Eccentric Connection: Shear + Tension: Bolt vs Weld questions?? 3

[EntryLevelEIT]

Hey folks,

I have a situation in which I am coping a W6 x 12 beam into another W6 x 12 beam. Due to eccentricity the connection will have shear and tension. I plan to use a double angle connection and have questions on both bolting and welding requirements.

BOLTED CONNECTION:
Per AISC equation J3-3a the (nominal tensile stress modified to include the effects of shearing, F'nt) must be less or equal to the (nominal tensile stress, Fnt) from table J3.2. In my situation F'nt is greater than Fnt, only due to a very low factored load, leading to a very low load shear stress fv.

1. If I increase my factored load, F'nt is less than Fnt and the equation is satisfed. Under any circumstance how can increasing the factor load (Pu) lead to satisfying equation J3-3a?

WELDED CONNECTION:
Per table 10-3 (All Welded Double-Angle connections)the minimum web thickness is .286" and a W6 x 12 only has a web thickness of .23".

1. Is there an equation or chart showing how to calculate the min. web thickness per weld size? I realize thickness should be slightly greater than the weld size.

2. With an eccentric connection, could I even use chart 10-3: as it is meant for "Simple Shear Connections"

 

[nutte]

A sketch of your connection would help. It's hard to visualize what you have based on your description. I'm guessing you really have a simple shear connection, and you need to check bending on the coped section.

As for the minimum web thickness, the commentary on page 10-12 will clear that up for you. As a rule, you should read all of the preceding information pertaining to any table or design aid you use.

 

[EntryLevelEIT]

Thanks Nutte,
Attached is a sketch...

 

[nutte]

You have a simple shear connection. Go through the checks for your connection type, and check bending on the coped section.

 

[EntryLevelEIT]

Under (10-7) Design of Simple Shear Connections, available strength for bolts references (Part 7). To no surprise of mine, part 7-(7-6) under Shear and Tension also does not list an equation for available strength and references Spec. section J3.7. After going through the AISC merry go round ,I am back to square 1 with equation (J3-3a)LRFD.

The only variables in equation (J3-3a) are Fnt, Fnv and fv. Per Table J3.2 A325 bolts Fnt=90 ksi and Fnv=48 ksi.
fv= ((shear/number of bolts)/(bolt area)). In my case the shear is 2.4K, with n=4, half inch bolts. fv equals ((2.4/4)/.19in^2)=3.16 ksi. Resulting in F'nt=109 > 90 ksi.

However if I had a shear of say 20 kips the equation is satisfied.((20/4)/.19in.^2)=26.3 ksi, =F'nt 51.23 ksi < Fnt 90ksi.

Can anyone explain why a shear of 2.4 kips is inadequate while a shear ok 20 kips seems to be ok? Or does this just mean my shear is so low that for avail strenght I can use FntAb??

 

[BadgerPE]

Yes. Your shear contribution is such that F'nt>Fnt which means that FntAb can be used. See Page 16.1-345 of the commentary.

"...offers the advantage that no modifications of either type of stress is required in the presence of fairly lagre magnitudes of the other type...."

So if you have a lot of tension and with little to no shear F'nt>Fnt and FntAb should be used.

 

[nutte]

Equation J3-3a means that F'nt cannot exceed Fnt. If the equation gives you F'nt greater than Fnt, you simply ignore that value and use Fnt. You'll see this same method used elsewhere in this specification, as well as others, to limit an equation to a certain value.

I still don't know why you're even checking tension on the bolts. The bolts are in straight shear. Hopefully you have a mentor that is guiding you through this stuff.

 

[BAretired]

Is this a single angle connection? How can you get two bolts in a 4" leg?

BA

 

[delagina]

W8 is the minimum depth for 2 bolt connection.

 

[EntryLevelEIT]

@ Nutte:
A similar example in my structural design book checks for the tension shear interaction. I am under the impression that a moment will be present, resulting in a tensile stress on the(4) bolts that are NOT directly beneath the 2.4K load.


@ BA:
For arguements sake I have currently have it as a double angle bolted conn. I am considering welding also, as I weight my options. If you mean how would they fit from a spacing standpoint?, I used Table J3.4 edge distance and section J3.3 for min. spacing.

 

[BAretired]

EntryLevelEIT,

I do not have access to the formulae or tables that you have referenced because I use Canadian standards, but the bolts appear too close.

I think you would be better to weld to the supported beam and bolt to the supporting beam. The hinge would be deemed to be at the web of the supporting beam.

I do not believe the bolts are in tension.

BA

 

[EntryLevelEIT]

I'll look into welding the angle. Just to clarify where I'm coming from with all this I attached an example from my book. The difference being that the beam is not coped and it connects to a column. The connection, however seems very similar to what I'm dealing with. Thanks for the help everyone.

 

[nutte]

What book is that from?

If your beam is simply-supported at each end, this load will result in a shear reaction on each end. There will be no moment at the support, and there will be no bolt tension.

 

[EntryLevelEIT]

The example is from "Steel Design" by Will Segui.
My beam was analyzed and sized simply supported with pin connections. I agree that only shear exists at the end of the beam. It is from that point to the web of the beam going in and out of the page "e" that a moment seems to be generated, regardless of how the connection is titled. Hence, one set of bolts is strictly in shear while the other needs to account for a resisting couple and tension is generated as shown in Figure 8.15.

I do have my own questions based on this approach. This implies the angle transfers the moment from 1 set of vertical bolts to the other. I don't see a check for the angle taking the moment. I realize that simple supports are assumed to never have a moment, which they don't because the simple support is the set of vertical bolts directly under the shear load of 2.4K.

By all means feel free to clarify the confusion.

 

[BAretired]

The moment is zero at the web of the supporting beam (going in and out of the page). The moment is 2.4*e at the two bolts in the supported beam where e is the dimension from the center of the web to the two bolts through the web of the supported beam.

BA

 

[connectegr]

SEE ATTACHED: Design example from 13th Edition Companion CD

Note that in the example of the welded connection angle with the outstanding legs weld, the weld provides only a return at the top. This is significant to maintain simple beam end rotation. BUT, I don't know in what condition a shop welded / field welded connection would be preferred.

The connections you show are standard simple shear connections. The capacities for these connections are tabulated in the AISC manual. The connection to the supporting member is straight shear. And although there is a small amount of eccentricity in the legs bolted to the beam web, this eccentricity is ignored if less than 3".

http://www.FerrellEngineering.com

 

[BAretired]

connectegr,

I tried to view your file but it appears I have the wrong version of Adobe Reader and in trying to download the latest version, something went wrong so I have not seen it.

I don't think you can ignore the eccentricity in this case because the bolts are too close together.

BA

 

[connectegr]

I have attached a sketch of my preference, for these tiny beams.

The outstanding legs have no eccentricity, and the back-to-back legs are eccentric as shown.

http://www.FerrellEngineering.com

 

[BAretired]

That is a much better detail.

BA