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?