Buckling with Prestress

[ThomasH]

Hello,

I have a problem that I can't quite figure out. I am working on a cable stayed bridge and I need to calculate the buckling modes for the main girder.

The problem is that I want to prestress the cables and then add a load that drives (varys) to cause the buckling. Prestressing is Load 1 and Live load is Load 2.

I have an example for the buckling setup, that won't work . And I can't figure it out from the manual. At least not yet. Help would be greatly appreciated.

SUBCASE 5
SUBTITLE = Prestress
TEMP(LOAD) = 1
SUBCASE 6
SUBTITLE = Live Load
LOAD = 2
STATSUB(PRELOAD)=5
STATSUB = 6
METHOD=1
BEGIN BULK

/Thomas

 

[ThomasH]

Hello again,

I had another look during the weekend and I think I solved it. If I am correct it was simpler and more straightforward then I first thought. It may be possible to do it more compact but as long as it works I am happy .

Hopefully it will work on the real model also. I have not tried that yet.

I anybode can confirm I am correct or otherwise correct me I would appreciate it.

SUBCASE 5
SUBTITLE = Prestress
TEMP(LOAD) = 1
SUBCASE 6
SUBTITLE = Live Load
LOAD = 2
SUBCASE 10
METHOD=1
STATSUB(BUCKLING) = 6
STATSUB(PRELOAD) = 5
BEGIN BULK

Thank you

/Thomas

 

[ThomasH]

Hello again,

I have now tested the approach on the "real" model. It is large so each run takes a while but a new problem occured. Nastran says that the stiffness matrix is not positive definite and stops. Is there anyway I can handle this?

/Thomas

 

[saplanti]

I am not expert on the issue but, just guessing, you need to define cables tension member only to solve the problem.

Do you use springs to take the thermal loads on the cables and differential movements in the system¿ In case they are not adequately sized for prestress this can be another problem. I would run the program without them to find out required forces and displacement and start sizing them from those and iterate. I do not know the abilities of the program that you are using, so this is the basic I guess with these iterations the cables are always in tension. But without assigning the cables tension only the buckling analysis will always cause non-convergence in the matrix analysis.

 

[ThomasH]

saplanti,
My description was not clear on this, but the preload is based on the thermal load in the cables. So, a better description of the current set-up is that Load 2 is the design load for the bridge.

Today my problem has more to do with numerical issues in Nastran. But that is discussed in another tread:

https://www.eng-tips.com/viewthread.cfm?qid=500040

Thank you.