TR.27.5 Spring Tension/Compression Specification
This command may be used to designate certain support springs as Tension-only or Compression-only springs.
General Format
SPRING TENSION
joint-list (spring-spec )
SPRING COMPRESSION
joint-list (spring-spec )
where:
spring-spec = *{ KFX | KFY | KFZ | ALL }
Description
Tension-only springs are capable of carrying tensile forces only. Thus, they are automatically inactivated for load cases that create compression in them. Compression-only springs are capable of carrying compressive forces only. Thus, they are automatically inactivated for load cases that create tension in them.
If no spring spec is entered or ALL is entered then all translational springs at that joint will be tension (or compression) only. This input command does not create a spring, only that if a support spring exists at the joint in the specified direction then it will also be tension (or compression) only. Refer to TR.27 Support Specifications to define springs.
For compression only springs the
ALL
parameter has special meaning. The compression
only spring is in the Y direction; the X and Z direction springs are
bi-directional. However when the Y direction spring goes slack, the X and Z
springs at the same joint are made inactive as well.
Since this command does not specify whether the spring is in the positive or negative direction from the joint, it is assumed in STAAD.Pro to be in the negative direction. For negative displacement the spring is in compression and for positive the spring is in tension.
If a
CHANGE
command is used (because of a change in list of
tension springs, supports, etc.), then the
SET NL
command must be used to convey to
STAAD.Pro that multiple analyses and multiple
structural conditions are involved.
- Refer to
TR.5 Set Command Specification
for explanation of the
SET NL
command. The number that follows this command is an upper bound on the total number of primary load cases in the file. -
STAAD.Pro performs up to 10 iterations
automatically, stopping if converged. If not converged, a warning message will
be in the output. Enter a
SET ITERLIM i
command (withi
> 10) before the first load case to increase the default number of iterations. Since convergence may not be possible using this procedure, do not set the limit too high. If not converged, a message will be in the output. - The principle
used in the analysis is the following.
- The
program reads the list of springs declared as
SPRING TENSION
and/orCOMPRESSION
. - The analysis is performed for the entire structure and the spring forces are computed.
- For the
springs declared as
SPRING TENSION
/COMPRESSION
, the program checks the axial force to determine whether it is tensile or compressive. Positive displacement isTENSION
. If the spring cannot take that load, the spring is "switched off" from the structure. - The analysis is performed again without the switched off springs.
- Up to
ITERLIM
iterations of the above steps are made for each load case. - This method does not always converge and may become unstable. Check the output for instability messages. Do not use results if the last iteration was unstable. You may need to include some support in each global direction that is not tension (or compression) only to be stable on every iteration.
- The
program reads the list of springs declared as
- A revised
SPRING TENSION
/COMPRESSION
command and its accompanying list of joints may be provided after aCHANGE
command. If entered, the newSPRING
commands replace all priorSPRING
commands. If not entered after a CHANGE, then the previous spring definitions are used. - The
SPRING TENSION
command should not be used if the following load cases are present: Response Spectrum load case, Time History Load case, Moving Load case. If used, theSPRING TENSION
/COMPRESSION
will be ignored in all load cases. - If the
SPRING TENSION
/COMPRESSION
command is used in a model with UBC, IBC or other such seismic load cases, each such load case must be followed by anANALYSIS
andCHANGE
command.
Notes
-
A spring declared as tension-only or compression-only will carry axial forces only. It will not carry moments.
-
The
SPRING TENSION
/COMPRESSION
commands should not be specified if theINACTIVE MEMBER
command is specified. -
Do not use Load Combination to combine these cases. Tension/Compression cases are non-linear and should not be linearly combined as in Load Combination. Use a primary load case with the Repeat Load command.
Example
SPRING TENSION
12 17 19 TO 37 65
SPRING COMPRESSION
5 13 46 TO 53 87 KFY
The following is the general sequence of commands in the
input file if the
SPRING TENSION
or
COMPRESSION
command is used. This example is for the
SPRING TENSION
command. Similar rules are applicable
for the
SPRING COMPRESSION
command. The dots indicate other
input data items.
STAAD …
SET NL …
UNITS …
JOINT COORDINATES
…
MEMBER INCIDENCES
…
ELEMENT INCIDENCES
…
CONSTANTS
…
MEMBER PROPERTY
…
element property
…
SUPPORTS
…
spring TENSION
…
LOAD 1
…
LOAD 2
…
LOAD 3
REPEAT LOAD
…
PERFORM ANALYSIS
CHANGE
LOAD LIST ALL
PRINT …
PRINT …
PARAMETER
…
CHECK CODE …
FINISH