R.J. Plank

Analyses of the effects of cooling and fire spread on steel-framed buildings

An existing computer model capable of predicting the response of three-dimensional semi-rigid steel framed buildings, including continuous floor systems, at elevated temperatures has been extended to include the possibility of extensive strain reversal within the material constitutive relationship. This allows the behaviour in cooling and the effect o f fire spread on steel-framed buildings to be investigated. A number of examples are presented to give an indication of the likely effects of cooling behaviour within the analysis. An initial study has also been made into the structural effects of fire spread from a single ignition point into adjacent bays of a two-dimensional steel building frame. It is shown that some of the distortions caused by the fire are increased by progressive fire spread, as compared with simultaneous burning across the same range of compartments.

The influence of shear connectors on the behaviour of composite steel-framed buildings in fire

Abstract A three-dimensional non-linear procedure is presented for modelling partial shear connection in composite steel-framed buildings in fire. The model has been incorporated within the computer program VULCAN which has been developed to model such structures. The accuracy and reliability of the model are demonstrated by analyses of four previously tested simply-supported composite beams, of which two were tested at ambient temperature, and a major full-scale fire test conducted in the Cardington composite test frame. The model is clearly capable of predicting the response of composite members and structures in fire with acceptable accuracy, although data is needed on the degradation of shear stud connectors at elevated temperatures. It is shown that, in statically determinate components, the local failure of shear studs can very rapidly alter the behaviour at the failure temperature, although whether this is important depends on the loading level. The inclusion of partial interaction is shown to make a small, but potentially significant, amendment to the analytical results in the high temperature range for the three-dimensional full-scale fire test.

Effective stiffness modelling of composite concrete slabs in fire

Abstract In this paper a modified layered slab element is developed based on the layered procedure previously developed for the modelling of composite slabs in fire. In the development reported here, the ribs forming the lower part of any slab cast onto metal decking are included in the slab modelling. The basic idea is to use the nominal thickness of the composite slab as the thickness of the slab element, and to use an effective-stiffness factor to modify the material stiffness matrices of plain concrete in order to take into account the orthotropic properties of the slab. A maximum-strain failure criterion is used in the modelling of concrete. Three standard fire tests on metal-deck composite slabs and one full-scale natural fire test on the Cardington composite building frame are modelled for validation. The validations indicate that the model proposed is clearly capable of predicting the fire resistance of this type of composite slab in fire with reasonable accuracy. It is evident from this study that the influence of the ribs across the bottom of the slabs is significant and should be accounted for. The calculation of effective stiffness factors, which are based on the theory of elastic beam bending, is adequate and efficient, and the maximum-strain failure criterion is simple and suitable for such problems.

A generalised steel/reinforced concrete beam-column element model for fire conditions

A generalised beam-column element is presented for three-dimensional composite structures at ambient and high temperatures. The element can model reinforced concrete and steel sections of different shapes. The element has been implemented in the software VULCAN, and has then been validated by comparing with existing theoretical and experimental results. The formulation currently includes both geometrical and material non-linearities and considers unloading for steel, cracking and crushing for concrete, and thermal expansion and degradation of material for both with elevated temperatures. The cross-section is divided into an appropriate number of segments so that non-uniform temperature profiles, variations of constitutive relationships and thermal strains across the section can be represented accurately.

AN ANALYTICAL MODEL FOR THE ANALYSIS OF COMPOSITE BEAMS WITH PARTIAL INTERACTION

Abstract The paper presents the derivation of a two-dimensional non-linear inelastic finite element model for the structural analysis of steel-concrete composite beams with flexible shear connection. The effects of slip between the steel beam and concrete slab and the non-linear nature of the force-slip characteristics of the shear connectors are included. The model can be used to predict load-deflection behaviour and the slip distribution along the length of the beam. It has been validated by comparing the results with published test data for simply supported and continuous beams.

The principles of sustainable construction

This article outlines the importance of immediate action to ensure sustainable development and explains why construction has such a major role to play. The broader issues are introduced but the focus is on those actions which the construction industry can take to make the biggest improvements, namely reducing energy use associated with both the building process and the operation of buildings throughout their life. It is also important that the useful life of the building is prolonged, opportunities are taken to reuse components and recycle materials when they are no longer needed and that materials are sourced in such a way that impacts are minimised. The challenge of reducing demolition waste and making positive use of other waste products are also considered. The specific information and relative importance of different issues discussed in this article relate mainly to the UK, but the principles are universal.

The influence of connection stiffness on the behaviour of steel beams in fire

Abstract Although steel beams are often designed as simply supported, practical connection details generally afford significant rotational stiffness. When exposed to fire, this can have the effect of improving the survival of the beam. The analytical approach described provides a means of studying this. Indicative studies using this method are included to illustrate the influence of certain key parameters, including the connection type, its temperature relative to the rest of the beam, and the proportions of the beam itself. The results are compared with current simplified design approaches. The studies are based on currently available test data for a limited range of connection details, and a simple means of estimating high temperature connection characteristics for more general conditions is suggested.

The Lateral-torsional Buckling of Unrestrained Steel Beams in Fire

Abstract A three-dimensional computer model which is capable of predicting the structural behaviour at elevated temperatures of skeletal steel frames has been used to investigate the ultimate behaviour of uniformly heated unrestrained beams. A series of different sections and spans has been studied for different loading patterns and load ratios. The predicted results indicate failure by lateral-torsional buckling in all cases. A comparison has been made with the limiting temperatures which are given in BS5950 Part 8, and in most cases the model predicts lower failure temperatures tha does the code. This is in contrast to the extremely accurate limiting temperatures which the Code specifies for restrained beams. A similar finding is obtained when the results are compared with limiting temperatures obtained from EC3 Part 1.2. It can be seen that failure temperatures generally depend on the ambient-temperature ratio of lateral-torsional buckling resistance moment to in-plane moment capacity, with the last slender cases failing at the lowest temperatures for any given load ratio.

Failure of steel columns in fire

Results of an analytical study of the performance of geometrically perfect steel colymns under fire conditions are presented. The analysis was based on the finite strip method and includes non-linear material characteristics as functions of temperature. Results obtained using different models of such functions are compared, demonstrating very significant variation in predicted behaviour. The influence of slenderness ratio was studied and shown to be a major factor controlling the failure temperature, whilst the effect of residual stress levels on column failure is little different from that at ambient temperature. Local buckling behaviour was also investigated. The results suggest that this mode of failure may be rather more critical for columns in fire than at ambient temperature. The effect of partial protection provided by blocking-in the column web was investigated. This shows that although there is relatively little improvement in failure temperature, significantly longer survival times are achieved using this form of construction. Although the study is not comprehensive, some useful indications are provided for future generation of practical design rules.

Component modelling of flexible end-plate connections in fire

This paper describes a component-based model for simulating the behaviour of flexible end-plate connections between beams and columns in steel framed structures in fire conditions. In this method, a simple steel connection was split into a number of active components for which mechanical properties are represented by non-linear springs. The behaviour of a steel connection is then determined by assembling the individual behaviour for each active component into a spring model. The component model presented in this paper is capable of predicting the behaviour of steel connections under varied loading conditions. It is also capable of predicting the tying resistance and critical components of failure for steel connections in fire. Compared with experimental test data, a good correlation with the simplified model has been achieved and this method, combined with finite element modelling, may be used to examine the performance of simple steel connections in fire conditions.