How to get your slope in an elevated slab

[dcarr82775]

For an elevated parking slab, primarily the slope is achieved either through constant thickness, or use of a flat bottom and varying thickness slab. Both have their good and bad points. What methods do you prefer?

I am leaning towards the varying thickness. In that scenario would the top mat also slope or would it typically be flat?

Thanks

 

[KootK]

This response is mostly a bump. I've always been curious about this myself. As the classy thing to do when bumping is to also add your two cents, I'll give that a whirl as well. By flat, I'll really mean mono-planar in what follows.

What methods do you prefer?

I prefer whichever method won't result in the contractor asking me to change it later on. I find that this tends to depend on the level of sophistication of the construction market and of the particular contractor. For commercial work on the west coast, it seems to go constant thickness following the desired slope. Of course, except for the simplest of layouts, there still winds up needing to be some build-up in certain locations. In the prairie regions, and particularly with residential work, it seems to go flat bottom with varying top slope most often.

In that scenario would the top mat also slope or would it typically be flat?

As a matter of practicality, I always keep the top mat parallel to the underside formwork. Chairing is just too complicated otherwise.

Some other thoughts:

1) From a purely technical perspective, I prefer the flat bottom scheme. I allow flexural bars to be cranked for a net change in slope up to 4% but, without fail, it makes me want to vomit. Especially with top steel.

2) From a purely quality control perspective, I prefer the flat bottom scheme. With cranked bars, placers need to get the cranks located just so which makes quality control even more difficult than usual. Additionally, when the bottom isn't flat, I find that columns often get poured to incorrect heights.

3) While I'm unable to locate it, I read something once that claimed that significant economy had been achieved in the past by forming the bottom of slabs as a dual planar, sloping trough. Imagine a pitched, V-shaped slab bottom with the low point in the middle of the drive lane. In this scenario, any additional sloping would be cast into the upper slab. This arrangement can be problematic, I think, for certain ramp layouts.

I would love to hear others chime in on the pros and cons of various construction methods. Same goes for you Dcarr.

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.

 

[dcarr82775]

In my perfect world the bottom slopes both ways resulting in a constant thickness slab. I always get asked to change it and I am never happy with what I believe Koot is referring to as cranked bars (bars across the folds or change in slope of the slab). Whenever I do it I always add another half inch to the reinf depth to account for the added construction tolerance problems. It is just a tough system to build and expect things to be built as nicely as you would on a flat slab.

Since I always get asked, this time I was looking at the flat bottom slab from the start. I have about 60-ft between drains so that is about 4-inches of slope to make up in the concrete. The weight doesn't add much to the foundations or the reinf in the slab if I slope the top steel, but like Koot, the chairs will be complicated. That pushes me towards using flat top and bottom mats, but then I have several inches of concrete 'cover' to the top steel on some columns lines. That screams big ugly cracks that scare folks, even though structurally things as fine. At that point I feel like I need to add another set of crack control top steel at some columns. That seems a bit too far out of the ordinary for what is a very ordinary covered parking slab.

I thought about the trough idea. Simple to form, a reasonable middle ground that allows for easy chairing of steel for the reinforcing that follows the profile of the forms. But I still get the problems over some columns where I get 3"+ of concrete cover.

So now I am thinking of going back to the constant thickness slab allowing for extra tolerance on the steel.

 

[hokie66]

My preference is flat bottom, and sloped top. I would also want to slope the top bars. I disagree about the complexity of chairing the steel.

 

[KootK]

I disagree about the complexity of chairing the steel.

Do tell. Is there a better way to taper the top steel away from the formwork other than to have chairs gradually increasing in height and needing to be located precisely?

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.

 

[hokie66]

No, but I don't think that is particularly challenging. Not too different from chairing PT tendons. Simpler actually, as you don't have different heights in the two directions.

 

[KootK]

The PT analogy is pretty persuasive, I agree.

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.