Asif Usmani - Academia.edu (original) (raw)

Papers by Asif Usmani

International journal of high-rise buildings, Mar 1, 2013

Many previous tall building fires demonstrate that despite code compliant construction fires ofte... more 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.

Fire Safety Science, 2008

Developments in design methodologies and practices for structures in fire has for the last decade... more Developments in design methodologies and practices for structures in fire has for the last decade focussed on the horizontal stability of structures in fire. In this field there have been a number of developments of significance including the acceptance of membrane action as a viable load carrying mechanism under the large vertical displacements of floor systems which often occur during a fire. This research has focussed on two scenarios-simply supported floor systems and laterally restrained floor systems. Despite the large horizontal forces which the supporting structure must resist as a result of the adoption of these mechanisms in addition to the original vertical mechanical forces which were applied, very little research has been carried out into the consequences to the surrounding structure of the adoption of these mechanisms. These consequences were illustrated sharply by the collapse of the world trade centre twin towers on September the 11 th 2001, where fires which raged inside of two tall steel structures ultimately brought about their collapse.

Journal of Structural Fire Engineering, 2015

OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The Open... more OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The OpenSees framework has been recently extended to deal with structural behaviour under fire conditions. This paper summaries the key work done for this extension and focuses on the validation and application of the developed OpenSees to study the behaviour of composite steel-concrete beams under fire conditions. The performance of the developed OpenSees are verified by four mechanical tests and two fire tests on simply supported composite beams. A parametric study is carried out using OpenSees to study the influence of boundary condition as well as composite effect of slab on the behavior of composite beams exposed to fire. The stress and strain along the beam section is output and compared with yield stress limit at elevated temperature to explain these influences in detail. The results show that the stress distribution in the web of the steel beam is more complex due to the support effects.

Fire Technology, 2014

This paper investigates progressive collapse mechanisms of braced steel frames subjected to vario... more 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.

Advances in Structural Engineering, 2014

OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The Open... more OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The OpenSees framework has been recently extended to deal with structural behavior under fire conditions. This paper summaries the key work done for this extension and focuses on the application of the developed OpenSees to study the fire-induced progressive collapse mechanisms of steel structures. The implicit dynamic analysis method (Newmark method) is applied and the influences of the load ratios, beam sizes and fire scenarios on the collapse behavior of frames are investigated. Single-compartment fire scenarios in the central bay and edge bay are considered, respectively. A total of four collapse mechanisms of steel frames are proposed by varying the three influencing factors. Most of the collapse of steel frames is 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 ...

Economic considerations dictate that building structures be able to resist extreme events, such a... more Economic considerations dictate that building structures be able to resist extreme events, such as a major earthquake or a fire, without collapse but with some structural damage. This makes it imperative for design to be based on nonlinear analysis that incorporates strength degradation. This study examines the complexities associated with modelling degradation of strength in structures for seismic and high temperature environments. It is shown that both dynamic and high temperature analyses are akin to displacement controlled static analysis. If appropriate numerical procedures are used, strength degradation does not result in dynamic instability often associated with this phenomenon. Inclusion of strength degradation as a material property can lead to results that are sensitive to model discretisation for both dynamic and high temperature loadings.

Fire induced geometrical changes produce large deflections in floor slabs and even if there is si... more Fire induced geometrical changes produce large deflections in floor slabs and even if there is significant loss of strength, the change of shape of the slab allows stability to be maintained by the slab acting as a tensile membrane. This of course depends upon the amount and arrangement of reinforcement and the end restraint conditions of the slab. If the slab is continuous over the supports and the continuity of reinforcement over the support can also be assumed then the "anchoring" of the tensile membrane action is not in doubt. There is however a mechanism by which even simply supported slabs can produce a degree of tensile membrane action. In this case although there are no lateral (or in-plane) reactions available at the supports, there is a self-equilibrating mechanism that occurs in the form of a "compressive ring" between the central and perimeter regions of the slab which provides restraint to the membrane tensions (rather like the rigid ring surrounding a trampoline). Some authors have used this to develop design methods for non-continuous simply supported slabs in fire; however there is a great deficit of knowledge regarding the quantification of this effect. One of the key parameters that may govern the magnitude of "membrane enhancement" available for simply supported slabs is the ratio of slab thickness to its "overlap length" from the edge of the support. This paper will present initial conclusions obtained from this study and discuss the implications for design.

