Collapse of tall buildings in multi-storey fires

Engineering Structures, 2012

Fire Safety Journal

During previous fire events such as the World Trade Centre Towers (WTC) 1, 2 & 7 in New York (2001), the Windsor Tower in Madrid (2005), and the Plasco building in Iran (2017), flames were observed to travel horizontally across the floor plate and vertically to different floors. Such fires are not considered as part of the traditional prescriptive structural design for fire. Recently, the Travelling Fires Methodology (TFM) has been developed to account for such horizontally travelling nature of fires. A dozen of studies have investigated the structural response of steel, concrete, and composite structures to a single-floor travelling fire. 5 out of 6 of the vertically travelling fire studies have been limited to the structures with a long span composite truss system as in the WTC Towers. The aim of this work is to investigate the response of a substantially different structural system, i.e. a generic multi-storey steel frame, subjected to travelling fires in multiple floors, and varying the number of fire floors, including horizontal and vertical fire spread. A two-dimensional 10-storey 5-bay steel frame is modelled in the finite element software LS-DYNA. The number of multiple fire floors is varied between 1 and 10, and for each of these scenarios, 5 different fire types are investigated. They include four travelling fire scenarios and the standard fire. In total, 51 fire simulations are considered. The development of deflections, axial forces, bending moments and frame utilization are analysed. 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. Results indicate that both the fire type and the number of fire floors have a significant effect on the failure time (i.e. exceeded element load carrying capacity) and the type of collapse mechanism. In the cases with a low number of fire floors (1-3) failure is dominated by the loss of material strength, while in the cases with larger number of fire floors (5-10) failure is dominated by thermal expansion. Collapse is mainly initiated by the pull-in of external columns (1-3-floor fires; 1-9-floor fires for 2.5% TFM) or swaying of the frame to the side of fire origin (5-10-floor fires). This study has assessed a different structural form compared to previous literature under an extensive range of multiple floor travelling fire scenarios. We find that although vertically travelling fires result in larger beam axial forces and initial deflections, simultaneous travelling fires result in shorter failure times and represent a more onerous scenario for the steel frame investigated.

International journal of high-rise buildings, 2013

Many previous tall building fires demonstrate that despite code compliant construction fires often spread vertically and burn over multiple floors at the same time. The collapses of the WTC complex buildings in 9/11 as well as other partial collapses like the ones of the Windsor Tower in Madrid and of the Technical University of Delft building posed new questions on the stability of tall buildings in fire. These accidents have shown that local or global collapse is possible in multi-floor fires. In most of the previous work involving multi-floor fires all floors were assumed to be heated simultaneously although in reality fires travel from one floor to another. This paper extends previous research by focusing on the collapse mechanisms of tall buildings in fire and performs a parametric study using various travelling rates. The results of the study demonstrate that vertically travelling fires have beneficial impact in terms of the global structural response of tall buildings in comparison to simultaneous fires. Contrary to the beneficial effect of the travelling fires in terms of the global structural response, it was noticed that higher tensile forces were also present in the floors compared to simultaneous multi-floor case. Designers are therefore advised to consider simultaneous multi-floor fire as an upper bound scenario. However, a scenario where a travelling fire is used is also suggested to be examined, as the tensile capacity of connections may be underestimated.

2013

The aim of this study is to investigate the effect of the fire load to a tall structure and draw conclusions for a more robust structural design. If a building is not properly designed and constructed to with-stand potentially catastrophic events due to risk factors posed by fires, such disasters can nullify the benefits gained from green construction. Leadership in Energy and Environmental Design (LEED) developed by the U.S. Green Building Council and other international certification organizations. One common aspect of most of these certification systems is a lack of consideration for the impact of risks such as fire hazard on sustainability. In order to design sustainable tall structures, their robustness against extreme fire scenarios must be adequately satisfied. In this study, a generic tall structure is modeled using the finite element software LS-Dyna. The two-dimensional model consists of line elements (beams and columns) as well as the shell elements (concrete deck). To simulate the building collapse, the explicit dynamic analysis is used with proper surface-based contact configurations.

The purpose of our submission is to investigate the extent to which present-day design of steel framed buildings are susceptible to total collapse when subjected to extreme fire events. We select a 50 storey structure in which 2 and 4 adjacent storeys located at different above-ground heights are, in separate scenarios, engulfed in raging fires. A total of 8 scenarios are analyzed, employing Newtonian mechanics and realistic energy dissipating properties of H-shaped columns and normal concrete floor slabs possessing secondary (shrinkage and temperature) reinforcement alone. The present Canadian building code is the basis for our column designs under loading conditions for which gravity loads control. Other attributes which would in practice participate in absorbing the kinetic energy of a crushdown upper block are excluded. Despite such conservative assumptions, it is shown that while partial collapse might indeed occur, there is no scenario that predicts a total collapse. These results should provide some comfort to code writers that present requirements to prevent the most catastrophic of failures due to fires, and might indeed suggest to fleeing occupants alternative routes to safety instead of the desperation cases of window jumpers noted during the WTC fire events 16+ years ago.

