Modeling Offsets

[HZStruct]

Hello,

I am modeling an existing, large storage structure with precast concrete columns, steel girders, steel purlins, and metal decking. I'm focused on modeling the steel framing members to determine member forces. The girders are fastened to the top of the precast concrete columns and the purlins are fastened to the top of the girders. I have run two models; one with member offsets and one without. The offsets are between the girder and top of column and between the purlin and top of girder. The model with offsets results in large axial force and smaller bending moments and the model without offsets has smaller axial forces and larger bending moments.

Is it accurate to model the roof framing without offsets between the purlins and girders? Is the change in results stated above to be expected when considering offsets in the vertical direction?

Thank you!

 

[STpipe]

That makes sense based on what you described. With the offsets you're getting eccentric moments at your member ends. Do you have screenshots/sketches of what you're trying to analyze?

You can see a good description of that effect in the link below:

https://wiki.csiamerica.com/display/tp/Effect+of+insertion+point+on+beam+reactions