Suspended Garage Slab Thickness 7

[Khashayar1]

Hi everyone!

I hope you are all doing well. I have a client for whom I am designing a suspended slab for his two-car garage space(5600mmx6600mm), beneath the garage would be a future theater room. I have designed the slab to be supported by two steel beams along the 5600 length (no interior column), these two steel beams (W310) will seat on a 6"x"8" pilaster (no pocket into the foundation wall). and I'm also proposing a composite slab comprised of a 76mm steel deck, with concrete topping and 15M rebars @ 250mm on both directions.

For estimating the total slab thickness, I have used the one-way slab design approach and rule of thumb of Ln/20, considering length of 5600, with 1.0 m width, have resulted in a 12" (~300mm) slab thickness, which I have a sense that it is over conservative. I would appreciate your thoughts about the appropriate thickness in this scenario. Do you think if I go with a two-way slab design I can achieve a more optimized slab thickness? or is 300mm just as it should be? any thoughts or suggestions is much appreciated!

Thanks!

 

[GeorgeTheCivilEngineer]

If you are using a composite deck system then the assumptions suitable for a RC slab are not applicable.

 

[Smoulder]

You have three spans of 2.2m.

 

[To Madeira]

My comments for what they're worth:

- I assume you mean W10x39 beams not W310? The beams would need to be dropped not flush assuming you have the headroom below.
- I would use a composite metal deck and avoid using 2x framing with plywood - stripping and removing the formwork through that small door would be a PITA.
- I don't think you need air entrained concrete if there is a roof but consider using lightweight concrete if that is readily available in your area.
- Make sure your slab design accounts for the 3000lb point load from a car jack requirement per ASCE 7-16 4.10.

I would avoid using a mix of metric and SI units in posts as well.

 

[BulbTheBuilder]

Looking at the spans, I have a feeling two-way slab is going to work for you. I am seeing more imperial units, so I am going to assume you're using American code. Garage is 40psf (LL) and as To Madeira (Structural) mentioned, don't ignore ASCE 7-16 4.10.

Concrete grade of 32mpa (4640psi; I simply multiply by 145 for my conversions) and assuming rebar grade of 60ksi, you can check with an 8" thick slab. Typically, you might want to do an initiate punching shear check before you get into the details to save yourself time. I also see "slopes towards door", if you intend to slope be sure to check the thickness for that area. E.g. You can design for say 8" and have it like 8.5" with 0.5" account for the slope.

Be careful not to "overestimate" the thickness when trying to achieve your slope. Shear capacity decreases with increasing thickness at a certain point. Good to always recheck with some quick simple calcs. ACI chapter 8 and 22 contains all the information you'll need.

 

[canwesteng]

I'm not sold on composite deck being appropriate for moving loads, but it will be much cheaper to use it as a form than to form and strip with plywood. Size the depth of the slab for the punching shear from ASCE, and then size the bar for it. Span the concrete one way between the 7-3 beams and you're probably looking at 5" of concrete

 

[jhnblgr]

Composite decking is used on bridges

 

[Khashayar1]

Thank you all for your informative input and suggestions! Based on the discussion, I am now tending toward the RC slab, and not the composite one. Initially, I wanted to go with composite to benefit from the induced composite action on the W-beams below to increase their bending capacity.

I'm in Canada, Ontario, so the building code I'm using will be NBC, and CSA-A23.3 for concrete, and CSA-S16 for steel. Some architects here use imperial, I'm more comfortable with metric, and that's why the draft plan I posted has a mix of units! will make sure to make it uniform before delivering it!

With regards to loading, I'm taking the minimum live load of 6 kPa as per the NBC (image below):

and I will consider the concentrated load of 36 kN over a 120mmx120mm area as per the table below:

However, I'm still not sure if in this case, we will see a two-way slab behavior or one-way. If we take the span ratio between steel beams, then I will have (5600mm/2200mm)>2, and I can assume one-way behavior will govern this floor system, right? Please see the partial plan below for an illustration of the 1m strip and the direction of load transfer that I'm assuming.

