Displacements, actions and reactions

During the static analysis phase, there are three basic groups of data that have to be calculated. They are node displacements, member and plate actions (forces and moments) and support reactions.

Node displacements

Node displacements are calculated for each load case being analysed and for every unrestrained degree of freedom in the structure. Each node may translate along or rotate about any or all of the three global axis directions. Restrained (fixed or deleted) degrees of freedom are automatically assigned displacements of zero except for those nodes that have prescribed displacements specified. In such cases those nodes are assigned the prescribed displacement only for the particular load case in which they were specified.

Member actions

There are twelve forces and moments that can be calculated for each member. Each end of a member is subjected to an axial force, a torsion, bending moments about its y and z axes and shear forces along its y and z axes.

The program is also capable of calculating forces and moments at user defined intermediate points along members. These intermediate values, however are not calculated during the analysis phase. Instead they are calculated as required when the graphical diagrams or output reports are produced.

For members that are pinned for rotation (hinged) at both ends, if lateral loads other than UDLs over their entire length have been applied then the post-analysis calculation of the intermediate deflections and moments may not be totally accurate. This is because the rotation of the member ends is unknown. In these cases, SPACE GASS estimates the end rotations assuming that the lateral loads are full length UDLs. This affects the graphical deflection and moment diagrams, the intermediate member deflection and moment reports, and any of the design modules that use the member intermediate deflections or moments. In order to get accurate intermediate deflections and moments with members that are pinned (hinged) at both ends and are subjected to other than full length UDLs, you should subdivide them. One intermediate node is enough, but you can add more if desired.

For more information on member actions, refer to Sign conventions.

Plate actions

Three forces and three moments are calculated for each plate node, making a total of 18 actions per triangular plate and 24 actions per quadrilateral plate. Two axial stresses, three shear stresses and three bending stresses are also calculated for each plate.

These are later used to calculate the 17 different force, moment and stress values for each plate that can be shown graphically as colored contours or included in text reports.

For more information on plate actions, refer to Sign conventions.

Reactions

External node reactions are the forces and moments exerted by the structure on the supports. They are calculated only for restrained nodes and are referenced by the global axes system.