I have used mixed mesh to put a gusset plate meshed with solid elements between beams but it seems that SW cannot capture the affect of it. As can be seen from photo 1, the bending stress is same as my hand calculation. Then I changed the plate gusset to hollow section so that the whole model can be meshed using beam elements (refer photo 2 & 3). But the results came up the same. What's wrong?. Thanks.

(NB: The load is applied to the face of the under-run bar beam)