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TR.32.8 Support Joint Displacement Specification

This command may be used to specify displacements (or generate loads to induce specified displacements) in supported directions (pinned, fixed, enforced, or spring).

General Format

SUPPORT DISPLACMENT
support-joint-list { FXFYFZMXMY | MZ } f1

Where:

ParameterDescription
f1 Value of the corresponding displacement.  For translational displacements, the unit is in the currently specified length unit, while for rotational displacements the unit is always in degrees.

FX, FY, FZ specify translational displacements in X, Y, and Z directions respectively.  MX, MY, MZ specify rotational displacements in  X, Y, and Z directions.

Description

There are two distinct modes of usage for this command.  If any ENFORCED specifications were used in the support command then the "displacement mode" is used; otherwise the "load mode" is used.  Despite the name of this command, if displacements are specified in spring directions, the displacement is at the joint not at the grounded end of the support spring.  Displacement cannot be specified in a direction that does not have a support or a spring.

  1. DISPLACEMENT  MODE - With this mode, the support joint displacement is modeled as an imposed joint displacement.  The joint directions where displacement may be specified must be defined (same for all cases) in the SUPPORT command, see TR.27.1 Global Support Specification. Any beam members, springs or finite elements will be considered in the analysis.  Other loading, inclined supports, and master/slave are all considered.  Any number of cases may have displacements entered.  However, all cases will have zero displacements at the enforced directions if no displacement values are entered for that case.  At inclined supports the displacement specification is assumed to be in the inclined direction.  Displacements may not be specified at slave directions.

    If some cases are to have spring supports and others enforced displacements at the same joint directions, then two PERFORM ANALYSES must be used with the CHANGE command in between.  The first perform analysis could have the SUPPORTS with springs, no enforced directions, and with the load cases without displacements.  The second perform analysis would then have SUPPORTS without springs but with enforced directions and the cases with displacements.

    Displacement Mode Restrictions:
    • The Support Displacement command may be entered only once per case.
    • Spring directions and Enforced directions may not both be specified at the same joint direction in the same Perform Analysis step.
  2. LOAD  MODE - With this mode, the support joint displacement is modeled as a load.  Only beam members (not springs or finite elements) are considered in computing the joint load distribution necessary to cause the displacement.  Other loading, inclined supports, and master/slave are also not considered.  These unconsidered factors, if entered, will result in displacements other than those entered (results are superimposed).  Only those cases with displacements entered will be affected.

    Load Mode Restrictions:
    • Support Displacements can be applied in up to four load cases only.
    • The Support Displacement command may be entered only once per case.
    • Finite elements should not be entered.
    • Inclined supports must not be entered.
    • Spring supports are not considered in calculating the load so their use will lead to displacements different from the input values.

Example

UNIT …
SUPPORT DISPL
5 TO 11 13 FY -0.25
19 21 TO 25 MX 15.0

In this example, the joints of the first support list will be displaced by 0.25 units in the negative global Y direction. The joints of the second support list will be rotated by 15 degrees about the global X-axis.