Elevated Concrete Slab - Garage Over Basement 1

[adamt83]

I'm designing a suspended concrete slab over a concrete basement walls for a house garage. 24'X24' plan dimension.

Ideally I would like to avoid beams, but I know locally (Missouri) it has been done with one or two steel W shape beams to support the slab. However, this is the first time I am designing/detailing one and want to be sure I cover my bases.

2009 IRC is the local code and the live loading for garages is 50 psf live load. Also for elevated slabs a 2000 lb concentrated load applied over 20 in. sq. area. Is it intended that these loads are applied simultaneously? I would think not as the concentrated load is presumably to simulate a jack load but would appreciate clarification.

I plan to use a ledge, say 4"-6" wide?, in the perimeter foundation wall for the slab to sit on. I would also slope the ledge from the back wall towards the front doors so that the resulting slab is sloped towards the doors for drainage. The top of foundation wall would be constant, say 2-3" above the top of the slab on the back wall and say 6" above the slab on front, but notched down so the slab could sit on top of the wall at the garage doors.

As a two way slab, I presume that a simple way to calculate the appropriate shear and moments would be to use PCA's Rectangular Concrete Tanks with the various coefficients for flat plates? I do have that book but don't have access to design software--want to keep this as simple/direct as I can by hand. What would be the best way to detail the connection of the slab to the wall ledge? Bend some vertical wall reinforcement into the slab and use a pinned edge condition for the slab? Or would the reinforcement dowels create more of a fixed condition?

If using a beam at midspan in one direction (or say 2 beams at 1/3 points) would I be able to design as a one way slab since the plan ratio would be 2 or greater? Any special detailing for the slab over the beam? Should I simply pocket the beam into the foundation wall the same 4"-6" as the ledge above for the slab?

I wasn't planning to use galvanized steel decking as a permanent form and for composite slab action, but would appreciate feedback here as well. I'm not all that well-versed on such design/detailing. I do want to keep things though fairly traditional/simple for the typical local residential builder/concrete flatwork contractor.

Finally I think my recommendation would be for sealing or epoxy coating the floor for extra protection to the slab from deicers. Since there is liveable space below I don't want want any problems with slab. Any other thoughts here? Maybe epoxy rebar?

Thanks in advance for the assistance.

 

[jdgengineer]

In our area it is somewhat common to frame these garages with wood framing. Typically there is a steel beam to reduce the span of the joists. We commonly see LVL joists @ 16" oc with pressure treated plywood then vapor barrier then 3-4" of concrete topping slab with #3 bars for reinforcement @ 12" oc each way. Both Boise Cascade and Weyerhauser have good technical bulletins on this construction practice. If you do go with wood framing don't use TJI joists.

I much prefer concrete slab option over wood framing but for whatever reason wood is common ($$$). With concrete option we typically use steel pandeck to support the slab. We do not use shear studs so we design the beams as no composite. (assuming small garage). Without the metal deck the slab needs to be shored during construction which most contractors don't like.

We apply the jacking force and live load force as non-concurrent. We use 50 PSF love load and 3000 PSF jacking force (IRC and IBC conflict so we use most conservative of both)

 

[rdubs]

In my area, it is quite common to use precast planks for this application. No support beams are required for your short span. The precast company's engineer will design the planks and provide the installation instructions. They may even set the planks for you. A topping slab would be applied by your contractor to finish the installation.

 

[XR250]

I usually do 1 1/2" or 2" composite deck on I-beams and design one-way with the jack and live loads non-concurrent.
Don't know why you would not want to use composite deck.

 

[Teguci]

I'll second the precast plank option. For parking garages over retail space, the recommended system is a plaza system. That's not going to happen here, but know that drainage and leaking is a serious concern and can be a big headache if its done wrong. Definitely argue for something more than just a seal coat. If you go with a 2-way slab over a simple span you're going to be pretty good on your compression side. Detail your edges so they don't put negative moment (and consequently, top cracks) into your slab and allow for the 2 way slab to freely shrink.

 

[HouseBoy]

We usually see precast plank used for this application.
8" plank with non structural topping spanning 24 ft is common.
I'd bump the slope up a bit if possible - min 4.5" and preferably 6" total slope over 24ft.
We have seen sloping planks with uniform thickness topping and level planks with varying thickness topping.
Had one recently where the guy wanted a car lift in the garage. Went to 10" planks for that one.

