Modeling a Chevron Brace

[pat2]

I am attempting to model a chevron brace in Visual Analysis, but the program is giving me an error message stating "The model has fewer than 25 degrees of freedom (DOF), modal analysis cannot be performed. You can split members or plates to get enough DOF for a modal analysis."

Brace information: The length and height of the brace is 33 feet and 18 feet respectively. The columns and braces are 6x6x3/8 tubes. The beam is W14x43. My model has pinned bases and the moments are released at the ends of the beam at the column locations, and at the end of the braces or diagonal members.

I would greatly appreciate any help in modeling the brace and directing me to a source. This is my first chevron brace design. It is being used in a low-rise industrial building.

Sincerely,

Paul (PAT2)

 

[shepherd]

Pat2,
I'm not familiar with that particular program, but it sounds like too many joint releases. First thing that comes to mind is did you release the torsional restraint of the braces or beam? If so, just fix the torsional restraint. IF torsional release isn't the issue, could be too many pins at one joint. At least one member framing into a joint needs to be fixed for analysis programs i've worked with. Hope this helps.

 

[3dboy]

first of all are you doing modal analysis or a static analysis?

 

[pat2]

shepherd and 3dboy,

Thank you for responding.

I am attempting to run a modal analysis.

I have tried to change around the member releases numerous times to see where the porblem lies with my assumptions.

I believe the brace should have the moment released at the top of columns and at the top of the braces at the point where the braces meet in the center of the beam. I assumed pin bases, but fixed thebases in the hope the model would run to no avail.


Paul

 

[3dboy]

pat2

have you contacted IES yet? They are typically very responsive to people.

Jeremiah

 

[dawn836]

well does the bracing members intersect with each other?
what you did at there intersection?
do you make 3d or 2d analysis?

 

[JAE]

I'm not familiar with that particular program, but the program message seems to indicate that you need more nodes in your model as it (the program) cannot develop the desired number or quality of mode shapes.

You could add intermittent nodes along the beam and columns to provide additional DOF for the modal analysis.

That said, I'm not sure why you are doing a modal analysis of a single brace model. Modal analysis should normally be performed on the entire structure together, not on a single brace component.

 

[pat2]

It ended up being a program limitation. I needed to break the members up to create enough degrees of freedom for the program to work.

Thank all for their help.

Paul