Most rules and investigations of the strength of structural members under fire assume that the me... more Most rules and investigations of the strength of structural members under fire assume that the member acts alone as an isolated structure. This matches the testing of individual members in a standard furnace test. The concept may seem appropriate where fire in a compartment effectively attacks only the individual structural members nearby. However, no account is taken of the interactions which inevitably occur with the surrounding structure. Where the complete structure is large and redundant, these interactions can completely change the structural response and effectively invalidate the design assumptions. This paper discusses the response of a structural element under fire within a highly redundant structure, such as a large building. The behaviour of the element under fire is strongly affected by the restraint provided by the surrounding parts which are not subjected to heating. A number of responses in quite simple structures are shown, to illustrate the roles of expansion, loss of material strength, the relative stiffness of adjacent parts of the structure, development of large deflections, buckling and temperature gradients. These aspects are illustrated with simple examples, and it is shown that there are several counter-intuitive phenomena in structures of this kind. The significance of these findings for the design of large buildings is explored briefly.

Fire Safety Journal, 2015

Fire, in the aftermath of an earthquake has evolved as a severely destructive force since the las... more Fire, in the aftermath of an earthquake has evolved as a severely destructive force since the last century [1]. Codes and regulations exist in countries situated in seismically active regions of the world in order to ensure safety of buildings and their occupants in the event of an earthquake; it is however rare to find regulations that also require the consideration of fire following an earthquake, thereby leaving this possibility to be dealt with entirely by emergency responders on an ad-hoc basis with little preparedness. Fire following earthquake (FFE) events in the past, although rare, have sometimes been as destructive as the original earthquake. The aim of this study was to carry out a set of full-scale loading tests on an earthquake damaged, reinforced concrete frame subsequently exposed to fire. The sequential loading was devised in the form of a three phase testing proceduresimulated earthquake loading facilitated by cyclic quasi-static lateral loads; followed by a compartment fire; and finally by subjecting the earthquake and fire damaged frame to a monotonic pushover loading to assess its residual capacity. The reinforced concrete frame was well instrumented with numerous sensors, consisting of thermocouples, strain gauges, linear variable differential transducers (LVDTs) and pressure sensors. A large database of results consisting of temperature profiles, displacements and strains has been generated and salient observations have been made during each stage of loading. This paper describes the experimental investigation and serves as a vehicle for dissemination of the key findings and all the important test data to the engineering community which could be used for validating numerical simulations for further advancing the knowledge and understanding in this relatively poorly researched area.

Proceedings of the Institution of Civil Engineers - Structures and Buildings, 2012

Fires are a relatively likely event following earthquakes in urban locations and in general are a... more Fires are a relatively likely event following earthquakes in urban locations and in general are an integral part of the emergency response strategies, which are focused on life safety in most developed economies. Similarly, building regulations in most countries require engineers to consider the effect of seismic and fire loading on structures to provide an adequate level of resistance to these hazards; however, this is only on a separate basis. To the authors' knowledge there are no current regulations that require buildings to consider these hazards in a sequential manner to quantify the compound loading and design for the required resistance. This paper provides a first and early report from a novel set of tests on a full-scale reinforced concrete frame subjected to simulated earthquake and fire loads. The results from the first test indicate that the test frame could withstand a pre-damage corresponding to a seismic performance level and subsequent 1 h fire exposure without ...

Thermal Science, 2007

This paper provides a "state-of-the-art" review of research into the effects of high te... more This paper provides a "state-of-the-art" review of research into the effects of high temperature on concrete and concrete structures, extending to a range of forms of construction, including novel developments. The nature of concrete-based structures means that they generally perform very well in fire. However, concrete is fundamentally a complex material and its properties can change dramatically when exposed to high temperatures. The principal effects of fire on concrete are loss of compressive strength, and spalling - the forcible ejection of material from the surface of a member. Though a lot of information has been gathered on both phenomena, there remains a need for more systematic studies of the effects of thermal exposures. The response to realistic fires of whole concrete structures presents yet greater challenges due to the interactions of structural elements, the impact of complex small-scale phenomena at full scale, and the spatial and temporal variations in ex...

Procedia Engineering, 2013

The traveling fire methodology provides more realistic fire scenarios for structural fire design ... more The traveling fire methodology provides more realistic fire scenarios for structural fire design by considering fire dynamics in large compartments which are beyond the validity or scope of conventional structural fire design codes. This novel methodology developed recently elsewhere has been implemented in the OpenSees software framework. In this work, effects of traveling fires on the thermal responses of a large composite structure are studied using OpenSees. Finite element analyses are performed to model the detailed heat transfer in the composite structure subjected to traveling fires. It is found that the traditional "equal area" concept is not applicable to evaluate the fire resistance of structures in traveling fires. Results show that traveling fires with larger sizes seem to be more detrimental to steel beams in terms of quicker failure time, while smaller traveling fires produce higher peak temperatures in the concrete slab. Large through-depth thermal gradients are created in the beam sections due to the heat sink effect of the concrete slab, with higher gradients produced by larger fires. The maximum thermal gradients in the concrete sections seem to be insensitive to the sizes of travelling fires.

Journal of Computing in Civil Engineering, 2015

Computational modeling of structures subjected to extreme static and dynamic loads (such as snow,... more Computational modeling of structures subjected to extreme static and dynamic loads (such as snow, wind, impact, and earthquake) using finite-element software are part of mainstream structural engineering curricula in universities (at least at graduate level), and many experts can be found in industry who routinely undertake such analyses. However, only a handful or institutions around the world teach structural response to fire (at any level) and only a few of the top consulting engineers in the world truly specialize in this niche area. Among the reasons for this are the lack of cheap and easily accessible software to carry out such analyses and the highly tedious nature of modeling the full (often coupled) sequence of a realistic fire scenario, heat transfer to structure and structural response (currently impossible using a single software). The authors in this paper describe how finite-element software can be extended to include the modeling of structures under fire load. The added advantage of extending existing finite-element codes, as opposed to creating fire-specific applications, is due to ability to perform multihazard type analysis, e.g., fire following earthquake. Due to its open source nature and object-oriented design, the OpenSees software framework is used for this purpose. In this work, the OpenSees framework, which was initially designed for the earthquake analysis of structures, is extended by the addition of new concrete classes for thermal loads, temperature distributions across element cross sections, and material laws based on Eurocodes. Through class and sequence diagrams, this paper shows the interaction of these classes with the existing classes in the OpenSees framework. The performance of this development is tested using benchmark solutions of a single beam with finite stiffness boundary conditions and a steel frame test. The results from OpenSees agree well with analytical solutions for the benchmark problem chosen and provide reasonable agreement with the test. The experience with OpenSees so far suggests that it has excellent potential to be the basis of a unified software framework for enabling computational modeling of realistic fires, and further work is continuing towards the achievement of this goal. The extensions made to OpenSees described in this work, in keeping with the open source ideals of the framework, have been included in the current OpenSees code and are available for researchers and practicing engineers to test, develop, and use for their own purposes.

Fire Technology, 2012

The collapse of the World Trade Center buildings on September 11, 2001 posed questions on the sta... more The collapse of the World Trade Center buildings on September 11, 2001 posed questions on the stability of tall buildings in fire. Understanding the collapse of the WTC Towers offers the opportunity to learn useful engineering lessons in order to improve the design of future tall buildings against fire induced collapse. This paper extends previous research on the modelling of the collapse of the WTC Towers on September 11, 2001 using a newly developed ''structures in fire'' simulation capability in the open source software framework OpenSees. The simulations carried out are validated by comparisons with previous work and against the findings from the NIST investigation, albeit not in the forensic sense. The column ''pull in'' that triggers the instability of the structure and leads to collapse is explained. The collapse mechanisms of generic composite tall buildings are also examined. This is achieved through carrying out a detailed parametric study varying the relative stiffness of the column and the floors. The two main mechanisms identified in previous research (weak and strong floor) are reproduced and criteria are established on their occurrence. The analyses performed revealed that the collapse mechanism type depended on the bending stiffness ratio and the number of floors subjected to fire and that the most probable type of failure is the strong floor collapse. The knowledge of these mechanisms is of practical use if stakeholders wish to extend the tenability of a tall building structure in a major fire.

Fire Safety Journal, 2001

This paper presents theoretical descriptions of the key phenomena that govern the behaviour of co... more This paper presents theoretical descriptions of the key phenomena that govern the behaviour of composite framed structures in fire. These descriptions have been developed in parallel with large scale computational work undertaken as a part of a research project (The DETR-PIT Project, Behaviour of steel framed structures under fire conditions) to model the full-scale fire tests on a composite steel framed structure at Cardington (UK). Behaviour of composite structures in fire has long been understood to be dominated by the effects of strength loss caused by thermal degradation, and that large deflections and runaway resulting from the action of imposed loading on a 'weakened' structure. Thus 'strength' and 'loads' are quite generally believed to be the key factors determining structural response (fundamentally no different from ambient behaviour). The new understanding produced from the aforementioned project is that, composite framed structures of the type tested at Cardington possess enormous reserves of strength through adopting large displacement configurations. Furthermore, it is the thermally induced forces and displacements, and not material degradation that govern the structural response in fire. Degradation (such as steel yielding and buckling) can even be helpful in developing the large displacement load carrying modes safely. This, of course, is only true until just before failure when material degradation and loads begin to dominate the behaviour once again. However, because no clear failures of composite structures such as the Cardington frame have been seen, it is not clear how far these structures are from failure in a given fire. This paper attempts to lay down some of the most important and fundamental principles that govern the behaviour of composite frame structures in fire in a simple and comprehensible manner. This is based upon the analysis of the response of single

