Abstract 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 structural elements under a combination of thermal actions and end restraints representing the surrounding structure.

Fundamental principles of structural behaviour under thermal effects (large composite buildings under compartment fires)

Post-fire mechanical properties of high strength structural steels S460 and S690

For the evaluation of damage to a structure after exposure to fire, the residual mechanical properties of structural materials need to be evaluated first. Many factors can affect the post-fire behavior of structural and reinforcing steels. In this paper, existing test data are collected from an extensive survey of the literature. A statistical analysis is conducted to analyze the effects of heat exposure on key parameters, such as the residual modulus of elasticity, residual yield strength, and residual ultimate strength, which control the full-range stress-strain curves of steel. A simplified stress-strain model is developed, which can be used for both structural and reinforcing steels after heating and cooling down to room temperature.

Stress-Strain Curves of Structural and Reinforcing Steels after Exposure to Elevated Temperatures

Fire is one of the most severe conditions to which structures can be subjected, and hence, the provision of appropriate fire safety measures for structural members is an important aspect of design. The recent introduction of performance-based codes has increased the use of computer-based models for fire resistance assessment. For evaluating the fire resistance of steel structures, high-temperature properties of steel are to be specified as input data. This paper reviews high-temperature constitutive relationships for steel currently available in American and European standards, and highlights the variation between these relationships through comparison with published experimental results. The effect of various constitutive models on overall fire resistance predictions is illustrated through case studies. It is also shown that high-temperature creep, which is not often included in constitutive models, has a significant influence on the fire response of steel structures. Results from the case studies are used to draw recommendations on the use of appropriate constitutive models for fire resistance assessment.

High-Temperature Properties of Steel for Fire Resistance Modeling of Structures

Mechanical properties of heat-treated high strength steel under fire/post-fire conditions

An experimental research programme was carried out during the years 1994-2001 in the Laboratory of Steel Structures at Helsinki University of Technology in order to investigate the mechanical properties of several structural steels at elevated temperatures by using mainly the transient state tensile test method The aim was to produce accurate material data for use in different structural analyses The main test results are public and they are available for other researchers In this paper the experimental test results for the mechanical properties of the studied steel grades S350GD+Z, S355 and S460M at fire temperatures are presented with a short description of the testing facilities A test series was also carried out for cold-formed material taken from rectangular hollow sections of structural steel S355J2H and these test results are also given in this report The mechanical properties of structural steel after cooling down have also been examined briefly and these test results are given in this report The test results were used to determine the temperature dependencies of the mechanical properties, ie yield strength, modulus of elasticity and thermal elongation, of the studied steel material at temperatures up to 950degreesC The test results are compared with the material model for steel according to Eurocode 3: Part 12

Mechanical properties of structural steel at elevated temperatures and after cooling down

Fire design model for structural steel S355 based upon transient state tensile test results

Behaviour of mechanical properties of different steel grades at elevated temperatures need to be well known to understand the behaviour of steel and composite structures in fire. Quite commonly simplified material models are used to estimate the structural fire resistance of steel structures. In more advanced methods, for example in finite element or finite strip analyses, it is important to use accurate material data to obtain reliable results. To study thoroughly the behaviour of certain steel structure at elevated temperatures, one should use the material data of the steel obtained by testing. Extensive experimental research has been carried out in the Laboratory of Steel Structures at Helsinki University of Technology in order to investigate mechanical properties of several structural steels at elevated temperatures by using mainly the transient state tensile test method. In this thesis a collection of test results of the behaviour of mechanical properties of different steel grades at elevated temperatures is presented with analysis of the test results. The tests have been carried out at Helsinki University of Technology during the past about 12 years. The aim of these tests has been to evaluate the accuracy of existing design values for the mechanical properties of structural steel and to support other different research projects aimed at studying the behaviour of steel or composite structures in fire. The results are presented with a comparison to the European design standard (EN1993-1-2) for structural fire design of steel structures, which is already the officially accepted standard in the EU countries and certainly will be largely in use in the near future. The results are quite well adopted and referred in other different research projects. The main aim of this research was to provide results to other researchers and design engineers in the field of structural fire engineering to improve structural fire safety in the future.

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Mechanical properties of structural steels at high temperatures and after cooling down

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel roof truss through self-tapping screws. At ambient temperatures, neither EN 1993-1-8 nor EN 1993-1-3 can be used directly for this type of connection. Besides, no design rules are available in EN 1993-1-2 for designing screwed connections in fire. A 3D Finite Element (FE) model for a single-lap shear screwed connection is developed using ABAQUS software. After the validation by tests, the model is used to predict the ultimate resistance of connections at both ambient and elevated temperatures. Further, the effects of edge and end distances on the connection resistance are investigated. Based on the analyses results, revised design equations for predicting the connection resistance are proposed. The design resistance is calibrated by testing and FE analyses results according to the procedure given in EN 1990 and the partial safety factor is derived.

Design of screwed steel sheeting connection at ambient and elevated temperatures

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel truss through powder-actuated shot nails or self-tapping screws. The lap shear behaviours of both shot nailed and screwed connections are studied in this paper using both testing and FE analysis at ambient and elevated temperatures. The studies for screwed connections show that four components of loading capacity, including bearing force between thin sheet and screw shank, frictional force between washer and thin sheet, frictional force between thin sheet and thick plate, and the bearing force by the tilting of thin sheet are identified and quantified. The studies for shot nailed connections show strong interactions among washer, shot nail, thin sheet and supporting plate. The protuberance feature developed during the nail driving process, which causes material in thick plate flowing upward, has a significant positive contribution to the loading capacity of the connection.

Jyri Outinen

Papers

Mechanical properties of structural steel at elevated temperatures and after cooling down

Fire design model for structural steel S355 based upon transient state tensile test results

Mechanical properties of structural steels at high temperatures and after cooling down

Design of screwed steel sheeting connection at ambient and elevated temperatures

Behaviour of shear connectors in cold-formed steel sheeting at ambient and elevated temperatures