Colin Bailey

Membrane action of unrestrained lightly reinforced concrete slabs at large displacements

Following full-scale fire tests on a steel-framed building, together with observations from real fires, it has been shown that membrane action, at large displacements, of composite floors comprising steel deck, concrete, and anti-crack mesh, is extremely beneficial to the survival of the building. It was therefore decided to review previous research conducted on unrestrained concrete slabs, under large displacements, at normal temperatures. It was found that the assumptions used to develop previous theoretical predictions for the load-carrying capacity, for a given vertical displacement, are only valid for square slabs and do not conform to test observations for rectangular slabs. A new theoretical approach is therefore presented which is valid for both square and rectangular slabs and conforms to the mode of behaviour observed in tests. The design method is shown to give excellent correlation with published test data. A prediction for ultimate collapse of the slab due to fracture of the reinforcement is also presented, which limits the allowable mechanical strain in the reinforcement. Comparison with available test data shows that this prediction is always conservative.

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 tensile membrane action of unrestrained composite slabs simulated under fire conditions

Abstract Following a major fire test programme on a full-scale, steel-framed building it was found that the composite flooring system, comprising lightweight concrete, anti-crack mesh reinforcement and steel deck, had a greater inherent fire resistance than suggested by current codified design methods. It was felt that this was due to tensile membrane action occurring in the slab at large displacements. This led to an independent test being conducted at the Building Research Establishment where a 9.5 m×6.5 m composite slab, with nominal horizontal restraint to its edges, was tested to failure. To simulate the behaviour of the slab in fire, the steel deck was removed, leaving the concrete and anti-crack reinforcement, before load was applied. Tensile membrane action was shown to occur, with the failure load being approximately double that calculated using the classic yield line theory.

Simplified and advanced analysis of membrane action of concrete slabs

When assessing structures under accidental loads, it is important to understand the membrane behavior of concrete slabs at large displacements. This study compares a simple analytical approach based on rigid-plastic behavior with a change of geometry, an advanced finite element model (FEM), and 14 tests on horizontally-unrestrained concrete slabs that reached vertical displacements up to 10 times the effective depth of the slab. Findings show that both analytical approaches predicted the membrane behavior of the slabs, comprising compressive membrane action around the slab’s perimeter and tensile membrane action in the central span region of the slab. The simple approach produced good predictions of the load-displacement response toward the end of the test, while the FEM produced reasonable predictions over the full history of the test. By considering the magnitude and pattern of the stresses within the FEM, the assumptions adopted within the simple approach were investigated and are discussed. The overall findings suggest that the simple approach can safely be used for predicting the load-carrying capacity, due to membrane action, of concrete slabs under large displacements.

Development of computer software to simulate the structural behaviour of steel-framed buildings in fire

Abstract This paper describes the development of computer software capable of simulating the structural response of steel-framed buildings in fire. The first part of the paper deals with the basic non-linear formulation of the program, which allows the three-dimensional response of steel members at elevated temperatures to be modelled. Recent extensions of the software, which allow the modelling of semi-rigid connections, lateral-torsional buckling, continuous floor slabs and strain reversal, are then discussed. Wherever possible, validation against test data or other independent computer software has been reported, together with illustrative examples.

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.

Efficient arrangement of reinforcement for membrane behaviour of composite floor slabs in fire conditions

Abstract At large displacements concrete floor slabs can support, by membrane action, a vertical applied load which is significantly larger than that calculated assuming simple flexural behaviour. In an accidental limit state, such as a building fire, membrane action at large displacements can be beneficial to the survival of composite floor slabs used in steel-framed buildings. By utilising membrane action significant cost savings can be achieved by identifying a large number of steel beams, which would have required applied fire protection using previous design methods, to be left exposed to the high temperatures experienced during a fire. This paper extends a previous derivation of a design method, which predicted the membrane load-carrying capacity of composite floor slabs in fire, but was limited to isotropic reinforcement. The extension to the method allows the specification of orthotropic reinforcement, presenting the designer with the tools to specify the most economical arrangement of reinforcement in the floor slab. It is shown, in the fire design example presented in this paper, that for a given area of reinforcement the membrane load-carrying capacity of a rectangular floor slab with an aspect ratio of 2 can be increased by 23% by placing more reinforcement in the longer span.

Changes in the stress in the femoral head neck junction after osteochondroplasty for hip impingement: A finite element study

The surgical treatment of femoroacetabular impingement (FAI) often involves femoral osteochondroplasty. One risk of this procedure is fracture of the femoral neck. We developed a finite element (FE) model to investigate the relationship between depth of resection and femoral neck stress. CT data were used to obtain the geometry of a typical cam‐type hip, and a 3D FE model was constructed to predict stress in the head–neck after resection surgery. The model accounted for the forces acting on the head and abductor muscular forces. Bone resection was performed virtually to incremental resection depths. The stresses were calculated for five resection depths and for five different activities (i) standing on one leg (static case); (ii) two‐to‐one‐to‐two leg standing; (iii) normal walking; (iv) walking down stairs; and (v) a knee bend. In general, both the average Von Mises stresses and the area of bone that yielded significantly increased at a resection depth of ≥10 mm. The knee bend and walking down stairs demonstrated the highest stresses. The FE model predicts that fracture is likely to occur in the resection area first following removal of a third (10 mm) or more of the diameter of the femoral neck. We suggest that when surgeons perform osteochondroplasty for hip impingement, the depth of resection should be limited to 10 mm.

The behaviour of asymmetric slim floor steel beams in fire

Abstract Computer software has been developed to predict the structural response of asymmetric slim floor steel beams, used with composite concrete floor slabs consisting of deep profiled steel decking. Comparisons between predicted behaviour and that recorded in standard fire tests, showed that the software is very accurate. By including the rotational stiffness of the beam-to-column connections, the fire resistance of the beam is significantly enhanced. This is mainly due to the connections retaining most of their strength during a fire, since they are fully encased in concrete as a consequence of this type of construction. The analyses presented in this paper indicate that it may be possible to increase the fire resistance of the steel beams from 60 to 90 minutes, by including the connection behaviour. The software has also been used to aid the design of a future large-scale fire test on the asymmetric slim floor system. Predictions of the structural response have been presented. These will enable the fire load and ventilation conditions in the test to be designed. In addition the software has been used to identify the minimum amount of fire protection that is required for the supporting columns.

The influence of the thermal expansion of beams on the structural behaviour of columns in steel-framed structures during a fire

Abstract In the UK, new design guidance is currently being developed for the behaviour of steel-framed buildings when subjected to fire. This is primarily based on recent research that considers the structural behaviour of all horizontal members, without applied fire protection, acting as a complete entity within the building. This guidance assumes that columns designed to current design procedures will always be adequate when used within this new design philosophy. For bare steel columns these existing design methods usually consist of applying some form of passive fire protection. Presented in this paper is an analytical investigation of the structural behaviour of columns when subjected to various structural and fire scenarios. The results from this study do not endorse the view that current fire design methods for columns are adequate. These design methods will require revision if instability of columns during a fire is to be avoided. In most cases this will result in the need for additional passive fire protection to be applied to the steel columns.