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Staad Pro coloured stress plots

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noratetoobig

Structural
Jul 7, 2010
18
I have recently created a 3d stiffness analysis worksheet in Mathcad (It includes member end releases, member rotations includes 3d manipulative graphics dispaly which also shows memnets and shears for selected members) and have been checking it with staad pro output. The results from my mathcad worksheet are near identical, if not exactly the same as the staad output (generally any differences are attributable to round off with respect to member properties - staad appears to sometimes slightly differing results depending on what method is used to extract properties data).

My problem is with respect to the sectional member stress plot. In many instances the plot 4 corner stress points do not match my mathcad results. Of course this could be considered a failure on my programs ability to calculate the correct stresses, however. Upon further investigation, it would seem Staad Pro is using incorrect forces at the section for the stress calculations. What I have found is; if I print out the section forces and stresses using the staad file list output, they are exactly the same as my program and give the exact 4 point stresses when combined. If I select the 4 points directly on the stress plot and list them on the Profile Stress Plot tab, what I have found is that the listed forces attributable to the calculated stresses are not correct and are for different sections than that selected (both MX and MY appear to be at different locations). The plot output also appears to be different to the forces shown when double clicking on the member in question and selecting the Shear Bending tab and imputing the section at which forces are required making sure the correct load comb is selected). at the section in question.

1, Has anyone noticed/experienced this as well.
2. Does anyone have an explanation apart from the Staad program using incorrect section forces ?

I include a pdf which shows what I am referring to.
 
 https://files.engineering.com/getfile.aspx?folder=5cb97fb8-be99-43e6-89d8-2f3ccd6d10ee&file=Staad_Output_Member_15.pdf
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