Fire Safety Journal, 2003

This paper uses a finite element model to investigate the stability of the Twin-Towers of the Wor... more This paper uses a finite element model to investigate the stability of the Twin-Towers of the World Trade Center, New York for a number of different fire scenarios. This investigation does not take into account the structural damage caused by the terrorist attack. However the fire scenarios included are based upon the likely fires that could have occurred as a result of the attack. A number of different explanations of how and why the Towers collapsed have appeared since the event. None of these however have adequately focused on the most important issue, namely 'what structural mechanisms led to the state which triggered the collapse'. Also, quite predictably, there are significant and fundamental differences in the explanations of the WTC collapses on offer so far. A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data (nature of the fires, damage to fire protection, heat transfer to structural members and nature and extent of structural damage for instance). There is however a consensus of sorts that the fires that burned in the structures after the attack had a big part to play in this collapse. The question is how big? Taking this to the extreme, this paper poses the hypothetical question, "had there been no structural damage would the structure have survived fires of a similar magnitude"? A robust but simple computational and theoretical analysis has been carried out to answer this question. Robust because no gross assumptions have been made and varying important parameters over a wide range shows consistent behaviour supporting the overall conclusions. Simple because all results presented can be checked by any structural engineer either theoretically or using widely available structural analysis software tools. The results are illuminating and show that the structural system adopted for the Twin-Towers may have been unusually vulnerable to a major fire. The analysis results show a simple but unmistakable collapse mechanism that owes as much (or more) to the geometric thermal expansion effects as it does to the material effects of loss of strength and stiffness. The collapse mechanism discovered is a simple stability failure directly related to the effect of heating (fire). Additionally, the mechanism is not dependent upon failure of structural connections.

Fire and Materials, 2004

It has become apparent from the series of fire tests carried out at Cardington, UK that the respo... more It has become apparent from the series of fire tests carried out at Cardington, UK that the response of steel-concrete composite structures in fire conditions is largely governed by the behaviour of the reinforced concrete floor slabs. This paper obtains a detailed understanding of the strength of heated concrete floor slabs when subject to the combined bending and membrane forces that typically result from restrained thermal expansion. To analyse such slabs the computer program FEAST is used. Initially the behaviour of the floor slab in the Cardington frame under pure bending and pure membrane forces is described and explained. The paper then explores the effect that interaction between bending and membrane behaviour has on the strength of the slab when it is uniformly heated, heated with a linear thermal gradient and heated with the non-linear thermal gradient observed during the Cardington tests. Finally, the effects of varying the area and location of the reinforcement in the slab are analysed.

Engineering Structures, 2014

The Pacific Earthquake Engineering Research (PEER) Center's Performance Based Earthquake Engineer... more The Pacific Earthquake Engineering Research (PEER) Center's Performance Based Earthquake Engineering (PBEE) framework is well documented. The framework is a linear methodology which is based upon obtaining in turn output from each of the following analyses: hazard analysis; structural analysis; loss analysis, and finally decision making based on variables of interest, such as downtime or cost to repair. The strength of the framework is in its linearity, its clear flexibility and in the consideration of uncertainty at every stage of the analysis. The framework has potential applications to other forms of extreme loading; however in order for this to be achieved the 'mapping' of the framework to the analysis of structures for other loading situations must be successful. This paper illustrates one such 'mapping' of the framework for Performance Based Fire Engineering (PBFE) of structures. Using a combination of simple analytical techniques and codified methods as well as random sampling techniques to develop a range of response records, the PEER framework is followed to illustrate its application to structural fire engineering. The end result is a successful application of the earthquake framework to fire which highlights both the assumptions which are inherent in the performance based design framework as well as subjects of future research which will allow more confidence in the design of structures for fire using performance based techniques. This article describes the PEER framework applied to structural earthquake design then follows the framework from start to completion applying suitable alternative tools to perform each stage of the analysis for structures in fire.

Engineering Structures, 2012

… European Conference on …, 2005

Following the events of September 11th 2001, understanding the performance of multistorey buildin... more 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.

