Axial load in gravity beams

[dcStrucEng]

I'm modeling a floor area in RISA-3D. The floor assembly consists of steel beams supporting a noncomposite concrete on metal deck slab (4" concrete on 3" deck, 7" total). I'm just modeling the beams and inputting the slab as uniform load on the beams. I'm setting the Lb-yy of the beams to approximately 2-ft for those beams perpendicular to the deck span since the deck is welded down to the top flange and I want to take advantage of the slab bracing the beams.

When I run the model, I get axial compression in some of the beams (most of the beams have pinned ends). I'm thinking that in some cases this is due to the system deflecting and forcing axial load into those beams. But in reality won't this axial compression be resisted by the slab (which isn't in the model)? And if so, should I just release the axial boundary condition to avoid putting compression into those beams?

Thanks!

 

[JoshPlumSE]

How much compression are you getting. If it is small and isn't causing problems with your model, then you could just leave it the way it is.

If it is large then you probably need to understand why it is getting into your members. Where it is coming from. It might represent a modeling mistake or a flaw in your system design.

If I had to guess I would say that you may have mistakenly turned on "Top of Member" offsets and the axial loads may be the result of those offsets. But, that is a wild guess.

 

[dcStrucEng]

Thanks Josh. Delving into the model further, the axial loads are occurring at locations where the beams are supporting CMU infill walls. It appears that RISA is connecting the walls and supporting beam and analyzing them as a composite deep beam. So the supporting beam is acting as the bottom flange of the deep beam which takes the axial tension. I reached out to RISA tech support to see if they have recommendations for disconnecting the wall and beam so the beam just takes the gravity load without transfering horizontal shear.

Now you could say: just input the wall as a line load on the beam. But, the top of the wall is supporting other steel beams framing into it perpendicularly, acting as a bearing wall.

 

[jayrod12]

I'd be inclined to tell you to release the axial loads in the members then and see if it performs as you're expecting.