Concept of Unbalanced Moment in Flat Slab
Concept of Unbalanced Moment in Flat Slab
(OP)
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.






RE: Concept of Unbalanced Moment in Flat Slab
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.
RE: Concept of Unbalanced Moment in Flat Slab
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.
RE: Concept of Unbalanced Moment in Flat Slab
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.
RE: Concept of Unbalanced Moment in Flat Slab
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.
RE: Concept of Unbalanced Moment in Flat Slab
DaveAtkins
RE: Concept of Unbalanced Moment in Flat Slab
RE: Concept of Unbalanced Moment in Flat Slab
RE: Concept of Unbalanced Moment in Flat Slab
The process of assuming a different moment is called 'redistribution of moments' and is limited to 10% in many codes.
RE: Concept of Unbalanced Moment in Flat Slab
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.
RE: Concept of Unbalanced Moment in Flat Slab
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.