Re-entrant corners in suspended slabs
Re-entrant corners in suspended slabs
(OP)
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
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
RE: Re-entrant corners in suspended slabs
When in doubt, just take the next small step.
RE: Re-entrant corners in suspended slabs
But diagonal bars can do the job too so it is really a matter of personal preference.
BA
RE: Re-entrant corners in suspended slabs
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)?
RE: Re-entrant corners in suspended slabs
When in doubt, just take the next small step.
RE: Re-entrant corners in suspended slabs
RE: Re-entrant corners in suspended slabs
Mike McCann
MMC Engineering
RE: Re-entrant corners in suspended slabs
So Hokie to clarify - you would put diagonal bars on top of the third layer? plus extra reo in the main mat?
RE: Re-entrant corners in suspended slabs
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.
RE: Re-entrant corners in suspended slabs
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.
RE: Re-entrant corners in suspended slabs
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.
RE: Re-entrant corners in suspended slabs
RE: Re-entrant corners in suspended slabs
The exact distribution of reinforcing steel is not important provided the total moment is handled by a combination of positive and negative steel.
BA
RE: Re-entrant corners in suspended slabs
RE: Re-entrant corners in suspended slabs
RE: Re-entrant corners in suspended slabs
RE: Re-entrant corners in suspended slabs
RE: Re-entrant corners in suspended slabs
BA
RE: Re-entrant corners in suspended slabs
Dik