Modeling Downstand Beam in Timber Structures 1: Torsion

Tips & Tricks

Modern buildings are designed with spaces tailored to personal wishes and dreams, expressing individual lifestyles. These requirements often include ceilings - whether in houses, office buildings or public buildings - that have an enormous span and no support, allowing for an optimal use of the space below.

However, this requires a very high stability level for the load‑bearing capacity and serviceability reasons. By extending the size of beam or plate cross‑sections, you can increase the stability, but the cost effectiveness decreases because of the additional consumption of material. A common solution for the large spans is to use timber or steel downstand beams.

To prevent transferring moments of the ceiling structure on the downstand beam, you can define ‘scissors releases’ in case of using ceiling beams. In order to consider the same effect also in the case of a flat ceiling (for example cross‑laminated timber), you can use the ‘Line Release’ option.

Using this function, it is possible to ‘release’ the φx degree of freedom, whereby the downstand beam is connected rigidly about the local x‑axis to the surface. In this way, you can represent the continuous effect of the surface very easily (without using coupling members).

The second part of this post will describe a semi‑rigid shear coupling between the surface and the downstand beam according to American standard ANSI/AWC NDS and Eurocode 5.

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RFEM 5.xx

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Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements