I am pouring a concrete slab on my first floor. Slab will be under south facing windows only. Slab will be 10' wide and 46' long. I want slab to hold heat, so not using light weight concrete. I will have pex for additional heat if needed.

I intend to use 2x8s on 16" center to support concrete. This will remain after pour. 3.5" floor will have 1/2" rebar grid on 12" centers. I calculate weight to be about 40lbs/ft^2. I don' think I need to put any temporary bracing up.

Am I on the right track? Are there any engineering reference data I could double check design?

regards,
Bob

How is this a suspended slab?
Don't understand what is to support the about 10 tons of concrete you have described.
Concrete is normally considered to weigh about 150 lbs per cubic foot (including forms) which would make it more like 50 lbs per sq. ft. at 3 1/2 inches.
These are just some facts for consideration but there many other questions to be answered.

Sorry for being unclear. The pour will be supported by the 2"x8"x10's on 16" centers. I suppose it may not technically be considered a suspended floor. I am not a civil engineer.

The woman who works for the lumber yard is telling me that she has to design the floor for the dead weight and the live weight. 50lbs dead and 40lbs live. I am not sure if this is the case because I think the rebar will support all of the weight.

In other words, once the concrete cures around the 1/2" rebar on a 12" grid the 2x8s are doing little work. I could probably remove the 2x8s and drive my truck over the floor without it flexing.

One other factor I should probably mention again is there will be Pex in the floor. Not sure if it matters much, but don't want to stress the pex tubing at all. Last thing I need is a leak in the floor.

The problem I have is she wants to use these very expensive lvls on the floor, and I think the 2x8 should support the whole pour with no problem at all. If there is a document *like a span chart* for pouring concrete that I could use to show here then this mess might g

Thanks for your help.

The interior ends of the 2"x8"x10' joists will be supported by a 4"x12" LVL that runs down the center of the basement ceiling. The other ends of the 2"x8"x10' will rest on a header on the outside wall - walkout basement. I will probably ask for 2"x8"studs on that wall and pack with fiberglass batts.

thank you.
Bob

Still don't understand very much about what you are doing. Are you placing ten tons of concrete on 2x8's (with plywood on top of 2x8's) and 2x8's that are resting on an existing floor? Is she looking at the load you are putting on that existing floor?
And just one fact about reinforcing steel-- it is placed in the areas of concrete where tensile stresses are high because while concrete is strong when subjected to compressive loads, it has only about one tenth of the compressive strength when under tension. In other words, the tensile strength of a concrete shape is about 1/10th its compressive strength.
If you believe that the load will be carried by the resteel, where does that load get transferred to a foundation?
Maybe I am too dumb to understand what you are doing. Have you ever seen these exact type slabs built? If you could remove all the 2x8's, wouldn't the bottom of the slab be sitting on the exiting floor? Or actually on top of the plywood form you will need on top of the 2x8's?
Is the lady at the lumber yard a professional engineer? (I am not-I am a retired highway and bridge construction inspector classified by a US state as a highway engineer). A bridge deck is in the neighborhood of 11 inches thick and spans the area between the steel girders typically less than 10 feet with large rebars especially near the bottom resisting the tendency of the slab to pull apart at the bottom because of the weak tensile strength. You might try driving your pickup on a 3 1/2 inch slab with 1/2 inch rebars with 30 feet of air below the slab but I wouldn't be along for the ride. Will you be submitting her calculations as part of a permit application? I hope so.

Hi Bob,

Honestly, I'm concerned. Unless the "lady at the lumber yard" is an engineer or architect then I think that you shouldn't just let her design a solution for you.

This isn't rocket science but it should be designed properly to accomplish your end goal.
It sounds like what you WANT is something like "PEX radiant heat for a section of your house".

Ok... well PEX doesn't take 3.5" of slab to work right. Our master bath has radiant heat - electric. I did it myself, under natural stone tiles and I only needed 1/2" of self leveling cement to cover the wires. PEX is bigger so you'd need more material but hardly more than 1" total - provided that your floor is pretty level.

In any case - in my opinion - it would be best to take a little step back and start with your end goal - the PEX heat, and explore options to get to that goal, without going overboard. The beauty of engineering is to properly meet the load calculations. Part of your solution might include adding joists, sistering joists, adding additional supports, etc. First things first - is to figure out the BEST PEX solution for you (unless you wish to electric... if your kWH cost is cheap then that might be worth thinking about).

Best of luck!

We are pouring a suspended floor that will support your pickup truck and its 13" thick with 1" thick rebar in it and is supported by the concrete foundation walls all 4 sides. I think what you're proposing sounds scary.

If you just want to use Pex to heat a floor, you can also just put pex in the 2"x8" joist spaces in the floor below. They make these metal reflectors that attach onto the pex and spread the heat under the subfloor. Its not as nice as a concrete floor, but its really difficult to install what you're suggesting after the fact.

As J17 suggests, electric in floor heating is a good solution too. It is cheaper to install and will be much more direct than pex under the subfloor. i.e. it will be warmer faster