International journal of high-rise buildings, Mar 1, 2013

Many previous tall building fires demonstrate that despite code compliant construction fires ofte... more 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.

Fire Safety Science, 2008

Developments in design methodologies and practices for structures in fire has for the last decade... more Developments in design methodologies and practices for structures in fire has for the last decade focussed on the horizontal stability of structures in fire. In this field there have been a number of developments of significance including the acceptance of membrane action as a viable load carrying mechanism under the large vertical displacements of floor systems which often occur during a fire. This research has focussed on two scenarios-simply supported floor systems and laterally restrained floor systems. Despite the large horizontal forces which the supporting structure must resist as a result of the adoption of these mechanisms in addition to the original vertical mechanical forces which were applied, very little research has been carried out into the consequences to the surrounding structure of the adoption of these mechanisms. These consequences were illustrated sharply by the collapse of the world trade centre twin towers on September the 11 th 2001, where fires which raged inside of two tall steel structures ultimately brought about their collapse.

Journal of Structural Fire Engineering, 2015

OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The Open... more OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The OpenSees framework has been recently extended to deal with structural behaviour under fire conditions. This paper summaries the key work done for this extension and focuses on the validation and application of the developed OpenSees to study the behaviour of composite steel-concrete beams under fire conditions. The performance of the developed OpenSees are verified by four mechanical tests and two fire tests on simply supported composite beams. A parametric study is carried out using OpenSees to study the influence of boundary condition as well as composite effect of slab on the behavior of composite beams exposed to fire. The stress and strain along the beam section is output and compared with yield stress limit at elevated temperature to explain these influences in detail. The results show that the stress distribution in the web of the steel beam is more complex due to the support effects.

Fire Technology, 2014

This paper investigates progressive collapse mechanisms of braced steel frames subjected to vario... more 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.

Advances in Structural Engineering, 2014

OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The Open... more OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The OpenSees framework has been recently extended to deal with structural behavior under fire conditions. This paper summaries the key work done for this extension and focuses on the application of the developed OpenSees to study the fire-induced progressive collapse mechanisms of steel structures. The implicit dynamic analysis method (Newmark method) is applied and the influences of the load ratios, beam sizes and fire scenarios on the collapse behavior of frames are investigated. Single-compartment fire scenarios in the central bay and edge bay are considered, respectively. A total of four collapse mechanisms of steel frames are proposed by varying the three influencing factors. Most of the collapse of steel frames is 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 ...

Economic considerations dictate that building structures be able to resist extreme events, such a... more Economic considerations dictate that building structures be able to resist extreme events, such as a major earthquake or a fire, without collapse but with some structural damage. This makes it imperative for design to be based on nonlinear analysis that incorporates strength degradation. This study examines the complexities associated with modelling degradation of strength in structures for seismic and high temperature environments. It is shown that both dynamic and high temperature analyses are akin to displacement controlled static analysis. If appropriate numerical procedures are used, strength degradation does not result in dynamic instability often associated with this phenomenon. Inclusion of strength degradation as a material property can lead to results that are sensitive to model discretisation for both dynamic and high temperature loadings.

Fire induced geometrical changes produce large deflections in floor slabs and even if there is si... more Fire induced geometrical changes produce large deflections in floor slabs and even if there is significant loss of strength, the change of shape of the slab allows stability to be maintained by the slab acting as a tensile membrane. This of course depends upon the amount and arrangement of reinforcement and the end restraint conditions of the slab. If the slab is continuous over the supports and the continuity of reinforcement over the support can also be assumed then the "anchoring" of the tensile membrane action is not in doubt. There is however a mechanism by which even simply supported slabs can produce a degree of tensile membrane action. In this case although there are no lateral (or in-plane) reactions available at the supports, there is a self-equilibrating mechanism that occurs in the form of a "compressive ring" between the central and perimeter regions of the slab which provides restraint to the membrane tensions (rather like the rigid ring surrounding a trampoline). Some authors have used this to develop design methods for non-continuous simply supported slabs in fire; however there is a great deficit of knowledge regarding the quantification of this effect. One of the key parameters that may govern the magnitude of "membrane enhancement" available for simply supported slabs is the ratio of slab thickness to its "overlap length" from the edge of the support. This paper will present initial conclusions obtained from this study and discuss the implications for design.

