Deformation load should not be included in mass source because its downward component creates an additional mass which will affect analysis results. At either end of an element, deformation load creates equal and opposite forces which are in equilibrium. Though net load is zero, the mass based on this mass source is not zero. Deformation load generates an upward internal force on one member end, and a downward internal force on the other. This downward component is then associated with an additional mass.

What should be specified as a mass source, however, are the external loads applied to a structure, including point loads, distributed loads, area loads, etc.

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