Updated: Apr 1, 2020
TIP - WHY DO WE DO MOMENT RE-DISTRIBUTION
What is redistribution of moments in a beam?
To understand what is moment redistribution, see the figure below.
Consider support A - Say after analysis, we got a moment of 100 and a steel of 2-T12+ 2-T20 is needed at support A. If we dont provide required steel, the beam will start to fail.However the moment crack forms,the stiffness or fixity reduces and the beam deflection increases.
This means the span moment will increase. If the steel at span is increased to take care of this new moment, the beam will not fail.
This is what is redistribution - An alternate load path!
Why do we have to redistribute in first place?
Consider that in a small house project,you have 20 numbers of beams and all of it except one needs only a max rebar dia of 16. Only one beam has a requirement of 20dia bar.
To avoid this , we can redistribute the moment from support to span thereby economise and avoid using a larger dia bar in support.
If you see the above figure, the useage of 20 dia bar can be avoided by redistribution in this particular case.
Note : If the moment in span was much higher before redistribution, then it may not be sensible to redistribute as the span will need 20dia bars anyways.
How much redistribution is possible?
The code IS-456-2000 allows moment redistribution of up to 30% in a beam.
Redistribution of moments may be done in accordance with 37.1.1 as per clause 22.7 of IS-456
If you read the clause , there are many conditions that needs to be satisfied if you are redistributing the moment. The main point is only 30% moment is allowed to be redistributed.
Note : Redistribution is not desirable in seismic zones and hence avoid for seismic combinations
Why is it restricted to 30% ?
Say we do 100% redistribution, that means we are making it like a simply supported beam.This means there should be no restraint.If we do that, then the frame will crack so badly that it becomes free at the joint like a simply supported beam. This will create durability,aesthetics and all kind of issues.
Though we say 30% redistribution,in reality redistribution may NOT even occur since the actual moment is less by around 50% (Considering a Factor of safety 1.5). So if the BM is 100 ,actual is only 50. Code is not letting you even reach there as 30% is only allowed. 70 is more than the unfactored moment 50. We also have a high Live load always considered if we follow the Live load code IS-875-Part 2
So in reality redistribution may not even occur unless the beam is over loaded.
Infact redistribution ability exists in all beams and all structural members even if you dont consider this in design and that is why many times a non engineered building also stands if the detailing is all proper!
A one way slab even if have slight deficiency of steel in short direction,may not have an issue as there is minimum steel provided in longer direction too. The slab will be able toredistribute load.
Redistribution purposely or by chance is a significant point to understand if you are a civil engineer aspiring to be a structural engineer.
How to do moment releases in ETABS and staad, how to learn etabs appreciating all these basics will be a separate discussion in another blog.However be sure that taking a training in etabs is not adequate unless you understand these points.
Before taking up online training in structural design and detailing ,ensure the content is rich and the structural design training covers structural scheme development,seismic analysis,basic principles of earth quake design,ductile detailing,stiffness modification, structural detailing and so on.