Punching Shear on Concrete Slab

[YoungPadawan]

Hi all,

I'm wondering if someone can shine the light on the problem that I am faced with. First, I do want to mention that I am quite new in Concrete Design. So here is my question:

I have a 200 mm thk concrete slab on grade that will be supporting a water tank. The tank has two cradles at each ends and the reaction that I calculated based on the weight of the tank filled with water is 62.5 kN. The contact area of the cradle to the concrete is 0.427 sq.m. I want to check the punching shear as well check flexure to properly design the reinforcement that I need. Could someone lead me to the right path to solve this problem.

The length of the concrete pad is 6430 mm and the the cradles are 4676 mm apart o/c, 877 mm from edge of concrete slab.

The width of concrete is 1766 mm and the cradle length is 1402 mm.

 

[KootK]

I'd be happy to help. You would need to provide a sketch or two as I'm having a hard time visualizing your situation.

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.

 

[YoungPadawan]

See link below. Any advice to approach the problem is much appreciated.

Note, I tried approaching it as a suspended slab supported by concrete column to check for punching shear, but I feel like I'm just lacking some more direction.

Thanks again @KootK

-YoungPadawan

 

[KootK]

Quality sketch. I've got some good news for you. I'd call this plain old one way beam shear rather than punching shear, owing to the considerable length over which the reactions are delivered. Much easier to evaluate. Depending on how you choose to go:

1) Combined footing design.
2) Beam on elastic-ish substrate.
3) Two independent footings with some feel good top steel & concrete between them.

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.

 

[YoungPadawan]

Thanks KootK!

If I analyze it as two separate footings (w/ concrete between them), will I not have the steel reinforcement at the bottom (COMP on top and Tension at the bottom) or T&B (some COMP and TENSION above N.A) and not just top?

I would like to know how one would analyze it as beam on elastic-"ish" substrate, do you have any recommended reading material?

Thanks again!

 

[JoshPlumSE]

I think the most appropriate way to design it is as a combined footing.

Two actually independent footings is possible. In which case you wouldn't need all of the concrete between them. I think what KootK is saying is that if you do fill the whole area between them with concrete (for ease of construction / forming), then it would wise to throw in some top steel as well. That's because it will start to mimic a combined footing with negative bending in that portion of concrete that's not really needed. And, the "feel good" steel will help prevent / control cracking.

Personally, I'd just design it as a combined footing.

 

[KootK]

If I analyze it as two separate footings (w/ concrete between them), will I not have the steel reinforcement at the bottom (COMP on top and Tension at the bottom) or T&B (some COMP and TENSION above N.A) and not just top?

If you used this approach, you would have only bottom steel required for flexure. That said, I'd throw in a top mat of temperature and shrinkage reinforcement (.0018 Ac) for the reasons that Josh mentioned.

I would like to know how one would analyze it as beam on elastic-"ish" substrate, do you have any recommended reading material?

ACI 336.2R-88 Suggested Analysis and Design Procedures for Combined Footings and Mats would be a good place to start. It will be considerable overkill in terms of theory for your situation. Most foundations texts books cover similar territory.

The reasons that I suggested the two independent footing approach are:

1) Based on your loading and proportions, I suspect that a combined footing approach would yield nearly the same answer anyhow.
2) It'll take you ten minutes. I like money.



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.

 

[YoungPadawan]

Thank you guys for helping me out and going above and beyond with your responses. This is has been a very knowledgeable experience.