Suspended slab porch cap spans

[CacheLanding]

Have a concrete porch cap on a wrap around porch that is 900sf in area, spanning a cold storage room below. Edges of porch cap to be supported by foundation walls. 1/2" rebar tying the cap to the walls 32"oc. Longest span is about 10'.

Layout can accommodate up to 11" thickness and I'm willing to up-size the rebar if necessary. I'd hoped that for cost reasons I would only have to pour the porch cap 6" thick with 1/2" rebar. Can anyone direct me toward span info regarding suspended slab porch caps?

 

[hokie66]

I don't even know what a "porch cap" is. Sounds like you need to find a structural engineer in your area to design this for you. Suspended concrete slabs should only be designed by those who know how.

 

[Ron]

Agree with hokie66...get a structural engineer involved. Reinforced concrete slabs are not designed from span tables like wood or steel. It's a bit more difficult than that.

What is a porch cap? Not familiar with that term.

 

[CacheLanding]

"Porch Cap" might be a regional term. I live in a very cold climate. Because of a deep frost line, virtually everyone just digs another 3' and builds on a full basement, 8" thick poured concrete. The porch (or front deck) is almost always poured into the full basement, creating a small room with 8" concrete walls on all four sides. The only difference between this room and the rest of the house is that the living space does not cover this room. This room is then "capped" with concrete which is usually about 6" thick. The reason is that the main floor joists are usually 12" deep, and there will generally be a 6" threshold step up into the front door. Once porch cap is on and basement slab is poured, you end up with a fully encased concrete room, with a vent or two in the exterior walls to allow air exchange. This is non-insulated space and usually used for food storage like underground cellars people have in other regions. Sometimes if the room is big enough (as in this case) there will be a concrete partition in that room and a safe room will be created.

Anyway, the walls under this porch will be left with about 3' of #4 rebar sticking out the top every 24". This will then be bent over and tied into the rebar grid for the cap to tie things all together. Subsequently, the cap spans a room below, but only needs a live load of 30lbs max.

 

[Ron]

Thanks for the explanation. I still suggest you get a local structural engineer involved to design the slab. You have code considerations and the area is fairly large. Your total load will be on the order of 100 psf, plus, if your porch is open, which from your description I believe it is, then snow drift loads will likely have to be considered.

 

[CacheLanding]

Fortunately the porch is covered, so additional snow loads are minimal.

Local engineer speced a 6" thickness with #4 bar.