The numerical modelling of composite floors exposed to fire

Nama, S. (2008). The numerical modelling of composite floors exposed to fire. (Unpublished Doctoral thesis, City University London)

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Abstract

This thesis deals with the influence of fire on the behaviour of steel concrete composite floors. A theory has been developed to calculate deflections during the fire and the ultimate strength ofthe composite floor under such conditions. The solution is based on the finite difference method. It takes temperature-dependent material properties into account. The method of analysis comprises two parts; the first is thermal analysis, enabling temperatures to be calculated as a fu~ction 'of fire exposure time. The second is strength analysis for calculating the strength of composite floor with material properties affected by temperature. For the heat flow analysis, the cross-section is divided into mainly rectangular elements. Sloping boundaries are approximated by triangular elements. The heat transfer from the fire to the surface is considered as well as heat conduction to the neighbouring points. At internal points, heat conduction to all the neighbouring points is considered. To calculate the deflections, the floor is divided into a two-dimensional mesh. The deflections are calculated for each mesh point based on orthotropic plate theory. The differential operators are replaced by the finite difference formulae. This reduces the governing differential equation into a system of linear algebraic equations. To calculate the plate rigidities, it is necessary to find curvatures for all mesh points in the two planes using finite difference operators. The thermal strains , are superimposed, on the mechanical strains associated with curvatures to finp the net strains, and then stresses are calculated using the non-linear temperature dependent stress-strain curves. Integrating the stresses, the internal stress resultants are calculated. The above method has been programmed in Visual Basic. To validate this method, a comparison with a number of fire tests has been carried out , for both thermal and mechanical behaviour. The temperatures at comparable points are generally close to each other. Comparisons have also been carried out for calculated mid-span deflections by this method and the published test results. The results show excellent correlation between the tests and the new method. A parametric study has carried out on floors with different boundary conditions when subjected to in-plane forces for two fixed and simple ends. Comparison of mid-span deflections between the fixed and simple end conditions has shown that fixed edges have better fire resistance than simply supported when not subjected to in-plane forces. It has found that in-plane forces had little effect on deflection rates at initial stages of the fire. These only appeared at later stages. When subjected to in-plane forces in one direction only the floor showed better response. The conclusion from the parametric study is that in-plane forces at different edges play a signific'ant role in the behaviour, as the surrounding structure provides restraint increasing the fire resistance ofthe structure within the fire compartment

Item Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Divisions: School of Engineering & Mathematical Sciences
URI: http://openaccess.city.ac.uk/id/eprint/8590

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