Strand7 Software: In Detail: Solvers: Load Influence
Strand7 load influence solver
The Load influence solver is used to determine the influence that a point load or point moment has on a specified response variable. For example, if a beam axial force response variable is assigned, the Load Influence solver will establish the effect that a point force or point moment at any node in the model has on this axial force. 
The Load Influence Solver performs the following steps: 

Theory The load influence solver works on the principal of Maxwell's Reciprocal Theorem. Coupled with response variables for all elements, influence lines and influence surfaces can be extracted. Maxwell's Reciprocal Theorem states: For two identical forces at points B and A on a linear elastic structure as shown below: 
The displacement at A caused by a unit force at B, is the same as the displacement at B caused by a unit force at A. This is identical to stating that all stiffness matrices relating displacement to forces are symmetric.
In a more general form the Reciprocal Theorem can apply to all structural causes and effects. Thus in any linear structure the effect on any response variable at A, be it a moment, a rotation, a shear force, due to a unit force at B is the same as the response variable at B due to unit force at A. Using this theory, the load influence solver determines the effect a unit load has on a particular response variable in the model, e.g. what displacement at point B will eventuate from a unit load at point A. All load and freedom case attributes are ignored for this solver except for response variables and restraints. Any number of response variables can be solved for in each load influence analysis. Each response variable will produce a result case in the load influence results. 
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