with beam elements and strain limits based fire design approach for steel beams and beam–columns

The use of advanced analysis techniques harnessing the capabilities of the currently available computational resources can lead to considerably more accurate fire design of steel structures relative to the currently adopted simple fire design methods in practice. In this paper, a structural steel fire design method by Geometrically and Materially Nonlinear Analysis with Imperfections (GMNIA) and using strain limits is put forward for the fire design of steel beams and beam–columns. In the proposed fire design method, the GMNIA of a steel member or structure is performed using computationally efficient beam finite elements and strain limits are adopted to account for the detrimental effects of local buckling on the ultimate capacities. To verify the accuracy of the proposed method, extensive numerical parametric studies are conducted through shell finite element modelling. The results indicate that the proposed approach provides accurate and safe ultimate resistance and limit temperature estimations for steel beams and beam–columns in fire, with a considerably higher level of accuracy and reliability relative to the provisions of the European structural steel fire design standard EN 1993-1-2.

Performance-based design for structures in fire: Advances, challenges, and perspectives

The Performance-Based Design approach provides a goal-oriented process to support safe, innovative, and resilient structural fire designs. However, adoption of this approach requires both a rigorous framework and appropriate analysis tools to assess the expected performance of structures in fire. The objective of this paper is to provide an overview of the Performance-Based Design approach, including the process and its specific elements, applied to structures in fire. First, the value of the approach is illustrated through a review of case studies. Then, the process is described with identification of the role of the structural fire engineer. Recent research in methods to enable the evaluation of the structural fire performance, conducted within the author's group at Johns Hopkins University, is discussed. Issues addressed include the coupling between the fire and thermal-structural models, the characterization of the material behavior at elevated temperature, numerical modeling of structures subjected to fire, probabilistic risk assessment, and cost-benefit analyses. The paper concludes with a discussion of some challenges and perspectives for the future of performance-based structural fire design.

Stability and design of stainless steel hollow section columns at elevated temperatures

This paper investigates the stability and design of stainless steel circular, elliptical, square and rectangular hollow section (CHS, EHS, SHS and RHS) columns at elevated temperatures. Nonlinear shell finite element models are employed to conduct comprehensive parametric studies whereby extensive benchmark structural performance data on the behaviour and resistance of stainless steel hollow section columns at elevated temperatures is generated. In total, 26,760 cold-formed and hot-rolled austenitic, duplex and ferritic stainless steel CHS, EHS, SHS and RHS columns at elevated temperatures are taken into account, considering various member slendernesses, cross-section geometries, cross-section slendernesses and elevated temperature levels. New flexural buckling design rules are put forward for stainless steel hollow section columns in fire, which consistently considers the elevated temperature strength at 2% total strain f2,θ as the reference material strength for all cross-sections classes. The accuracy, safety and reliability of the proposed new design rules are assessed for a wide range of cases. Comparisons are also made against the results obtained through the column fire design rules of the European structural steel fire design standard EN 1993-1-2 [1]. It is demonstrated that the proposed new design rules furnish more accurate, safe-sided and reliable flexural buckling resistance predictions for stainless steel hollow section columns at elevated temperatures relative to the column fire design provisions of EN 1993-1-2 [1].

Local buckling response and design of stainless steel hollow sections at elevated temperatures

AbstractThe structural behaviour and design of stainless steel hollow sections in fire, including stainless steel (i) circular hollow sections (CHS), (ii) elliptical hollow sections (EHS), (iii) square hollow sections (SHS) and (iv) rectangular hollow sections (RHS), are explored in this paper. Shell finite element models of stainless steel CHS, EHS, SHS and RHS able to replicate their structural response at elevated temperatures are created. Numerical parametric studies are performed on cold‐formed and hot‐rolled austenitic, duplex and ferritic stainless steel CHS, EHS, SHS and RHS subjected to (i) pure compression, (ii) pure bending, (iii) combined axial compression and bending and (iv) combined bending and shear in fire, covering different cross‐section slendernesses and elevated temperature levels. Calibrated against the benchmark structural performance data obtained from the numerical parametric studies, the new design proposals for predicting the ultimate resistances of stainless steel CHS, EHS, SHS and RHS in fire put forward in Quan and Kucukler [1,2] are set out. It is demonstrated that relative to the design recommendations provided in the European structural steel fire design standard EN 1993‐1‐2, the proposed cross‐section design methods provide more accurate and safe‐sided ultimate resistance predictions for stainless steel CHS, EHS, SHS and RHS at elevated temperatures.

Resistance and behaviour of carbon steel and stainless steel columns with axial and rotational end‐restraints in fire

AbstractDue to the presence of neighbouring structural elements, carbon steel and stainless steel columns are typically axially and/or rotationally restrained at their ends in fire. Despite the significant influence of the axial and rotational end‐restraints on the behaviour, the fire design rules for steel columns provided in the European structural steel fire design standard EN 1993‐1‐2 were originally developed considering the structural response of axially unrestrained steel columns with pin‐ended support conditions. Thus, EN 1993‐1‐2 may provide inaccurate ultimate resistance predictions for axially and/or rotationally restrained steel and stainless steel columns in fire. Considering this, this paper investigates the behaviour and design of carbon steel and stainless steel columns with axial and/or rotational end‐restraints at elevated temperatures. A number of carbon steel and stainless steel I‐section columns are considered, taking into account different axial and rotational end‐restraint stiffnesses, room temperature loading ratios, member and cross‐section slendernesses. A new design method for the fire design of axially and/or rotationally restrained steel and stainless steel columns is put forward. The accuracy of the proposed new design method is extensively verified against the results from nonlinear shell finite element modelling.

