headed studs and composite beam

[a2mfk]

If you have a composite beam with 2" of camber, your total deck and slab thickness is 6-1/2", your headed stud installed height is 5", you have 1-1/2" clear distance when the beam is level. But prior to deck and slab installation, the HS will theoretically be projecting 1/2" above the finished floor elevation near the center of the span due to the beam camber (5" stud + 2" camber = 7" > 6.5" slab thickness).

We have a calc'd slab weight deflection of @ 3/4"-1", so by the time they are finishing the concrete the beams will have deflected to where the top of the studs will be 1/2" or more below the T.O.S.

Its been a while but this all sounds OK to me.

Thoughts?

 

[WillisV]

You should generally only camber about 80% of the dead load to avoid just such an issue (the possibility of heads poking out of the concrete). This issue was somewhat clarified in the 2010 AISC Specification (I3.2c(1)(2)) by noting that there shall be at least 1/2 in. of specified (emphasis added) cover over the top of the headed steel anchor. The "specified" term added as there have been lots of questions over what happens if this distance ends up being more like 1/4" or close to zero. Obviously if the head pokes through it is not acceptable, but if it does not, this language indicates that less than 1/2" of cover in the final condition is acceptable, though I would certainly give my best shot at not ending up there through better detailing (thicker slab) or reduced camber.

 

[a2mfk]

Willis- thanks a lot. 80% of the dead load anticipated to be in place when the slab is being placed would just be the slab weight, beam weight, and deck weight?

I think we may back the camber off to 1-1/2" to be safe then.

Thanks!

 

[a2mfk]

OK, its fine tuned a little bit. We have a 2.2" deflection using the beam strength only in non-composite, for slab and deck weight, plus beam self weight. When you specify 2" of camber, I have understood you can anticipate some camber loss just from handling and shipping. So I think 2" is looking pretty good.

80% of slab and self weight = 1.79".

So if we start with the studs 1/2" above the theoretical level FF, once there is a couple of inches of concrete on the deck they will be below the T.O. slab.

 

[WillisV]

I would tend to call that 1.5" instead of 2", but its up to you. That is a fairly flexible beam you have, double check vibrations etc.

 

[a2mfk]

Thanks Willis!

 

[hokie66]

Dead load deflections of steel members can be calculated fairly accurately. With members that are going to deflect a lot, you can require the slab to be cast to a constant thickness rather than level.