Saturday, August 23, 2014

What is Redistribution of Moments

The concept of ‘Redistribution of Moments’  which is found in  structural design  is often  not well understood, specifically among undergrads.

Furthermore it is mostly confused with another widely used concept of  ‘Moment Distribution’  which is an essentially a method used to find the BM & SF in structurally indeterminate Beams & Frames.

The Redistribution of Moment is subjected only to moments which derived only by elastic analysis methods.
Moment Redistribution is meant to be that transferring of derived moment from one place to another while not altering the total height of the bending moment diagram. However  it meant to be reduce the BM of one point and in practically that point become plastic and yield which meant there will be a rotation.

Let me clarify this by an example

Let we assume that we got a following Bending Moment envelop from an elastic analyst of the structural element.

source : ALLEN , RF Design to BS 8110

If we do the RF design for this BMD , we may provide the adequate RF for hogging BM of 300kNm and sagging BM of 280 kNm. In that case we neglect that the ability of the element  to withstand the structural effects as an one unit AND also neglect that this envelop was derived  by several load cases. Note that the height of the  BM  is  580

Let assume that this BM envelop was derived by two other BM diagrams.

source ALLEN, RF design to BS 8110

The  height of the each BM diagrams are 400 & 480 respectively.

So we do not want  need necessarily  to design the BM height of 580 that gives by the envelop.

Instead we reduce the maximum BM by some percentage ,let syt 70% and redefined the BM envelop as follows

source ALLEN RF design-to BS 8110

The total BM height is 475 and it is nearly equal to critical elastic BM height of 480.

That is Redistribution of Moments simply explained.


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