Vehicular floating plaza slab 2

[MacGruber22]

Does ACI or anyone else have a slab design guide for floating plaza slabs? I am expecting maximum axle load of about 24 kips (AASHTO H-15 truck), which concerns me a bit. Seems to me that the drainage board under the slab will not be rigid enough to prevent flexure in the slab.

"It is imperative Cunth doesn't get his hands on those codes."

 

[MacGruber22]

Bump. Why did someone star my post?

"It is imperative Cunth doesn't get his hands on those codes."

 

[TehMightyEngineer]

I imagine because someone has a similar question.

I'm not familiar with floating plaza slabs, can you provide a drawing/sketch?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[MacGruber22]

"It is imperative Cunth doesn't get his hands on those codes."

 

[TehMightyEngineer]

Interesting; I see what you mean. I imagine that as long as the drainage board has sufficient compressive stiffness to avoid displacement then the slab will transmit loads without curvature/flexure. I would think that the compressive strength of the drainage board would be well documented; similar to how rigid foam insulation gives a compressive strength for similar reasons.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[MacGruber22]

Yup - the stiffness is on my list of thing to investigate. I was hoping there would be an ACI design guide or similar that recommended some specs for the drainage board/tile.

One product I was looking at:

https://laticrete.com/~/media/datasheets/lds2900.ashx

That compressive strength of the system is very high - I am surprised. 432,000 lbs/ft^2

It may be a non-issue with something similar to the above.


"It is imperative Cunth doesn't get his hands on those codes."

 

[MotorCity]

Assuming the precast below is capable of supporting the load, I see it as being similar to a slab on grade. I would assume the precast is rigid, and design the slab based on the spring constant of the drainage board (assume it is the subgrade modulus as if you were designing a slab on grade). Unless you know the parameters on which the compressive strength of the drainage board is based, I don't think it will offer much help. In other words, does it assume that the load "crushes" the material by 1/16"? a 1/8". Similar to the bearing capacity of wood, it should be based on some "crushing distance" of the material.

 

[MacGruber22]

The subgrade modulus method was on my mind. This article provides some info:

http://www.constructionspecifier.co...tion-and-waterproofing-to-structural-support/

I won't have rigid insulation, only the composite drainage board, some sort of protection board, and the waterproofing - it seems the "470 pci" tested value for the assembly with insulation may work as a lower bound for me. I will have to search around a bit more.

"The authors have found hardscape/substrate interactions governed by compressive and flexural stiffness are often at the root of these types of problems (Cracking, spalling, and joint deterioration). Avoiding them requires understanding the behavior of not only the individual component materials, but also the resultant assembly. The authors have found the industry literature lacking in this regard. To fill in some of the data gaps, the authors undertook a laboratory test program to evaluate representative plaza materials and assemblies for compressive strength and stiffness."

It seems the manufacturer's aren't doing their part.

"It is imperative Cunth doesn't get his hands on those codes."

 

[wannabeSE]

In the Laticrete link there is a section through a typical assembly. The drain board looks like it has a profile similar to tiny metal deck. Wouldn't the concrete topping need to crush before the drain board deformed.

 

[MacGruber22]

Wouldn't the concrete topping need to crush before the drain board deformed

No? I don't understand the logic. What is inherently wrong with the discussion above treating the drainage board as a bed of springs?

"It is imperative Cunth doesn't get his hands on those codes."

 

[wannabeSE]

The picture on the first page of the link makes the drain board look like a cup cake tin. So it is like making concrete cupcakes with a topping slab. The concrete cup cakes will need to crush before the drainage board deforms.

I am assuming that the deformation of thin piece of drainage board below the concrete cup cakes is negligible. The deformation of the water proofing membrane may be higher than the thin piece of drainage board below the concrete cupcakes.

 

[MacGruber22]

Ah..I understand now. That is a good point, and seems like that style of drainage board would be better suited for heavier point loads. Thanks, wannabeSE.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

Bump. Why did someone star my post?

That was me. I meant to simply flag the thread for notification as it's a subject that's near and dear to me as well. However, after a loooong day of arguing about retaining wall joints, my motor skills were shot and I starred you by accident. You've had a couple of stars taken away from you in the past so I figured it just amounted to karmic scale re-balancing anyhow.

My advice, beyond what has already been covered.

1) I feel that something is missing from the SOG formulation: the flexibility of the underlying structure. I was taught to keep the structural slab at least 4x the depth of the topping slab to create a favorable stiffness ratio and keep the topping from doing any meaningful flexural work. I know of no rigorous basis for this.

2) It's pretty hard to keep these things from cracking long term. I like a mat of galvanized rebar at the top of the topping.

3) In your particular case, you may have topping spanning over the curvature discontinuity that occurs at the joint where precast plank ends meet. That's a good place for some topping top steel. And a control joint.

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.

 

[MacGruber22]

Hey there, KootK. Thank you, sir.

1. Definitely something to talk about (didn't want to go crazy with the first post). At this point I guess I should reveal my hand - It is an existing structure that used to have a plaza-style topping slab, and some bozo decided at some point (probably because the topping wasn't serviceable anymore) to rip it all up and lay asphalt down. The precast structure is actually similar to masonry dox plank (not nearly as old) which is 12" deep with a composite structural topping (2" that needs to be verified). As is common, the planks are keyed on their sides to interlock pieces (helpful for your concern). My plan thus far is to shore the entire soffit, remove the asphalt and chip the bonded topping mostly away (there is a lot of water intrusion; the topping may be fairly damaged). Rebuilding will consist replacing damaged plank with cast-in-place, restore a new thicker composite topping (sloped), and the rest of assembly above. Plank spans are 25'-0" bearing on cast-in-place frame. Based on your ratio of 4/1, I am almost there at 14".

2. Absolutely. Generous contraction joints, probably 10 to 12 feet on center. Our firm tends to use epoxy mesh for garage repairs, and the local contractors are used to that. In fact, times when I don't ask for it, they tend to be using it. They don't seem to understand why that is not OK to randomly substitute. Anyways...

3. Yes. Those are the locations I plan on laying out the contraction joints first, and working from there.

"It is imperative Cunth doesn't get his hands on those codes."