Numerical Analysis of Flexural Performances of Composite Steel-Timber Beams under Fire Conditions

Recently, a novel type of composite structure, composite steel-timber (CST) structure, has attracted much attention by combining steel and timber in an effective way to form composite structural components, which unitises the advantages of high strength and excellent ductility of steel and decent sustainability and fire resistance of timber. However, the existing research is lacking, especially in structural fire design and analysis. In this study, based on the sequentially coupled method, the commercial finite element software ABAQUS was used to numerically simulate the dynamic performances in the temperature field and the flexural behaviours in the displacement field for a typical CST beam with a steel element embedded within the Glulam and connected by adhesives and bolts under standard fire for two hours. In the numerical simulations, the temperature distributions within the CST beam were explored, and the flexural performances of the beam in the displacement field were examined. Through the comparative analysis, the temperature distributions in the embedded steel beam and the surrounding Glulam beam under one-hour standard fire verified the advantages of this type of CST beam in structural fire design. Specifically, under a 2-hour standard fire, the surrounding Glulam could still protect the embedded steel beam from sustaining too high temperatures, so as to retain most of its material properties and help maintain the bearing capacity of the whole structure and improve the refractory limit. Parametric studies on the fire resistance of the CST beam were also conducted by adjusting the bolt spacing and the protection thickness of the Glulam. The obtained results indicated that reducing the bolt spacing and the thickness of the Glulam protection layer would have an adverse effect on the temperature distributions in the embedded steel element to a large extent, and would eventually lead to its rapid heating and strength loss and the final failure of the whole CST structure.

Thermal and Structural Response of Beam-end Shear Connections During a Large Compartment Fire Experiment

The role of steel connections is essential in structural fire design and analysis for steel-framed composite structures. The current structural design provisions provide strength reduction factors of load-carrying members and their end-connection elements at elevated temperatures, based on small-scale experiments under uniform heating conditions. The realistic thermal and structural evolution in member connections, especially as part of full-scale floor assemblies exposed to a large compartment fire, has not been well characterized. A large compartment fire experiment was recently conducted on a 9.1 m by 6.1 m composite floor assembly as part of a two-story steel-framed building. The test assembly had a total of ten shear-tab (fin-plate) connections subjected to combined fire and mechanical loading. This paper presents the measured thermal response of these connections to fire and comparison with the corresponding Eurocode 3 predictions with two methods (1) incorporating the beam bottom flange temperature at midspan and (2) the section factor method. The results show that the Eurocode 3 methods conservatively predict the maximum temperature during heating and the cooling rate but overestimate the high-temperature strength of connections when estimated using the section factor method, showing that the Eurocode 3 simplified approaches are not meant to provide the details of the failure mode for connections. This study suggests that estimating the strength of connections using strength reduction factors may not guarantee a safe structural fire design. In addition, this paper estimated the total axial force (from slab and beam) at the composite connection via using the strain gauge measurements close to the column bases which were not exposed to fire. It suggests realistic axial load and rotational demand on the shear connection due to restraints to thermal elongation or contraction of supported members should be considered in future design guidance as should designing and detailing the connections for ductility to withstand the inelastic deformation demands during the heating and the cooling phases.

Simulation and cross-section resistance of stainless steel SHS and RHS at elevated temperatures

This paper investigates the structural response and design of stainless steel square and rectangular hollow sections (SHS and RHS) at elevated temperatures. Finite element models able to replicate the behaviour of stainless steel SHS and RHS members at elevated temperatures are developed and verified against the results from physical experiments, which are then used to perform extensive numerical parametric studies to generate a broad range of benchmark structural performance data on the behaviour of stainless steel SHS and RHS at elevated temperatures. In total, 13860 cold-formed and hot-rolled austenitic, duplex and ferritic stainless steel SHS and RHS with a wide range of cross-section properties and subjected to various loading conditions at different elevated temperature levels are considered. A cross-section design method for stainless steel SHS and RHS under different loading conditions at elevated temperatures is proposed, considering the recent design recommendations in [1] for the local buckling assessment of stainless steel plates at elevated temperatures which will be included in the upcoming version of the European structural steel fire design standard EN 1993-1-2. Relative to the current local buckling assessment rules of EN 1993-1-2, the higher accuracy, safety and reliability of the new proposals in the estimations of the ultimate cross-section resistances of stainless steel SHS and RHS at elevated temperatures are demonstrated.

Numerical validation of structural fire design for steel framed car park buildings

The installed automatic sprinkler systems in car park buildings can control the fire spread between vehicles, leading to unique localized vehicle fire scenarios. An advanced structural fire design methodology specific to steel-framed car park buildings was proposed by Linus Lim [11] based on the overall performance of structures under localized fire. This paper aims to assess the accuracy and reliability of this design methodology through numerical modelling using ABAQUS. The validated approach of establishing finite element models for steel-framed structures is adopted to simulate an eight-bay car park frame under two different fire scenarios following the design procedure. The overall structural fire performance of simulated structures indicates that even though the structural beams generated plastic deformation in two fire scenarios, none reached failure in the simulation. The comparison of load between simulation and design shows that the structural fire design methodology overestimated the load on the structural components inducing a conservative design result, which can be used for the car park building structural fire design. It should be pointed out that the neighbouring beams at a lower temperature could sustain the additional load from the continuous bending beam right above the fire, resulting in better overall performance of steel beams in localized fire. The assumption of load redistribution between different beams was validated in simulation.

News: Steel Construction 2/2023

AbstractECCS NewsSave the Date: Webinar on ”Fire Design of Steel Structures“Eurosteel 2023 conference – the most recent developments in steel structuresNewSkin Project 1/04/2020 – 31/03/2024In Memoriam