Structural Fire Engineering Research Articles

Fire Testing of Grade 304 Stainless Steel Plate Material Under Transient‐state Conditions

AbstractOwing to its superior performance at high temperatures, stainless steel is usually considered as a potential replacement for mild steel in a wide range of structural fire engineering applications. When it comes to the fire design of steel structures, time‐dependent deformations occurring in the adequately‐heated steel material, called “thermal creep” need to be somehow considered in the design calculations. In EUROCODE 3‐1‐2, for example, the thermal creep strains are implicitly included in the transiently‐determined stress‐strain curves for steel at elevated temperatures. Using the same approach, this paper investigates the fire‐temperature mechanical properties of Grade 304 stainless steel by conducting transient tensile tests at temperatures of up to 900°C on standard coupons taken from a 3.0 mm thick plates. To simulate structural fire conditions in the transient tests, the temperature of the sample is steadily increased when a constant tensile load is applied. From these tests, strain‐temperature curves are obtained, wherein the strain contains both mechanical and thermal components. The strain‐temperature curves with the same heating rate of 10°C/min are then converted to elevated‐temperature stress‐strain curves suitable for fire design applications.

A numerical study on the structural performance of a ductile connection under fire conditions

AbstractThe component‐based model of a novel ductile connection has been incorporated into the software Vulcan in order to facilitate global frame analysis within a performance‐based structural fire engineering design process. This paper reports on the validation and verification of the model, as well as the applications of the model in order to investigate the effects of the ductile connections on the structural responses of long‐span frames at high temperature. Firstly, three single‐beam models with the novel connections at both ends, connected to rigid supports, are used to verify that the component‐based connection model has been correctly incorporated into Vulcan, via comparisons against detailed finite element modelling with Abaqus. The structural performance in fire of long‐span frames with the novel ductile connections has been compared with the performance of the same frames with idealized rigid, idealized pinned and conventional end‐plate connections, initially using a limited sub‐frame model. Results show that, compared with the above mentioned three connection types, the ductile connection provides much higher axial and rotational ductilities to accommodate the deformations generated by the connected beams as their temperatures rise. As part of this process, these connections are instrumental in greatly reducing the axial forces to which the surrounding structure is subjected. Finally, parametric studies varying several key parameters have been carried out, in order to optimize the design of the ductile connection to enhance its performance subject to catenary action at very high temperature to prevent potential connection fracture and progressive collapse.

Rheological modelling of high‐temperature stationary creep tests of Grade S275JR steel

AbstractThe paper presents a new validation study of a unified rheological model, in reproducing creep strains from a recent experimental study performed on grade S275JR coupons. The objective of this research is to further test the modelling capabilities of the proposed rheological model and its validity for application in general structural fire engineering analysis. The rheological model is composed of two serially connected Kelvin‐Voight elements, used to represent the material's sensitivity to strain‐rate‐governed changes of yield strength, temperature and heating rate. The paper presents the performance of the model in stationary‐creep conditions.

Potential of Surrogate Modelling for Probabilistic Fire Analysis of Structures

The interest in probabilistic methodologies to demonstrate structural fire safety has increased significantly in recent times. However, the evaluation of the structural behavior under fire loading is computationally expensive even for simple structural models. In this regard, machine learning-based surrogate modeling provides an appealing way forward. Surrogate models trained to simulate the behavior of structural fire engineering (SFE) models predict the response at negligible computational expense, thereby allowing for rapid probabilistic analyses and design iterations. Herein, a framework is proposed for the probabilistic analysis of fire exposed structures leveraging surrogate modeling. As a proof-of-concept a simple (analytical) non-linear model for the capacity of a concrete slab and an advanced (numerical) model for the capacity of a concrete column are considered. First, the procedure for training surrogate models is elaborated. Subsequently, the surrogate models are developed, followed by a probabilistic analysis to evaluate the probability density functions for the capacity. The results show that fragility curves developed based on the surrogate model agree with those obtained through direct sampling of the computationally expensive model, with the 10–2 capacity quantile predicted with an error of less than 5%. Moreover, the computational cost for the probabilistic studies is significantly reduced by the adoption of surrogate models.

