Deflection Limits of Composite Floor Systems 1

[marinaman]

I have a question regarding composite steel framed floors. We have guidance in the code and AISC regarding individual member deflections, but I do not see anything about system deflections. Here’s what I mean:

Let’s say we have a composite floor on a 30’ x 30’ grid. If the beams and girders are both designed to meet L/360 for combined dead and live loads, each would be designed to allow 1” of deflection. Which sounds good initially….but if the beams are 10’ on center, the system deflects 2” at mid span of the middle beam (beam ends go down 1” with the girder, then at beam mid span, there is another 1” of deflection)

This seems too high.

How do you guys deal with this in design? What are your limits?

 

[KootK]

1) I believe that the code deflection limits are only meant to limit member curvature appropriately to avoid damage to any directly attached finishes. As such, I feel that you're correct in your concern for system/absolute deflection in situations where that would matter. And it often does. Glass, infill stud walls...

2) When I look at system deflection, I've usually got a few things working in my favor at the girders:

a) Whenever possible, I make the girder spans shorter than the infill spans. This limits girder deflection via the L^4 business.

b) When I look at girder deflections, I take live load reductions for that. Since the girders collect a lot of load, this helps.

c) For many occupancies, vibration concerns tend to govern girder design. As such, you often have considerably more stiffness in the girders than the L/360 etc.

d) Design for concrete ponding also tends to favor a fairly stiff system.

3) I'm not shy about laying claim to large portions of the dead load being installed prior to the installation of things that might be damaged by deflection.

4) For many occupancies, I'll do my serviceability checks with about 50% of the live load if I feel that to be reasonable given potential consequences etc. As an example, spaces designed for residential occupancy at 40 PSF will, statistically, rarely see more than 6 PSF.

 

[JLNJ]

You do need to be extra careful with unshored composite systems. The temptation is to skinny the beams down as far as possible, resulting in excessive dead load deflection before the concrete cures.

You will need to use your judgment as to whether or not the system DL deflection is a problem for your Owner. You also need to decide if you want the contractor to hold the slab thickness or add DL by bringing the floor up to level with extra concrete (see KootK's concrete ponding note above).

 

[JoshPlumSE]

A agree with what's previously been written by KootK and JLNJ.

One thing to add, however. For 30 ft beams (or longer), it is often good / possible to camber out much of the dead load deflection. That's helped me occasionally where total deflection was an issue.

 

[IceNine]

For Unshored construction, AISC Design Guide 3 has the recommendation that the total accumulated deflection in a bay due to dead load be limited to L/360, not to exceed 1".
So this would be the total of beam + girder deflection. I include 10 to 15% extra load for the additional concrete weight and have them finish the floor level.