iTFM: improved travelling fires methodology for structural design and the effects on steel framed buildings

Abstract

Accidental fire can be disastrous, especially in buildings. Most fire deaths occur due to the toxic effects of smoke before any structural collapse. However, the effect of fire on structural stability is critical in regard to safe evacuation and safe access for fire-fighters, financial losses, and lost business. This is particularly the case in tall buildings where extended evacuation times are required due to phased evacuation practises.

The understanding of fundamental mechanisms of whole building behaviour in fire has significantly increased over the last decades, in particular after the full-scale tests of various multi-storey buildings carried out in Cardington between 1994 and 1999. However, most of the current understanding and consequently the design codes are based on the assumption of uniform fire conditions in a compartment. While this assumption may be suitable for small enclosures, fires in large open-plan compartments have been observed to travel. Examples of such fires include the World Trade Centre Towers 1, 2 & 7 (2001), Windsor Tower fire in Madrid (2006) and the recent fire at the Plasco building in Tehran (Jan 2017). All of these buildings ultimately either partly of fully collapsed.

Current design standards do not account for travelling fires. The standard and parametric time-temperature curves are based on small scale tests, and assume uniform burning of fire and homogeneous temperature distributions in a compartment. In the recent years a new design concept of the Travelling Fires Methodology (TFM) has been developed by G. Rein to account for the travelling nature of fires in large compartments. This design methodology considers non-uniform temperature distribution in the compartment and a wide range of burning floor areas. In this thesis the Travelling Fires Methodology is improved to account for more realistic fire dynamics and then applied to investigate the structural response of a multi-storey steel frame using finite element software LS-DYNA. This thesis is presented in a manuscript style: each chapter takes the form of an independent paper, which has been published or submitted to a journal for publication. A final chapter summarizes the conclusions, and suggests potential areas of future research.

Firstly, an improved Travelling Fires Methodology (iTFM) that accounts for better fire dynamics is presented in Chapter 2. Equations are introduced to reduce the range of possible fire sizes taking into account fire spread rates from real fires. The analytical equations used to represent the far-field temperatures are presented in continuous form. The concept of flame flapping is introduced to account for variation of temperatures in the near-field region due to natural fire oscillations. iTFM is then used to analyse the effect of non-uniform heating associated with travelling fires on the thermal response of structural members and identification of the location of peak temperature along the fire path. It is found to be mainly dependent on the fire spread rate and the heat release rate. Location of the peak temperature in the compartment is found to mostly occur towards the end of the fire path.

Full-scale testing of real structures is complex, expensive and time consuming. This is especially the case for structures with large compartments. There has only been a limited number of full-scale tests on real buildings carried out worldwide (e.g. Cardington tests). As a result, computational tools are commonly used to assess the structural response of complex buildings under fire conditions. However, they have to be validated first. Therefore, in Chapter 3, prior to the study of the effects of iTFM on the structural response, explicit solver of finite element software LS-DYNA used for the analyses in Chapters 4-7 is benchmarked for structural fire analyses against other static numerical codes and experiments. Four canonical problems that encompass a range of thermal and mechanical behaviours in fire are simulated. The parameter sensitivity study is carried out to study the effects of various numerical parameters on the convergence to quasi-static solutions. The results confirm that when numerical parameters are carefully considered not to induce excessive inertia forces in the system, explicit dynamic analysis using LS-DYNA provide good predictions of the key variables of structural response during fire.

Finally, the structural response of a two-dimensional multi-storey steel frame subjected to uniform design fires and iTFM (presented in Chapter 2) occurring on a single floor and multiple floors is investigated in Chapters 4, 5, & 7, and Chapters 6 & 7, respectively. Fire type and the location of the fire floor in the frame are varied. The analyses and comparison of structural response mechanisms is presented in Chapter 4. Uniform fires are found to result in higher compressive axial forces in beams compared to small travelling fires. However, results show irregular oscillations in member utilization levels in the range of 2 - 38% for the smallest travelling fire sizes, which are not observed for the uniform fires. Beam mid-span deflections are similar for both travelling fires and uniform fires and depend mainly on the fire duration, but the locations in the frame where these displacements occur are found to be different. Chapter 5 extends the study presented in Chapter 4 and compares the results in the terms of the limiting temperature criteria and various structural limit states. Critical fire scenarios are found to occur on the upper floors of the frame where column sections reduce in size. Also, results show that depending on the fire scenario higher level of fire protection for different members within the frame may lead to either enhanced or worse structural response and/or resistance.

During previous fire events, e.g. the World Trade Centre Towers (WTC) 1, 2 & 7 in New York (2001), flames were observed to not only travel horizontally across the floor plate but also vertically to different floors. In this thesis, the effect of vertically travelling and multiple floor fires on the structural response of a two-dimensional multi-storey steel frame is investigated in Chapter 6. The number of fire floors, and horizontal and vertical fire spread are varied. Results show that the largest stresses develop in the fire floors adjacent to cool floors, and their behaviour is independent of the number of fire floors. All, the fire type, the number of fire floors, and the location of the fire floor, are found to have a significant effect on the failure time (i.e. exceeded element load carrying capacity) and the type of collapse mechanism (Chapter 7). In the cases with a low number of fire floors (1 to 3) failure is dominated by the loss of material strength, while in the cases with larger number of fire floors (5 to 10) failure is dominated by thermal expansion. Collapse is observed to be mainly initiated by the pull-in of external columns or swaying of the frame to the side of fire origin.

Analyses presented in Chapters 4 to 7 highlight that in the structural design for fires it is important to consider more realistic fire scenarios associated with travelling fires as they might trigger previously unnoticed structural mechanisms. Results on the multi-storey steel frame indicate that, depending on the structural metric examined, both travelling fires and uniform fires can be more severe than the other. A single worst case fire scenario under which a structure could be designed and deemed to be safe cannot be established. For different fire exposures failure is found to occur on different range of floors subjected to fire. Therefore, in order to ensure a safe fire resistance design of buildings with large enclosures, a range of different fires including both travelling fires and uniform fires need to be considered.