Slip critical connection for overhang 2

[kmead]

I am designing a 3-story building with a large roof overhang. I have two major girders cantilevering 40 feet and pick up infill beams. Since the roof has downward dead load and snow load, as well as upward wind load, do I need to consider load reversal and design the connections of the beams to girders as Slip Critical connections? Normally, on a small overhang I don't worry about it, but I want to make sure with this large overhang that I'm covering all my basis.
Any help would be greatly appreciated.

 

[spats]

Yes, I would design the connections as slip-critical.

 

[slickdeals]

Which connection arenyou wanting to make sc?

 

[kmead]

Spats, thank you for the quick reply.
Slickdeals: I am thinking all beam to girder connections should be SC. The girder to column connection will be a welded connection to the top of the column cap.

 

[hokie66]

I don't understand the logic. What advantage is gained by making the beam connections slip critical? Aren't they just simply spanning between the big cantilevered girders?

 

[kmead]

hokie66:
I would think since the beams and girders will experience load reversal from gravity and wind uplift, the connections would go into bearing once the gravity load is applied, but the wind uplift reverses the direction of the bolt bearing. I'm not sure if my logic is 100% but that's how I see it. Like I said, normally I don't worry about it with small overhangs, but with a 30 psf wind uplift on the overhang, I want to cover all my basis.

 

[slickdeals]

Let me put the question in a different way. Assuming these infill beams were in the edge of a building, say in Florida, where you would have significant uplift, would you design these beams as a slip critical connection?

I am not sure you would need to do SC connections just because it is a infill beam on a large overhang.

 

[nutte]

While there is some slop in the bolt holes, there's not much. The bolts will have to be banged and prodded to be inserted in the first place. Further, the loading condition that would lead to a 30 psf uplift is a rare occurrence. If that happens, and if the beam slides upward 1/16", what's the harm?

 

[dik]

Any connections that can see a 'frequent' load reversal/change are usually slip critical.

Dik

 

[kmead]

nutte: I see your point but I also want to make sure I comply with one of the requirements for the SC connection, which is what dik indicated in his response. My understading is that anytime you have load reversal you have to design the connections as SC.
This is sounding to me like one of those "engineering judgements" and I will err on the conservative side.

Thank you all for the replies.

 

[nutte]

To dik's comment: full wind load is not a "frequent" occurrence.

And where do you find the requirement that anytime there's load reversal you have to design the connections as SC? Bolted vertical bracing members are often designed with bolts in the bearing condition. These members usually have to resist both tension and compression, so there's definite load reversal. Similar to your infill beams, this load comes from an infrequent load occurrence.

 

[kmead]

Section 4.3 of RCSC Specifications. Also, in the AISC 360-05 Section 4.3
I think I'm convinced now that SC connections will not necessary, but again, this was an unusally large overhang and the project is really high profile and I didn't want to overlook anything.

Thanks again for the explanation.

 

[nutte]

What follows is off-topic. The trouble with blanket requirements like this is when they get combined with other blanket restrictions, ultimately leading to ridiculous scenarios. You want to make the connection slip critical, because it’s high profile, because of the load reversal (albeit infrequently), because it’s a big overhang. OK, no big deal.

Then you size the beam. W8x10 is needed for strength, but it deflects too much. Make it a W12x14. No big deal.

It’s time to put the beam shear on the drawing for the fabricator/connection designer. But this is a big building, high profile, with lots of complicated framing. This is a simple area, an afterthought. Don’t put a load on, and rely on the 50% tabulated load value in your general notes. That method has been around a long time, and our general notes have worked for 20 years, so why change them now?

Remember how the beam should have been W8x10 based on strength? Well, the shear requirement is going to be based on W12x14.

So now you have a small beam, with a thin web, with a ridiculously high load. It’s framing into a heavy girder, so it needs a deep and long cope. Oh yeah, there are steel joists framing to this beam, so the W12x14 is 2.5” higher than the girder. Cope gets even deeper.

And the bolts have to be slip critical, so you’re now using only 70% of their bearing strength.

Each one of these decisions wasn’t too bad on its own, but when put together, you’ve got a problem. The connection designer is going to spend a lot of time on this minor connection, diverting attention from the important connections, like the moment connection at the girder cantilever. This is not good practice. Your firm may not be guilty of all of these, but a lot of firms are.

 

[dik]

Nutte, just to clarify things a tad... I didn't say frequent full wind load... the fact that the load is frequently variable/reversable is good enough reason for me... I wouldn't worry about a sustained full load... snug tight is OK for that... and, I noted 'usually', not always...

Dik

 

[hokie66]

I don't consider roof members to be subject to 'load reversal' just because they have to be designed for both gravity loading and uplift. Regardless of whether it is an overhang, any roof in a high wind area will see both up and down loading, and you don't design all roof framing with slip critical connections.

 

[spats]

As usual, hokie is right on. I was the first responder to this question and, for the record, I thought kmead was talking about the main cantilevered girder connections, not the filler beams. If he were bolting the girders, I would still recommend slip critical.