Composite slab thickness 2

[Tdog67]

Fellers,

When we design elevated slabs for an office bldg we always use a nominal thickness for the concrete slab such as 4", 4.5" or 5". I've seen other dwgs for floors like this where they use 5-1/4" total slab thickness. this requires the use of a bent plate on the perimeter which is much more expensive. Is there a reason to use a dimension such as this? Is it possibly because of the studs used in composite construction?

thanks

 

[Lion06]

There are possibilities, but I can't think of any that would really dictate a dimension like that. Some that come to mind are slab capacity, composite beam capacity, unshored deck capacity. I don't think it would be because of the studs - you can usually get those to work without some odd slab thickness. That being said, the slab thickness provided is rarely what is provided - mostly because of ponding.

 

[csd72]

I would say that it depends on the beam spacing and the type of floor load (particularly point loads).

Beam spacing is dictated by the spanning capacity of the decking under the given thickness of wet concrete. Slab thickness is then dictated by the required permanent load capacity.

I would say the 5.25" comes from someone being overzealous with refining the slab thickness.

 

[WillisV]

Fire-rating. In order to get a 2hr rating without spraying the deck, a slab requires 3 1/4" of LW concrete cover or 2 1/2" of NW concrete over the deck. So with 2" deck you are looking at a 5 1/4" total depth LW slab or a 4 1/2" NW slab.

 

[WillisV]

Example - see p61 of the catalog linked below - "2 hr rating - unprotected deck". Its based on UL requirements.

http://www.nuconsteel.com/pdf/vulcraft_steel_deck.pdf

 

[JKStruct]

agree with WillisV. We use 5 1/4" and 6 1/4" total slab thicknesses all the time for rated construction with LW concrete. It's not uncommon.

Regarding your bent plate comment, you need something as a pour stop around the perimenter anyway, regardless of thickness. And depending on the edge condition, something stronger than a light gage pour stop might be required. We use bent plates around the perimeter on most of our projects. Again, it's not uncommon.

 

[Tdog67]

Bent plates are more expensive than just angle, but isn't LW concrete more expensive than NW concrete? Also don't you use the perimeter angle as your transfer strut from the floor diaphragm to the vertical bracing or frames?

 

[jike]

Willis:

Just a slight correction: unprotected deck 3 1/4" LW over deck or 4 1/2" NW over deck. The 2 1/2" NW is for protected deck (spray-on, etc.).

As JKStruct said these slab thicknesses and bent plate pour stops are not uncommon in rated construction. It is done all the time.

Tdog67: LW is more expensive than NW but it is less concrete. The selection is often a matter of what costs less. In floor construction, pour stops are not usually used to transfer diaphragm forces. Diaphragm forces usually get transfered thru headed studs into framing.

 

[WillisV]

jike - yup, thanks.