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Prying Action

Prying Action

Prying Action

(OP)
I'm working late and having a brain fart....
I have a simple connection with a beam whose top flange is bolted to the bottom flange of another beam (hanger-type).
Beams are 90 degrees to each other. The bolt gage on the lower beam is 3" and the distance between the bolts in the orthogonal direction is 2". So the bolt pattern is 3"x2".

Is the "p" distance in AISC's prying calc in this situation half of the 2"(or = 1") or could it be considered 2" since it is "the distance tributary to the bolt row". ????
I hope I am not completely embarrassed as I read the answers here blush
(P.S. dont blame me for the bolt pattern...its existing)

RE: Prying Action

I would use 2" since, as you stated, that is the trib length per pair of bolts.  

One thing that stands out to me, though, is the lack of torsional restraint at the support for the lower beam.  Maybe you can add stiffeners and that kills two birds with one stone?

RE: Prying Action

(OP)
Is this the man formerly known as "EIT"?

I dont think I can add stiffeners, its a monorail.

what is the logic behind killing the two birds with stiffeners.
Keep in mind, I am not question you, just been working since 7:30 am...with some breaks.  

RE: Prying Action

Yes, this is StructuralEIT.  I was thinking if you could add stiffeners between the bolts then bending of the flanges (and therefore prying) is taken off the table.  The second bird to shoot down with that stone would be the stability issue.  It seems like this is a support at which the beam is not braced torsionally.  I just realized that I might have been unclear, but I'd be looking to add the stiffeners on the lower beam.

I've never dealt with a monorail, so I have no idea how much room you might have, but even if you could get partial depth stiffeners in and leave room below them that would help.

RE: Prying Action

(OP)
Yes, I agree 100%.
If this were just a beam-to-beam, stiffeners would be default IMO.
Of course 2" isnt much room for that with bolts in there too.

Monorails typically have no stiffeners in this situation. There isnt much room for them at all and often times these places move trolleys and hoists from monorail to monorail as needed....hate to move one and have it hit the stiffeners!

RE: Prying Action

isn't prying the same as "heel and toe" ?  when you have an offset tension load, like on the leg of a angle, reacted by a fastener on the base ( or the leg of a Tee reacted by two fasteners) ?  I can see that you might use 1/2 the distance from the fatener to the edge, we (aeros) typically use 2/3 (triangular distribution, peaking at the edge).

RE: Prying Action

(OP)
Gents-
Running across some weird numbers as I finish this up.
If I assume the bolts are A307, I actually get a higher allowable than if they were A325 bolts.
In designing new, this would probably never come up as one would most like choose a bolt and run with it.
Anyone ever run into this???

RE: Prying Action

(OP)
Problem with that is it is only good for A325, A490.  

RE: Prying Action

Are there four bolts here?  On the top beam, are there bolts on each side of the beam web?  (Doesn't sound right, with a 2" gage.)  A sketch would help.  I also don't understand how you get a higher capacity with A307 bolts over A325 bolts.

Ignoring the top beam, the bottom beam has 4 bolts.  I would use p equal to 1" (half the row spacing) plus the distance from the end of the beam to the bolt (assuming you're near the end of the beam).  If you're not near the end of the beam, use an end distance of 6 times the bottom beam flange thickness, and add this to your 1".

RE: Prying Action

(OP)
nutte- again, the bolt pattern is existing = not my fault its goofy. And yes, believe it or not the gage is 2" on the supporting beam and 3" on the supported.
I am in the middle of the beam, as you mention.
I decided to use p=2.0"
AISC suggests using p no greater than the beam gage on the supported beam, which in this case would be 3".
P=2.0" gave me adequate results.

I am pretty confident in my numbers and, A307 bolts give a very slightly higher capacity then A325's.
 

RE: Prying Action

To be complete, you should check prying for both beams (top one has gage=2", row spacing=3"; bottom one has gage=3", row spacing=2").  Now maybe one of these has such a thick flange that you can tell by inspection it won't control.

I'm curious about the A307 bolts being stronger.  Feel like posting your calculation?