Most rules and investigations of the strength of structural members under fire assume that the me... more Most rules and investigations of the strength of structural members under fire assume that the member acts alone as an isolated structure. This matches the testing of individual members in a standard furnace test. The concept may seem appropriate where fire in a compartment effectively attacks only the individual structural members nearby. However, no account is taken of the interactions which inevitably occur with the surrounding structure. Where the complete structure is large and redundant, these interactions can completely change the structural response and effectively invalidate the design assumptions. This paper discusses the response of a structural element under fire within a highly redundant structure, such as a large building. The behaviour of the element under fire is strongly affected by the restraint provided by the surrounding parts which are not subjected to heating. A number of responses in quite simple structures are shown, to illustrate the roles of expansion, loss of material strength, the relative stiffness of adjacent parts of the structure, development of large deflections, buckling and temperature gradients. These aspects are illustrated with simple examples, and it is shown that there are several counter-intuitive phenomena in structures of this kind. The significance of these findings for the design of large buildings is explored briefly.

Fire Safety Journal, 2015

Fire, in the aftermath of an earthquake has evolved as a severely destructive force since the las... more Fire, in the aftermath of an earthquake has evolved as a severely destructive force since the last century [1]. Codes and regulations exist in countries situated in seismically active regions of the world in order to ensure safety of buildings and their occupants in the event of an earthquake; it is however rare to find regulations that also require the consideration of fire following an earthquake, thereby leaving this possibility to be dealt with entirely by emergency responders on an ad-hoc basis with little preparedness. Fire following earthquake (FFE) events in the past, although rare, have sometimes been as destructive as the original earthquake. The aim of this study was to carry out a set of full-scale loading tests on an earthquake damaged, reinforced concrete frame subsequently exposed to fire. The sequential loading was devised in the form of a three phase testing proceduresimulated earthquake loading facilitated by cyclic quasi-static lateral loads; followed by a compartment fire; and finally by subjecting the earthquake and fire damaged frame to a monotonic pushover loading to assess its residual capacity. The reinforced concrete frame was well instrumented with numerous sensors, consisting of thermocouples, strain gauges, linear variable differential transducers (LVDTs) and pressure sensors. A large database of results consisting of temperature profiles, displacements and strains has been generated and salient observations have been made during each stage of loading. This paper describes the experimental investigation and serves as a vehicle for dissemination of the key findings and all the important test data to the engineering community which could be used for validating numerical simulations for further advancing the knowledge and understanding in this relatively poorly researched area.

Proceedings of the Institution of Civil Engineers - Structures and Buildings, 2012

Fires are a relatively likely event following earthquakes in urban locations and in general are a... more Fires are a relatively likely event following earthquakes in urban locations and in general are an integral part of the emergency response strategies, which are focused on life safety in most developed economies. Similarly, building regulations in most countries require engineers to consider the effect of seismic and fire loading on structures to provide an adequate level of resistance to these hazards; however, this is only on a separate basis. To the authors' knowledge there are no current regulations that require buildings to consider these hazards in a sequential manner to quantify the compound loading and design for the required resistance. This paper provides a first and early report from a novel set of tests on a full-scale reinforced concrete frame subjected to simulated earthquake and fire loads. The results from the first test indicate that the test frame could withstand a pre-damage corresponding to a seismic performance level and subsequent 1 h fire exposure without ...

Thermal Science, 2007

This paper provides a "state-of-the-art" review of research into the effects of high te... more This paper provides a "state-of-the-art" review of research into the effects of high temperature on concrete and concrete structures, extending to a range of forms of construction, including novel developments. The nature of concrete-based structures means that they generally perform very well in fire. However, concrete is fundamentally a complex material and its properties can change dramatically when exposed to high temperatures. The principal effects of fire on concrete are loss of compressive strength, and spalling - the forcible ejection of material from the surface of a member. Though a lot of information has been gathered on both phenomena, there remains a need for more systematic studies of the effects of thermal exposures. The response to realistic fires of whole concrete structures presents yet greater challenges due to the interactions of structural elements, the impact of complex small-scale phenomena at full scale, and the spatial and temporal variations in ex...

Procedia Engineering, 2013

The traveling fire methodology provides more realistic fire scenarios for structural fire design ... more The traveling fire methodology provides more realistic fire scenarios for structural fire design by considering fire dynamics in large compartments which are beyond the validity or scope of conventional structural fire design codes. This novel methodology developed recently elsewhere has been implemented in the OpenSees software framework. In this work, effects of traveling fires on the thermal responses of a large composite structure are studied using OpenSees. Finite element analyses are performed to model the detailed heat transfer in the composite structure subjected to traveling fires. It is found that the traditional "equal area" concept is not applicable to evaluate the fire resistance of structures in traveling fires. Results show that traveling fires with larger sizes seem to be more detrimental to steel beams in terms of quicker failure time, while smaller traveling fires produce higher peak temperatures in the concrete slab. Large through-depth thermal gradients are created in the beam sections due to the heat sink effect of the concrete slab, with higher gradients produced by larger fires. The maximum thermal gradients in the concrete sections seem to be insensitive to the sizes of travelling fires.

