best software for designing an elevated residential garage slab?

[Engineerataltitude]

Most of my 30+ yr practice a has been residential and low rise commercial and light industrial. The only elevated garage slabs over conditioned space I've done over the years have been single car garages. I now have a big luxury home with a 24ft x 24 ft elevated garage over conditioned space. They'd rather not have posts if possible.

I will need to figure out whether it is more cost effective to put some very stiff framing under a 3 1/2" reinforced concrete slab or design a self-supporting reinforced concrete slab.

I'm a bit out of my area of expertise here.

Any recommendations for software that could model these options well? Any reference which could help me with connection details, etc?

 

[JoelTXCive]

Concrete Bending by IESweb will help with a slab design. It has a 30day free trial and is very easy to use. (I have no affiliation with them, but I recommend their products all the time.)

You can be up and running in under an hour.

https://www.iesweb.com/cb/index.html

They have several videos on youtube too.

 

[XR250]

3 1/2" is a bit thin for a garage slab. You need to support jack loads.
In my locale, it would slab on composite metal deck on I-beams.
I would find out what the contractor is comfortable with and start there.

 

[dik]

In these environs, non-structural SOG for residential garages is typically 4" min and for structural slabs is normally 5" min plus whatever supporting structure you have.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

Minimum for bridge slabs is typically 6", except for precast, which is typically higher strength concrete than CIP.

The AASHTO bridge design spec has loading (including for "maintenance vehicles", which would be for a large pickup) and distributions for wheel loads for slabs and slab on beams.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[CrabbyT]

I'd forget the software and get a quote for some hollowcore planks. Way less liability, and they'll provide the details.

Not to mention, you'd probably sleep better at night not having to think about concrete quality, or whether or not they placed the concrete correctly.

https://www.hollowcore.net/load-tables

If that's no good, maybe frame the floor with steel and put a non-composite deck on top. Like a 1.5C16 or something. Vulcraft makes load tables for their decks.

 

[jayrod12]

I'm with Crabby. All of these that we do are hollowcore with a thick topping and a good waterproofing membrane on top of the plank, but below the topping. Keep the topping well reinforced and get them saw cutting the topping as early as possible as well to keep the crack widths to a minimum.

 

[XR250]

As I previously stated, I would highly recommend having a conversation with the Contractor to find out his comfort level with all the options presented before you go designing something.

 

[Engineerataltitude]

Excellent guidance. Very appreciated.

 

[Engineerataltitude]

Did a google search for hollowcore planks and there are many brands. I'm in CA, can anyone recommend a quality brand in this area?

 

[KootK]

I'm in CA, can anyone recommend a quality brand in this area?

You don't really want a "brand" for this. Select a comfortable plank thickness based on CrabbyT's table and let the contractor or owner shop the project around to different suppliers for a competitive bid.

 

[KootK]

Also, were you to do this in CIP, I wouldn't bother with software. I'd do it as simple span beams (steel or concrete) and one way spanning slab (CIP or steel deck). Those things can all be designed simply and reliably using the ASIC/CRSI manuals and steel deck supplier catalogs.

 

[driftLimiter]

Here is one vote for concrete over metal deck! No formwork needed, the precast option also is good with this. 24x24 is going to be a pretty substantial amount of concrete. Steel framing beneath is likely going to be an economical solution. Probably could get away with 1 or two intermediate girders.

 

[DoubleStud]

I think what OP was saying, wood framing 3.5" waterproof concrete topping.

OP, I would keep it wood framing + steel beams with waterproof concrete topping. Just read the IBC and design it per that for garage. Design for 40 psf LL OR 3K point load. I would make sure the joist hanger (if needed) has at least 1.5 kip capacity. I assume 2 hangers share the 3k point load if the wheel is close to the hanger. Put thicker plywood + waterproofing.

Metal deck on steel beams also a good option.

 

[KootK]

Ah yes... Weyerhauser has a pretty good document for topping over engineered wood, including recommendations for durability and dealing with wheel/jack loads: Link

For long term durability, I don't love any of these systems -- including concrete on steel deck -- without a topping over membrane setup. Some "second best" alternatives that seem to be deemed acceptable for residential applications:

1) Wood. Use pressure treated.

2) Concrete on steel deck:

a) Consider the steel deck to be sacrificial (non-composite) and design slab rebar to resist ULS loads.

b) Get your hands on some ventilated deck which may have cost implications.

 

[Engineerataltitude]

The project is still in design, so client doesn't have a GC yet. It is on the east side of the Sierra, about 250 miles from So Cal sources for pre-cast products. Maybe Reno or Las Vegas could be options. Transporation costs for precast hollowcore planks might be daunting. Especially for a relatively small amount. Will make some calls. If I spec those on my plans, I will end up having to tell the owner/GC how to get them. So I better be ready.

In the past (in 2006), for a 17ft x 22ft elevated garage slab with no posts, I spec'd 4" thick 4000 psi concrete w/ #3@8" E.W. over 3/4" 24" O.C. APA rated sturd-i-floor exterior T&G ply over W12x35 steel beams at 5'-6" o.c. (spanning 17ft) with flush framed 2x12's at 12". Modeled this in RISA 2D to limit deflection to limit concrete cracking. (These days most GC's I work with don't like to use sawn 2x12's for floors because they aren't very straight, I would probably spec microllams now)

Client for this project kind of panicked when he saw the W-BMs. Asked if I would look at other options that might be cheaper. Tha's what I'm doing now.

Thank you for the Weyerhaeuser doc on elevated garage slabs.

While I could design a concrete deck, I'm not confident there is local expertise to place it correctly in a residential application. That is usually a commercial thing around here...uses a different kind of GC and subs. I'll have to think about that option.

Never thought about the steel deck option. That might have some merit to pursue.

Thanks for all the ideas!