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

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Abstract

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.

Introduction

Fire disasters cause many thousands of deaths and huge economic losses each year. Several steel structure buildings around the world have experienced partial or total collapse triggered by fire. The terrorist attack and catastrophic collapse of the World Trade Centre buildings in 2001 has led to increasing concern about structural robustness worldwide. Prior to this disaster, structural fire engineering has mainly focused on the behaviour of individual members and connections. Very few full-scale fire tests have been performed on steel frames. Among these, the Cardington fire tests play a significant role in today's understandings of actual structural system behaviour in real fire. A number of numerical studies have been conducted based on Cardington fire tests, including the work of Bailey and Moore [1], Elghazouli et al. [2], Sanad et al. [3], Wang [4], O'Connor [5], Lamont et al. [6] and Foster et al. [7]. A key finding of the numerical studies is that two-dimensional frame analysis is only capable of capturing the load transfer mechanism of skeletal frames under fire. Three-dimensional models are needed in order to establish the actual behaviour of composite frames, in which three-dimensional flexural bridging, catenary action in slabs and beams, and tensile membrane action in slabs play a crucial role in providing enhanced fire resistance.

Although no collapse occurred during the Cardington fire tests, in principle the designer is concerned with preventing fire-induced disproportionate collapse of buildings. As a result, there is a growing body of literature that recognizes the importance of collapse assessment of steel frames under fire and a comprehensive review can be found in Porcari et al. [8]. Takagi [9] identified the governing factors in fire-induced structural system collapse using structural sub-assembly models and carried out a probabilistic assessment to determine the most significant factor. Sun [10] investigated the progressive collapse mechanisms of a two-dimensional moment resisting frame with different bracing systems under edge bay fire scenario and central bay fire scenario. Lange et al. [11] examined two possible failure mechanisms for tall buildings subjected to multiple floor fires. Agarwal and Varma [12] assessed structural robustness of two types of ten-storey steel buildings under fire, one with a gravity frame and rigid core in the form of a concrete shear wall, and one with gravity frame and perimeter moment resisting frame. Results indicated that gravity columns were most likely to reach critical temperatures first, due to the highest utilization ratios. Jiang and Li [13] performed progressive collapse analysis of eight-storey moment resisting steel-framed buildings. The severity of various fire locations is investigated and the influences of different fire protection levels are examined.

The research to date shows that important progress has been made in understanding the collapse mechanisms of steel structures under fire. It is important to note that the reliability of modelling attempts heavily depends on the accurate representation of the material behaviour. The behaviour of structural steels under high temperatures has been studied in extensive research work, and a comprehensive review of the high-temperature test data and constitutive models available can be found in Kodur et al. [14], Luecke et al. [15] and Kodur and Harmathy [16]. The severe deteriorating effects of high temperatures have also been well recognized by design codes. It is common practice for researchers to base their numerical work on simplified experimental approximate curves of stress-strain relationships given in ASCE [17] and EN 1993-1-2 [18]. However, despite this progress, far too little attention has been paid to damage and fracture induced by large deformations in steel, and the design codes might turn out to be non-conservative in severe fire.

While continuum damage mechanics has been extensively used in describing the damage mechanisms at ambient temperatures [[19], [20], [21], [22], [23], [24]], the development of damage models for steel has not quite been extended to elevated temperatures. Existing work that has dealt with thermo-mechanical damage coupling focuses on metalworking [25,26] and thermo-mechanical fatigue [[27], [28], [29]]. In structural fire engineering, there still remains a lack of research in careful modelling of steel property deterioration as the structures experience severe fire and undergo large structural deformation in accidental events. To fill this gap, a coupled thermal-mechanical damage model for predicting steel deterioration behaviour under simultaneous mechanical loads and elevated temperatures has been developed and validated in Lu et al. [30]. The present paper will continue to verify the applicability and effectiveness of the coupled damage model for use in structural fire engineering, and employ the coupled damage model in assessing the susceptibility of steel-framed office buildings to progressive collapse.

Section snippets

Presentation of the coupled thermo-mechanical damage model

This section briefly summarizes the coupled thermo-mechanical damage model proposed in Lu et al. [30]. The coupled thermal-mechanical damage model has been developed based on an enhanced Lemaitre damage model proposed by Bouchard et al. [31] and extended to take into account the high-temperature effects. Two damage components, associated respectively with mechanical damage process (Eq. (1)) and thermal damage process (Eq. (2)), were introduced first (D represents the non-decreasing scalar

Applications of the coupled damage model in fire analyses of steel frames

This section starts with the application of the coupled thermo-mechanical damage model to fire analysis of two-dimensional simple frame systems. Following this, the applicability of the proposed damage model is further verified by simulating the complex three-dimensional structural system tested in the Cardington fire test 7.

Modelling of steel office buildings subjected to compartment fire

The previous section has validated the accurate and conservative predictive capabilities of the coupled damage modelling approach at the structural level. Therefore, the proposed model can be used with confidence to predict the robustness of steel-framed structures under fire. This section introduces the structural design and numerical modelling aspects of typical steel-framed office buildings subjected to a corner compartment fire.

Coupled damage progressive collapse analyses of office buildings

The primary objective of this section is to provide a check of the steel office buildings for satisfying robustness requirements under fire, with the structural fire resistance quantified in the time domain. Coupled-damage numerical simulations are conducted to evaluate the vulnerability of steel-framed office buildings against fire-induced progressive collapse and to assess the role of damage accumulation in eventual structural collapse. Fire occurs in the corner compartment on the ground

Summary

In this paper, the newly developed thermo-mechanical damage coupling model [30] has been used in numerical simulations performed with the software ABAQUS dedicated to progressive collapse analyses of steel-framed office buildings under localised fire. The single compartment fire scenario represents one of the most typical fire events that might occur during the service life of office buildings. The case studies presented show that the damage model approach produces more conservative results,

Author statement

Weimiao Lu: Conceptualization, Methodology, Software, Validation, Writing – original draft preparation, Ashraf Ayoub;: Conceptualization, Methodology, Supervision, Writing- Reviewing and Editing, Project administration and Cedric D'Mello: Conceptualization, Methodology, Supervision, Writing- Reviewing and Editing, Project administration.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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