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*9.14 USE OF MOMENT-AREA THEOREMS WITH
STATICALLY INDETERMINATE BEAMS
The reactions at the supports of a statically indeterminate beam can
be determined by the moment-area method in much the same way
that was described in Sec. 9.8. In the case of a beam indeterminate
to the first degree, for example, we designate one of the reactions
as redundant and eliminate or modify accordingly the corresponding
support. The redundant reaction is then treated as an unknown load,
which, together with the other loads, must produce deformations
that are compatible with the original supports. The compatibility
condition is usually expressed by writing that the tangential deviation
of one support with respect to another either is zero or has a pre-
determined value.
Two separate free-body diagrams of the beam are drawn. One
shows the given loads and the corresponding reactions at the sup-
ports that have not been eliminated; the other shows the redundant
reaction and the corresponding reactions at the same supports (see
Example 9.14). An MyEI diagram is then drawn for each of the two
loadings, and the desired tangential deviations are obtained by the
second moment-area theorem. Superposing the results obtained, we
express the required compatibility condition and determine the
redundant reaction. The other reactions are obtained from the free-
body diagram of beam.
Once the reactions at the supports have been determined, the
slope and deflection may be obtained by the moment-area method
at any other point of the beam.
9.14 Use of Moment-Area Theorems with
Statically Indeterminate Beams
EXAMPLE 9.14
Determine the reaction at the supports for the prismatic beam and load-
ing shown (Fig. 9.66).
We consider the couple exerted at the fixed end A as redundant
and replace the fixed end by a pin-and-bracket support. The couple M
A
is now considered as an unknown load (Fig. 9.67a) and will be deter-
mined from the condition that the tangent to the beam at A must be
horizontal. It follows that this tangent must pass through the support
B and, thus, that the tangential deviation t
ByA
of B with respect to A
must be zero. The solution is carried out by computing separately the
tangential deviation (t
ByA
)
w
caused by the uniformly distributed load w
(Fig. 9.67b) and the tangential deviation (t
ByA
)
M
produced by the
unknown couple M
A
(Fig. 9.67c).
B
w
A
L
Fig. 9.66
A
A
w
w
B
B
AB
M
A
M
A
t
B/A
0
B''
B'
(t
B/A
)
w
(t
B/A
)
M
(a)(b)(c)
Fig. 9.67
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