Cast-in-place Conc + Steel Elevated Parking Structure

[slapjack]

I have a 2 level elevated steel parking structure with cast-in-place concrete slab on metal form deck. The structure will be open and exposed to environment. AISC Steel Design Guide 18 (Steel Framed Open Deck Parking Structures) recommends the following:

1) Use of normal weight concrete
2) Use of 4,500 psi concrete (minimum)
3) 2"-3" light gauge vented galvanized metal deck
4) Metal deck CAN NOT be counted on for reinforcing the slab - slab must contain sufficient reinforcing (rebar)
5) Reinforcing (rebar) must be protected (epoxy coated)
6) Total slab thickness of 5"-6"

This seems a bit excessive for a little ol' parking structure with no heavy traffic, just light weight cars and trucks for retail parking. Are these recommendations normal for this type of structure or over-kill? I've reviewed structural drawings for similar structures and very few of these recommendations applied. Anyone have any input?

Thanks!

 

[concretemasonry]

What seems excessive? - We currently have two parking ramps that have recently closed, repaired temporarily and are slated for demolition unless they are totally repaired. Parking decks can be subjected to severe conditions (temperature movement, moisture and unusual loading conditions).

Nothing ecessive about 4500 psi normal weight concrete. - Around here (MN) you cannot buy anything less than 4000 psi or 4500 psi (air entrained) for a simple driveway. The good supliers will refuse.

Galvanized forms will not rust.

Epoxy coated bars are common wherever there is the possibility of corrosion.

The slab thickness is dictated by the span, loadings and cover.

 

[JAE]

Galvanized forms will not rust.

Yes they will. Over time. Galvanizing won't last forever under wet/dry conditions.

Slab thickness is also thicker to keep tensile stresses down where cracking under service loads is kept to a minimum - thus helping to keep cracks small or non-existant and thus inhibit moisture infiltration into the slab.

 

[concretemasonry]

JAE -

I guess I should never say never either. - Thanx

Dick

 

[dik]

What spans for the slab... 5-1/2" to 6" could be OK for about 15'+/- spans. Use proper joints and I use a 25psf alternating load; this represents a realistic maximum live loading. This in conjunction with regular 50psf live load for maximum uniform loading. Don't rely on epoxy coated bars to any extent. Consider 1-1/2" min cover over top rfg and a good sealer. Use a proprietary topping in drive isles and ramps and good expansion joints. You can benefit from composite construction. Provide the client with an annual maintenance program; provide hose bibs and a good spec'd sealer. Good Luck.

Dik

 

[dik]

Forgot to add... use min slump; for parking slabs I often spec 3" max... not, 3" +/- 1-1/2".

 

[slapjack]

The current design has the slab spanning 9'-0" between composite castellated steel beams.

Thanks for all the explanations and recommendations. As far as a sealer/proprietary topping - any suggestions?

 

[dik]

For sealer, I usually use a siloxane type and for traffic wearing surface, there are several that include an abrasive grit. It's not a concrete topping, but an epoxy like material or similar. What type of climate? The castellated beams are spanning 60'+/-? or do you have intermediate columns. The 9' is 'car spacing'? With that span, a 5" slab should be OK... just make sure you have proper cover and good slopes and proper drainage traps... preferably 1-1/2% min. CSA produces a good design guide for parkades in Canada... we have the climate and some areas use high de-icing salts.

Dik

 

[slapjack]

This structure will be located in that Atlanta, Georgia USA area... so there won't be a lot of cold weather like up north. The castellated beams are spanning approximately 60', but deep in size - 30 inches. The 9' spacing just works out with the column grid. I'll do a little research on the toppings. Thanks for the advice and info!

 

[JAE]

slapjack,

Check this company product out:

http://www.neogard.com/

 

[JedClampett]

Think of it this way: Do you want to be deposed and defend why you didn't follow the clear recommendations of the organization responsible for writing the steel codes? How will you answer?
You'd be surprised how big a deal something like this can be made to sound like. And it doesn't have to be a design problem. Shoddy construction can be turned into a design issue when lawyers are involved.