Truss element for brace modeling

[Zonghu Pei]

Dear STKO team,
I am modeling a BRB-precast concrete frame. Here I attach an image to show the model scheme. I used zeroLength element to simulate the rigid coonnection between brace and frame. For the brace which has two different section, it was given truss element.
However, this model did not work. I was wondering if I used unproper element or modelling method in this model. What should I do to correct is? Thank you!

[STKO Team]

You cannot discretize a truss element otherwise it will be singular.
You either model it as a single element (but you can't because you are assigning different section in the central part), or you can use beam elements, with beam-end releases

[Zonghu Pei]

Thanks for your reply！I used the truss element because it does not consider the out-plane deformation. You said that truss element may not suitable for my model. So here I have a question. If I use beam element to model the BRB(buckling restrained brace), what should I do to avoid the out-plane deformation?

[Zonghu Pei]

Hello STKO team,
I have changed my model. In this model, I use a single truss element for brace. However, it still doesn't work. Is there any other things that I should pay attention to when use truss element?

[STKO Team]

Here it is:

brace frame.zip

(610.58 KiB) Downloaded 171 times

Issues:

• The torsion material is too high (1.0e20). It makes all other numbers look like 0 to a PC.... . When you use a stiff number you are following a penalty approach (as in the Penalty constraint handler). Therefore you need to compute a number which high enough, but not too high!
• The truss MUST be discretized with only 1 element, otherwise the system will be singular

  

  truss.png (55.51 KiB) Viewed 2291 times

• Don't use the lagrange constraint handler in the first analysis (it's diffucult to handle if you don't know the theory behind Lagrange Multipliers). I switched to Penalty

[Zonghu Pei]

Thans for your help！ I will keep studying this software。

[STKO Team]

You're welcome