topic Re: Steel bracing - advanced bar properties - instability type 1 error in Robot Structural Analysis Forum

Steel bracing - advanced bar properties - instability type 1 error

skellyRSA2023 — Wed, 09 Sep 2020 23:15:43 GMT

Hi,

i was wondering could someone help me, i am trying to model a concrete shear wall and steel bracing frame to resist a lateral load applied as per the attached model. However everything is fine until i use the advanced bar properties tool to define the internal bracing members of the steel bracing as truss bars to take axial loads only and not take moment into the members, however it shows up an instability type 1 warning each time i run the calculations. Can anyone advise what i am doing wrong? Many thanks in advance.

Re: Steel bracing - advanced bar properties - instability type 1 error

Krzysztof_Wasik — Thu, 10 Sep 2020 08:24:14 GMT

Hi @skellyRSA2023

Nodes 100 and 1010 once connected to truss element can move in Y direction.

Instability type 1 can be removed by defining UY support in nodes 100 and 101

Re: Steel bracing - advanced bar properties - instability type 1 error

skellyRSA2023 — Thu, 10 Sep 2020 10:31:38 GMT

Krzysztof, Thanks for getting back. By blocking the Uy support direction does this not defeat the purpose of the using the advance bar properties. I have merged the diagonal bars into two bars, and then just defined the internal diagonal bracing and horizontal beams in the bracing bay as truss members and its seems to work OK without errors wthout to much of a difference in results. see attached. Is there any examples of a bracing bay being designed in RSA which i may refer to?

Re: Steel bracing - advanced bar properties - instability type 1 error

Krzysztof_Wasik — Thu, 10 Sep 2020 10:48:28 GMT

Hi @skellyRSA2023

If you will define crossing braces using 4 independent truss elements system in statically unstable in direction perpendicular to bracing plane (rotation is releases in each bar end). This results in Instability type 1 error.

If you will define 2 crossed elements (working independently - not connected in the middle) system is stable and usually for tension bars this solution is used, so your second model does not report warnings

If braces should be connected in the middle, fictitious support should be used for truss bars, or regular bars with releases defined should be used.