Steel Frames In Fire Research Articles

Plastic redistribution capacity of stainless steel frames in fire

Stainless steel frames with compact cross-sections are capable of redistributing internal forces at room temperature and forming plastic failure mechanisms, which is reflected in the new generation of structural codes for stainless steel by allowing to carry out global plastic analyses on certain types of stainless steel if the joints conforming the structure are classified as full-strength joints. Nevertheless, the requirements for the fire design of stainless steel structures in current standards is still primarily based on the resistance of individual members, disregarding strain hardening effects and the redistribution of internal forces, mainly because the influence of the performance of stainless steel joints on the response of the frames under fire situation is yet unknown. On this basis, this paper presents a numerical study focused on stainless steel frames with compact rectangular hollow sections, both at room temperature and at elevated temperatures, to analyse the influence of full-strength joints on the frame's response by means of two joint modelling techniques, and to assess the plastic redistribution capacity of stainless steel frames, especially under fire situation. In addition, an exhaustive revision of the failure criteria for stainless steel frames under fire situation is carried out and a new failure criterion is proposed based on the results derived from the parametric study. The results demonstrate that stainless steel frames are also capable of redistributing internal forces and forming plastic collapse mechanisms under fire situations, suggesting a new procedure to predict the response of these structures under fire situation more accurately.

Thermal creep of shear tab connection assemblies under transient-state temperatures of fire

Purpose This study aims to investigate the thermal creep behavior of steel frame assemblies with shear tab connections subjected to transient-state fire temperatures. Different key parameters are investigated to study their effect on the global response of the steel frames in fire. Design/methodology/approach Finite element (FE) models of connection assemblies are first analyzed using Abaqus under transient-state temperature conditions and validated against experimental work available in the literature. Upon acquiring the validated conditions, parametric studies are carried out to study the effect of key geometric and heating parameters on the overall response of the frame assembly to fire temperatures. Thermal creep material is also incorporated in the analyses through a user-defined subroutine, and a comparison between including and excluding creep material is illustrated to show the effect of thermal creep on the structural behavior. Findings The results reported herein indicate that having a rigid column increases the thermal-induced axial forces, thus increasing the development of thermal creep strains. Slow heating rates can cause axial stress relaxation in the restrained beam and increase the mid-span deflection and consequently the development of beam catenary action. The results also show that reaching higher initial cooling temperatures and having longer cooling phase durations result in more tensile forces at the end of the cooling phase. Originality/value Previous studies were limited to isolated steel connections under steady-state conditions. This study investigates the creep behavior of shear tab connection assemblies under transient-state conditions of fire when creep effects are explicitly considered. This can provide a rational and realistic assessment of the steel behavior in fire events.

Effect of earthquake damage on the behaviour of composite steel frames in fire

Fire loading following earthquake loading is possible in any building in a seismic-prone area. However, most design approaches do not consider fire following earthquake as a specific loading case. Moreover, seismic design philosophies allow a certain degree of damage in structural elements which make structures more vulnerable when subjected to post-earthquake fire. This study uses three-dimensional numerical models to investigate the effect of earthquake damage on the fire resistance of composite steel-frame office buildings. A total of two types of earthquake damage, fire insulation delamination and residual lateral frame deformation, are investigated. It is concluded that earthquake damage can significantly reduce the fire resistance of composite buildings, with delamination of fire protection having the greatest effect. The results of this study can be used by designers to improve the post-earthquake fire resistance of composite buildings.

10.30: Mechanical response of steel structures in fire: Influence of different material and creep models

ABSTRACTThe paper deals with the influence of viscous creep of steel on the behaviour of steel structures during fire. Two stress‐strain relationships for structural steel, combined with Harmathy's explicit model of viscous creep at elevated temperatures, are presented in the paper: (i) bilinear stress‐strain relationship and (ii) modified Poh's stress‐strain relationship. The presented models have been implemented in self‐developed software Pozar for numerical analysis of mechanical response of planar steel frames in fire. Further on results are validated against experimental ones published by Rubert and Schaumann, 1985. The numerical results, obtained with stress‐strain relationship according to the Eurocode 3, are added for comparison. It is shown that all tested models give satisfactory results for small load ratios. Poh's stress‐strain relationship in combination with Harmathy's model of viscous creep and newly proposed coefficients give the best results among the tested models with explicitly considered viscous creep. However, none of the considered models give good agreement with the experiment at high values of load ratio. Furthermore, the comparison between the different material and creep models is conducted for three heating regimes, namely linear heating regime and fast and slow natural fire development. A prominent influence of the employed material model on the final midspan displacements is observed.

