Technical>Static

Wood slabs and vibrations – Sizing criteria

What are the sizing criteria for a slab made of timber? Is it sufficient to consider only the stresses (bending moment and shear) and the deformations (instantaneous and at infinite time)?

The answer to this question is simply “no”. Especially on important span (over 4m) the most important criterion to be respected during the sizing phase is vibrations which belongs to SLS or Serviceability Limit States.

A problem on vibrations does not lead to collapse or to critical situations such as an excessive deformation. Its consequences concern rather a very delicate issue related to the “well-being” of the occupants who will perceive certain frequencies as annoying. It is evident that the criterion is extremely subjective and therefore the limit values ​​required by the various regulations derive from statistical surveys on samples of people.

Eurocode 5 in paragraph 7.3 shows the method for the correct sizing for a very simple but equally frequent case: single span and uniform load distributed over the length. How should we behave, however, if these conditions are not met? Furthermore: are the limits proposed by Eurocode 5 sufficient? Ergodomus relies on a finite element solver that allows the calculation of the eigenmodes with considerable precision so as to allow the correct sizing of the slab while optimizing the sections at the same time. We also use Austrian and German regulations that are much more complete for this specific area as an additional regulatory reference.
The following images show some examples for a single and double span floor.

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