Question on the purpose of Moment Redistribution

[Enhineyero]

Hi all, I would like to ask a simple question regarding moment redistribution in the design of RC beams. 1st i will layout what is my basic understanding on why Engineers use this:

1.)To take into account the scenario when the beam is loaded in its ultimate stress level and hinging will occur the supports on the beam and thus plastic behavior will start to function on the beam

2.) To reduce the rebar near the column to avoid congestion.

So my question is

1.) other than reasons that i stated, is there any other purpose of Moment redistribution.

2.) If i designed my beams based on the elastic moments and shears and not the redistributed moments and shears. is my design approach acceptable, or would it pose any problems? because the code does not specify that we design beams based on redistributed forces and moments, but we MAY. in other words its just optional to redistribute the forces (that is how i interpreted what is written on the BS code, i think ACI has a similar provision)

 

[ishvaaag]

I think the historical reason for what redistribution was introduced was failure of performance the negative reinforcement rebar; this quite likely was in the plain rebar era ... so a risky situation appeared when too little positives' reinforcement was standing. In fact this reason survives in some codes diffused in some minimum steel for one way ribbed floors -sometimes to extravagant amounts from an a rational look to redistribution- and beams and slabs to supports (mixed with of course final safety hanging from supports).

In any case in Spain like you say of UK it stands a permissible thing, and not a recommended or mandatory one.

Other main reason is that there being bigger the solicitations at supports, as the load grows out of flexural cracks some bigger rotations than those from elastic fixity or deformation can be expected at the supports than midspans, hence it may be thought that some degree of moment redistribution can lead, average, to a more accurate portrait of the relative demands in supports and midspans at the limit states.

 

[asixth]

Helps the engineer lighten the reo rate. In the days of patterned live loading where a moment envelopes are created such that the ultimate negative moment and ultimate positive moment are never co-existant, I allow some of the negative moment re-distribute back to the positive moment region if it allows me to lighten the reo.

 

[PicoStruc]

Another reason is...

In real life, rebar may be badly placed on field in a way that you cannot develop the right development length in negative moment zone. Another exemple, is that top slab rebar with hook located in a wall (thin) cannot be fully developped at the wall face location.

Remember that your software that simulate 100% fixity don't account for these things.

For that reason, a lot en enginner add a lot of rebar in the positive moment to be sure to have a reserve and allows moment redistribution.

 

[NS4U]

I think everyone's response above answers your first question. Here's my opiniong on your second question:

Yes it is perfectly safe to design based on elastic load distribution, with one big BUT...

... if you are going to design your RC beam for the elastic moments and shears, then you need to design your column or spandrel beam to take the negative moments created. another way of putting this is- you need to follow the load! just as you can't clear up congestion at column by taking advantage of moment redistrib. without adding more positive reinforcement. whatever assumption you make when sizing the beam you need you need to make sure the entire structure is sized to carry the load (based on your assumptions) to the ground.

Regardless of what analysis assumption you use (elastic or plastic) as long as provide a complete load path for that assumption then you'll be fine. There is a technical term for what i am talking about but can't recall what it is. i think may be lower bound theory.

 

[slickdeals]

@N4SU: Are you thinking of "shakedown" analysis?

 

[Enhineyero]

Thanks for all your meaningful reply...

 

[ahmedhegazi]

I would like to add up something,
Elastic analysis for continuous members(Slabs or beams ) gives low positive reinforcement when pattern loading is not considered ,pattern loading is required by all codes ,thus i normally increase positive reinforcement by 25% to 40% to account for pattern load case