Moment resistance of welded connections

[jfeng810]

Hello everyone! There currently is a temporary situation where a W column is supporting a W beam. The bottom flange of the W beam is welded to the top of the W column via a square plate and 1/4" longitudinal and transverse welds. There is a resulting Moment at the top of the column, that basically needs to be resisted by the weld. I want to check to make sure the weld is adequate to be able to check fixity of this connection. I'd like to get input to see if my logic is correct to basically multiply the weld metal strength by the section modulus. Please see attached, thanks!

https://imgur.com/a/QjjMkQh

 

[JLNJ]

You can't just figure some sort of section modulus this way. Your load paths are not properly direct and therefore the joint is not really "fixed". The welds are also loaded in an improper manner to be even moderately effective. The arrangement certainly does not meet the normal standard for a moment connection.

Certainly there is SOME moment capacity. If you loads are very, very small it might be OK. What are you trying to accomplish?

 

[jfeng810]

We know this is not ideal for a true moment connection, but we are trying to get a temporarily loaded condition to work as a fixed connection. When modeling the entire structure as pinned, we can't get it to work. However, fixed does. If we can prove the weld (although not ideal) is adequate to consider this connection fixed for a temporary condition. It is a small frame with small loads that was not constructed to plan, trying to analyse condition to get it to work without a complete re build.

Thanks for the input!

 

[canwesteng]

Well you need to check each part of the connection. How does moment get from the flanges of the beam to the weld itself? This probably causes deformation, which affects the stress distribution in the weld. As is you haven't done enough to check this, and it's probably a decent undertaking, but you can probably resist some moment with your connection.

 

[sbisteel]

Fillet welds transfer force through shear in the throat, so you cannot use the full tensile strength of the weld metal.

Also, welds are usually analyzed as line elements to determine maximum forces in the weld group (i.e. section modulus would be in in^2). Then either a moment capacity or required throat thickness are determined.

Lastly, all you are calculating here is the moment capacity of the weld group. There are clearly some stiffness issues with the arrangement you have, particularly the column orientation.

Outside of proving that the frame would be stable under self weight, I wouldn't put too much faith in the fixity of this connection

 

[jfeng810]

Thanks for the input. FYI, orientation of W column is drawn incorrectly.

Starting to think analyzing as fixed is too much of an undertaking.

 

[KootK]

Very recently, I proposed what I thought to be some pretty good detailing to a similar condition in this thread: Link

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[KootK]

- For small amounts of moment, you can probably make this work.

- An option is shown below with all of your welds in shear. For small moments, you probably could make this work without stiffeners. Given that you'll be out there anyhow though, I'd just include the stiffeners and make the problem much simpler for yourself and robust in the field. The stiffeners will help with beam rollover anyhow.

- My colleagues are correct, there's a whole load path that needs to be evaluated here. You either need to be able to evaluate it or you need to be comfortable enough with the situations that you can judgment call parts of it.

- You definitely don't want to be trying to deal with your moment via moments in the welds. Fillet welds are terrible when used that way. The mechanism shown below keeps all of the welds being used in shear.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[JAE]

Those calculations show that you don't understand how weld groups resist moment about the weld group axis.

Flex in the beam flanges, or possibly in the cap plate, can cause bending across the throat of the welds themselves, a very brittle and unsafe condition.
If you are all students then talk to your professor to get some help on this (you said it was in a "class").





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