Shoring required for topping slab?

[dylansdad]

I have a lightweight topping slab set to be placed above an already cured and stressed post tensioned slab.
This topping slab is to facilitate drainage above a party deck and will receive pavers, etc. for a finished floor.
The structural slab supporting this topping slab is a 13" two-way PT slab designed for a live load of 100 psf.
My question is can this lightweight topping slab be placed without shoring the 13" PT slab?

 

[phamENG]

Was this designed as a composite system? In other words, does the topping contribute to the strength of the final construction?

 

[KootK]

If the base slab is stressed and cured, as you say, I don't see that you'd need the shoring. Presumably the slab is already designed to carry the topping (albeit the dry weight) and some snow/construction load.

 

[dylansdad]

Thanks for the quick response. There is no composite action and yes the slab was placed and tensioned around 6 months ago.
My initial thoughts were no shoring should be required.

 

[Celt83]

...(albeit the dry weight)...

So I've seen this mentioned a bit and have looked into it a couple times but all the research I've done on the subject indicates that the wet weight and the dry weight are relatively the same because the water is either captured in the hydration of the cement or encapsulated within micro-voids of the concrete which leaves only a small percentage of the water in the mix that can actually evaporate off.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[jayrod12]

But you have to account for them piling wet concrete in one location prior to moving it around. So if they're putting a 2" topping on the entire thing, you could end up with a localized piling of 6 or 8" deep. although in that scenario, you likely wouldn't have the standard 50psf construction load so you'd probably still be within the design loads for the PT slab.

 

[phamENG]

I agree with Celt - my research revealed the same. As long as the topping slab was considered in the original design, I see no reason to shore it.

 

[Ingenuity]

Your 13" thick PT slab for LL of 100 psf only spans 26'? L/D is pretty small.

You are certain it is a post-tensioned slab?

 

[dylansdad]

Ingenuity,
It is definitely post tensioned and 13" thick.
The general spacing for columns left/right is 27'.
There are lower floors post tensioned and 9" thick.

 

[KootK]

That's good to know about the wet vs dry weights guys, thanks. I'll concern myself with that no longer.

 

[DTGT2002]

It's been said a number of times, but this topping slab is explicitly noted with unit weight assumptions and maximum depth in the structural drawings? If yes, I agree that there is nothing to be concerned with from a loading point of view and your 100 psf live load rating exceeds the generally accepted 50 psf for construction loading in the concrete formwork industry, which is expected to help cover placing activities, not using a powered buggy.

A question would be how is the placing to occur, but even if by powered buggy, 75 psf is recommended, which is also within your rating.

Is the topping explicitly shown in the drawings or associated calculations?


Daniel

 

[dylansdad]

DTGT2002,
Yes this topping slab is explicitly shown as being placed above the post tensioned slab. It is used to facilitate
drainage for a roof observation deck. It will be placed via pump.
Thanks for all replies and comments.