Proceedings of The 2011 International Conference on Advances in Structural Engineering and Mechanics (ASEM’11), Seoul (South Korea), 2011

The paper deals with the problem of understanding and evaluating the structural response of steel buildings to fire and outlines a general framework for the structural fire safety design of high-rise building. Among all building typology, the fire design of high-rise buildings is particularly challenging with respect to both non-structural and structural design aspects: the enhanced design difficulties in providing i) a safe and prompt vertical evacuation of the building and ii) an effective vertical compartmentalization for ...

Structures Congress 2010, 2010

The WTC 7 investigation by the National Institute of Standards and Technology (NIST) identified several factors that alone, or in combination, led to fire-induced failures of the floors, and subsequently, total collapse of the 47-story WTC 7 building. At the present time, a sensitivity study is being conducted to determine the relative contribution of the identified factors, which includes the presence or absence of shear studs on girders, connection types, asymmetric framing, and bay span lengths. The technical basis for the identified structural factors is presented in this paper.

… European Conference on …, 2005

Following the events of September 11th 2001, understanding the performance of multistorey buildings during large-scale fires has assumed greater importance. These events have highlighted the possibility of large uncontrolled fires lasting for several hours (WTC-7). Owners of high-rise buildings are seeking assurance that integrity can be maintained during similar elevated temperature situations. This work is part of a much larger study to evaluate the performance of high-rise steel-framed structures in the event of large uncontrolled fires, using primarily a computational approach. Given a building and its operating conditions, different fire scenarios are established. The choice of scenarios is established on the basis of probability of occurrence and also as a function of damage potential. Computational fluid dynamics models are used to predict critical conditions within predetermined areas of the building. Emphasis is given to establish a proper thermal boundary condition for the structural elements. A three dimensional numerical model of the structure provides the basis for a structural finite element analysis to be carried out under combined static and thermal loading. Full investigation of the temperatures and stresses generated on structural members due to the design fire chosen are considered. Particular attention to detail is given to those members that are thought likely to contribute to total collapse through localised failure. This is done by combining CFD codes with finite element models. This paper will present a selection of results from the aforementioned investigation, with particular emphasis on the conditions that cause total collapse for the chosen case study.

Fire Technology, 2014

This paper investigates progressive collapse mechanisms of braced steel frames subjected to various fire scenarios using OpenSees, an open-source object-oriented software developed at UC Berkeley. The OpenSees framework has been recently extended to deal with structural behavior under fire conditions by authors. This paper summaries the key work done for this extension and focuses on the application of the developed OpenSees to study the effect of different bracing systems on the fireinduced progressive collapse resistance of steel-framed structures. The study considers two types of bracing systems (vertical and hat bracing) and different fire scenarios such as single and multicompartment fire on the ground floor and second floor. Four collapse mechanisms of steel frames in fire are found through parametric studies. These are general collapse characterized by the collapse of the heated bay followed by lateral drift of adjacent cool bays, global collapse of the whole frame due to the buckling of ground floor columns, local and global lateral drift modes of collapse caused by catenary action developed in the heated beams under large deflections. All the collapse mechanisms are triggered by the buckling of the heated columns. The thermal expansion of heated beams at early heating stage and their catenary action at high temperature have great influences on the collapse mechanisms. The vertical bracing systems has positive effects on increasing the lateral restraint of the frame against local or global drift, while when arranged at edge bays of frames they negatively contributes to the spreading of a local damage to a global collapse in the form of sequential buckling of adjacent columns through load-transfer mechanisms. For a more realistic arrangement of vertical bracings inside the frame, the bracing acts as a barrier to restrain the spread of local damage to the rest of the frame. Instead, using hat bracing can effectively optimize the load-transfer path through a more uniform redistribution of loads in columns and enhance the resistance of structures against progressive collapse. The application of vertical bracing systems alone on the steel frames to resist progressive collapse is proved to be unsafe and a combined vertical and hat bracing system is recommended in the practical design.

2004

A two-dimensional finite-element model is developed that provides some insight into the behavior and collapse of high-rise steel buildings with open web floor systems. For one prescribed temperature distribution that corresponds to a two-story, quarter-span fire, the diagonals of the heated trusses buckle inelastically, causing considerable sag in the fire floors. This behavior puts a high tension demand on the truss connections to the perimeter column, which remains at moderate temperatures in this model and does not experience buckling. Our analysis is based on temperatures and material properties that were selected for illustrative purposes. Therefore no claim is made as to its applicability to any specific structure.