| Product(s): | SACS |
| Version(s): | All. |
| Area: | Post Proccesor |
Community Question:
Hello, Bentley Why does the torsional rotation/deflection in SACS/STAAD show higher values compared to an FEA model in Ansys?
I have a model Sacs model in which I have a deflection of 72 inches and Staad 72 inches, but if I compare the deflection against what I get by the FEA model of 14 inches this doesn’t Add up. Which of these two deflections is wrong?
Next is resented the result using STAAD / SACS
Next is presented the Ansys FEM Result
Solution:
The origin of the difference in the results can be expressed with the following.
Sacs / STAAD make a linear analysis using bar elements (Linear Analysis - without warp effect) with which we analyze a system that is directly proportional to the type of loads and deformations in nodes, while a finite element analysis (using solids) considers the rigidity of the system and whit this, the warp effect. The last one is who defines the difference between these two types of analysis, When we talk about rigidity we mean the characteristics of the system such as shape, material, connections, and supports. That is why we cannot reach the same result.
On another hand when you use ANSYS whichever platform you prefer APDL or Workbench, you need to be sure of the kind of element to use for this comparison. If your intention is to compare the result from Staad / Sacs (without warp effect) against Ansys FEA, you need to select the type of element beam188 or beam189 and work with elements bars, This is the correct form to do it.
Below are the results of this comparison using this element without considering the warp effect
Displacement = 1941 mm = 76.41 inches
As can you see, when you select the adequate type of element the displacement matches with the results.
On the other hand, if your intention is to compare the deflection considering the warping effect, unfortunately, this is not possible using SACS, but you can consider this effect in "STAAD" by including the card set warp, this helps to get the same deflection in the FEM model using solid element and beam element.
As a SUm up, Sacs and Staad consider pure torsional (St Venant's) resistance but ignore warping resistance while a 3D model using solid elements in ansys takes into account the torsional and warping resistance effects
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