Moment redistribution in ACI code

[drasticxxxx]

Hello,

According to ACI 318 code we allow to make moment redistribution up to 20 percent.
but the amount of reduction of negative moment will not equal to increment in positive moment,because if we reduce the negative moment at one support and keep the other support the same value, the positive moment will increase an amount depend on the values of negative reduction and span length, but unfortunately I don't remember this simple equation we took in the college(which depend on elastic theory)


so can anyone share an example please?

 

[KootK]

Here's what you do:

1) Draw moment diagram.
2) Draw line connecting negative peaks at either end.
3) Adjust one or both peaks as you see fit and adjust the line at the same time.
4) Make the vertical distance between the line and the positive moment trough at mid-span the same for both before and after cases.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RPMG]

Wow. There's an actual method. I was, absolutely, not taught anything about this subject, but this makes sense.

I've always used a program and replaced reaction forces with points loads of approximately P=0.8*R, then reiterated when necessary.

 

[PicoStruc]

What Koot explained is really the way to go.

It is based on the fact that at the maximum positive moment and the corresponding negativement moment projected on that straight line (that connect both negative moment ) equals W*L^2 / 8

I Always calculate that way to boost positive moment in two-way slab design to account bad embedment/development in rebar at wall boundary/support in that span!!.

 

[drasticxxxx]

Thank you guys

really appreciated

Two last comment I would like to add

1) this method will be less accurate when the maximum positive moment is not in the center of the beam
2) if I have a wall support and the negative moment form both sides is different in the beam , I think that I should go with moment redistribution on left side for example without adjusting the right side(if it is not needed),is this right?

Thank you all

 

[KootK]

1) this method will be less accurate when the maximum positive moment is not in the center of the beam

If you follow the procedure above for all locations along the beam, the new moment diagram that you wind up with will be entirely accurate at all locations along the beam. At it's heart, the method is just superposition. If you take 50 kN*m off of one side of the beam, it's the same as adding a linearly varying moment diagram with a peak at -50 kN*m to the original moment diagram. You are correct that the peak positive demand may not occur exactly at mid-span. That is generally the case in continuous members, however, even when you're not redistributing moments.

2) if I have a wall support and the negative moment form both sides is different in the beam , I think that I should go with moment redistribution on left side for example without adjusting the right side(if it is not needed),is this right?

You can do whatever you like in this regard. The wall will need to be designed to take the moment differential across the joint however (step in the moment diagram). I prefer to design walls and columns for the non-redistributed moments when those are higher than the distributed moments. Axially loaded members are less forgiving with regard to moments than purely flexural members.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[drasticxxxx]

Thank you