• Influence lines for reactions
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Qualitative Influence Lines and Loading Patterns for
an Multispan Indeterminate Beam
The MUller Breslau Principle, used previously to draw the influence lines for statically
determinate structures, can also be extended to define the influence lines for
indeterminate structures. This principle simply states that the influence line for afunction
is proportionally equivalent to the deflected shape of the structure when it undergoes a
displacement as a result of the application of the function.
For indeterminate structures, an understanding of how complex structures deflect and
react when acted upon by a force is required in order to draw accurate diagrams.
To determine the influence line for the support reaction at A, the Miiller Breslau Principle
requires the removal of the support restraint and the application of a positive unit
deformation at this point that corresponds to the direction of the force. In this case, apply
a unit vertical displacement in the direction of Y
A.
A
I
B
C
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The resulting deflected shape, due to the application of the unit deformation, is then
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 Fall '10
 Hosteng
 Force, Shear Stress, influence line

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