Most planks are 4 ft wide. Another one we had recently was tough site access so they used 2 ft wide planks using lighter equipment.

 

[adamt83]

Thanks for the responses thus far. I will look into composite deck and the precast planks. If anyone has any resources to share, please post.

If I stick with my original plan, any further comments to the questions in the original post?

Teguci, would you mind elaborating on "Detail your edges so they don't put negative moment (and consequently, top cracks) into your slab and allow for the 2 way slab to freely shrink."? Do you have any details to share? Would you recommend NOT pinning the slab to the wall and possibly include expansion material around the slab perimeter to freely move?

Thanks!

 

[jdgengineer]

Do you all really see pre-cast concrete for a single-family residential garage? We have never run across that solution. Wouldn't that require a crane that may otherwise be unnecessary?

 

[HouseBoy]

Yes, many times.
One guy wanted a bowling alley under his auto court. Another wanted a rifle range.
Most people are building on a hillside and it is sometimes cheaper to add the extra space than it is to buy the gravel fill and support the extra weight (of the gravel).
PLUS- they get extra space!! (think man-cave)

 

[KootK]

The Canadian concrete code has a rather handy coefficient method for the design of rectangular plates. If you'd consider it, let me know and I'll fire up the scanner. What are the dimensions of your space?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jdgengineer]

Interesting. We've done a lot of big basements before, but haven't framed above with precast concrete. I'll check with our local contractors, but I don't think it's common in our area.

 

[KootK]

I just noticed that you already provided the plan dims. Sorry about that.

Is it intended that these loads are applied simultaneously?

Non-concurrent for me as well. In all honesty, I rarely even bother with CIP slabs. They're rather insensitive to the concentrated load.

I plan to use a ledge, say 4"-6" wide?,

Sure. The detailing sounds well thought out.

As a two way slab, I presume that a simple way to calculate the appropriate shear and moments would be to use PCA's Rectangular Concrete Tanks with the various coefficients for flat plates?

Could do if they have a condition that matches your scenario suitably well. I'd give some kind of attention to deflection as well as strength though.

What would be the best way to detail the connection of the slab to the wall ledge? Bend some vertical wall reinforcement into the slab and use a pinned edge condition for the slab? Or would the reinforcement dowels create more of a fixed condition?

That would work. I more commonly provide separate dowels to bend into the slab. I'd be inclined to design the slab pinned but detail the reinforcing for fixity and crack control.

If using a beam at midspan in one direction (or say 2 beams at 1/3 points) would I be able to design as a one way slab since the plan ratio would be 2 or greater?

Yeah, that would be appropriate. It might also allow you to design the slab using ACI's direct design method which is a breeze.

Any special detailing for the slab over the beam?

You'll want some slab top steel over the beam for both strength and crack control.

Should I simply pocket the beam into the foundation wall the same 4"-6" as the ledge above for the slab?

Yup. I'm sure it's fine but would recommend a few things:

1) Nominal top steel in the beams at the ends.
2) A bearing check at the ledge.
3) Beware of proper anchorage of your bottom bars at the support. You may need to terminate hooked or otherwise anchored bars beyond the centerline of the support to meet code. That can be tough with larger diameter bars. No big deal, just needs a little love.

I wasn't planning to use galvanized steel decking as a permanent form and for composite slab action, but would appreciate feedback here as well.

Finally I think my recommendation would be for sealing or epoxy coating the floor for extra protection to the slab from deicers. Since there is liveable space below I don't want want any problems with slab. Any other thoughts here? Maybe epoxy rebar?

Do you actually use deicers in MO? Might need to bone up on my geography some. In a warm climate, residential application, the durability of the slab doesn't concern me all that much. Probably just plain rebar and low permeability concrete for me. My main concern would be building envelope concerns related to the space below. That would tend to steer me away from the use of metal deck as I'd be concerned about water getting through the concrete, pooling over the deck, and causing corrosion or staining there. They make a breathable version of deck but I'm not sure how well that works. And I'm a bit leary of the slab doing its breathing into occupied space. All that said, XR250 seems to have used deck successfully in similar applications and he's a lot closer to you geographically. He's also a pretty sharp guy despite having recently fired me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[HouseBoy]

Check it out!

