# Unit dummy force method

﻿
Unit dummy force method

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 such as trusses, beams or frames having members interconnected at the nodes. Let the consistent set of members' deformations be given by $mathbf\left\{q\right\}_\left\{M imes 1\right\}$, which can be computed using the member flexibility relation. These member deformations give rise to the nodal displacements $mathbf\left\{r\right\}_\left\{N imes 1\right\}$, which we want to determine.

We start by applying "N" virtual nodal forces $mathbf\left\{R\right\}^*_\left\{N imes 1\right\}$, one for each wanted "r", and find the virtual member forces $mathbf\left\{Q\right\}^*_\left\{M imes 1\right\}$ that are in equilibrium with $mathbf\left\{R\right\}^*_\left\{N imes 1\right\}$:

:$mathbf\left\{Q\right\}^*_\left\{M imes 1\right\} = mathbf\left\{B\right\}_\left\{M imes N\right\} mathbf\left\{R\right\}^*_\left\{N imes 1\right\} qquad qquad qquad mathrm\left\{\left(1\right)\right\}$

In the case of a statically indeterminate system, matrix B is not unique because the set of $mathbf\left\{Q\right\}^*_\left\{M imes 1\right\}$ 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:

* External virtual work: $mathbf\left\{R\right\}^\left\{*T\right\} mathbf\left\{r\right\}$
* Internal virtual work: $mathbf\left\{Q\right\}^\left\{*T\right\} mathbf\left\{q\right\}$

According to the virtual work principle, the two work expressions are equal:

:$mathbf\left\{R\right\}^\left\{*T\right\} mathbf\left\{r\right\} = mathbf\left\{Q\right\}^\left\{*T\right\} mathbf\left\{q\right\}$

Substitution of (1) gives

:$mathbf\left\{R\right\}^\left\{*T\right\} mathbf\left\{r\right\} = mathbf\left\{R\right\}^\left\{*T\right\} mathbf\left\{B\right\}^\left\{T\right\} mathbf\left\{q\right\}$

Since $mathbf\left\{R\right\}^\left\{*\right\}$ contains arbitrary virtual forces, the above equation gives

:$mathbf\left\{r\right\} = mathbf\left\{B\right\}^\left\{T\right\} mathbf\left\{q\right\} qquad qquad qquad mathrm\left\{\left(2\right)\right\}$

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, it is important to note that 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 arises from the interepretation that the coefficients $B_\left\{i,j\right\}$ in matrix B are the member forces in equilibrium with the unit nodal force $R^*_j = 1$, by virtue of Eq.(1).

General systems

For a general system, the unit dummy force method also comes directly from the virtual work principle. Fig.(a) shows a system with known actual deformations . 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, noting that the work done by the virtual reactions in (b) is zero because their displacements in (a) are zero:

: $R^* imes r = 1 imes r$: The desired displacement

* The internal virtual work done by the virtual stresses is::where the virtual stresses must satisfy equilibrium everywhere.

Equating the two work expressions gives the desired displacement:

:

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Flexibility method — In structural engineering, the flexibility method is the classical consistent deformation method for computing member forces and displacements in structural systems. Its modern version formulated in terms of the members flexibility matrices also… …   Wikipedia

• Slope deflection method — The slope deflection method is a structural analysis method for beams and frames introduced in 1915 by George A. Maney.Citation|first=George A.|last=Maney|year=1915|title=Studies in Engineering|publisher=University of… …   Wikipedia

• Virtual work — on a system is the work resulting from either virtual forces acting through a real displacement or real forces acting through a virtual displacement. In this discussion, the term displacement may refer to a translation or a rotation, and the term …   Wikipedia

• List of structural engineering topics — This page aims to list all articles related to the specific discipline of structural engineering. For a broad overview of engineering, please see List of engineering topics. For biographies please see List of engineers.compactTOC NOTOC AA frame… …   Wikipedia

• Energy principles in structural mechanics — express the relationships between stresses, strains or deformations, displacements, material properties, and external effects in the form of energy or work done by internal and external forces. Since energy is a scalar quantity, these… …   Wikipedia

• Mathematics and Physical Sciences — ▪ 2003 Introduction Mathematics       Mathematics in 2002 was marked by two discoveries in number theory. The first may have practical implications; the second satisfied a 150 year old curiosity.       Computer scientist Manindra Agrawal of the… …   Universalium

• Glossary of contract bridge terms — These terms are used in Contract bridge[1][2] , or the earlier game Auction bridge, using duplicate or rubber scoring. Some of them are also used in Whist, Bid whist, and other trick taking games. This glossary supplements the Glossary of card… …   Wikipedia

• List of U.S. Marine Corps acronyms and expressions — This is a list of acronyms, expressions, euphemisms, jargon, military slang, and sayings in common or formerly common use in the United States Marine Corps. Many of the words or phrases have varying levels of acceptance among different units or… …   Wikipedia

• bridge — bridge1 bridgeable, adj. bridgeless, adj. bridgelike, adj. /brij/, n., v., bridged, bridging, adj. n. 1. a structure spanning and providing passage over a river, chasm, road, or the like. 2. a connecting, transitional, or intermediate route or… …   Universalium

• Sniper — For other uses, see Sniper (disambiguation). Sniper US Army sniper team shooting from within a room in Afghanistan with M24 SWS, 19 October 2006. Occupation Names Sniper Activity …   Wikipedia