A new methodology using beam elements for the analysis of steel frames subjected to non-uniform temperatures due to fires

Non-uniform heating in structures under fire involves the appearance of 3D-phenomena and typically requires the use of complex models built with finite elements shell or solid. Although different procedures have been developed to model the complex thermo-mechanical phenomenon, there is no simple, accurate, and low-cost computational methodology involving the space–time variation of the temperature and displacement fields that opens the path advancing more easily towards modeling more complex structural problems in a fire situation. To overcome this knowledge-gap, this paper presents a new methodology that fulfills those conditions, making it possible to carry out more complex analyses that require many simulations in a short time and at low computational costs. The new methodology to obtain the thermo-mechanical response to non-uniform heating and mechanical loads is general, simple, accurate, and avoids using complex and high-cost finite elements, simplifying the structural modeling, and reducing the computational analysis cost. As a result, complex structural fire engineering problems such as probabilistic and optimization analysis can be handled much more easily, representing a significant step toward the generalized application of performance-based approaches to deal with fire effects on structures. The procedure uses simple but advanced Timoshenko’s beam-type finite elements and represents the non-uniform temperature space–time field through a mean value of the temperature and the two mean values of the section thermal gradients which are variable in time during the fire. The methodology is satisfactorily validated with results (experimental and numerical) of the Cardington frame test and captures 3D-phenomena such as buckling, flexural–torsional buckling, and warping.

Effectiveness of coupled thermo-mechanical damage modelling in steel structural fire engineering

This paper explores the effectiveness and applicability of a newly developed thermo-mechanical coupled damage model for use in structural fire engineering. The damage-affected structural response of frame systems is accurately reproduced in this paper, which demonstrates the capability of the coupled damage model in giving a realistic representation of steel deterioration behaviour under combined actions of fire and mechanical loads. The coupled damage model is then used to assess the performance of a three-dimensional low-rise five-storey building and mid-rise ten-storey building with office occupancy under fire. Results provide a check of the office buildings for satisfying robustness requirements and give insight into the reasons that cause structural collapse. Compared to the conventional numerical approach, the consideration of coupled thermo-mechanical damage accumulation results in a substantial decrease in the time to collapse. A key finding from this study is that estimation of ultimate failure time by incorporating the coupled damage model with the recommended damage parameter sets has the potential to be utilized as a useful tool in helping designers to determine how much time is realistically available for evacuation before progressive collapse occurs in these types of buildings.

Fundamental Behavior of Timber Concrete-Composite Floors in Fire

Timber-concrete composite (TCC) floors are becoming more commonly used in mass timber construction due to their increased stiffness and strength relative to nonconcrete composite timber floors. Current building codes do not recognize the contribution of the composite action of TCC floors to their combined strength and fire performance. Extensive structural testing is required for structural engineers to design fire protection in timber buildings that use TCC floors using either alternate means and methods or structural fire engineering. This paper presents two experimental studies conducted to examine (1) properties of shear connection systems used in TCC floors through ambient temperature direct shear tests, and (2) the behavior of TCC floors through large-scale fire tests performed at the National Research Council (NRC) in Canada. The main objective of this research is to collect data on the fundamental behavior of TCC floors in a fire. The results indicate that TCC floors have improved fire performance when compared with mass timber floors. In addition, the research quantifies the force-slip behavior of the shear connectors used and benchmarks the flexural behavior of TCC floors at ambient and elevated temperatures to existing analytical models.

Tisova Fire Test – Fire behaviours and lessons learnt

One of the largest unknowns in structural fire engineering design is the fire itself, particularly in large internal environments, where real fires have been observed to travel around the floor plate. Several methodologies have been proposed to characterise these fires, most of which rely on the separation of the fire exposure into two so-called fields - near and far – allowing simplifying assumptions to be made. These models are now being used in the structural fire design. However, there is a paucity of validation data for these models with only a few tests reported in the literature. The Tisova Fire Test was conducted on the ground floor of a 4-storey concrete frame building, with concrete and composite deck floors. The fire compartment had a total area of ca. 230 m2, a height of 4.21–4.28 m, and a uniform fuel bed across the floor at 40 kg/m2. This paper presents temperatures, spread rate and the observed behaviour of the travelling fire, comparing the current design guidance with the low temperatures observed in the experiment. The paper also examines the experimental issues and the planning of these experiments with a view to helping others plan for and execute large scale fire tests in the future.