Behavior of Sway Two-Bay, Two-Story Composite Steel Frames in Fire

The experimental results of three full-scale two-story, two-bay, composite steel frames under furnace loading are described briefly. During tests, the composite beams and steel columns were exposed to fire, and only the beam-to-column connections of the tested frames were protected. The furnace temperature, composite steel beam temperature, steel column temperature, horizontal displacements of the columns, and vertical deflections of the composite beams as well as the complete deformation process of the test frames observed during the heating phase and the cooling phase are given. By introducing the slope-deflection equations and joint equilibrium conditions at each joint, the critical temperatures of the buckling of two-story, two-bay, composite steel frames are determined for different furnace scenarios; therefore, this method can consider the effect of the axial force at different levels. The calculated critical temperatures agree well with the experimental results.

Progressive failure modelling and ductility demand of steel beam-to-column connections in fire

A numerical procedure has been developed to model the sequences of failure which can occur within steel beam-to-column connections under fire conditions. In this procedure two recent developments, a static–dynamic solution process and a general component-based connection element, have been combined within the software Vulcan in order to track the sequence of local failures of the connections which lead to structural progressive collapse in fire. In particular the procedure developed can be used to investigate the structural behaviour in fire, particularly the ductility and fracture of different parts of the steel-to-steel connections, and the influence of the connections on the progressive collapse resistance of steel frames in fire.In the component-based connection model, a connection is represented as an assembly of “bolt-rows” composed of components representing different zones of mechanical behaviour whose stiffness, strength, ductility and fracture under changing temperatures can be adequately represented for global modelling. The potential numerical instabilities induced by fractures of individual connection’s components can be overcome by the use of alternate static and dynamic analyses. The transfer of data between the static and dynamic analyses allows a seamless alternation between these two procedures to take place. Accuracy and stability of the calculations can be ensured in the dynamic phase, provided that the time steps are set sufficiently small. This procedure has the capacity of tracking the sequence of local failures (fractures of connection components, detachment and motion of disengaging beams, etc.) which lead to final collapse.Following an illustrative case study of a two-bay by two-storey frame, the effect of ductility of connections on the collapse resistance of steel frames in fire is demonstrated in two case studies of a generic multi-storey frame. It is shown that the analytical process is an effective tool in tackling the numerical problems associated with the complex structural interactions and discontinuous failures which can affect a steel or composite frame in fire, potentially leading to progressive collapse. It can be seen that both tensile and compressive ductility in the connections make a contribution to the fire resistance of the beams. Preventing the detachment of steel beams in fire can be achieved by inducing greater ductility into their connections. Combined with appropriate component-based connection models, this procedure can be adopted in performance-based fire-resistant design to assess the ductility requirements of steel connections.

Effect of Bracing Systems on Fire-Induced Progressive Collapse of Steel Structures Using OpenSees

This paper investigates the effect of various bracing systems on the fire-induced progressive collapse resistance of steel-framed structures using OpenSees. Two types of bracing systems (vertical and hat bracing) and various fire scenarios (single and multi-compartment fires) are considered. Four collapse mechanisms of steel frames in fire are found through parametric studies. General collapse is characterized by the collapse of the heated bay followed by lateral drift of adjacent cool bays. Global collapse of a whole frame is due to the buckling of ground floor columns. Another two lateral collapse modes (local and global) are caused by catenary action developed in the heated beams under large deflections. Vertical bracing systems have 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 contribute to the spreading of a local damage to 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. It is found that 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 practical design.

An Experimental Investigation of Structural Fire Behaviour of a Rigid Steel Frame

Some experimental studies have been conducted on structural fire behaviour of steel sub-frames in order to investigate the effects of thermal stress due to the axial restraint from columns to a heated beam on the behaviour of connections, and its influences on the connected beam and robustness of steel frames. However, the object connections were not beam-to-beam connections but beam-to-column connections with fin plates, end plates, and web cleats. This paper discusses, on the basis of experimental results, structural fire behaviour of a rigid steel frame with fully-moment-resisting beam-to-beam connections with splice plates and HSFG bolts, and beam-to-column connections with full penetration welds. The structural behaviour in the test was also analysed with finite element analysis using Bernoulli-Euler beam elements. The test results indicated that the moment-resisting connections in the rigid steel frame have sufficient load-carrying capacity, but failure may occur in the connected beam due to inadequate shear resistance of the beam web in fire. The critical temperature of the steel beam could be approximated on the basis of its inherent resistance at elevated temperature and initial effects, because the thermal stress disappeared at the fire limit stage. This study was also intended to provide experimental data to help understand the fundamental behaviour of rigid steel frames in fire.

