Cold Formed Steel Joist to Wide Flange

[BS2]

I have a situation where I designed cold formed steel channels to be supported by a wide flange beam on one side only. I specified a Simpson L70 connector in an effort to reduce the amount of torsion induced in the wide flange. The steel shop drawings I received showed a shear tab connection (see attached). I told them to supply the connection per the drawings. However, the steel was fabricated with the shear tab per the shop drawings. With a 3.5" eccentricity, it seems that a significant amount of torsion will be induced in the beam. The beam spans approximately 25' and the joists are only for ceiling framing.

Any thoughts on this situation? Are there any problems using the Simpson L70 connector I originally specified? Or should I accept the shear tabs as adequate? Thanks for your help!

 

[hokie66]

I can't comment about the Simpson connector because I don't know what it is. But the shear tab as shown would have been my choice for this connection. The compatibility torsion will be resisted by the bolt couple into the cold formed member.

 

[dik]

As long as the bolts can transfer the shear and the moment caused by the (shear * distance) from the bolt holes to the beam c/l, then the connection should be OK. As Hokie notes, the plates would likely have been my choice, except don't know what the Simpson connector is. Might have used Buildex TEK 5 for BAR stock.

Dik

 

[KootK]

It appears that the bolt holes are actually short slotted which would impact their ability to resist moment without a fair bit of beam rotation occurring first. The original clip detail would have required coping the joist flanges, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BS2]

Thank you all for the comments. The Simpson L70 is simply a right angle bracket with four screws on each side.

KootK - Yes, the original detail required flange coping. And yes, I agree, part of the reason for my concern was the short slots.

To add a little bit more detail about this topic, another reason I originally detailed the connection with the light gauge clip was because we are framing under an existing wall in one area and because of the clearance, the joists will have to be dropped a few inches.

 

[cliff234]

The L70 is a connector for wood construction. Simpson makes similar connectors for CFS. The detail provided on the shop drawings looks like a standard shear tab connection for structural steel. I would not use this detail if it required coping the CFS joist - and I would be cautious about connecting CFS framing with 1/2" diameter bolts.

 

[KootK]

One thing that you've got going for you is that it probably doesn't much matter where your CFM joists get located. So if you want to bypass the shear tabs altogether and use the clips, so be it. Just shift your joist layout over some.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BS2]

cliff234 - I believe the Simpson L70 is both a wood and CFS connector.

KootK - Exactly. I figured they could shift the whole line of joists over 8".

I am contemplating the idea that the tightened bolts will provide some frictional resistance to torsion even though they are in slotted holes. I know that we can't guarantee that friction will work in all cases, but in this case, there are quite a few bolts providing clamping pressure. Alternatively, I am considering providing an external brace. Does anyone have a good reference for compatibility torsion?

Thanks again for the help!

 

[dik]

an advantage of using TEK5 fasteners... make your own hole and it's snug...

Dik

 

[BSVBD]

My first choice, also, would have been the shear tab. However, I frequently specify TEK3 or TEK5 screws to a plain shear tab without holes. Some variables to consider would be the joist reaction(s) and shear tab thickness. I also would have designed the supporting beam for the torsion, if it is a new beam.

You could STILL use TEK screws fastened MUCH closer to the beam web, to reduce the torsion distance, IF you are comfortable (as you originally should have been) with the coped top flange of the CFS joists.

Simpson L70 "IS" used for both wood and CFS.

As you have already considered, it's still your call to bypass the tabs and connect per original design.

** If you use your original design of coped joists, wouldn't fastening with TEK screws to the already fabbed shear tab, at a distance equivalent to that of the Simpson L70 pre-punched holes, accomplish the same connection, with equal torsion (that may not have originally have been considered - nor would have been necessary), with a much more solid result, without spending the extra money of the additional clip, "appeasing" the fabricator, showing how you, the designer can quickly adapt to an alternate? I see a "win-win" here!

Clark-Dietrich also has several series of "Easy-Clips" that "could" be considered for future reference.

Considering this is only for ceiling framing, is torsion and moment resistance REALLY that much of an issue?

 

[KootK]

Does anyone have a good reference for compatibility torsion?

I'm not sure that I've ever seen a steel specific reference to it. Usually it's a hot topic in concrete owing to the ubiquitous continuity and how much of a pain in the butt it is to design stuff for torsion. The theory's simple enough though. Any tendency to twist can either be resisted by the girder in torsion or by the beams in flexure. In many cases, including this one, the beams in flexure is probably the stiffer load path. That, especially, given that your beam end support connections are probably simple shear connections that do not engage the flanges and are thus pretty flexible with regard to torsion. While your beam may be able to resist some meaningful torsion, it's all for not if you can't provide viable torsion reactions at the ends of the beam.

A design based on compatibility torsion would likely entail:

1) designing the the shear tab and bolts assuming that the tab was essentially a flexural extension of the joists that was simply supported at the girder web. The bolts become a moment connection after a fashion.

2) giving some consideration to the fact that, while the girder will likely rotate fairly freely to match the joist end rotation at the girder mid-span, that may well be less true near the girder supports where there will be more resistance to girder twist.

3) In concrete, you'd design the girder for a nominal, cracked "compatibility torsion". That, because torsional cracking of an unreinforced concrete member is brittle and may compromise shear capacity. In steel, we tend not to worry about that and just assume that there is no torsion on the girder.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.