Experimental and numerical investigation of the ductile fracture of structural steel at elevated temperatures

Fracture of steel at ambient temperature has been extensively studied. Such studies developed efficient micromechanical fracture models, which can be used to predict the behaviour of steel structures and components up to failure under extreme monotonic or cyclic loading, e.g. extreme deformations due to loss of column scenarios or large earthquakes. Considerable research has been also carried out on the high-temperature fracture of steel under creep conditions but its findings have not yet been applied to structural fire engineering practice, though ductile fracture has been reported as a common mode of failure for steel structural elements under fire conditions. This paper reports results of an experimental program carried out to investigate fracture of the S275 structural steel grade at temperatures ranging from 20 °C to 450 °C. A series of tests, including tensile and shear tests, are executed to extract information on fracture of structural steel for a wide range of stress triaxiality values. Moreover, numerical simulations of the tests are carried out using MSC Marc to get further insight for the role of stress triaxiality, Lode angle parameter, and temperature. Based on both the experimental and numerical results, a fracture model proposed in the literature is calibrated to predict fracture of structural steel for the temperature range 20 °C - 450 °C.

Observational Analysis of Fire-Induced Spalling of Concrete through Ensemble Machine Learning and Surrogate Modeling

Despite ongoing research efforts, we continue to fall short of arriving at a consistent representation of fire-induced spalling of concrete. This is often attributed to the complexity and randomness of spalling as well as our persistence in favoring traditional approaches as a sole mean to examine this phenomenon. With the hope of bridging this knowledge gap, this paper demonstrates how utilizing surrogate modeling via data science and machine learning algorithms can provide us with valuable insights into fire-induced spalling. In this study, nine algorithms, namely naive Bayes, generalized linear model, logistic regression, fast large margin, deep learning, decision tree, random forest, gradient boosted trees, and support vector machine, are applied to analyze observations obtained from 185 fire tests (collected over the last 65 years). The same algorithms were also applied to identify key features that govern the tendency of fire-induced spalling in reinforced concrete columns and to develop tools for instantaneous prediction of spalling. The results of this comprehensive analysis highlight the merit in utilizing modern computing techniques in structural fire engineering applications given their extraordinary ability to comprehend multidimensional phenomena with ease, high predictivity, and potential for continuous improvement.

Probabilistic fire demand model for steel pipe-racks exposed to localised fires

This paper proposes a new method to build a probabilistic fire demand model (PFDM) to investigate the structural behaviour of a steel pipe-rack located within an industrial installation and exposed to a localised fire. The PFDM will serve to develop fire fragility functions to be used either in a fire risk assessment or in a fully probabilistic structural fire engineering (PSFE) framework. The cloud analysis (CA) was exploited to build a PFDM based on different engineering demand parameters (EDP) – intensity measures (IM) pairs. In particular, the analysis was applied to a prototype steel pipe-rack integrating an industrial plant in Italy. In order to cover a wide range of plausible fire scenarios and to introduce uncertainties in the fire model, 539 fire scenarios were examined by varying the fire diameter, the fire-structure distance and the fuel. The selection of the fire diameters was based on parametric analyses quantifying liquid flow through orifices and pipes. The thermal impact of the pool fires on the structure was analysed using the LOCAFI localised fire model and the thermo-mechanical response of the pipe rack was evaluated by means of finite element analysis. Based on the structural analysis outcomes, it was found that the interstorey drift ratio (ISDR) – maximum average heat flux impinging the structure (HFavg) EDP-IM pair was the most efficient and was also characterised by the highest relative sufficiency among the other pairs. Moreover, it has to be noted that for this type of case study, the CA revealed to be a viable option to build a PFDM.

A Study on Development of the Guideline (Draft) for Performance-based Structural Fire Engineering Design in Building Structures

A building fire can cause severe loss of life and damage to the property and is regarded as a social disaster. Therefore, employing the fire resistant structures appears to be a potential solution of the stated problem. In this study, the guideline (draft) of the structural fire engineering design, using scientific and engineering methods for evaluation of the structural stabilities in a fire, was developed. Further, by application of the guideline when the desired building is under construction, the rational solution for the improved fire safety and cost competitiveness is expected.

A computationally rigorous approach to hybrid fire testing

Real fire incidents have shown that the structural systems of high-rise buildings perform much better during a fire attack than what classical structural fire analysis predicts. The reason behind this phenomenon is the empirically established fact that beneficial interaction mechanisms evolve between the fire-exposed and the fire-unexposed parts of structural systems. However, there is no computational method available to reliably quantify such interaction mechanisms for design purposes. Pure numerical methods are too uncertain, because they cannot be validated, and isolated structural member testing does not allow considering the interaction. Hybrid fire testing, however, can precisely fill this fundamental methodological gap when set up as an extended finite-element analysis technique. Here, we provide the first computationally rigorous method for hybrid fire testing. We have validated the novel method with several proof-of-concept tests covering the entire temperature-range relevant for structural fire engineering. In contrast to other approaches, only our method can deal, so far, at the same time suitably, accurately and robustly with the experimental and computational challenges of temperature-dependent material behavior. We therefore place hybrid fire testing on a sound scientific basis enabling existing fire test facilities to do more realistic (hybrid) assessments of the fire performance of structural systems.