 

[XR250]

Most people are building on a hillside and it is sometimes cheaper to add the extra space than it is to buy the gravel fill and support the extra weight (of the gravel).
PLUS- they get extra space!! (think man-cave)

I usually sell them on a pseudo tornado shelter and the fact that the foundation walls do not have to resist the gravel backfill (I assume that is the weight you were talking about). Also, washed stone is expensive

 

[Tahoebrian5]

I do a lot of raised garages on hill side lots in the Tahoe area. Common practice around here is 4" conc slab over moisture barrier over plywood over LVL joists at 12" o.c. This allows you to spread the concentrated load to 2 joists. The joist space allows for insulation.

 

[adamt83]

Thanks to KootK and everyone else for the input.

A few things before I dive into this soon:

1. I checked with a few suppliers in our area and sounds like prestressed plank is not really an option locally, at least not yet.
2. I'll do some more digging on the metal deck, seems as there are certainly pros but the corrosion concerns with livable space below.
3. KootK thanks for the detailed response....a few things:

1) The beam I was thinking of adding would be a steel beam. So other than bearing check on the pocket, anything else I am missing?
2) Would shear studs welded to the beam protruding into the slab likely be necessary for either scenario with our without metal deck?
3) Designing the slab as pinned makes sense to be conservative on the slab itself. Can you elaborate on "detail the reinforcing for fixity and crack control"? Makes sense to me that even if we design as pinned it may in reality function with some fixity and some negative moment would be present at the support. Detailing for that is where I don't have enough experience to be comfortable. Can you elaborate or provide a resource/detail?

Thanks!

 

[KootK]

1) with a steel beam, most of the requirements that I mentioned go away except for the bearing check that you mentioned and the need for slab top steel over top of and perpendicular to the beam.

2) Shear studs aren't necessary for either scenario. In fact they'll be quite uneconomical if field installed because there would be so few studs relative to the cost of mobilizing the stud welding equipment. For the slab without metal deck, you'll need some way to provide lateral restraint to the beam top flanges.

3) For something of this scale, I'd probably just use #5 @ 12" hook dowels with 24" horizontal and vertical legs. That'll give you a connection and some nominal crack control around the edges.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[adamt83]

Thanks again KootK. Been preoccupied on a few other things the past several days; finally back on this/just now really getting into design.

I am proceeding with 2 steel beams, splitting the slab into thirds and designing as one-way. Also, no metal deck after some discussions with the owner and contractor.

Can you explain how you recommend providing lateral restraint to the top flanges of the beam? Is this just using web stiffeners to reduce the unbraced length of the beam for LTB? Or am I thinking the approach on this totally wrong?

 

[KootK]

No sweat Adam.

Can you explain how you recommend providing lateral restraint to the top flanges of the beam? Is this just using web stiffeners to reduce the unbraced length of the beam for LTB? Or am I thinking the approach on this totally wrong?

I just meant that the slab has to provide lateral restraint to the top flange of the beam somehow. Most commonly attachment to a metal deck accomplishes that. Some options here:

1) Put some shear studs on top of the beam.

2) Turn the slab down at the beam such that the top of the beam is encased.

3) Weld some little pieces of angle or bent rebar to the top of the beam.

4) Install some vertical bolts in the top flange that project into the slab some.

I'm just not sure if straight friction between the slab and beam top flange is reliable enough to be called sufficient bracing. It's a bit ironic really. This system is so old school that I'm not actually sure what's best. Shear studs seem simplest but I worry that, if they have to be field installed, you'd incur a steep mobilization penalty on the stud welding machine. Another good conversation to have with your contractor I suppose.

Web stiffeners alone don't cut down the unbraced length for LTB unless they're attached to something.





I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[adamt83]

Thanks KootK. Not sure I totally understand the concept, as before I talk to the contractor I want to be able to explain the need for it in layman's terms. I don't think he would understand how with gravity loads you need to provide a shear connection between the slab and beam. I understand the top flange is in compression, but don't totally get the "slip" that occurs between the beam and slab especially if the top flange is stiffened to prevent rotation. If I were designing as a truly composite system I think it would be easier to justify/explain, but in my case I plan to design each (slab and beam) independently.

Your idea about bolts drilled through the top flange seem to be the easiest to install; second would probably be using either angle or rebar welded to the top flange and projecting into the slab. However, if I can get away with nothing that would be even better. I know several similar single family residential slabs have been built over the years around here with no such provision. Now, that doesn't mean they were designed properly, but it does speak to the expectations of the owner and contractor.

Thanks again.