Rapportnummer Trätek I 8808057
In Sweden, the loadbearing walls of single family houses must at present be designed with respect to fire i n order to provide the necessary degree of safety against the spread of fire. One of the preconditions in order that the fire resistance of an external wall should be secured is that its loadbearing capacity should be maintained.
With the aim of studying the structural behaviour of wood studs, a test series was carried out on axially loaded wood studs. I n these tests , the effect of fire on the wood studs was simulated by removing layers from the stud by planing it on the side which was supposed to be exposed to fire. Different support conditions, such as the use of cellular rubber sole plate sealing strips and inclined base or inclined roof truss rafter, were studied. Six of the specimens consisted of solid 45 x 120 mm timber studs, and two were light weight studs comprising timber flanges and webs of wood fibre board. In all cases the studs were joined by short pieces of timber corresponding to the sole plate and top plate. During the tests, the specimens were placed between rigid, non rotating support plates, the intention being to reproduce conditions in a single family house construction.
In the theoretical part of the investigation, two analytical models are studied. In one of the models it is assumed that the ends of the stud are pinjointed. The structural behaviour cannot be described satisfactorily by this model. In the other analytical model, the stud is assumed to be placed between rigid end plates as in the tests. The end surfaces of the stud are idealised as cylindrical convex surfaces, enabling a rolling motion to take place as loading and deformations proceed. Theoretical solutions are derived for different loading cases and boundary conditions. In order that it should be possible to apply these, ideal radii are determined with the aid of the test results to describe the geometrical shape of the end surfaces. An approximate expression for the ideal radius is formulated in order that it may also be used for other cross sections and lengths.
With the aid of parametric studies, the influence due to inclination of the base and the interaction between axial force and transverse load is studied. The results show that the two load components can be superimposed by applying linear interaction.
Fire tests can thus be carried out separately for axial and transverse loading. By determining the loadbearing capacity for a certain fire resistance period, it will be possible to design a timber stud under fire exposure conditions for arbitrary loads and load combinations.
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