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Bolt Prying 2

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CardsFan1

Structural
Mar 6, 2018
49
connection-page-001_suauui.jpg


I am evaluating this connection and it has significant prying. The detail shows thick washers on the top side of the flange.
based on my understanding of prying, I don't see how this eliminates prying. If it does, how do you determine the thickness?

Does anyone have any experience with using thick washers to eliminate prying?
 
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Never heard of that one before. Doesn't make sense to me on any level. I'd check prying for each connection component, regardless. Even if the detail was correct, for some unknown reason, it's probably not a good policy to rely on the detailer to spec the right washer and then the erector to follow through 100% w/ the correct bolt assembly, when it could come back on you.
 
Washers don't do it... you have to use a thicker cap plate... else design for prying action.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
There is also prying in the bottom flange of the beam that should be checked. AISC Steel Manual gives equation for checking this or you can use the design guide for HSS members
 
I don't think a thicker cap plate resolves the issue for the thin beam flange.

I think that gussets on either side of the bolts or welding a doubler plate to the underside of the flange may resolve the prying force, but I can't find a reference that confirms that.
 
prying is due to (relatively) thin plates reacting offset loads, producing out of plane bending.

Gussets resolve the stresses produced by this loading (without changing the loading itself) ... just clarifying "resolve the prying force".

another day in paradise, or is paradise one day closer ?
 

Correct... I was only addressing the cap plate... my oversight.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
My guess is that somebody didn't quite understand the prying checks, ran through the "minimum thickness to preclude prying" equation and put in a washer that matched that thickness. NOT the way to do it.

You have to resist a moment in the connection somewhere. It's either in the leg of the angle/stem of the tee/web of the I (which is accomplished by way of thickness and the ability to resist to the moment without forming a hinge) or a couple between tension in the bolt and bearing of the leg/flange it's in. This induces additional tension in the bolt - prying. Has to be checked on both the cap and beam flange as others have stated.

For light loads you may be able to prove that it limits prying. If the thick washer shifts the effective reaction point of the bolt closer to the web, it effectively reduces the moment arm in the flange which would, in turn, reduce prying. To take advantage of that I'd think you'd need to pretension your bolts.

 
Yes, I understand it does not change the load or the bending stress in the beam. I will check web crippling, yielding, etc.
The loads on the connection require a 1 1/8" beam flange to resist the prying.

If I include the prying force in the bolt design, the 1 1/4" bolts are overloaded by a factor of 3.

Im trying to understand what is the best way to deign the connection without increasing the
W24X68 beam to a W24x162 so I can get a 1.22" flange thickness.

AISC Part 9 states "change the beam flange thickness or the geometry" to eliminate or reduce the prying.

I could add more bolts. That should reduce the force on each bolt. But then I need to start adding gussets to the cap plate or make the cap plate a lot thicker.
 
connection-page-001_bnyjmq.jpg


I think the original sketch I posted was too small to see what is going on.
 
What are your thoughts on adding another gusset in the beam web on the other sides of the bolts?

Does that eliminate the prying force in the flange and the bolts?
 
That is much easier to see.

It will at least reduce it. I'm surprised the existing gussets don't eliminate it enough to make it work. Though to properly analyze it I think you're getting into yield lines.
 
That detail doesn't strike me as a very stiff or strong "moment" connection. If the designer doesn't understand prying, what else don't they understand?
 
Concerning the use of washers to reinforce connections against prying forces, the "Handbook of Structural Steel Connection Design & Details" by Tamboli discusses this method in detail, check out 2.5.2.
 
Nice. Thanks, sbi. That's a good looking book, too.
 
Thank you all for the comments. I will purchase a copy of this book.
Not that I need an excuse to buy more books!
 
There is also a newer version of this book but the table of contents is different. Do you know if the same procedure is covered in the newer version?

Maybe I have to buy them both! :)
 
I don't follow this current trend of HSS (say less than 8" sq) moment connections. I did up a 3D model for a project with some HSS frames recently to argue about their lateral load distribution. The silly HSS frames were not stiff enough to attract much load at all. The concrete floor diaphragm went into torsion, and the loads went to stiffer elements elsewhere. It seemed to me the EOR had looked at these frames assuming a very simple load distribution. Maybe this was an odd case?
 
Brad - these are really common roof level connections for wind resistance here on the east coast. I've used them for residential, too, where the loads were pretty small (small garage with a loft above, etc.). Anything more than that and you're right - way too flexible.

 
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