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.

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 .

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

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.