The implications of compartment fire non-uniformity for the membrane action of reinforced concrete slabs
Maintaining structural stability is an integral component of building fire safety. Stability must be ensured to provide adequate time for safe egress of the buildings occupants, fire fighting operations and property protection. Structural fire engineering endeavours to design structures to withstand the effects of fire in order to achieve this objective. The behaviour of reinforced concrete in fire is not as well understood as other construction materials, such as steel. This is in part due to the complexity of concrete material behaviour and also due to concrete’s reputation of superior fire performance. Concrete technology is, however, continually evolving; structures are increasingly slender, more highly stressed and have higher compressive strengths. A more robust understanding of concrete’s behaviour in fire will enable predictions of the implications of changing concrete technology and also help to properly quantify the fire safety risk associated with concrete structures. A fundamental key to understanding structural fire performance is the relationship between the thermal environment induced by the fire and the structure. Significant thermal variation has been found experimentally to exist within fire compartments. Despite this the design of structures for fire almost universally assumes the compartment thermal environment to be homogeneous. In this thesis the implications of compartment fire non-uniformity for concrete structural behaviour is investigated to assess the validity of the uniform compartment temperature assumption. The investigation is conducted using numerical tools; a detailed review of the necessary background knowledge, material modelling of reinforced concrete, finite element modelling of reinforced concrete structures and compartment fire thermal variation is included. The behaviour of a two-way spanning reinforced concrete slab is used as a structural benchmark. The membrane behaviour exhibited by two-way spanning RC slabs at high temperatures has been previously studied under uniform thermal conditions. They therefore are an ideal benchmark for identifying the influence of non-uniform thermal environments for behaviour. The relationship between gas phase temperature variation and concrete thermal expansion behaviour, which is fundamental to understanding concrete high temperature structural behaviour, is first investigated. These preliminary studies provide the necessary fundamental understanding to identify the influence of gas phase temperature variation upon the membrane behaviour of reinforced concrete slabs. The individual influences of spatial and temporal variation upon slab membrane behaviour are investigated and the behaviour under non-uniform thermal variation contrasted with uniform thermal exposure behaviour. The influence of spatial variation of temperature is found to be strongly dependent upon the structural slenderness ratio. The tensile membrane action of slender slabs is particularly susceptible to the distorted slab deflection profiles induced by spatial variation of gas temperature. Conversely the compressive membrane behaviour of stocky slabs is found to be insensitive to the deformation effects induced by spatial variation of temperature. The influence upon slender slabs is demonstrated under a range of temporal variations indicating that the thermal response of concrete is sufficiently fast to be sensitive to realistically varying distributions of temperature. Contrasting behaviour induced by uniform and non-uniform thermal exposures indicates that uniform temperature assumptions provide both conservative and unconservative predictions of behaviour. The accuracy of the uniform temperature assumptions was also found to be dependent upon the type of fire, for example, fast hot and short cool fires. Additionally, the sensitivity of structural performance to deformations caused by spatial variation of temperature demonstrated in this thesis challenges the purely strength based focus of traditional structural fire engineering. Spalling is an important feature of concrete’s high temperature behaviour which is not currently explicitly addressed in design. The incorporation of spalling into structural analysis is not, however, straightforward. The influence of spalling upon behaviour has therefore been dealt with separately. A spalling design framework is developed to incorporate the effects of spalling into a structural analysis. Application of the framework to case studies demonstrates the potential for spalling to critically undermine the structural performance of concrete in fire. It also demonstrates how the framework can be used to quantify the effects of spalling and therefore account for these in the structural fire design addressing spalling risk in a rational manner.