Elevated Slab for porch 1

[jimtheengineer10]

A client is installing a foundation for a house that will have a concrete slab for a front porch. The porch extends out 6' from the house and is 22' wide.

He would like to have the area under the porch slab to be part of the full basement. In order to do this he will need to install an elevated slab. Can the galvanized steel decking span 6' or does a steel beam need to be installed at the 3' midpoint? The slab would have an 8" frost wall to bear on and the foundation wall between the house and the porch can be 12" or 14" wide in order to provide 4"-6" of bearing for the slab up against the house.

 

[JAE]

Can the galvanized steel decking span 6' or does a steel beam need to be installed at the 3' midpoint?

If you are an engineer you simply calculate the required decking, reinforcement, slab thickness, etc. to span the required distance and make a decision as to what you need.
Why are you asking for a straight-up design solution here?


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[jayrod12]

Is this an area where cold weather is a concern?

Personally not a fan of slab on deck in these situations unless the deck is sacrificial formwork

 

[jimtheengineer10]

Yes cold weather. The area below the elevated slab will be in the basement and enclosed. There will be a door going into the actual basement. Basically you have a rectangular foundation with a U-shaped bumped out on the front which the elevated slab will sit on top of.

 

[XR250]

Download a Vulcraft catalog which gives allowable spans for steel deck.

 

[jayrod12]

It's likely they'll want a thermal break between the slab and the warm part of the building. They'll also need to insulate the underside of the slab, which of itself isn't a big deal, however it removes the chance of being able to review the underside of the slab periodically to figure out whether the deck is holding up to the possibility of trapped water.

I'd be doing a cast-in-place structural slab personally. I'd provide a stand-off galvanized steel angle detail where it connects to the interior alcove walls, which would minimize the cold strike to just the connection between the slab and the foundation wall on the outside. Spray foam the entire underside of slab with 6-8" of insulation.

 

[HouseBoy]

Curious about several things:
What is the significance of the front most wall being called a "frost wall"?
Won't that simply be a basement foundation wall that happens to be supporting the structural slab? (i.e. it is doing much more than providing frost protection.)

What does the OP intend for the metal deck?
Is it a concrete form or is it intended to be composite decking?
Assuming it is the latter, that gets into the slab design and may be investigated as JAE and XR250 have indicated.
If it is working as form work, that too can be explored.

Regardless of whether it is form or composite decking, I think thorough galvanize protection of the metal is important.

Regarding thermal separation, that's a big deal too. Most research (that I've seen) shows that barrier separation is critical to preventing thermal transmission.

Getting more to the OP question of whether a steel deck can span 6 feet - that depends on the depth and gauge of the decking, the type of decking, what the tension reinforcing material is and whether it is desired to use shoring or avoid it. In short, yes it can if it is designed properly.