Joist Girder to Girder Connection 2

[MillR]

I have not done a girder to girder connection with bar joists. Normally the end of a girder is supported by a steel column, and that detail is standardized by SJI. Has anyone done this detail? Do the joists need to be the same depth?

Thanks in advance.

 

[milkshakelake]

Just standard clip angles. It's a regular steel to steel connection. If it's supporting joists, it should be lower than joists. If it's not, it should be full height up to the deck in order to receive lateral bracing.

 

[MillR]

Thanks milk. I am usually reluctant to have the support be shallower than the supported for steel beams and this is 56" supported by 24" so I started overthinking.

 

[milkshakelake]

Ok, that's a different story. I don't want to give a false assurance, and maybe you're not overthinking it. 56" girder supported by 24" girder doesn't sound right to me, unless the 24" is a very short beam (shear controlled). Maybe a sketch would help? Technically it should still be okay as long as the clip angle can handle the massive shear forces, but the dimensions are very unusual.

 

[KootK]

I am usually reluctant to have the support be shallower than the supported for steel beams and this is 56" supported by 24" so I started overthinking.

I'm not sure that is overthinking.

If you're doing a side connection rather than running the seat over the top, a I expect, then that connection will create some shear eccentricity that you probably want to translated into an end moment in the incoming girder. And that usually means that you want to engage both the top and bottom chords of the incoming girder axially so that you can form a force couple to resist that moment.

I'd be tempted to replace the 24" girder with a dropped, 24" wide flange that either needs no additional bracing or just some intermittent discrete bracing (not the roof deck).

 

[MillR]

The short girder is only 20'-0" long, the long girder is 62'-0" supporting 60' joists. I decided to match depths in that bay. It doesn't even look right otherwise. The depth issue got me thinking about the normal bottom chord to column detail can't happen at a girder. Joist people do your magic.

 

[milkshakelake]

@KootK If it's connected to the web, there's no real eccentricity. Technically maybe a tiny bit, from the moment arm being center of the web to the face of the clip angle. There could be accidental eccentricity from fixity of the clip angle, which is always the case. A beam being deeper doesn't change that since the shear is through the web.

 

[CANPRO]

As others have noted, there may be more favorable framing options that could simplify this. Below is a possible solution - essentially building a beam web into each member. Deep bolt group to handle the eccentricity and good engagement of the top and bottom chords of the 56" deep truss.

 

[milkshakelake]

@CANPRO That's a fantastic idea, I was only thinking about the top part of the plate. This will solve shear problems at the connection. Thanks for the sketch, you helped more than OP. I'm going to use this idea for similar situations.

Here's an idea using KootK's dropped girder but eliminating eccentricity: