Keywords: fire, composite beam, steel, concrete, coupled heat and mass transfer.

This paper presents a fire analysis of steel-concrete composite slabs with interlayer slip. Fire analysis is divided in two reasonable, mathematically independent, phases. In the first phase the temperature field over the cross-section of the composite beam is determined. Based on irreversible thermodynamics the first phase determines the time-dependent change of temperatures of vapour pressure and free water content in the concrete as coupled heat and mass transfer (of free water, a mixture of dry air and water vapour), considering evaporation of free water and liquefaction of water vapour as well as dehydration of chemically bound water. The model proposed by Tenchev et al. [1] is employed. The corresponding system of partial differential equations was solved using the finite element method. In the second part of fire analysis we considered the mechanical behaviour of a steel-concrete composite deck. In order to analyse such a composite structure with sufficient accuracy, a new strain based finite element was developed [2]. The model considers the exact kinematical equations of the beam where axial and flexural deformations are considered while neglecting the shear deformation. Interlayer slip between the concrete slab and steel beam is allowed while delamination is not possible. The non-linear stress-strain relations for steel and concrete at elevated temperatures and the rules for the reduction of material parameters due to an increased temperature are taken from the European standard EC2 [3] and EC 3 [4]. The use of model presented is shown with a numerical example where the composite slab is exposed to standard fire ISO 834. From the results it can be concluded that (i) the steel sheet as a diffusion barrier has a small influence on the distribution of temperatures, while it has a significant influence on the arrangement and magnitude of pore pressure, (ii) to sustain sufficient fire resistance additional reinforcement bars are required in the composite slabs and (iii) the mechanical behaviour of a composite slab with additional reinforcement bars subjected to a fire is similar to the behaviour of a reinforced concrete deck.

1

R.T. Tenchev, L.Y. Li, J.A. Purkiss, "Finite element analysis of coupled heat and moisture transfer in concrete subjected to fire", Num. Heat Transfer Part A, 39, 685-710, 2001. doi:10.1080/10407780152032839

2

T. Hozjan, "Nonlinear analysis of composite planar structures exposed to fire", University of Ljubljana, Faculty of Civil and Geodetic Engineering, Doctoral thesis, 2008 (in Slovene).

3

Eurocode 2, "Design of Concrete Structures, Part 1.2: Structural fire design", European Committee for Standardization, 2004.

4

Eurocode 3, "Design of Steel Structures, Part 1.2: Structural fire design (draft)", European Committee for Standardization, 2004.

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