Prying Action

[ToadJones]

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
(P.S. dont blame me for the bolt pattern...its existing)

 

[Lion06]

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?

 

[ToadJones]

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.

 

[Lion06]

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.

 

[ToadJones]

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!

 

[rb1957]

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).

 

[ToadJones]

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???

 

[delagina]

you can use this spreadsheet to verify your calcs.

 

[ToadJones]

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

 

[nutte]

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".

 

[ToadJones]

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.

 

[nutte]

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?

 

[dhengr]

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.

 

[ToadJones]

Well...I did it really quickly by hand....
excuse the handwriting and sloppiness.

 

[ToadJones]

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.

 

[nutte]

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.

 

[ToadJones]

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

 

[connectegr]

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

 

[rb1957]

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

 

[ToadJones]

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