I am trying to calculate the torsion in a rectangular bar.

I have found closed form calculations in Shigley, Roark, and a NZ structural code which all agree (within 10% or less).

However, the solid works simulation of the same geometry is higher:

b = 4"

t = 1"

L = 3"

T = 10,000 in*lbf

Tau (Shigley) = 8.63 ksi

Tau (Roark) = 8.63 ksi

Tau (NZS 4404) ~9 ksi

Tau (FEA) = 11.65 ksi

The FEA results are 35% higher.

Has anyone every experienced this

Results are attached below: