Modern composite steel frame structures possess a high degree of redundancy. This allows them to survive extreme fires without collapse as there are many alternative loadpaths which can be used to transfer load away from the fire affected part of the structure as demonstrated in the Broadgate fire. Subsequent tests carried out on the Cardington frame showed that it was not necessary to apply fire protection to all steel beams. It was possible to leave selected secondary beams without fire protection. In the event of a fire this results in large deflections due to thermal expansion and material degradation, however, in a fire where servicability requirements do not need to be met this is acceptable so long as life safety is ensured. The weakening beams and large deflections result in a change in the load transfer mechanism with load being carried through tensile membrane action in the slab. This thesis presents a method for calculating the membrane load capacity of composite floor slabs in fire. Extensive numerical modelling at the University of Edinburgh has shown that the temperature distribution through a structural member greatly effects the deflection and pattern of internal stresses and strains. Theoretical solutions were produced to calculate the structural response of laterally restrained beams and plates subject to thermal loads. The theoretical deflections and internal forces were shown to compare well with those from numerical models. To determine the membrane load capacity of concrete floor slabs in fire a three-stage design method was developed. Initially the temperature distribution through the slab was calculated for the design fire. From this the deflection of the slab and resulting stress and strain distributions in the steel reinforcement due to the thermal loads were calculated for the design fire. From this the deflection of the slab and resulting stress and strain distributions in the steel reinforcement due to thermal loads were calculated using equations from the theory developed previously. Failure of the slab was defined based on a limiting value of mechanical strain in the reinforcement, this strain corresponded to a limiting deflection. The load capacity of the slab at the limiting deflection was calculated using an energy method. When compared against results from numerical models the ultimate load capacity was shown to be accurately predicted. None of the fire test carried out on the Cardington structure reached failure. Although demonstrating the inherent strength of such buildings this was also a major short coming as it was not possible to define the point of failure. the design method developed was used to calculate the membrane laod capacity of four of the six Cardington tests. All four tests were shown to have had a significant reserve capacity with none being close to failure.

Over the last decade it has become increasingly clear that the traditional methods of fire safety design can be unnecessarily conservative and therefore expensive. In 1995 a series of fire tests were carried out at Cardington, UK on a full-scale eight storey steel-concrete composite building. These tests produced an extensive body of data about the response of such structures to fire conditions and it is intended that this data be used to develop a clearer understanding of the structural behaviour involved. This thesis presents a method of analysing the behaviour of structures such as the Cardington frame using the commercial finite element package ABAQUS, with the addition of user defined subroutines; applies the method to two of the Cardington tests and analyses the results. FEAST, a suite of computer programmes that defines the behaviour of shell finite elements using a stress-resultant approach, was programmed for use with ABAQUS. The FEAST suite consists of two main programmes. The first, SRAS, is designed to model the behaviour of orthotropic plate sections at elevated temperatures. The second, FEAI, interfaces with the finite element package ABAQUS and allows realistic models of the behaviour of whole structures in fire conditions to be obtained. Phenomena modelled by FEAST include non-linear thermal gradients, non-linear material behaviour and coupling between membrane and bending forces. FEAST was used to analyse the behaviour of the Cardington Restrained Beam Test and the Cardington Corner Test. In both cases it was possible to produce a comprehensive set of results showing the variation of forces, moments and deflections in the structure under fire conditions. In addition, a number of parametric studies were performed to determine the effect of factors such as slab temperature and coefficient of thermal expansion on the behaviour of the structure. Special attention was given to the role of tensile mebrane action. The results showed that the behaviour of the heated structure was very different to that of an unheated structure. The response of the structure was shown to be very strongly governed by restrained thermal expansion and by thermal gradients. Degradation of material properties were found to have only a secondary effect on the structural behaviour.

How well do calculations methods prescribed in today's design codes and standards represent conditions in natural fires? Can the temperature and behavior of a steel member in fire be predicted from these calculations? A literature review of structural fire codes, full scale fire tests, published fire test data, the function and selection of design fires, mechanical and thermal behaviors of structural steel, and numerical calculation methods for the temperature of steel members was conducted as a foundation to analyze whether a not a structural fire engineer can answer these questions. Through comparisons of published data from four natural fires tests performed at the Cardington test facility in the United Kingdom to numerical calculations based upon prescribed methods from Eurocode 3 and the Swedish Design Manual, time-temperature curves were developed to demonstrate the variation in temperature of the recorded data in the natural fire tests at Cardington to the equivalent members being analyzed with numerical calculation methods. When available, fire compartment characteristics were replicated during numerical calculations to ensure the highest correlation between the recorded and calculated results. An Excel tool was created to rapidly calculate and produce the resulting time-temperature curves as well as yield strength, modulus of elasticity, and load carrying capacity using a variety of input parameters including design fire data and steel member selection. The goal of the Cardington fires study was to provide comparisons of published natural fire data to results of numerical calculation methods from the codes. Additional comparisons were developed using a US Office design to show the effects of changing compartment and design parameters on the steel temperature, yield strength, elastic modulus and load carrying capacity. Differences found in temperature of steel members between the published Cardington data and numerical calculations proved the difficulty of predicting the behavior of a structural steel beam throughout an entire length of a fire or even until failure. Discussion of results addressed the selection of design fires, input parameters, structural layouts of office buildings, heating and cooling phases of steel members, and failure criteria.