Concept of Unbalanced Moment in Flat Slab

[ali07]

Just have a situation where we are checking flat slab for transfer of unbalanced moment to shear. There are different span lenghts. I am just wondering that this unbalanced moment at support can be ignored if you designed for more moment at midspan. Like if we have more moment capacity at midspan then is it really necessary to check for unbalanced moment again.

 

[UKNZEng]

Your assumptions sound right assuming it is a typical flat slab with nothing too unusual. You have to be realistic about how accurate your analysis of a flat slab really is. Even the best FE analysis is still only an approximation, so I wouldn't get carried away with too much analysis.

As long as you provide a sensible reinforcement layout for the total demand you will be fine. I wouldn't worry too much if you are a little short here or there, you can redistribute your moment around. If you are a little short at your support you can push the demand to the mid span where you have more capacity. The BS allows upto 15% moment redistribution.

 

[PanamaStrEng]

A good chunk of the unbalanced moment has to be resisted by shear capacity in the supports. In my opinion, this has very little to do with how much flexural capacity you have or how much moment redistribution you are allowed by code to use.
I don't think you can neglect it. I would not. When talking about shear, it is better to be on the safe side.
Also, even if the spans were of the same length, remember to check if you are required by code to do unbalanced live loads, which may also produce a similar effect.

 

[JAE]

I don't think you can neglect it. I would not. When talking about shear, it is better to be on the safe side.

I think PanamaStrEng is correct. Shear is an abrupt failure that shouldn't be taken lightly and it is also correct that extra negative moment doesn't affect the unbalanced moment taken by the shear. Be careful.

 

[ali07]

I personally don't want to ignore this. But just need to know that by ACI or CSA, what are the max. percentage of redistribution of moment is possible.

Actually i was thinking this only if for some reason desired strength havn't achieved but the concrete is still acceptable as per code.

Is 15% as per BS is same as in ACI or CSA.

 

[DaveAtkins]

I also agree it should not be ignored, but the column will only attract as much moment as it can accept. If you have nominal reinforcing extending up out of the column into the flat slab, the top of the column will take some moment, then become a hinge and the unbalanced moment will stay in the flat slab.

DaveAtkins

 

[ron9876]

Punching shear in flat plates is a very serious matter. Both the equivalent frame method and the method used to calculate the moments/shears and related shear stresses are approximate. If you get close to the actual conditions for the flexural design the slab will work as long as the slab/column connection works. If you get very far away from the normal assumptions for these approximate methods then you are moving into the unknown and in my opinion that is risky for punching shear.

 

[PanamaStrEng]

I agree with DaveAtkins in that the column will only attract as much moment as it can accept. However, you may end up with a brittle shear failure before the hinge forms, because the slab still has to resist a part of the unbalanced moment by shear mechanism. Additionally, the formation of this hinge may result in a lot of unnanticipated cracking, right? I´ve seen some older designs where continuous slabs where designed on the basis of simple spans (or very close to that), with little top reinforcement over the columns, but with column capitals for shear. There is a lot of flexural cracking on the top of the slab in some places. It may be safe, but I question the serviceability aspects of those designs.

 

[csd72]

The further away the assumed design moment distribution is from the elastic moment the more likely is is to undergo significant cracking.

The process of assuming a different moment is called 'redistribution of moments' and is limited to 10% in many codes.

 

[ali07]

csd72
Appreciate guys for help.
Just want to know even for example 10% moment redistribution in flat slab would be dangerous to assume.
Actually we are checking existing design, where midspan reinforcement is far more than at support(support is fine too). But i was worried to assume 10-15% redistribution so that slab would be fine for full loading. Does slab tend to become pin for redistribution or it is safe to assume redistribution.
I am always conservative in my design, especially where failures are abrupt. But this was an existing situation, where dismantling of slab would cause millions to client, that's why i tend to check slab based on allowable code limits that it can work and perform without cracks.

 

[rapt]

Redistribution is only allow for ductile members and ductile failure modes.

For the slab design you can redistribute up to the code maximum depending on the ductility of the slab, but for punching shear design no redistribution is allowed. If the column can attract the moment then you must design the connection for that moment.