Elevated slab construction.

[OneEyedMan]

Are there any texts that reference formulas and practice for elevated slabs? Are there so many variables that only an engineering firm can calculate the design? Asking for a friend.

 

[billconner]

I think I could design one but would never get building department sign off without a registered design professional signing off.

 

[readhead]

I have built lot’s of them but there are lots of variables. I would definitely ask an engineer.

 

[LopezBart]

If the slab stiffness is ignored in the floor support design, a slab like this is just a very heavy dead load and the design is not unusual and you may be able to find prescriptive solutions. Note that increasing the floor stiffness (reducing deflection) will help prevent cracking in the slab, but such cracks are difficult to avoid.

If the slab's resistance to bending is part of the design (likely along with cast in place girders and lots of rebar), you have a much more complex issue, both in design and construction, and you're looking more at commercial practice rather than residential, and engineers will definitely be involved.

Note that hydronic heating often uses poured concrete or "gypcrete" (a lightweight "concrete") for the floors, and these are not necessarily on grade. It does mean lots of girders supporting the floor - a 4" concrete slab is order 50 lbs/square foot of additional dead load. From personal experience, it does make for a quiet and solid feeling house.

 

[billconner]

How clear do you need the space under to be? Not so hard if you can live with supports - walls and or beams - 10' or less on center. 8" formed or block walls 10: on center and steel form deck. No rebar required.

Also check out ICF floors. First on that pops up is Amdeck by Alleguard. They seem to engineer and sell whole system. No experience, just have read.

 

[larry4406]

What size slab?

We do structural concrete slabs in garages with integral grade beams. The dirt is a one shot mold for the concrete and its loose fill. Over time, the dirt below the slab compresses and sinks away leaving the structural slab floating by its perimeter support as it was designed.

If the dirt below the slab were to be fully removed (as some folks later do by making an entrance from the basement) they create a room below the slab. Or you provide robust form work to support the concrete until it sets up.

 

[bluedog225]

And 8” or so can be done with card board boxes designed for the purpose.

 

[wssix99]

Asking for a friend.

lol

Are there any texts that reference formulas and practice for elevated slabs? Are there so many variables that only an engineering firm can calculate the design?

Generally, yes. An engineer is needed to design a custom slab. Little things can turn a simple slab into a relatively complex engineering problem.

There are systems that offer pre-engineered solutions that may cover simple scenarios, like LiteDeck: https://liteform.com/products/litedeck/litedeck-steel-rib-system.html

I have personally not seen anything pre-engineered that could take the load of vehicles, so an elevated garage slab will probably need to go back to the engineers.

 

[Rusted Nut]

Elevated Slabs for Dummies, Page 36....

 

[OneEyedMan]

Elevated Slabs for Dummies, Page 36....

Exactly. I would very much enjoy learning every detail that goes into designing a steel and concrete structure. In lieu of traveling back to my young self to sell the virtues of pursuing an engineering career, I would like some hard fast parameters.

 

[OneEyedMan]

What size slab?

We do structural concrete slabs in garages with integral grade beams. The dirt is a one shot mold for the concrete and its loose fill. Over time, the dirt below the slab compresses and sinks away leaving the structural slab floating by its perimeter support as it was designed.

If the dirt below the slab were to be fully removed (as some folks later do by making an entrance from the basement) they create a room below the slab. Or you provide robust form work to support the concrete until it sets up.

I had looked at doing a ground formed slab by backfilling a foundation with sand and using extra rebar. A FB find of pan decking changed my outlook. Proposed project is 12’ span, 80’ length, now using pan decking on 4x6 ledger angle anchored to 8” walls. #6 rebar on 1’ grid. Not sure it’s enough, was told 6” slab would work but thinking 8”. I don’t know if extra is enough or not. Point load could justify the extra though not a driveway area.

 

[readhead]

This is not the time for guessing. A 12’ single span with 6-8 inches of concrete and rebar is going to be pretty heavy. It will require some serious decking. You might consider beams for the 12’ span and running the decking the eighty foot way. The ledge angle can be a lighter, the decking and rebar can be lighter and make sure you are using galvanized Conform decking.

 

[mike93lx]

And 8” or so can be done with card board boxes designed for the purpose.

Can you elaborate?

 

[bluedog225]

Can you elaborate?

Referring to void boxes and beams on piers. Op was short on details.

 

[OneEyedMan]

This is not the time for guessing. A 12’ single span with 6-8 inches of concrete and rebar is going to be pretty heavy. It will require some serious decking. You might consider beams for the 12’ span and running the decking the eighty foot way. The ledge angle can be a lighter, the decking and rebar can be lighter and make sure you are using galvanized Conform decking.

Not a bad idea to rotate the orientation 90 degrees. I can get 14” H beam to span 12’. One project manager I talked to, not an engineer but the guy doing the building, said they did 8’ spans with this decking with 6” concrete and #4 bar and were good for a 10k forklift to drive on. Not engineering terms, but something to start at.