And on another note, regarding the slope towards doors that BulbTheBuilder_EI mentioned, I'm thinking to add a general note saying the slope is to be provided by arch finishes, and not the structural slab sloped, so that I can assure I have a consistent structural thickness throughout. What do you think?

 

[To Madeira]

The aspect ratio is more than 1:2 so the slab would behave as a one way slab.

 

[JAE]

You can set the bottom (soffit) of the slab - the formwork - flat. Then have the finishers slope the slab how you want it.
It makes for a little more control/management during concrete placement but would provide you with a simple and clean way to get a sloped floor.

The flat soffit makes the formwork and beams simple to place. The design would be based on the thinnest depth unless you want to economize by determining shear/moment/rebar with different depths, etc. but that seems like a lot of work for little gain.

You can still go with a composite beam-slab interface with this using welded studs, etc.

Another concern is water penetration/rusting. With a home theater underneath (or any occupied space for that matter) you need to have some type of waterproofing.

Two options I've used:
1. Build the slab as I've described above and use a top surface traffic bearing waterproof membrane system. There are many out there - we've used Autogard by Neogard (

https://www.neogard.com/coating-systems/traffic-coating-systems/auto-gard-systems/auto-gard).

There are many others. This is an elastomeric coating with sand grit that looks like a paint product. When the waterproofing wears thin due to tire wear you can "see" the damage and repair it on the spot.

2. Place your structural slab, then top with a waterproof coating system and then cover with a traffic bearing concrete topping that is sloped to drain. The topping would be a typical slab with control joints and joint sealant. The waterproof layer would need to have some tie-in to several drains throughout the floor area so that when water infiltrates the topping (it will) then the water would travel to the drains and the waterproofing would be flashed into them. There are many systems like this that also include insulation layers - see the attached brochure for a Carlisle system.

The problem with option 1 is that you have to insulate the slab on the bottom of the slab. Vapor control might be a problem. The problem with option 2 is that if the waterproofing ever leaks, you don't know where exactly the leak is and large portions of the traffic topping must be removed to find the location.

 

[hokie66]

Mixing units never goes well, in design or construction. Thus the confusion of some here.

 

[BulbTheBuilder]

With regards to loading, I'm taking the minimum live load of 6 kPa as per the NBC (image below):

6kN/m2 is roughly 120psf. For such a load, deflection won't control slab thickness rather punching shear. I still believe 300mm (12") is going to work. You might even get 275mm (10.8") thick slab to work for your spans.

However, I'm still not sure if in this case, we will see a two-way slab behavior or one-way

If you design for a flat plate, you'd want to design it two way and check for check shear both two-way (punching) and one-way.

And on another note, regarding the slope towards doors that BulbTheBuilder_EI mentioned, I'm thinking to add a general note saying the slope is to be provided by arch finishes, and not the structural slab sloped, so that I can assure I have a consistent structural thickness throughout. What do you think?

Better make it bold, bigger, and placed on a portion of plan that can easily be seen. You don't want it somewhere at note #7 at the bottom of other generic and mundane notes.

Another concern is water penetration/rusting. With a home theater underneath (or any occupied space for that matter) you need to have some type of waterproofing.

This is one important factor to consider. Most problems for parking are often corrosion. I recently attended a webinar on a similar topic. Don't take it lightly... the cost of remedy of the aftermath can be expensive if not thought of during the initiate phase of design and construction.

 

[BridgeSmith]

The slabs for our highway bridges, supporting 72 kip trucks with 16 kip wheel loads are 8" thick, with spans up to 9'-9". They're reinforced with #5 bars at 9" spacing longitudinally, and #5 at 6" transverse. Punching shear is not even a design check.

 

[hokie66]

This is just a one way slab supported by walls and beams, with nominal top reinforcement at edges.

Not sure why all the concern about waterproofing. A good trafficable coating, sure, but isn't there a roof over?

 

[JAE]

hokie66, yes probably a roof...but I was concerned about water/salt dripping off the cars as they drive in.
In the midwest USA and other areas where salt is used on roadways it can be a killer to garage slabs suspended over an occupied space.

Concrete is simply a hard wet sponge.

 

[hokie66]

Yes, that would be location driven.