topic Re: Only tension elements for bracing problem in Robot Structural Analysis Forum

Only tension elements for bracing problem

Anonymous — Sun, 28 Jul 2019 16:37:29 GMT

Hi, I am new in using Robot.

When I add only tension elements for my structure bracing, No convergence of nonlinear problem appear.

Hoping someone can tell me whats wrong, I tried using all solutions from forums but doesnt help. Any advice for improving my model is welcome. I attached file below. Please help me to figure out whats wrong with model.

Only tension elements-bracing errors

Anonymous — Sun, 28 Jul 2019 16:35:04 GMT

Hi, I am new in using Robot.

When I add only tension elements for my structure bracing, No convergence of nonlinear problem appear.

Re: Only tension elements for bracing problem

Muhammed.OPERA — Mon, 29 Jul 2019 14:15:47 GMT

Hi @Anonymous

The same problem you face here is explained in the Autodesk Webinair: Instability warnings during model analysis by Mr. Artur & Mr. Rafal exactly at the 5th minute of the video, you can watch it here: https://www.youtube.com/watch?v=5CCLzG12JQ4&list=PLY-ggSrSwbZqow_60fiqJwS69mg1nQMzk&index=16

He said that you should make the diagonals and posts as truss members and the upper and lower chord should be continuous (one piece) and make a release to it (pinned pinned release) not truss bar.

Note: truss bars shouldn't have releases too.

Re: Only tension elements for bracing problem

Muhammed.OPERA — Mon, 29 Jul 2019 14:29:30 GMT

That method from the webinair would solve the instability but causes a moment in the upper and the lower chords.

So, i have another approximate to solve both of that and won't affect the results much.

As you can see in the picture:

I have made the diagonals and the posts as truss members, roof and wall bracing too.

But, fot the upper and lower chord members i have made a custom release for them as an elastic release with partial stiffness coefficients in Mx (start), My (start & end) and Mz (start & end).

Then, i have made that partial elastic pinned pinned release to every member of the chords.

As i design the purlines as continuous, i have made a release to it's start and end only (at the 1st and last truss).

That will solve your instabilities.

Re: Only tension elements for bracing problem

Anonymous — Mon, 29 Jul 2019 16:37:33 GMT

Thanks, but my primary problem is that No conergence of nonlinear problem appears when i try to calculate it with only tension elements. I tried with compresion-tension bracing elements and had no problem.

Re: Only tension elements for bracing problem

Simau — Tue, 30 Jul 2019 15:20:22 GMT

Hi @Anonymous

The main problem is the 2 purlins at the top of the roof. I deleted one row of them then the model converge.

Set all cases that don't act alone as auxiliary

Set upper and lower chord as simple bares (not truss)

Set roof diagonals as "neglected" so as not to interfere with load distribution

Model attached