For the dimensioning of door/window headers to ceiling and wall girders it is favourable if the horizontally running board layers and the narrow sides are bonded. By bonding the narrow sides the horizontal board laminates are firmly attached to each other so that flexibility does not have to be considered. In these cases the dimensioning occurs on a calculated cross section whose dimensions result from the height of the door/window header to ceiling hS and an effective reference thickness dI.
For the effective reference thickness d´ the following applies:
with dy,i: The thickness of the horizontal lamellae joints on the narrow side of the horizontal board layers has the effect that the individual lamellae are not fixed but rather flexibly bonded together.
The flexibility results from the shear deformation of the cross points of the horizontal and vertical board lamellae. The deformation image of the cross points is comparable to that which appears when a wall board is under stress from a horizontal header load in element planes. Accordingly the flexibility and thus the effective shear strength of the door/window header to ceiling drop S can be derived from the panel shear strength Dxy
of the wall elements:
: Panel shear strength
: Height of the door/window header to ceiling
The calculated cross section is presented in the diagrams above. For further calculations the number of boards laid over each other in the horizontal board layers are to be determined. Only full boards may be taken into account. The stress in the board layers is to be calculated with consideration given for the shear deformation, for example, after the shear analogy process.