I get different stress values when I analyse an envelope in STATIK and when I do a special analysis with FAGUS using that same envelope. Why?

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On various occasions we have received queries related to the differences in the results that are obtained in STATIK, regarding the stresses, when we obtain them analysing an specific envelope and when we obtain them carrying out an especial analysis with FAGUS using the same envelope.

The differences on the results happen because STATIK, unless we are explicitly doing a non-linear calculation, does a linear calculation for the behaviour of the material, so it is not affected by the behaviour of the concrete in tension that we have entered in the analysis parameters. This means that it will take the elastic modulus entered for the material into account but not if, for example, we decide that material does not work in tension. FAGUS, however, will do an analysis of each cross section of the bar and it will do it considering the behaviour that we indicate for the material that each section is made of.

If we enter a doubly-supported beam with rectangular cross section (to clearly see the linear behaviour) in STATIK, subjected to its self-weight:

- ENVELOPE INCLUDING ONLY SELF-WEIGHT (SW): Linear behaviour -> we must have equal stresses on both the top and the bottom fibre (because it is a symmetric cross section with respect to its C.O.G, rectangular in this case)

We must point out the fact that, despite what we could intuitively expect, the first diagram represents the tensile stresses (hence, positive) of the bottom face while the second one represents the compressive stresses (negative) of the top face.

- ESPECIAL ANALYSIS WITH FAGUS FROM THE PREVIOUS ENVELOPE: Different defined behaviour for compression and tension -> we must have different stresses on the top and bottom faces.

 

If we take any intermediate cross section of the bar, we will be able to see the cross sectional analysis that FAGUS carries out. This analysis is the one done for each of the cross sections the bar is made of. In this case we are going to take the central cross section to see the same values that are shown on the figure above: