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Unformatted text preview: 16.21 Techniques of Structural Analysis and Design Spring 2004 Unit #8 - Principle of Virtual Forces Raul Radovitzky March 18, 2005 Principle of Virtual Forces (PVF) Consider a compatible displacement field u i in a deforming structure B of volume V and its associated strain field: 1 ij = ( u i,j + u j,i ) (1) 2 The displacement field necessarily satisfies the displacement (essential) bound- ary conditions: S on i u (2) u i = u where u i are the imposed displacements on the displacement part of the boundary S u . We multiply ( 1 ) by an somewhat arbitrary virtual stess field ij to be further defined below and integrate over the volume of the material. 1 ij ( u i,j + u j,i ) ij dV = 0 (3) 2 V In addition, we multiply ( 2 ) by a set of somewhat arbitrary boundary virtual tractions t i : ) i ( u i u t i dS = 0 (4) S u 1 V Adding up ( 3 ) and ( 4 ) we obtain: 1 ij ( u i,j + u j,i ) ij dV + ( u i u t i dS = 2 ) i 0 (5) V S u We will require the virtual stress field and the external virtual tractions to...
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