Ponding of Concrete on Composite Steel Deck Floor

[KootK]

When I've used composite floor decking over steel beams in the past, I've typically cambered the joists to avoid ponding issues with the concrete topping.

On my latest project, we're going to go with unshored, uncambered steel beams beneath the composite deck. Therefore, the concrete will pond to some degree.

How have others gone about accounting for the additional dead load due to concrete ponding in this situation? I've read a couple of articles on the subject and it sounds quite daunting.

The steel decking deflects and increases ponding; the steel infill beams deflect and increase ponding; the supporting girders deflect and increase ponding... on and on it goes. And it's iterative to boot.

Is there a simpler yet still rational way to deal with this?

Also, where the ponding occurs, flexural capacity will also increase as result of the increased effective depth of the system. Is this typically utilized?

Thanks for your help.

KootK

 

[SteelPE]

I seem to remember a seminar given by the AISC where they stated that the ponding of concrete didn't matter in most cases when using composite construction. That is, you get additional concrete but you usually end up with additional capacity due to the additional concrete so it is a wash. So the lecturer said don't worry to much about it.

You also have to remember that each beam will have a natural camber and that the fabricator is suppose to place the camber in the upwards direction, so even if you don't specify camber you will always end up with at least a little.

 

[KootK]

Thanks steel PE. Even with the moment capacity perhaps a wash, one would steel need the increased load for the sake of load take downs to other elements. Contractors typically add 10% to the volume of concrete do they not? Perhaps that's a reasonable estimate of the extra weight?

 

[WillisV]

Add 10% of the concrete wet weight.

 

[Hoaokapohaku]

Only contractors unsuccessful at bidding routinely add 10% to concrete quantities, unless the quantity is very small.

 

[Ron]

It's all progressive. If you computed deflections for serviceability based on the assumed concrete depth, they will be off. If your design is "stretched" you'll be way off in deflections.

Don't count on any increase in capacity due to increased section.

WillisV's approach is good. You might want to increase the percentage to 15 if your design is stretched.

Another consideration...place the concrete in two lifts. You can put stud rails in the bottom of the flutes to tie together the two lifts.

 

[spats]

Right or wrong, I've just always added an extra 1/2" of concrete to the pre-composite dead load, and don't account for it in the post-composite properties. This is for floors that normally have 2"-3" of concrete above the deck.

 

[ooox]

The issue of ponding is comprehensively addressed in the Steel Construction Institute's P300 Document, 'Composite Slabs and Beams using Steel Decking: Best Practice for Design and Construction.'

Their recommendations include limiting the residual deflection of the decking soffit to span/180 but not more than 20mm. This can be increased to span/130 where the design allows for ponding.

They also recommend allowing for an increased weight of concrete where the predicted deflections are greater than 10% of the overall slab depth.

I normally allow an extra 15-25mm of wet concrete for the construction stage and limit deflections to 25mm.

Another important check during the construction stage is for beams running parallel to the ribs where the decking/fixings may not be adequate against LTB.

 

[WillisV]

Note that my 10% number was not just made up, it is based on the paper by Ruddy referenced below which is also referenced in AISC Design Guide 3 (Serviceability). If you look at the study 10% is a reasonable rule of thumb that takes into account an assumed placement sequence and the iterative nature of the problem.

For further corroboration AISC discusses this same issue and recommends the same percentage in the composite beam design example for the 14th Ed. Design Examples on page I-9
(

http://www.aisc.org/designexamples)

Ruddy, J., "Ponding of Concrete Deck Floors," Engineering Journal, V. 23, No. 3, 1986.

 

[MiketheEngineer]

Shore it during the pour

 

[shobroco]

If the concrete guy is paying for the concrete on his fixed price bid, you're more likely to have a problem of decking ribs showing through the floor than too much concrete.