Instability for simply supported beam

[dccd]

I have assigned pinned pinned condition for this simply supported beam with UDL applied on top. However, I kept getting error ***WARNING - INSTABILITY AT JOINT DIRECTION = MZ . What's the cause and how to solve it ??

 

[JoshPlumSE]

I was going to say that it was a torsional instability. Because all simply supported beams have torsional instability if neither of the supports can resist a beam torsions. It would be an inconsequential instability if the beam has zero torsional moment (as most SS beams do).

However, your post says that it's an MZ joint instability. It could be that you've doubled up your member end releases and your supports..... What I mean by that is if you have a Pinned support at both ends then you should NOT release the bending moment at the beam ends because it will cause an instability to be reported. It would be an inconsequential instability, of course, but programs like this usually report it as an instability.

 

[dccd]

I have no assigned end member release. I have just assigned pinned pinned support condition. And the error popped up. How to resolve this ?

 

[JoshPlumSE]

Sorry, I'm not a STAAD expert. I used it for a short time 25 years ago! But, that's about it. I did tech support for one of their competitors for a number of years. So, I thought I should take a shot at answering your question. Unfortunately, I'm just not knowledgeable enough about this program to give any definitive answers.

The only instability I can see in your model is a torsional instability. I would refer to this as a MX joint instability. So, I'd try correcting that instability and see if that corrects the problem. To me, the simplest way to do this is to switch to FIXED supports, but use bending moment releases at the ends of the beam. You'll get the desired behavior, but will be less likely to get weird instabilities.

 

[SethGuthrie]

I agree with Josh, fix one end in rotation about x. I assume this is a SPACE (3D) analysis.

 

[dozer]

First, it's a warning, not an error. As Josh said it's most likely inconsequential. Second, I recreated your model and the warning I got was that the direction of the instability was MX, which makes sense. Not sure why yours is saying MZ.

Try this. Replace one of your pinned supports with this: "2 FIXED BUT MY MZ". The "2" may or may not be right for your model. It's just the number of one of your support nodes.

If it fixes the problem then my guess is you're using an older version of STAAD that has a bug in it's warning reporting. If it doesn't fix it then you've got a really funky model. You'll need to post it so we can see what's going on.