Elevated Concrete Slab - Garage Over Basement
Elevated Concrete Slab - Garage Over Basement
(OP)
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.
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.






RE: Elevated Concrete Slab - Garage Over Basement
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)
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
Don't know why you would not want to use composite deck.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
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!
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
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)
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
Non-concurrent for me as well. In all honesty, I rarely even bother with CIP slabs. They're rather insensitive to the concentrated load.
Sure. The detailing sounds well thought out.
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.
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.
Yeah, that would be appropriate. It might also allow you to design the slab using ACI's direct design method which is a breeze.
You'll want some slab top steel over the beam for both strength and crack control.
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.
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.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
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!
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
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?
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
The easiest to install in new construction are shear studs welded to the top of the beam. This is done in the shop and is shipped to site ready to go.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
It's the slip implied by the deflected shape shown below for lateral torsional buckling. As Jayrod mentioned, stiffeners don't do much of anything to help that. In many cases, I suspect just plain old friction between slab and beam would be enough to restrain the top flange but that's not a risk I'd be willing to take myself. Not for the cost a few studs. If the slab soffit is constructed at all lower than the top of the beam, the beam flange may well be keyed into the slab in a way that prevents movement. I'd like get a look at the underside of these other projects that you mentioned to see what's up there.
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.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
Post installed anchors might be the simplest but I'd also consider just having holes drilled in the top flange and adding bolts with a nut above and below the flange as a way to get the top flange well anchored by the slab. Generally I think 2% to 5% of the flange force is recommended to be sufficient to provide the needed bracing.
Thinking about costs, I'd weigh the cost of the bolts (or any other strategy) verses the additional steel weight assuming something like $2 per pound. That ought to be a reasonable estimate.
RE: Elevated Concrete Slab - Garage Over Basement
RE: Elevated Concrete Slab - Garage Over Basement
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.
RE: Elevated Concrete Slab - Garage Over Basement
I'll probably proceed with shear studs. Besides just making an assumption on type/length/spacing, can anyone point me to the appropriate procedure for design?
If I'm still hell-bent against studs, I guess one other possibility I could pursue would be since Lb > Lr is to calculate the critical elastic moment, Mcr. I could then use that Mcr as Mn. Basically, use LTB as the limit state for flexure rather than yielding, correct?
RE: Elevated Concrete Slab - Garage Over Basement
In practice, I'd just provide 3/4" diameter studs at 24" o/c and sleep easy.
In theory, I suppose that one technically ought to:
1) Use AISC appendix 6 to work out the strength demand on the studs such that they can serve as bracing.
2) Design the studs per AISC to meet that strength demand.
Normally you consider stiffness as well but with the stiffness being provided by a monolithic concrete slab here, there's no need/point.
Yup. That should shake out of the code specified moment calculation procedures however. No need to reinvent anything.
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.