Suspended slab saw cut 3

[rowingengineer]

Back ground: A dumb contract decided a flat slab basement suspended slab needed to be saw cut. The saw cuts are on grid, the columns are on grid, the saw cuts are 40-45mm deep. The cover specified for the project was 30mm, thus assuming all the negative moment steel has been cut in one direction, and possibly 50% or more in the other direction. The slab is still fully propped at the moment; total depth of slab is 300mm, size 30m (100’) x 60m (200’).

The slab depth is sized for punching shear. Don’t have enough reo in the slabs to span from column to column without the continuity of the slab over the columns.
Problem: The client wants to keep the slab but isn’t too concerned about deflections for this level. Says he is willing to live with 3-4 time the original predicted deflection.

Possible solutions:
1. Provided carbon fibre strips to the top of the slab to replace the negative reo, as well as installing some shear studs/bolts. I however have concerns about the durability of this solution with regards to the carbon fibre on top of the slab and in such large amounts. Am considering putting it on the underside, but this would mean $$$$$$.

2. Provide a tensioned system to the underside of the slab. However have limited head height thus not very efficient.

So does anyone have any ideas/thoughts?


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[hokie66]

With no top reinforcement at the columns, I don't know how you would overcome punching shear issues. Even with post-installed shear reinforcement, the flexural reinforcement has to be developed.

Any chance you can reveal the identity (at least a clue), of this contractor?

 

[Tomfh]

Top cover can vary wildly so you should check exactly how much reo has been cut rather than assuming it.

 

[dik]

Do you have enough distance from the punching shear failure surface to the sawcut to develop the top rebar. If not, you may have a real problem <G>... I already think you have a bit of one.You likely have enough bearing; it's a matter of trying to determine what the punching shear capacity is. If the slab thickness was predicated on shear cap, then you may have to do something at the column to increase the shear perimeter.

Dik

 

[Ron]

RE...I agree with Tomfh...cover can vary a lot. Check it first.

You might consider a load test to see if the slab is reacting as designed. It might have quite a bit more capacity than anticipated.

 

[OzEng80]

I take it extra support (intermediate retrofit columns) is out of the question? Might be able to get away with skinny steel columns on the carparking grids (assuming it is a basement carpark).

What about a bonded structural topping (basically another slab poured over the top) - if your floor levels are ok and the substructure can take the additional weight.

I would also like a hint as to who the genius with the concrete saw was!

Good luck.

 

[hokie66]

That's the idea, OzEng80. A bonded topping with new top bars. Should be a lot simpler and more economical than the alternatives.

I wouldn't try to load test this thing. With punching shear a big concern, it could be dangerous, and to me wouldn't prove anything.

If the top bars are low enough to not be affected by the sawcuts, that in itself is cause for rejection of the slab.

 

[rowingengineer]

Bloody good ideas!

OzEng80 and everyone, I owe you a beer . The bonded slab is definitely worth putting it through the wringer to see if it stacks up. It feels more economical than my solutions and gets the tick in the box for the shear problem which I and everyone (as it seems) was concerned about.

As for hints, a few of my colleagues are members of the forum, so I have made a few changes to the storey, just so I couldn’t be accused of unprofessionalism ect. Because as you can imagine this is very sensitive subject for all involved, there is fair bit of egg on people faces. So sorry guys but I will have to keep it under my hat until the storm blows over.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[civeng80]

Pretty silly putting a sawcut in a suspended slab. Defeats the whole purpose of doing structural computations for dept of slab.

But just on the sideline, for large suspended slabs is it fair to say that crack control is solely from the reinforcement in the concrete (apart from any construction joints !) ?

 

[hokie66]

I think that is a fair statement.

 

[rowingengineer]

large suspended slabs, depends on what you define as large. normally I wouldn't pour such a big slab without a contraction (expansion) joint, however This slab had details used to reduce restraint at edges and there are no walls. however I don't think i need to worry about shrinkage cracks, in the slab now, with something like 360m of saw joints.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it