Re-entrant corners in suspended slabs 2

[demayeng]

Hi,

In slabs on ground, we put in diagonal bars at re-entrant corners to prevent shrinkage cracks propagating from the stress concentration at the corner.
With re-entrant corners in suspended slabs (say a 90 degree wall under) there is a similar stress concentration due to the bending under vertical loads - how do you normally deal with this stress concentration? do you put extra reo both ways in the top or do you add diagonal bars? Or do you just let relaxation happen and don't worry about it?
Just wondering what standard practice is..

Thanks for your help

 

[rowingengineer]

I normally put bars in both ways in the top steel, and also reinforce the wall for this moment so it done not crack up.

When in doubt, just take the next small step.

 

[BAretired]

Putting bars both ways at a reentrant corner is preferable in my opinion. It controls cracking in any direction away from the corner and is more consistent with the typical slab reinforcement.

But diagonal bars can do the job too so it is really a matter of personal preference.

BA

 

[demayeng]

Thanks guys

I guess putting bars both ways means you don't have an extra layer on top affecting your cover.. but then diagonal bars would be more efficient as they are directly normal to the crack you are trying to prevent..

How much extra reinforcement do you put in - is it a standard arbitrary amount (like for slabs on ground)?

 

[rowingengineer]

The amount required for the moment you generate at that point in each direction, or minimum crack control whatever requires more reo.


When in doubt, just take the next small step.

 

[hokie66]

There are some locations which you just know are going to crack. I go with the belts and braces approach, bars both ways and diagonal. But the diagonal bars in a suspended slab should always go in the third layer, not outside the main bars.

 

[msquared48]

I always attributed the difference between diagonal and main bars to the visual perception abilities of the drunk.

Mike McCann
MMC Engineering

 

[demayeng]

The bending moment out of Slabs (FEA program) right at that point would be impractical to reinforce against (see attached file for what i mean). My boss reckons that he hasn't worried about this before and that moment relaxation will remove the need for extra reo. The only reason the question has come up is because FEA can see that at that point the slab is trying to bend away in two different directions.

So Hokie to clarify - you would put diagonal bars on top of the third layer? plus extra reo in the main mat?

 

[hokie66]

demayeng,
To clarify, call the bottom layers 1 and 2. Call the top layers 3 and 4. Put all the diagonals just above 2, and just below 3. Sometimes I do increase the orthogonal bars, depending on how determined I am to control a crack at that point.

Your boss is correct about the FEA output in that the stress will be there whatever you do with reinforcement. You have to know when to be concerned about hot spots and when not. But I still like to control (not necessarily prevent) cracks that I know will occur.

 

[rowingengineer]

Demayeng,
This is a problem with using FEA, If rapt was in the area he would give you a complete run down on the situation, of how FEA can give you a bum steer, ie hot spots. However, since I am here I will give it a stab. so you have more of an idea of what Hokie66 is corretly pointing out.

The moment you see is because of the elastic analysis you are using, where i believe you would assume the same stiffness for the total slab. but at the point of the high moment you will have cracking, the reo is only there to limit the width of crack. Hence the stiffness should be reduced, thus less moment will end up there. Also the wall will not be able to handle the load as shown in your FEA and will propably allow some rotation, you can probably rejig you wall stiffness to account for this problem.

The next trick in the box is that the FEA results should be distributed/averaged over an area, how big you ask. I do not know the exact formula, I generally avoid modelling slabs in FEA, I just hate wood armer equations. But I would be thinking something like the width you would distribute a point load over, like 1m (3 feet)????





When in doubt, just take the next small step.

 

[asixth]

I provide trimmers around re-entrant corners in suspended slabs. I generally match the larger top or bottom bar that I am using and use that bar to trim around the entire opening.

Your model looks like the re-entrant corner is located over a negative moment region so I'm guessing that the section is going to be very heavily reinforced with top reinforcement so there probably isn't a need to provide additional trimmer bars.

 

[dik]

I often consider the slab wall location to be fully fixed since the re-entrant corner generally provides an equivalent stiffness several times great than the slab framing into it.

 

[BAretired]

If the typical top mat has 30 bars each way, it would be reasonable to provide 18 bars continuous and 12 bars hooked at the slab edge in each direction.

The exact distribution of reinforcing steel is not important provided the total moment is handled by a combination of positive and negative steel.

BA

 

[demayeng]

Thanks for all the replies. Sounds like it's a bit of a grey area that hasn't actually caused major problems in the real world to be a significant issue. Possibly the high amount of negative reo we generally put in suspended slabs combined with a small amount of relaxation at the corner is sufficient.

 

[apsix]

Attached is the current IStructE/Concrete Society advice;

 

[dik]

Thanks for the article... Dik

 

[vandede427]

Be weary of designing for extreme stress concentrations in a finite element model. I'm not saying they don't exist, but there's nothing wrong with distributing it over a longer length or greater area. It's what will happen anyway realistically, not just theoretically.

 

[BAretired]

Should that be weary or wary?

BA

 

[dik]

Vandeed427... I've always been of the opinion that if something works, but doesn't work theoretically... then maybe the theory is wrong.

Dik