How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Gopalraj Manitraj — Tue, 30 Jul 2013 19:56:02 GMT

Oh sorry; I thought there were other errors. I was able to break the beams at the brace points and get it to work.

Thank you for all your help.

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Seth Guthrie — Tue, 30 Jul 2013 19:49:48 GMT

After properly segmenting the beams and columns at the brace intersection points, I no longer see any errors when running the file. If you are still having an issue after that you might want to post the output file or cite the errors.

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Gopalraj Manitraj — Tue, 30 Jul 2013 18:40:17 GMT

Can you explain the errors that you mentioned earlier?

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Gopalraj Manitraj — Mon, 22 Jul 2013 16:58:28 GMT

My understanding was this would reduce the length/height of the beam/column accordingly; and hence I had not gone with this approach. I will correct this in the analysis.

Can you expand on the other errors in the analysis? Please also suggest corrections for these errors.

Thank you.

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Seth Guthrie — Sat, 20 Jul 2013 03:10:44 GMT

1. I recommend a space frame if you are really dealing with corner columns in frames for both directions.

2. Check out the SELECT OPTIMIZED command.

3. If you look at the Post Processing screen you'll see the knee braces flying off into space. This happens because the beams and columns are bypassing the nodes leaving the braces unsupported. As such the whole analysis is not right.

There are a number of ways to split the beams where the braces intersect. I used Geometry - Break Beams at Selected nodes (with all of the beams, columns, braces and nodes on the right selected).

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Gopalraj Manitraj — Sat, 20 Jul 2013 01:27:27 GMT

1. Actually I am designing Plane frames. Does this change the approach or do I just insert the correct "K" values?

2. Right. Thank you for confirming this. I assumed it was related to this but was not sure.

Is there a quick way of assigning the selected members from the output; directly to the input file; instead of going through each member and assigning the same in the input command file?

3. There are no loads applied directly to the truss members/bracing (there are wind loads on the column to which the bracing is attached); and no self-weight has been specified. I have shared the model in the Secure File upload. The file name is 13-556_Frame 21_bracing.

Re: How do I design a moment frame in the weak axis? Staad is "failing" sections that were designed in a previous command for "Select Members". Why?

Seth Guthrie — Sat, 20 Jul 2013 00:55:16 GMT

1. You don't have to do anything special to have the weak axis of the column checked in a space frame model. Do be careful assigning the K factors, however.

2. Once the member size is changed, a subsequent analysis may result in larger forces for the modified member because the stiffness matrix has changed. This could fail the member requiring yet a larger size. A few iterations is usually enough to level things.

3. Make sure you don't have any loads applied directly to the truss members. We sometimes recommend a weightless material (or just don't assign self weight) to prevent bending in truss members. If that's not the cause of your issue then please share your model (there is a Secure File upload link on the right if you don't want to share publically)