Review of Current Practice in Probabilistic Structural Fire Engineering: Permanent and Live Load Modelling

Probabilistic analysis is receiving increased attention from fire engineers, assessment bodies and researchers. It is however often unclear which probabilistic models are appropriate for the analysis. For example, in probabilistic structural fire engineering, the models used to describe the permanent and live loads differ widely between studies. Through a literature review, it is observed that these diverging load models are based on surveys conducted between 1893 and 1976 and that widely adopted assumptions, such as the rule for combining permanent and live loads into the total load effect, are commonly adopted based on precedent. The diverging current models however relate to mostly the same underlying datasets and basic methodologies. Differences can be attributed (largely) to specific assumptions in different background papers, which have become consolidated through repeated use in research papers and adoption in background documents to codes. By reviewing the studies underlying currently applied probabilistic load models in structural fire engineering, a consolidated probabilistic load model is proposed in this paper. It is concluded that the total load effect is ideally described by KE·(G + Q), with KE the model uncertainty for the load effect, G the permanent load, and Q the imposed load. The model uncertainty KE can be described by a lognormal distribution with mean equal to unity and coefficient of variation (COV) of 0.10. The permanent load is preferably modelled by a normal distribution with mean equal to the nominal permanent load, and a COV which can either be assessed on a project specific basis, or can be set to 0.10 for a first assessment. For common occupancies (office, residential), the live load is preferably modelled by a Gamma distribution. The mean live load can be taken as 0.2 times the nominal, and the live load COV can be taken as 0.60 for large load areas (> 200 m2) and 0.95 for smaller load areas (< 100 m2). Comparison between the failure probabilities of steel and concrete columns subject to fire, considering the proposed consolidated model and two currently commonly used models, indicates that relative differences of the probability of failure can be in the order of 10%. Live load models for evacuation routes and warehouses require specific study and are outside the scope of the review.

Probabilistic structural fire engineering using incremental fire analysis and cloud analysis

Probabilistic structural fire engineering is an approach to designing structures for fire conditions, which allows the estimation of an annual rate of exceeding various levels of building response. This is done by combining the annual rate of exceeding various fire exposure conditions with probabilistic relationships between structural response and fire severity. One available method that has been used to quantify this relationship is incremental fire analysis (IFA). Another approach which is well-established in earthquake engineering but has so far not been explored in fire engineering is cloud analysis (CA). This paper compares how well both methods estimate structural response under fire conditions and their associated probabilities of exceedance of a specified level of structural response. The paper demonstrates the inner workings of the two methods and their application and also proposes recommendations for their application to structural fire engineering. It is demonstrated with the help of a case study of a composite steel beam exposed to a suite of fires, generated by considering fuel load and ventilation as random variables. CA is found to be computationally less demanding than IFA, yet both approaches produce similar probability of exceedance of various levels of structural response.

Autonomous Fire Resistance Evaluation

AbstractThe structural fire engineering community has been slowly evolving over the past few decades. While we continue to favor a classical stand toward evaluating fire resistance of structures th...

Structural fire resistance of partially restrained, partially composite floor beams, II: Modeling

This paper is Part 2 of a two-part investigation of the structural fire resistance of a partially composite, one-way spanning steel floor beam assembly with partial translational and rotational end restraint provided by shear tab connections to a supporting frame. A preceding paper (Part 1) described a pair of experimental tests on structurally identical composite floor specimens (one protected and the other unprotected) that are subjected to fire via a large modular furnace. In this paper, those experimental results are used to validate numerical models that conservatively capture the thermo-mechanical response of the specimen assemblies to fire. Models are developed to maximize simplicity toward implementing performance-based design of these assemblies in practice. Thermal analysis of the steel is performed using a multiple lumped mass approach, which can be implemented via spreadsheet or a simple, non-iterative programmed solution. Thermal analysis of the slab via a simple one-dimensional heat flow model is compared against a more detailed computational solution. Two types of structural finite element analyses were performed: one composed of shell elements (more complex) and another composed of fiber-beam elements (simpler). The results of all models show conservative agreement with the experimental data. These models are used to analyze the tested assemblies for variations in applied load and fire protection thickness to demonstrate the utility of the ASTM E119 standard fire test as a validation baseline for performance-based structural fire engineering evaluation.

