suspended concrete slab with steel beam over metal deck

by Dave
(Newfoundland, Canada)

Question:

I'm planning a small two story garage with a living space (recording studio) in the basement with a suspended concrete floor on the main level.

The main level will house a workshop/garage and there will be occasions when a vehicle will be parked on the main level. The ground is rear sloping so it will have a ground level walk out area on one side of the basement. The finished inside dimension of the garage is 24' x 24'.

I've gotten a couple of quotes on constructing the suspended concrete floor using steel decking and beams for a clear span in the living space. The prices seem outragious to me...one was for $13,000 and the other $18,000.

There is a couple of things you could help me with. What size steel beams and decking do I need to span 24 feet. Can I support the steel decking with wooden beams and if so what size wooden beams would I need?

I know there's various combinations of decking and beams that will support the same loads so I'm looking for the most cost effective combination. Thanks Dave

Answer:
Hi Dave,
Wood is a much weaker material than concrete and steel, the idea of wood carrying metal deck with concrete on top of the deck is the equivalent of a dog carrying a horse.

Let’s figure out if you’re getting a fair estimate for your project. A 24ft x 24ft garage floor is 576 square feet. Typical floor thickness like this will be 6 inches thick.

Since concrete is sold in cubic yard, we need to convert the 576 square feet to cubic yard. Without going into detail, the calculation goes like this: 576 x 0.5 /27 = 10.67 cy. You can call up any concrete ready mix supplier at your area and ask how much the 11 cubic yards of concrete cost. Here in Salt Lake City, Utah it costs about $105/yard. With fees and taxes concrete it will cost $1,300.

To get the necessary strength you need and in order to park your vehicle on your suspended slab you will need rebars. The reinforcements needed are #4 bars to be spaced at 12 inches on center each way. This translate into 70 #4 x 20ft long, with tax and chairs to hold the rebar in place while concrete is being poured this item will cost $800.

The metal deck needed will be 20ga with at least 2 inch depth. This type of deck runs about $200 for each 3ft sheet. With tax and fees this translates to $1800.

Since metal deck can only span 10 ft or less, the 24ft need to be divided to 3 spaces of 8ft long. This would mean you need 2 steel beams, W12x26 should work that span 24ft. These steel beams will cost about $800.

You will probably need equipment to lift the steel beams in place. You will need steel columns to support the steel beams and concrete footings to support the steel columns. Also you will need concrete pump to pump the concrete from the cement trucks to the metal deck. These can run additional $2-$3000. We haven’t touched the walls or any other part of the garage yet.

The numbers above add up to about $7000, remember this is only material. The numbers mentioned are for illustrative purpose, you can call your local supplier to get an idea how much real material cost and taxes in your area. I hope this can provide idea how to determine whether an estimate is reasonable.

Hope this helps
Good Luck