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.

1. 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.
2. 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 (with i > 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.    
3. The principle used in the analysis is the following.
   • The program reads the list of springs declared as SPRING TENSION and/or COMPRESSION.
   • 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 is TENSION. 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.
4. A revised SPRING TENSION / COMPRESSION command and its accompanying list of joints may be provided after a CHANGE command. If entered, the new SPRING commands replace all prior SPRING commands. If not entered after a CHANGE, then the previous spring definitions are used. 
5. 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, the SPRING TENSION / COMPRESSION will be ignored in all load cases.
6. 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 an ANALYSIS and CHANGE command.

Notes

1. A spring declared as tension-only or compression-only will carry axial forces only. It will not carry moments. 

2. The SPRING TENSION / COMPRESSION commands should not be specified if the INACTIVE MEMBER command is specified.

3. 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.