I design a combined structure made of timber materials with different creeping parameters. How can I perform the serviceability limit state design according to EN 1995‑1‑1?


In RFEM and RSTAB, the simplified design from [1], Chapter 2.2.3, have been implemented for the automatic load combinations. This means that strictly speaking, only structures concerning the final deformation may be analyzed, in which materials with identical creep behavior occur since the creep deformations are considered in a simplified way on the load side. If the structure is a mixed structure made of wood with different creep properties or in combination with steel, the final deformations must be determined according to [2] Amendment to 2.2.3 as follows:

'(4) If a structure consists of structural components or components with different creep properties, the long-term deformations should be calculated according to (1) due to the quasi-permanent combination of actions with the final values of the mean values of the corresponding elasticity, shear, and displacement modules. The final deformation ufin is then calculated by superposition of the initial deformation due to the difference of the characteristic and quasi-permanent combinations of actions with the long-term deformation.'

However, this requires a superposition of results from different load combinations, which cannot be implemented automatically in RFEM and RSTAB.

If the different creep properties are to be taken into account, the load combinations must be created manually, and the stiffness must be reduced according to the creep coefficient.
The procedure is described using the example of a timber-concrete composite floor presented on the Info Day 2017. Below this FAQ, you can find the link for this.


Creep coefficient Deformation coefficient Creep ratio Long-term deformations Final deformation


[1]   Eurocode 5: Design of timber structures - Part 1-1: General - Common rules and rules for buildings; EN 1995-1-1:2010-12
[2]   Eurocode 5: Design of timber structures - Part 1-1: General - Common rules and rules for buildings; EN 1995-1-1:2004/A2:2014


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