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

Influence of ply configuration and adhesive type on cross-laminated timber in flexure at elevated temperatures

This paper describes experiments on cross-laminated timber (CLT) beams exposed to uniform non-charring temperatures under sustained loading. Two different ply configurations and two different adhesive types were examined under sustained loads of both 30 and 50% of the ultimate ambient temperature flexural capacity. It was found that the adhesive type has a significant influence on the magnitude of the deterioration in structural stiffness during heating. From image correlation analysis this influence was attributed to increased shear strains along the adhesive lines between timber plies for specimens bonded with a polyurethane (PU) adhesive, when compared to those that used a melamine urea formaldehyde (MF) adhesive. It was also found that considerable deflections that were measured during heating were irrecoverable during cooling of the CLT, suggesting that these deformations were driven by creep of the timber – and possibly also the adhesives

Predictive testing for heat induced spalling of concrete tunnels – The influence of mechanical loading

This paper describes Phase II of a project being undertaken to develop a predictive test method to investigate heat-induced explosive spalling of concrete, with a specific focus on concrete used in tunneling applications (but obviously applicable to other applications). The test method seeks to allow careful control of the thermal and mechanical transient conditions influencing the occurrence of heat-induced concrete spalling, thus enabling convenient, representative, repeatable, and comparable testing to be carried out on various concrete mixes under various potentially relevant conditions. Phase I of the project focused on establishing suitable thermal exposures to use for testing based on the thermal exposures which a sample would be exposed to during a standard furnace test (cellulosic or modified hydrocarbon) in the Promethee testing facility at CERIB in France. The work described in this paper deals with establishing suitable mechanical loading conditions for a spalling test, the focus in the current work is to enable provision of a representative test for precast segmental concrete tunnel linings (as opposed to sprayed or cut-and-cover tunnel linings). With small adaptations the spalling test method could be adjusted to suit other applications. This paper focuses on the motivation for developing the testing method and outlines the testing to be carried out. Tests are currently underway, and the full suite of results will be presented at the conference

BRE large compartment fire tests – characterising post-flashover fires for model validation

Reliable and comprehensive measurement data from large-scale fire tests is needed for validation of computer fire models, but is subject to various uncertainties, including radiation errors in temperature measurement. Here, a simple method for post-processing thermocouple data is demonstrated, within the scope of a series of large-scale fire tests, in order to establish a well characterised dataset of physical parameter values which can be used with confidence in model validation. Sensitivity analyses reveal the relationship of the correction uncertainty to the assumed optical properties and the thermocouple distribution. The analysis also facilitates the generation of maps of an equivalent radiative flux within the fire compartment, a quantity which usefully characterises the thermal exposures of structural components. Large spatial and temporal variations are found, with regions of most severe exposures not being collocated with the peak gas temperatures; this picture is at variance with the assumption of uniform heating conditions often adopted for post-flashover fires

Structural capacity in fire of laminated timber elements in compartments with exposed timber surfaces

In compartment fires with boundaries consisting of exposed mass timber surfaces – for example in compartments with exposed cross-laminated timber (CLT) walls or floors – the thermal penetration depth, i.e. the depth of timber heated to temperatures significantly above ambient behind the char-timber interface, during fire exposure may have a significant influence on the load bearing capacity of structural mass timber buildings, particularly in the decay phase of a real fire. This paper presents in-depth timber temperature measurements obtained during a series of full-scale fire experiments in compartments with partially exposed CLT boundaries, including decay phases. During experiments in which the timber surfaces achieved auto-extinction after consumption of the compartment fuel load, the thermal penetration depth continued to increase for more than one hour, whilst the progression of the in-depth charring front effectively halted at extinction. A simple calculation model is presented to demonstrate that this ongoing progression of thermal penetration continues to reduce the structural load bearing capacity of the CLT elements, thereby increasing the potential for structural collapse during the decay phase of the fire. This issue is considered to be most important for timber compression elements. Currently utilised structural fire design methods for mass timber generally assume a fixed ‘zero strength layer’ depth to account for thermally affected timber behind the char line; however they make no explicit attempt to account for these decay-phase effects

Effects of exposed cross laminated timber on compartment fire dynamics

A series of compartment fire experiments has been undertaken to evaluate the impact of combustible cross laminated timber linings on the compartment fire behaviour. Compartment heat release rates and temperatures are reported for three configuration of exposed timber surfaces. Auto-extinction of the compartment was observed in one case but this was not observed when the experiment was repeated under identical condition. This highlights the strong interaction between the exposed combustible material and the resulting fire dynamics. For large areas of exposed timber linings heat transfer within the compartment dominates and prevents auto-extinction. A framework is presented based on the relative durations of the thermal penetration time of a timber layer and compartment fire duration to account for the observed differences in fire dynamics. This analysis shows that fall-off of the charred timber layers is a key contributor to whether auto-extinction can be achieved

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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo