Element System Element System

Both beams and generalized plates have a special direction: along the length for a beam and normal for a generalized plate. Thus for a beam, the local X axis will be from the first end to the second one, and for a plate the local Z axis will be normal to the surface. One of the other two directions can be chosen arbitrarily and the third will be defined by a cross product of the two vectors.

By default, MOSES has two additional vectors which will be used to complete the local system definition: a primary direction and a secondary one. Normally, the primary direction defines the direction that an axis "points". For beams this is the Z axis, and for generalized plates it is the X. The secondary direction is used if the special direction and the primary direction are parallel. The primary and secondary vectors, SAV1 and SAV2, are defined with the -DIR_PLATE or -DIR_BEAM options of a &DEFAULT command.

The SAV1 and SAV2 vectors are "part system" vectors. This makes the business of connecting parts special since here one is dealing with two parts. The local system of a part connector will be discussed below.

For a beam the local system is constructed as follows:

and for a generalized plate

For both beams and generalized plates, the default behavior can be changed with two options on the element definition command:


     -DIR_LOC, SAV1(1), SAV1(2), SAV1(3), SAV2(1), SAV2(2), SAV2(3)

The -REFN option replaces the default SAV1 vector with a unit vector from the element origin (first point specified) to the point defined by *REFNOD. The -DIR_LOC option can be used to completely redefine both the SAV1 and SAV2 vectors. If less than 4 numbers are specified with this option, then only SAV1 will be redefined.

The local system definition described can be altered again for both beams and generalized plates. In particular, for generalized plates, the -DIR_LOC option can be used with only the string NODES following. This instructs MOSES to use the vector from the local origin to the second node as the SAV1 vector. For beams, one can specify the option:

     -CA, CHANG

This option rotates the local system about the local X axis an angle of CHANG (degrees). The rotation of the system is about the beam x axis, positive towards the beam negative Y axis (right hand rule).

If for a beam one wants the strong axis of beams to be "horizontal", the SAV1 vector should be the vector defining "vertical" in the part system. In other words, if Z is the part "vertical" then SAV1 would be 0, 0, 1. Alternately if Y is the part "vertical" then SAV1 would be 0, 1, 0. In either case, SAV2 simply handles the special case and can be chosen to either suit one's fancy or to conform to some existing practice. The relationship of the local coordinate system is shown in the following figure.