Bar Joist Frame

[ToadJones]

I have a frame that basically consists of two columns with a 24" deep bar joist running between.
The bar joist has a bottom chord extension welded over to the column flange.
For lateral frame stiffness, should the bar joist be considered pinned at the column even though the bottom chord extension is there?
The extension is fully welded at the column...not the type where the extension is only for lateral stability of the joist.

 

[ToadJones]

...also, the roof is mental decking with a rubber roof.

 

[ishvaaag]

I would say that how I would look the matter would depende on what the (structurally-wise) current intent is. Certainly is not pinned and the making of the thing may require some inspection on how it is made and so on. Depending on the loadings and construction the issue can be more or less relevant, and as well for lateral frame stiffness. If to be used for frame stiffness, a proper construction should be in place etc.

 

[JAE]

It would be fixed.

 

[msquared48]

I agree with fixed. The joist can develop a moment at the connection.

I was wondering though, what is mental decking? Is this a psychiatrist's office?

Mike McCann
MMC Engineering

 

[SAIL3]

I don't think it would make much difference either way...as I would expect the moment capacity of the bottom chord alone would be negligible...

 

[msquared48]

The moment is not in the bottom chord alone.

The moment is in force couple of the top and bottom chords in that both chords are welded to the column and can develop T/C.

Mike McCann
MMC Engineering

 

[dhengr]

Toad:

That type of detail has been used for years, and has generally worked fairly well, without a lot of failures attributed to it. Maybe not in a full blown moment frame analysis, but certainly to provide some moment resistance at the top of the column. However, it was probably used with a bit less than full consideration of all the things that might be going on, from a loading standpoint. A 24" lever arm is certainly plenty to provide a significant moment at the top of the column, assuming the forces imparted to the top and bottom chords of the bar jst. don’t overstress it. This detail was done many times without the full knowledge of the bar jst. designer, so he didn’t include those forces in his design, and therein lies the potential problem. Fully informed of this condition the truss designer can include this condition in their design. Does the bar jst. rest on top of the column and then the bot. chord frame into the column flg. as part of the jst. design, or just sorta as an added, different sized, angle btwn. the bot. chord and the column. That might be a hint of its inclusion as a consideration in the bar jst. design. Does the bar jst. bear on a beam, which is on top of the column, and then the bot. chord is extended into the column? Obviously, that is quite a different detail in terms of moment transmission.

 

[ToadJones]

Well, I am quite familiar with using trusses as part of a lateral frame.
In this case the top of the bar joist frames into the top of the column as does the bottom chord. I consider this to be the same as the truss I mentioned earlier. My concern there was that the bar joist stiffness relative to the column wasn't sufficient to consider the column fixed at the top (I was looking to avoid doing a full frame analysis)

In another case the bar joist bottom chord does not extend to the column. With the roof diaphragm in place, I would consider this column to be pinned at the top, agree? In other words, the column is not "free-to-sway" at the top.

Maybe ironically so, but considering the bar joist fixed at the top in conjunction with the other column loads causes the column to "fail" under gravity loading.

 

[paddingtongreen]

My guess! I think it is only a sway brace and not considered in the vertical load paths.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[RFreund]

I would think that if the top and bottom chords are welded to the column then this is a fixed connection (T-C couple as mentioned earlier) and it may fail in combined stresses from gravity load. Typically a note is used specifically saying not to weld the bottom chord only extend it to the column for this very reason (I believe).
Look at "Designing with Vulcarft" manual it has good design examples.

You could check to see if the truss and or connection fails/yields prior to the required moment. Or check the stiffness of the bottom/top chord.

Also I would agree that the if the top chord is only connected and the bar joist connected to the roof diaphragm than yes this is a pinned column.

EIT