ponding allowance for concrete on metal deck

[bjb]

what kind of allowance should be made for additional concrete as the metal deck deflects when the concrete is poured? The kind of floor I'm talking about has steel beams, metal floor deck and concrete fill. It's been a while, but I seem to remember allowing for an additional 5 psf of dead load due to ponding concrete.

 

[JedClampett]

The metal deck catalogue should give maximum spans for the type and gauge of deck to prevent the wet concrete from deflecting the deck at all (or very much). If it doesn't meet the criteria, you need to shore the deck.

 

[bjb]

I don't have a catalog in front of me (at home), but I always make sure my deck spans are below the maximum unshored construction spans. There has to be some deflection in response to the applied load, and some of the previous firms I worked for made some kind of allowance for it. Adding another 5 psf for design of the filler beams is about 1/2" of extra concrete over the fuill span, which is probably conservative.

 

[Taro]

I always add additional dead load (and seismic mass) to account for the deck deflection. One deck catalog on my shelf states that the construction deflection limit without shoring is L/180 or 3/4". Another catalog states that a 4 psf deflection allowance is used. 5 psf seems like a good number to use.

 

[tweedledee]

I allways add the weight of an additional 1/2" of concrete to allow for this.

 

[bjb]

Thanks for the replies.

5 psf is what I remember, but I was curious as to what others did. Most of my multi-story buildings have been designed with wood for a long time now, and I've gotten a little rusty (no pun intended) with steel framed floors.

 

[concretedoc]

Be careful in using old rules of thumb for this. When combining Grade 50 steel with LRFD design, the depth of the steel framing flexural members can be reduced enough to cause more deflection than expected. There's a good article that describes the phenomena at

http://www.concreteconstructiononline.com.

Enter "deflection" as the key word, and "Ryan" as the author. There are also some older articles on elevated slabs (around 1992) by Suprenant and Tipping that describe some of the problems in trying to keep floors flat but with a reasonably uniform thickness. It's not easy.