 

[readhead]

Get beam height out of your head. You can span 12’ with a 10” beam if it is specified correctly. You really need to talk to an engineer for the best and most cost effective design.
Is there a permit involved for this project? If so you will probably be required to provide engineering.

 

[readhead]

Just for information. There are thirty seven 14” beams listed in the AISC manual. Again, not the time for guessing.

 

[C-S-H]

now using pan decking on 4x6 ledger angle anchored to 8” walls.

This is probably a more complicated design than the slab design itself.

 

[firebirdparts]

Exactly. I would very much enjoy learning every detail that goes into designing a steel and concrete structure. In lieu of traveling back to my young self to sell the virtues of pursuing an engineering career, I would like some hard fast parameters.

So first you need to pick a geometry. I don't even know if I can even list them. Like for instance:
- reinforced concrete with drop beams
- reinforced concrete without drop beams
- post tension with drop beams
- post tension without drop beams
- steel supported, red iron
- steel supported, trusses

Then, as a separate parameter, you have:
- point supported, slab continues
- point supported, slab ends
- line supported, slab continues
- line supported, slab ends

So I think, if you want rules of thumb you need to go through this and pick just one thing. then you can look for examples of that one thing and you can actually do a pretty good job of thinking about how your situation compares to the example. In my opinion, anyway. I grew up on a farm - You can do a lot just by building something stronger than necessary. That becomes VERY HARD to do with reinforced concrete. It's considered forgiving, but it's unbelievably heavy, and you've seen several fatal errors that you can probably remember.

 

[firebirdparts]

Here's one example, go to Walmart and look at the ceiling. That's truss supported and the slab only spans a few feet. really strong.

 

[firebirdparts]

Here's another example - everybody will remember this. This is not a slab, but it does show that you can convince yourself to build a pedestrian bridge 175 feet long that weighs 1000 tons. Seriously, 1000 tons. Concrete can be a very stupid choice sometimes, as it was here. they all agreed to it somehow.

Florida International University pedestrian bridge collapse - Wikipedia

en.wikipedia.org

 

[Rusted Nut]

I had looked at doing a ground formed slab by backfilling a foundation with sand and using extra rebar. A FB find of pan decking changed my outlook. Proposed project is 12’ span, 80’ length, now using pan decking on 4x6 ledger angle anchored to 8” walls. #6 rebar on 1’ grid. Not sure it’s enough, was told 6” slab would work but thinking 8”. I don’t know if extra is enough or not. Point load could justify the extra though not a driveway area.

The correct gauge of pan deck will span 12’, I’ve poured lots like that; but not making an engineering judgement here. An 8” slab will be weaker than a 6” if not correctly reinforced, and you would have to reshore a 8” slab, possibly a 6” as well.. Contact a structural engineer..

 

[billconner]

The other option is to have concrete planks delivered. Flexicore is a kind of brand name that's become generic. Maybe more properly prestressed precast hollow core panels.

 

[abfish]

My company shop has a 5" thick fiber-reinforced slab. It has no cracks after ten years. We often have a dump truck or truck tractor in the shop for service.

It's like building a road- what's under the surface is as important as the surface course. Properly compacted, select fill is the key.

 

[readhead]

No fill involved. This is a raised floor slab.

 

[readhead]

May I ask what the situation is and why this situation wasn’t handled at the plan stage.

 

[carlaisle]

You haven't shared your friend's required design loads or most other pertinent information. It's a 12' by 80' project. It's not a driveway area, but does have an implied, unspecified point load concern.

An equivalent to the joist span tables that exist for lumber doesn't really exist for elevated slabs/reinforced concrete because there are too many variables. You can easily clear span 12' with reinforced concrete. The required deck thickness, beam spacing (if any), bar diameter and spacing/placement, etc. are all functions of the design requirements.

Bill's suggestion of precast flown in via crane can be very cost competitive when you add up the labor savings. And don't discount the ICF floor forms. They're a little pricey, but they save a ton of time.

If you can get your friend to cough up some more pertinent details we'll probably be able to offer some more pertinent help.

 

[rharman]

When I hear "elevated slab", I'm envisioning an open, usable space underneath. Is that correct?

If that's the case, does your "friend" really want his wife, kids, or anyone, underneath that thing without it being professionally designed/engineered and constructed?

Maybe I'm misunderstanding the premise but this just screams "DUMB IDEA".

 

[reader2580]

Pre-stressed concrete is commonly used for garage floors with a room underneath. No idea on the cost. My parents bought a two year old house in 1981 with a garage like this that has another garage underneath. My father made the space into a shop along with an area to store all the lawn and garden stuff.

If you do this make sure a rubber membrane is laid over the pre-stressed concrete before the concrete cover layer is added. My parents didn’t have this and water got into the space below. A few years ago they had the finish layer of concrete removed and a membrane add with new concrete finish layer. It cost over $20,000 to fix.