elevated concrete/steel beam garage slab design! 1

[patrox777]

I've got a project to design a concrete slab on steel beams with tube steel columns. It is a residential garage so I've opted to use wide flange beams in liue of joists to avert excess vibration.

A 6" concrete slab on metal corrugutated sheets supported by wide flange beams probably space them close at 3 foot on center... it's a square garage 23' x 23'...with no interior walls... so I'll have a beam line across the centerline of the garage...

Question is what concerns should I have about 'wheel loads' on the beams..?? a concern or no?

thanks,

 

[JAE]

The slab should spread the loads out adequately so a wheel load on the beam is not a specific concern. However, the appropriate loading for the garage should be provided for. AASHTO has bridge and truck loadings so these are quite excessive. Most parking garage designs, per code, use 50 psf uniform load as one load case and a concentrated load case as follows:

2000 lbs over an area of 20 sq. inches (per the IBC 2000)

These two load cases are for garages with passenger cars only.

 

[JAE]

You should also look at punching shear in the concrete slab, but a 6" slab should be adequate for a car.

 

[patrox777]

Yes... purely residential slab load... I agree... a small 3 foot beam spacing should be fine... I think I'll use 75 psf loading though... just in case...

Punching shear- yes.. could be a possibility.. will look into it...


the walls and roof will be loading the perimeter beams only... I will design for that..

 

[Lutfi]

Do not forget to add steel in the slab even though you are using metal deck. Consult metal deck catalog for their recommendations. Watch also for the negative bending in the slab over the steel beam. Big issue is the placement of the steel in the slab.

Good luck

 

[JAE]

....right - Lufti is right - be sure to reinforce with rebar as I would be concerned that water brought into the garage on the car would seep down and eventually rust the deck....galvanized deck is a must here. Also, some type of floor sealer would go a long ways to inhibiting moisture migration down into the slab.

If you are in the north country, where salts on the roads are used, this is even a more critical situation. In most houses in the north, the garage is an on-grade slab where this isn't an issue. But with a structural slab - concern yourself with the long term servicability performance of the slab.

 

[patrox777]

Thanks ya'll- I'm in Austin Texas... yes a six inch thick concrete slab with w6x6-w2.9xw2.9 on a galvanized steel deck 0.6c28 holding at least 400 psf.

Beams at 3'0 centers...12' spans...

the underside will be exposed... think the beams and columns should be galvanized?

this forum is great..thanks for all the info!!! great !!!

 

[JAE]

Great town, Austin - I spent 9 years down the road in San Antonio. Just a bit hot during the summer - but you are hitting some good weather ahead.

 

[haynewp]

I don't think the concentrated load would be a problem for the deck, you can find the effective widths to use on page 36 of "Designing with Steel Joists & Girders and Deck" manual. There is also an example on page 39.

I am curious why you are using such a close beam spacing?

 

[GFT]

Elevated residential garage floors are quite common in my area and I have designed a lot of them. There are several good methods.

1. As previously noted, a 50 psf live load is usually adequate. Some times it is necessary to jack-up a vehicle in the garage so, although it rarely controls, be sure to check the 2000 lb wheel load requirement. Some of my clients have elaborate shops in the lower level and on occasion I've design the floor beam for hoisting loads.

2. Advise the client that condensation on the underside of the slab may become a problem if the floor is located in a cold climate. Condensation can be controlled when proper attention is given to ventilation, vapor barrier, and insulation.

3. Use a two-way slab or hollow-core planks if your client wants a column free space. A two-way slab of reasonable thickness can easily span the 23'x23' area, but either use a central drain or give it adequate slope because long term creep may cause a puddle to develop. 8" thick hollow core planks (my first choice and preferred method) are terrific for spans of about 27' or less. I usually use a 2 1/2" topping but never count on the topping structurally. When using hollow core planks most of the work is in detailing the edges to make sure the slab is properly tied to the walls. Use idiot proof details.

4. Use a single W8 central beam and design the slab to span one-way, if a column can be tolerated. I have used composite steel deck on several occasions when the owner insisted, but I prefer formed concrete slabs. I usually use studs on the beam to get composite action, but design the beam as unshored. The slabs are usually about 6" thick with #4 or #5 @ 12" BW supported on 1" slab bolsters. There are two ways to get slope to the garage doors. You can either keep the bottom flat and slope the top surface, or keep the slab uniform in thickness but slope the wall chase. I prefer the later.

5. Many carpenters shore the one-way slabs using telescopic aluminum beams that can be rented from a vendor who rents forms and equipment related to concrete construction. The plywood forming can be salvaged for use elsewhere on the house. Be sure to protect it from the concrete by covering it with poly before placing the concrete.

6. Open web joists are usually not a good choice because they reduce the headroom of the lower level, and thin slabs usually don't stand up well when exposed to deicers and long term moisture.

7. Watch your headroom in the lower level. It is very disappointing to the client to spend a lot of money on an elevated slab only to find that lower level is unusable.

8. An epoxy floor coating is the perfect finishing touch.