Journal of Computing in Civil Engineering, 2015

Computational modeling of structures subjected to extreme static and dynamic loads (such as snow,... more Computational modeling of structures subjected to extreme static and dynamic loads (such as snow, wind, impact, and earthquake) using finite-element software are part of mainstream structural engineering curricula in universities (at least at graduate level), and many experts can be found in industry who routinely undertake such analyses. However, only a handful or institutions around the world teach structural response to fire (at any level) and only a few of the top consulting engineers in the world truly specialize in this niche area. Among the reasons for this are the lack of cheap and easily accessible software to carry out such analyses and the highly tedious nature of modeling the full (often coupled) sequence of a realistic fire scenario, heat transfer to structure and structural response (currently impossible using a single software). The authors in this paper describe how finite-element software can be extended to include the modeling of structures under fire load. The added advantage of extending existing finite-element codes, as opposed to creating fire-specific applications, is due to ability to perform multihazard type analysis, e.g., fire following earthquake. Due to its open source nature and object-oriented design, the OpenSees software framework is used for this purpose. In this work, the OpenSees framework, which was initially designed for the earthquake analysis of structures, is extended by the addition of new concrete classes for thermal loads, temperature distributions across element cross sections, and material laws based on Eurocodes. Through class and sequence diagrams, this paper shows the interaction of these classes with the existing classes in the OpenSees framework. The performance of this development is tested using benchmark solutions of a single beam with finite stiffness boundary conditions and a steel frame test. The results from OpenSees agree well with analytical solutions for the benchmark problem chosen and provide reasonable agreement with the test. The experience with OpenSees so far suggests that it has excellent potential to be the basis of a unified software framework for enabling computational modeling of realistic fires, and further work is continuing towards the achievement of this goal. The extensions made to OpenSees described in this work, in keeping with the open source ideals of the framework, have been included in the current OpenSees code and are available for researchers and practicing engineers to test, develop, and use for their own purposes.

Fire Technology, 2012

The collapse of the World Trade Center buildings on September 11, 2001 posed questions on the sta... more The collapse of the World Trade Center buildings on September 11, 2001 posed questions on the stability of tall buildings in fire. Understanding the collapse of the WTC Towers offers the opportunity to learn useful engineering lessons in order to improve the design of future tall buildings against fire induced collapse. This paper extends previous research on the modelling of the collapse of the WTC Towers on September 11, 2001 using a newly developed ''structures in fire'' simulation capability in the open source software framework OpenSees. The simulations carried out are validated by comparisons with previous work and against the findings from the NIST investigation, albeit not in the forensic sense. The column ''pull in'' that triggers the instability of the structure and leads to collapse is explained. The collapse mechanisms of generic composite tall buildings are also examined. This is achieved through carrying out a detailed parametric study varying the relative stiffness of the column and the floors. The two main mechanisms identified in previous research (weak and strong floor) are reproduced and criteria are established on their occurrence. The analyses performed revealed that the collapse mechanism type depended on the bending stiffness ratio and the number of floors subjected to fire and that the most probable type of failure is the strong floor collapse. The knowledge of these mechanisms is of practical use if stakeholders wish to extend the tenability of a tall building structure in a major fire.

Fire Safety Journal, 2001

This paper presents theoretical descriptions of the key phenomena that govern the behaviour of co... more This paper presents theoretical descriptions of the key phenomena that govern the behaviour of composite framed structures in fire. These descriptions have been developed in parallel with large scale computational work undertaken as a part of a research project (The DETR-PIT Project, Behaviour of steel framed structures under fire conditions) to model the full-scale fire tests on a composite steel framed structure at Cardington (UK). Behaviour of composite structures in fire has long been understood to be dominated by the effects of strength loss caused by thermal degradation, and that large deflections and runaway resulting from the action of imposed loading on a 'weakened' structure. Thus 'strength' and 'loads' are quite generally believed to be the key factors determining structural response (fundamentally no different from ambient behaviour). The new understanding produced from the aforementioned project is that, composite framed structures of the type tested at Cardington possess enormous reserves of strength through adopting large displacement configurations. Furthermore, it is the thermally induced forces and displacements, and not material degradation that govern the structural response in fire. Degradation (such as steel yielding and buckling) can even be helpful in developing the large displacement load carrying modes safely. This, of course, is only true until just before failure when material degradation and loads begin to dominate the behaviour once again. However, because no clear failures of composite structures such as the Cardington frame have been seen, it is not clear how far these structures are from failure in a given fire. This paper attempts to lay down some of the most important and fundamental principles that govern the behaviour of composite frame structures in fire in a simple and comprehensible manner. This is based upon the analysis of the response of single

Fire Safety Journal, 2003

This paper uses a finite element model to investigate the stability of the Twin-Towers of the Wor... more This paper uses a finite element model to investigate the stability of the Twin-Towers of the World Trade Center, New York for a number of different fire scenarios. This investigation does not take into account the structural damage caused by the terrorist attack. However the fire scenarios included are based upon the likely fires that could have occurred as a result of the attack. A number of different explanations of how and why the Towers collapsed have appeared since the event. None of these however have adequately focused on the most important issue, namely 'what structural mechanisms led to the state which triggered the collapse'. Also, quite predictably, there are significant and fundamental differences in the explanations of the WTC collapses on offer so far. A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data (nature of the fires, damage to fire protection, heat transfer to structural members and nature and extent of structural damage for instance). There is however a consensus of sorts that the fires that burned in the structures after the attack had a big part to play in this collapse. The question is how big? Taking this to the extreme, this paper poses the hypothetical question, "had there been no structural damage would the structure have survived fires of a similar magnitude"? A robust but simple computational and theoretical analysis has been carried out to answer this question. Robust because no gross assumptions have been made and varying important parameters over a wide range shows consistent behaviour supporting the overall conclusions. Simple because all results presented can be checked by any structural engineer either theoretically or using widely available structural analysis software tools. The results are illuminating and show that the structural system adopted for the Twin-Towers may have been unusually vulnerable to a major fire. The analysis results show a simple but unmistakable collapse mechanism that owes as much (or more) to the geometric thermal expansion effects as it does to the material effects of loss of strength and stiffness. The collapse mechanism discovered is a simple stability failure directly related to the effect of heating (fire). Additionally, the mechanism is not dependent upon failure of structural connections.

Fire and Materials, 2004

It has become apparent from the series of fire tests carried out at Cardington, UK that the respo... more It has become apparent from the series of fire tests carried out at Cardington, UK that the response of steel-concrete composite structures in fire conditions is largely governed by the behaviour of the reinforced concrete floor slabs. This paper obtains a detailed understanding of the strength of heated concrete floor slabs when subject to the combined bending and membrane forces that typically result from restrained thermal expansion. To analyse such slabs the computer program FEAST is used. Initially the behaviour of the floor slab in the Cardington frame under pure bending and pure membrane forces is described and explained. The paper then explores the effect that interaction between bending and membrane behaviour has on the strength of the slab when it is uniformly heated, heated with a linear thermal gradient and heated with the non-linear thermal gradient observed during the Cardington tests. Finally, the effects of varying the area and location of the reinforcement in the slab are analysed.

Engineering Structures, 2014

The Pacific Earthquake Engineering Research (PEER) Center's Performance Based Earthquake Engineer... more The Pacific Earthquake Engineering Research (PEER) Center's Performance Based Earthquake Engineering (PBEE) framework is well documented. The framework is a linear methodology which is based upon obtaining in turn output from each of the following analyses: hazard analysis; structural analysis; loss analysis, and finally decision making based on variables of interest, such as downtime or cost to repair. The strength of the framework is in its linearity, its clear flexibility and in the consideration of uncertainty at every stage of the analysis. The framework has potential applications to other forms of extreme loading; however in order for this to be achieved the 'mapping' of the framework to the analysis of structures for other loading situations must be successful. This paper illustrates one such 'mapping' of the framework for Performance Based Fire Engineering (PBFE) of structures. Using a combination of simple analytical techniques and codified methods as well as random sampling techniques to develop a range of response records, the PEER framework is followed to illustrate its application to structural fire engineering. The end result is a successful application of the earthquake framework to fire which highlights both the assumptions which are inherent in the performance based design framework as well as subjects of future research which will allow more confidence in the design of structures for fire using performance based techniques. This article describes the PEER framework applied to structural earthquake design then follows the framework from start to completion applying suitable alternative tools to perform each stage of the analysis for structures in fire.

Engineering Structures, 2012

… European Conference on …, 2005

Following the events of September 11th 2001, understanding the performance of multistorey buildin... more 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.