Rectification of “restrained vs unrestrained”

SummaryFor furnace testing of fire‐resistant floor and roof assemblies in the United States, the ASTM E 119 standard (and similarly the UL 263 standard) permits two classifications for boundary conditions: “restrained” and “unrestrained.” When incorporating tested assemblies into an actual structural system, the designer, oftentimes a fire protection or structural engineer, must judge whether a “restrained” or “unrestrained” classification is appropriate for the application. It is critical that this assumption be carefully considered and understood, as many qualified listings permit a lesser thickness of applied fire protection for steel structures (or less concrete cover for concrete structures) to achieve a certain fire resistance rating if a “restrained” classification is confirmed, as compared with an “unrestrained” classification. The emerging standardization of structural fire engineering practice in the United States will disrupt century‐long norms in the manner to which structural behavior in fire is addressed. For instance, the current edition of the ASCE/SEI 7 standard will greatly impact how designers consider restraint. Accordingly, this paper serves as an exposé of the “restrained vs unrestrained” paradigm in terms of its paradoxical nature and its controversial impact on the industry. More importantly, potential solutions toward industry rectification are provided for the first time in a contemporary study of this paradigm.

Recommendations for performance-based fire design of composite steel buildings using computational analysis

Adoption of performance-based approaches in structural fire engineering is now largely recognized as having the potential to deliver benefits in terms of safety, resilience, sustainability and cost for the built environment. However, there is no systematic design guidelines for defining performance objectives, design fire scenarios, and methods of analysis. This paper focuses on steel framed buildings with composite floor slabs and provides an in-depth analysis of the structure under fire using performance-based fire engineering supported by computational modeling. First, a set of performance objectives and associated fire scenarios are defined. The scenarios include single- and multi-compartment fires as well as fire as a secondary event following a column loss. Then, response of the structure and effects of design changes are studied using different modeling approaches. Single slab, single slab with restraint, and full building models are investigated to understand the influence of continuity in the boundary conditions notably on the development of tensile membrane action. The full building model exhibited the most favorable behavior, while the single panel model without horizontal restraint was the most conservative. Best practices for computational modeling of a steel framed building under fire are discussed and recommendations are provided. The paper demonstrates an iterative design procedure, where systematic design changes are introduced to address the observed failure modes. It is shown that performance-based fire design can harness advanced computational models to examine the fire-structure behavior for multiple design alternatives and hazard scenarios.

Flame extension and the near field under the ceiling for travelling fires inside large compartments

SummaryStructures need to be designed to maintain their stability in the event of a fire. The travelling fire methodology (TFM) defines the thermal boundary condition for structural design of large compartments of fires that do not flashover, considering near field and far field regions. TFM assumes a near field temperature of 1200°C, where the flame is impinging on the ceiling without any extension and gives the temperature of the hot gases in the far field from Alpert correlations. This paper revisits the near field assumptions of the TFM and, for the first time, includes horizontal flame extension under the ceiling, which affects the heating exposure of the structural members thus their load‐bearing capacity. It also formulates the thermal boundary condition in terms of heat flux rather than in terms of temperature as it is used in TFM, which allows for a more formal treatment of heat transfer. The Hasemi, Wakamatsu, and Lattimer models of heat flux from flame are investigated for the near field. The methodology is applied to an open‐plan generic office compartment with a floor area of 960 m2 and 3.60 m high with concrete and with protected and unprotected steel structural members. The near field length with flame extension (fTFM) is found to be between 1.5 and 6.5 times longer than without flame extension. The duration of the exposure to peak heat flux depends on the flame length, which is 53 min for fTFM compared with 17 min for TFM, in the case of a slow 5% floor area fire. The peak heat flux is from 112 to 236 kW/m2 for the majority of fire sizes using the Wakamatsu model and from 80 to 120 kW/m2 for the Hasemi and Lattimer models, compared with 215 to 228 kW/m2 for TFM. The results show that for all cases, TFM results in higher structural temperatures compared with different fTFM models (600°C for concrete rebar and 800°C for protected steel beam), except for the Wakamatsu model that for small fires, leads to approximately 20% higher temperatures than TFM. These findings mitigate the uncertainty around the TFM near field model and confirm that it is conservative for calculation of the thermal load on structures. This study contributes to the creation of design tools for better structural fire engineering.