RE: Prying Action

Toad:
I don't have the copy of the AISC manual you guys are using on Hanger-Type Connections.  And, only God and AISC people who worked on this particular section of the code know the whole truth, as they see it...  but my guess or thoughts on the matter are:  (1.)  One would normally think that the supported beam is the lighter beam in terms of flange thickness and thus the beam to check.  And, if that's not the case, because of the funny bolt gages and spacings you indicate (I'm not sure I see your picture either), are you checking the critical flg. condition?  Mightn't the relationships btwn. 'a' & 'b' lengths, flg. thick., and 'T' & 'Q' forces on the other beam give you a more critical condition?  Which flg. is actually causing the prying?  I don't know the answer to that.  (2.)  The reason that A307 bolts give a slightly better result than A325's might be that they relax, under prying action, a bit more than the A325's, and thus allow a slight redistribution in flg. plate moments under the bolts and at the beam web, thus a slightly improved flg. stress, or different flg. deformation.  This would be buried someplace in one of the AISC multiplying or adjustment factors, or one of their equations, and would not be particularly obvious to us.  Oh!, the mysteries of steel design.

RE: Prying Action

(OP)
dhengr...
My thoughts were the same. I was thinking (while buried over my head in jobs) that the weaker grade bolt might elongate more under the same loading....or something along those lines, and this might be relieving some of the "prying action".
In any case, this is better than the prying action I receive from my wife after coming home smelling like IPA from picking up take out.  

RE: Prying Action

4.50 kips versus 4.48 kips?  That's round-off error.  I get the same answer for both grades of bolt, 4.47 kips.

RE: Prying Action

(OP)
I'm not about to start arguing round-off....point is, the prying results are the same, and that is interesting, no?

RE: Prying Action

I don't understand where the bolt grade applies to the prying capacity of the flange.  The variable is the bolt diameter.  I have attached an old sample prying calculation, per ASD 9th Edition.  The first part is not relevant to your condition, as it determines a tension per bolt based on a moment.  But for a given tension per bolt the prying calculation should be the same.  The calculation determines a minimum thickness for a determined tension.  But, this could be reversed.  

And A325 bolts definitely have a greater tensile strength, than A307.   

http://www.FerrellEngineering.com

RE: Prying Action

i'm curious ...
what's tc ?
and why is it dependent on the allowable load of the bolt ?

RE: Prying Action

(OP)
nutte-
do you agree, or am I screwing something up?  

RE: Prying Action

(OP)
Connect-
You asked "I don't understand where the bolt grade applies to the prying capacity of the flange."

Isn't the prying check for the bolt itself?
Does not the prying mechanism induce more tensile load to the bolt?  

RE: Prying Action

Toad, the results are the same because bending on the flange controls, not tension on the bolt.

RB, tc is the plate thickness required to fully develop the bolt tensile strength with no additional prying forces induced.  A stronger bolt requires a thicker plate to fully develop the bolt's strength.

RE: Prying Action

Toad, the prying check is for both tension on the bolt and bending on the flange.

RE: Prying Action

I would select the bolt for the magnified tension on the bolt, or prying on the bolt.  And then determine the minimum flange thickness for the same tensile force.   

http://www.FerrellEngineering.com

RE: Prying Action

(OP)
Wow.
Pretty asinine on my part.
Apologies to all for wasting your time.  

RE: Prying Action

I don't know the exact machinations that AISC went through to arrive at their methods and formulas for this problem.  But, it seems to me that there are two modes of failure here; either flg. plate bending and stresses, or flg. plate deformation which might cause prying, bolt bending plus tension and possibly failure of the bolts or a bolt.  I believe AISC assumes the flg. plate moments are the same at the beam web and under the bolt line, one +M and one -M.  And, if the flg. plate is stiff enough this bending will control and impart primarily tension to the bolts.

You keep throwing me curve balls and then laughing at me when I miss them.  I thought you were talking about a larger difference.  I'd call that round-off error too.  And flg. plate bending is controlling your design, so bolt strength makes no diff.  Your answer will be the same for either .75" dia. bolt.

RE: Prying Action

(OP)
I'll admit, in doing new design, I try to take prying out of the mix with geometry and plate thickness.
I dropped the ball here by simply getting tunnel vision on the bolt strength.
Quite pathetic really.
I'd like to blame it on the fact that it is an existing structure evaluation for which I have been asked to make an assumption on bolt strength that lead to my being a fool, but that would be more pathetic. blush
I usually check flange bending by other methods.

By the way, bolts will be replaced

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