Modelling the structural response of reinforced concrete slabs exposed to fire: validation, sensitivity, and consequences for analysis and design

Abstract

Structural fire design represents one important aspect of the design of reinforced concrete buildings. The work presented in this thesis seeks to elucidate the structural behaviour of reinforced concrete slabs during exposure to heating from below, as would occur in the case of a building fire, with a particular focus on structural fire modelling using finite element analysis. The focus in on validating finite element models against experimental results and quantifying the sensitivity of model outputs to relevant thermal and mechanical input parameters. A primary goal of the work is to provide recommendations to structural fire engineering analysts and designers considering the performance-based design of reinforced concrete slabs for structural fire resistance using available finite element software. A critical review of the available knowledge of the structural fire response of reinforced concrete structures in general and concrete slabs in particular is presented, along with an awareness as to the importance of understanding structural response of concrete structures exposed to fires. Current techniques for structural fire design of concrete structures are reviewed, and shortcomings highlighted. Available experimental data are presented, and various finite element models of these slabs are developed and interrogated to identify important aspects for understanding, as well as for future improvement of similar studies (both experimental and numerical) with the intention of supporting future progress in structural fire engineering, in particular as regards performance based structural fire design of concrete slabs. A range of thermal and mechanical parameters that are potentially important and influential in the structural fire design of reinforced concrete slabs is then studied, including: fire scenario, thermal properties of materials (thermal conductivity and specific heat), heat transfer parameters (coefficient of convection and emissivity) and assumptions, restraint conditions at the supports, variations of span-to-depth ratio, reinforcement detailing, as well as plan aspect ratio are all investigated; their influence on the structural fire response of reinforced concrete slabs is studied and discussed. A key issue in validating finite element models against experimental results lies in defining the temperature inputs to the structural finite element models correctly. Variation of available thermal and mechanical input parameters, as recommended in Eurocodes, influences the predictive performance of thermal and structural finite element models, however these are not the main contributing factors in obtaining a credible prediction of response from the finite element models. The most challenging aspect in performing heat transfer analysis for fire furnace tested reinforced concrete slabs lies in defining the correct thermal boundary condition. For simply supported one-way spanning and two-way spanning slabs, increasing slab’s thickness (lowering span-depth ratio) does not improve fire resistance rating for the slabs when both limiting deflection criteria and limiting tensile plastic strain are set as acceptance criteria. Two-way slabs with higher span-depth ratio have better fire resistance ratings, judging from the overall trends and magnitudes of mid-span deflections. The formation of plastic hinges is likely to occur for one-way spanning slabs modelled with finite rotational spring stiffness at supports, but not for two-way spanning slabs. A yield line mechanism in two-way slabs means that the behaviour is more complex as compared to the simple flexural mechanism for one-way slabs. In one-way slabs, plastic hinges potentially occur at the location where top reinforcement is curtailed, highlighting the importance of properly understanding the nuances in response of concrete slabs in fire. Investigation of the influence of aspect ratio in two-way spanning slabs confirms that slabs with lower aspect ratios have better structural fire resistance than slabs with higher aspect ratios when both limiting deflection criteria and limiting tensile strain in reinforcing steel were used as the performance indicators. A combination of both limiting mid-span deflection criteria as well as limiting tensile plastic strain is recommended for specifying acceptance criteria for both one-way and two-way slabs, since it gives more accurate and comprehensive assessment on the structural response of the slabs under exposure to severe heating from below.