Deformation-reversal in component-based connection elements for analysis of steel frames in fire

A previous paper covered the principles of practical modelling of common steel beam-to-column connections in frameworks at elevated temperatures, using a component-based approach to represent the main structural actions generally grouped at bolt rows. This is needed for the structural modelling which is a necessary part of performance-based structural fire engineering design based on analysis of a range of natural fire scenarios. This paper extends the treatment of components to the effects of change of temperature during an analysis. This includes the treatment of reversal of displacement, which is essential in order to allow components within the whole model to change their temperatures, even when temperatures are increasing monotonically. Reversal curves, and their Intersection Points with the zero-force axis, are used to link the states of deformation of any component on adjacent force–displacement curves at different temperatures. A desirable feature of this treatment is that it encompasses the effects of cooling after heating, which has been observed in full-scale tests to cause failure of connections as beams contract. A simple sub-frame model including beams, columns and endplate connections, has been analysed using the component-based model, in order to illustrate how the connection and its components behave, and how failure develops when the members and connections are subjected to a heating–cooling sequence.

Principles of a component-based connection element for the analysis of steel frames in fire

In this paper an approach is developed for practical modelling of the in-plane behaviour of connections between steel structural elements in global 3-dimensional frame analysis when structures are subject to heating by accidental fires. Because of the interactions between high deflections, thermal expansion and material weakening at high temperatures, connections are subject to considerable variation of both normal forces and moments in fire, so that it is insufficient to represent their behaviour simply in the moment–rotation terms which are usually adequate at ambient temperature. In order to analyse this behaviour reliably, use of the component-based approach is the only feasible method (short of creating highly complex finite element models) which can be included in non-linear, full-frame or subframe, numerical modelling. In previous work non-linear “spring” formulations have been developed and validated for the principal components which occur in the most commonly used steel-to-steel joint details, particularly for endplate connections, taking into account the local geometry, temperature and loading history. The principles governing the creation of connection elements, presenting just two nodes to the connected structural members, from assemblies of such components, are described for connections subject predominantly to two-dimensional structural actions.

Research of Resistance Capability and Equivalent Floor Live-Load of Steel Frame under Fire

The increasing of equivalent floor live-loads is used to describe the weakening of ultimate carrying capacity of the structure by analyzing variety of the beam’s ultimate moment and ultimate shearing force and the carrying capacity and stability of the column of steel-frame in fire, the formulas of the beam’s ultimate shearing capacity, the carrying capacity and stability of column and the equivalent floor live-loads have been derived based on these. The variety rules of the carrying capacity of beam and column of the steel-frame with different thickness of the protection layers and the equivalent floor live-load have been analyzed in the example, the results showed that different equivalent floor live-load may be used to analyze the infection of fire temperature to the carrying capacity of structure in the computations of steel-frame in fire according to different time and fire temperatures of structure members, the mechanics computations of structure would be predigested and the variety of ultimate carrying capacity of steel-frame also could be estimated.

The Effects of Geometric Nonlinearity on Elasto-Plastic Analysis of Plane Steel Frames in Fire

In this paper, the co-rotational total Lagrangian forms of finite element formulations are derived to make elasto-plastic analysis for plane steel frames either under increasing external loading at ambient temperature or under constant external loading at elevated temperatures. Geometric nonlinearity and thermal-expansion effect are taken into account. A series of programs are developed based on the formulations. To verify the accuracy and efficiency of the nonlinear finite element programs, a couple of numerical benchmark tests are carried out. And the results are in a good agreement with solutions from literature. The effects of nonlinear terms of the stiffness matrices on the computational results are investigated in detail. It is demonstrated that the influence of geometric nonlinearity on the incremental step by step finite element analysis for plane steel frames in fire is limited.

The Effects of Geometric Nonlinearity on Elasto-Plastic Analysis of Plane Steel Frames in Fire

The Effects of Geometric Nonlinearity on Elasto-Plastic Analysis of Plane Steel Frames in Fire

Creep of the Plane Steel Frame at Elevated Temperature

In this paper, co-rotational total Lagrangian finite element formulation is derived, and the corresponding numerical model is developed to study creeping behavior of plane steel frames in fire. Geometrical nonlinearity, material nonlinearity, high temperature creeping, and temperature rising rate are taken into account. To verify accuracy and efficiency of the numerical model, four prototypical numerical examples are analyzed using this model. Results are in a great agreement with solutions in literatures. Then the numerical model is used to analyze creeping behavior of the plane steel frames when temperature is lowering. The numerical results have significant contribution to resistance and protection for steel structures against disastrous fires.

