A992 Gr 50 vs A570 Gr 50

I am modelling a very long lintel beam carrying a low roof, masonry, and a higher roof.  Roof structure is composed of open-web joists.  The low roof joists will be seated on the bottom flange.  To model torsion induced by the slight eccentricity of the joist seats I am using a very short plate of the flange thickness, offset by rigid links to the elevation of the bottom beam flange.  All of this works great and I get good, reasonable results with expected torsional moments, vertical and horizontal deflections.  My problem arises when I use the very same W-shape in either A992-50 or A572-50 steel.  The A992 shape results in a 2% overstress (102% usage), the A572-50 shows a max usage of 54%, with torsion governing the result.  Even A-36 has a max usage of 91%.

I am using the materials supplied with the program, not custom materials.  The only rational explanation I can find for the result is the Poisson ratio for A992 is shown at 0.3, and 0.26 for the other steels.  Is this accurate?

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