Sloping composite beam 1

[MIStructE_IRE]

I have a roof with a 10 degree pitch. The client now wants it done in concrete filled metal deck supported on steel beams. That’s no problem. I’m wondering though is there anything stopping me going composite here given that its sloped? I feel like I should be considering some effect but can’t figure out what that might be.

Thanks all!

 

[dik]

are the beams vertical or sloped... I usually use vertical beams with slope created by angle sections and not composite.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[MIStructE_IRE]

The beams are sloped as is the metal deck slab above them.

 

[le99]

I don't understand the question "is there anything stopping me going composite?". I should only think of the effects to be considered in selecting a "composite system" rather than a "non-composite" system - it affects beam size, deflection and stability of the slab (sliding).

 

[CELinOttawa]

All that matters is the anchoring.

You may be having a St. Venant's issue, as in: Stroking out between local versus global. Locally the pan must be developing composite action with the concrete, globally the elements must be anchored against sliding...

Sketch?

 

[dik]

Go for it... about 30 years back did a hangar in Lesoto and they sloped the windows to match the roof slope... really weird.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoshPlumSE]

The only thing that is different from a traditional composite beam is that you will have some axial force from the slope. As long as that axial force is pretty low, then I wouldn't think it would affect your design at all.

How low the axial force has to be is a matter of engineering judgement though. I might look at the elastic stresses from the axial load that is imposed on the concrete slab (assuming perfect composite behavior) and make sure that it's less than 10% of the ultimate strength of the concrete. That's just something off the top of my head, though.

 

[Deker]

I don't see any reason why you couldn't consider composite design for a sloped diaphragm. You'll have a gravity demand in addition to lateral seismic/wind demand. Check diaphragm shear, collectors, and shear studs accordingly. This article provides guidance for dealing with the gravity/lateral interaction effects on the shear studs: Link.

 

[KootK]

1) 10 degrees is a pretty paltry slope and, as such, I don't foresee any problems.

2) The steel deck probably needs to nominally do some diaphragming to keep everything from sliding downslope prior to the concrete setting up.

3) There is indeed an axial demand on the studs that would be additive, on one side, to the flexural demand. That said, for a beam that is not a drag strut or chord, I would deem that additional axial demand to be so small relative to the flexural demand as to not warrant explicit checking.

4) For more serious slopes, I'd probably be paying attention to:

a) Is it practical to pour the concrete at such slopes.

b) Has deflection induced concrete ponding been checked using the true, sloped length of the beam.

I feel as thought the "serious slope" limit might be more in the neighborhood of 30 degrees.