The Unit dummy force method provides a convenient means for computing displacements in structural systems. It is applicable for both linear and non-linear material behaviours as well as for systems subject to environmental effects, and hence more general than Castigliano's second theorem.

Discrete systems

Consider a discrete system, which can be computed using the member flexibility relation, which we want to determine.

We start by applying N, one for each wanted r:

In the case of a statically indeterminate system, matrix B that satisfies nodal equilibrium is infinite. It can be computed as the inverse of the nodal equilibrium matrix of any primary system derived from the original system.

Imagine that internal and external virtual forces undergo, respectively, the real deformations and displacements; the virtual work done can be expressed as:

According to the virtual work

contains arbitrary virtual forces, the above equation gives

It is remarkable that the computation in (2) does not involve any integration regardless of the complexity of the systems, and that the result is unique irrespective of the choice of primary system for B. It is thus far more convenient and general than the classical form of the dummy unit load method, which varies with the type of system as well as with the imposed external effects. On the other hand, Eq.(2) is for computing displacements or rotations of the nodes only. This is not a restriction because we can make any point into a node when desired.

Finally, the name unit load in matrix B, by virtue of Eq.(1).

General systems

For a general system, the unit dummy force method also comes directly from the virtual work. These deformations, supposedly consistent, give rise to displacements throughout the system. For example, a point A has moved to A', and we want to compute the displacement r of A in the direction shown. For this particular purpose, we choose the virtual force system in Fig.(b) which shows:

• The unit force R* is at A and in the direction of r so that the external virtual work done by R is the desired displacement

• must satisfy equilibrium everywhere.

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