Parking Deck PT Slab - Slope for Drainage

[VT17]

Hello,

I'm curious as to how the slope of a PT slab for the top level of a parking deck should be constructed/analyzed for adequate water drainage. Per the following thread,

https://www.eng-tips.com/viewthread.cfm?qid=154500,

the advice I like the most is keeping the slab thickness uniform in lieu of having a thicker slab at high points. My question is would the formwork then be laid out in triangle shapes so the drainage profiles can be achieved (see image for diagram)? I can't think of another way where a uniform slab thickness is maintained while allowing for proper water drainage. When it comes to modeling, would it still be reasonable to model the slab as a perfectly level slab (I'm using ADAPT if that makes any difference)? Any other thoughts or suggestions as to how people handle the slab thickness for the top level of a parking deck?

 

[Just Some Nerd]

I'd definitely recommend avoiding trying to slope the slabs (from beneath). The way my workplace does it (also aus) is to model a uniform slab with the minimum thickness, add some additional self-weight for the concrete you're ignoring, and off you go with the design.

 

[CURVEB]

I work in the US and have seen slopes achieved using many different configurations, including what you described above. It is important that you establish the manner of drainage, because as you said if they want to keep the soffit level and just slope the top surface to the drains you will have more thickness than a flat slab on average. I'd also recommend establishing your concrete cover requirements as it relates to the PT and study how that may affect the profiles of your tendons. Further, if you are using the flat soffit/sloped top strategy it will change the overall thickness of your floor structure which may have an impact on architectural requirements.

One simple way to handle this is to base your design on a "minimum" slab thickness which is effectively your thickness at the drain. All of your strength and serviceability calculations are based on this thickness, as well as your detailing for the rebar and tendons. The added concrete to create slope is treated as a superimposed load. If it is cast monolithic with the "structural" portion of the slab, technically you would have some added strength and stiffness benefit but since you don't have the same amount everywhere I think most engineers would say it is conservative to ignore this "benefit".

If you have a builder already involved in the project I'd recommend discussing this with them.

 

[rapt]

With PT design, you cannot ignore the "extra" bits and simply design based on the minimum thickness. The height of a tendon relative to the concrete centroid at any location will affect the capacity by changing the Secondary moment effects.

Most builders would prefer to us a flat soffit with varying depth of concrete.

 

[la belle vie]

Can you make the PT slab thickness uniform and add topping with some minimum thickness that is sloped to drain? This is what I’ve usually seen. The topping could be lightweight.

 

[hokie66]

Topping is unnecessary. It is not difficult to build falls into the top slab surface. But it is preferable to slope the whole slab in one direction to external gutters, rather than internal drains, which are more prone to clogging.

 

[dik]

Just the top level? with our climate, in these environs, most levels have a marked slope for drainage. Pretty much as Rapt, notes... I've seen toppings with a membrane work...

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

-Dik

 

[VT17]

Thanks everyone for your input so far. I would agree that if a slab is to be monolithically cast with varying thickness, the analytical model should reflect that to capture the secondary moment effects appropriately. I think I've figured out how to do this in ADAPT if I am to go this route. If a topping slab is used, then I just need to worry about the additional weight.

Topping is unnecessary. It is not difficult to build falls into the top slab surface. But it is preferable to slope the whole slab in one direction to external gutters, rather than internal drains, which are more prone to clogging.

Hokie66, what do you mean by falls into the top slab surface? Also, I don't think for this particular project they're going to go with external gutters but I'll consider it.

Just the top level? with our climate, in these environs, most levels have a marked slope for drainage.

Dik, I'll coordinate this with the architect. When I wrote this I imagined the top level only, but most likely it would apply to all levels.

 

[dik]

With road salts, etc... good drainage is necessary to reduce corrosion issues... and good expansion joints... you can either have a good joint, or a cheap one... good joints are pricey.

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

-Dik

 

[Just Some Nerd]

With regards to the extra bits...does anyone have experience/seen an example of the design difference between modelling the thickness correectly and choosing not to? I can't say I've encountered a company that doesn't just use the minimum thickness (admittedly, mostly D&C contractors that aren't always bright bulbs), and like CURVEB said it's generally just assumed to be conservative but I'm wondering now if everyone is mistaken. For some companies I'm not sure how they'd even go about accounting for this in terms of workflow, because of software limitations (3D software such as RAM Concept doesn't have falls, and 2d software while capable of accounting for walls aren't great for large non-uniform slab layouts)

----------------------------------------------------------------------

Why yes, I do in fact have no idea what I'm talking about

 

[jerseyshore]

dik is right on about sloping all levels and using drains. Cars bring in a lot of water with them and if you live in any type of climate with snow, the salt is coming in with it.

I recently checked out a 2 or 3 story parking garage below grade (below an apartment building) and the corrosion was significant. Not quite as bad as a normal exposed parking garage, but enough to cause concern and much more than you would expect for a structure that is totally protected from the elements.

 

[rapt]

And relatively thin variable thickness toppings added to the top to provide slope do not remain connected for very long!

 

[JohnRwals]

If you plan to pour topping concrete later, do you add 'intentionally roughened surface with 1/4" amplitude' requirement
to the Spec or/and structural drawings/notes/details?
I wonder how topping concrete overlay can do composite action or monolithically in this situation.

 

[hokie66]

VT17,
Building in the falls just requires competent use of a laser level to adjust the thicknesses over the slab. Rapt has identified the reason not to use a topping...it won’t stay put. And anyway, the same profiling operation is required for a topping as for monolithically placed, so topping just duplicates effort.

I would try harder to eliminate internal drains. If you have a hydraulics engineer on the project, I think he would agree.

 

[dik]

I've only been involved in the design of 20 or 30 of them... including parking layout and entrances and exits... One of my first employers specialised in RC parking garages and used 'excessive' cover long before it became trendy... You should be looking at min slopes of 1-1/2% or so...

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

-Dik

 

[Teguci]

Keep the outside edges level because architects. High point to drain should be 3/16 inch per foot, minimum. Don't think about it as triangles but as a twisted quads - no valleys except over beams. Precasters get concerned but forms shouldn't care. Will want about 3 to 4 ft of wash area to slope away from interior columns and beams to drain (can't have the drains in these areas anyhow). Not sure how I'd configure the ramp.

 

[dik]

hyperbolic parabaloids???

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

-Dik