Creep of the Plane Steel Frame at Elevated Temperature

In this paper, co-rotational total Lagrangian finite element formulation is derived, and the corresponding numerical model is developed to study creeping behavior of plane steel frames in fire. Geometrical nonlinearity, material nonlinearity, high temperature creeping, and temperature rising rate are taken into account. To verify accuracy and efficiency of the numerical model, four prototypical numerical examples are analyzed using this model. Results are in a great agreement with solutions in literatures. Then the numerical model is used to analyze creeping behavior of the plane steel frames when temperature is lowering. The numerical results have significant contribution to resistance and protection for steel structures against disastrous fires.

Instability of Statical Solutions of Steel 2D Frames in High Temperature

AbstractThis work is intended as an attempt to stability analysis of static equations of flat steel frames in fire. High temperature during fire adversely affects on structural steel elements by activating irreversible processes (plasticity, creep). Rheological property can be described in simplified form by power term. Steel properties is represented by physical equations which contains three terms: linear (elasticity), nonlinear and rheological (related to creep). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear Plastic Hinge Analysis of Three-Dimensional Steel Frames in Fire

A beam-column model for simulating the inelastic behavior of 3-D steel frames is presented. Yielding of the steel section is modeled by 2 nested stress-resultant surfaces and takes into consideration the softening effect of steel material at elevated temperature. Derivation of the inelastic stiffness of a 3-D beam-column element is explained. Degradation of the of the material at elevated temperatures is formulated according to the effective yield strength concept. Application of the method is demonstrated by analyzing and studying the behavior of steel members and a 3-D multistory frame exposed to fire.

Assessment of the Fire Resistance Test with Respect to Beams in Real Structures

Historically fire resistance design of structures has been based upon single element behavior in the standard fire resistance test. Engineers have always recognized that whole frame structural behavior in fire cannot be described by a test on a single element. However, it is only in relatively recent years since the Broadgate phase 8 fire (SCI, 1991) and the subsequent Cardington frame fire tests that researchers have fully nvestigated and understood the behavior of whole structures in response to fire. The University of Edinburgh in collaboration with Imperial College and Corus (formally British Steel) have played a major role in this field of research through the UK Department of Environment and Transport in the Regions (DETR), Partners in Technology (PiT) project. Significant progress towards a complete understanding of the structural behavior of highly redundant composite steel frames in fire has been achieved (Edinburgh University, 2000). It has been shown that the fire resistance test has little relevance to the behavior of structural elements as part of highly indeterminate structures typical of modern, composite steel frame buildings. The test methods are particularly inadequate when the end conditions during service are unknown and the beam is tested as simply supported, i.e. no consideration of restraint is made. By including the effect of restraint in a simple beam model the temperature at which "runaway" occurs is greatly increased. The reasons for this, i.e. the changing load-carrying mechanisms involved as catenary action develops as a result of the deflected shape have also been explained.

Visco-Elasto-Plastic Analysis of Steel Frames in Fire

A finite element formulation for the analysis of two-dimensional steel frames either at ambient or elevated temperature is presented. The formulation includes both geometric and material nonlinearities, and a creep model. A new feature is the manner in which strain reversal is incorporated into the program. The program can analyze steel frames subjected either to (1) increasing external loads at ambient temperature or, (2) constant external loads at elevated temperatures. The program then gives predictions of (1) a collapse load or (2) critical temperature, whichever the case may be. This program has been applied to a number of experimental and analytical benchmark tests. Excellent agreement has been obtained in all these cases. Following on these benchmark tests, creep effects on the buckling of heated columns have also been studied. It shows that creep generally starts to be dominant beyond 400°C. In connection to this, the current simplified method for calculating the critical temperature in Eurocode 3...

Strength and Stability of Steel Frames in Fire: Rankine Approach

A simple analytical approach based on the Rankine principle has been developed to determine the ultimate resistance of steel frames in fire. The proposed Rankine approach gives an approximation of the frames' fire resistance through a simple interaction between two idealized structural behaviors—strength and stability. Here, the strength and stability of the structures are evaluated using the rigid-plastic and the elastic buckling analyses, both incorporating the thermal effects. The proposed approach is first verified using a finite-element model. The verification studies include the effects of column and frame slenderness ratios, beam-column stiffness ratio, steel grades, initial sway imperfections, and initial residual stresses. These studies indicate that frame slenderness ratio is an important parameter governing the behavior of simple frames in fire, and the performance of the proposed approach is related to it. The Rankine approach is then applied to a series of 18 test frames from the literature. ...