The mechanical response of timber changes in a significant way depending on the three directions characterising the material, and this is the reason why it is considered orthotropic. As it is widely known, the major strength is in the same direction of the grain, therefore structural timber elements are mostly exploited in that direction.
The Xlam production process consists in gluing massive wooden boards in crossed layers, therefore the direction of grain rotates 90 degrees for every layer. Can this material be considered orthotropic? And what happens when it is mistakenly laid 90 degrees rotated from what was expected by the project?
In the present paper we will try to answer these questions by analysing a statically and geometrically determined model, in which only the mechanical properties and the orientation of the material will be modified.
By means of the finite elements software Dlubal RFEM, we generated a perfectly rectangular 400×200 cm slab, with supports on both the short sides and on one meter of one of the long sides.
Applying a 3kN/m2 uniform load, we analysed the variation of support reactions and displacements in the events of:
The panel has been modelled as a ‘surface’ element, to whom the 200mm thickness and the C24 orthotropic mechanical features were assigned. The main direction must be parallel to the long sides (y-axis of the global reference), and consequentially the surface’s orientation was correctly rotated 90 degrees from the global reference (as demonstrated by the position of the local x-axis in the centre of the surface).
As expected, the maximum displacement takes place in the middle of the non-supported long side of the slab.
Even the Xlam panel was modelled as a ‘surface’ element, to whom a separately-evaluated stiffness matrix was assigned, containing both geometric data (regarding number and thickness of layers) and mechanical characteristics (for more information about the definition of the matrix, see ‘Xlam: Evaluation of the correct stiffness matrix’). In this case, we modelled a 5-layers panel composed of wooden boards (class C24, thickness 40mm and 0-90-0-90-0 orientation).
Having only three layers parallel to the local x-axis, in this direction the slab is less stiff than case 1. In fact, a slight decrease in support reactions corresponds to a marked increase in displacements (of more than 60%, compared to the homogeneous panel – 2,3 vs 1,4 mm).
On the contrary, having two layers in the same direction as the local y-axis, in that direction the slab is slightly more stiff than the previous one, as proved by the faint increase in support reactions (5,45 vs 5,14 kN/m).
This last example shows the serious consequences of laying a Xlam slab with the main layers perpendicular to the slab’s planned direction. The previous case’s slab was rotated 90 degrees, to have this orientation: 90-0-90-0-90.
As you can see from the following pictures, a slab like this is very unlikely to be verified because, having only two resistant layers in the direction of the long side, displacements in the middle will be much greater than in case 1. or 2. (5,5 vs 2,3 vs 1,4).
Based on the results discussed so far, the following considerations may be made:
Type of panel
Support reaction in x (kN/m)
Support reaction in y (kN/m)