Minimum Bolt Spacing Code Compliance (AISC)

[ahypek]

This may seem like a silly question but it's something that I have been thinking about for years and I'd prefer to ask those more experienced with this subject.

As we all know, AISC explicitly states that a minimum bolt spacing requirement of 2-2/3(d) is required -> 3d preferred.

As far as I know, they have omitted any reference to load orientation as it relates to the minimum spacing. In addition, they have chosen to use the general word spacing as opposed to gage or pitch.

We know that this minimum spacing is intended for constructability and to add some inherent bearing/tearout strength although the latter is not guaranteed to be adequate.

My question, although it may seem silly, am I not in compliance with AISC360 if I choose to space (perpendicular to the load path) my bolts at a smaller distance than required? Especially knowing that workable angle gages do not always comply with this requirement.

If I am in compliance, can someone reference me to text in the AISC360 that would protect me, as an engineer, from building code violation.

 

[humanengr]

I would not use less than minimum, which may take into account fabrication. Punching those holes too close to each other could be an issue.

 

[Archie264]

If I've understood you correctly, in that scenario isn't the material resisting the load the material in between the bolt holes, therefore by reducing that spacing the material resisting the loading is also reduced?

 

[nutte]

Are you looking at the workable gage table for angles, then using a bolt that is too large to say "see, their own value doesn't comply"?

 

[ahypek]

humanengr, I work in a fabrication + structural engineering firm so that is the first thing I take into account.

Archie264, I am trying to restrict bolt spacing perpendicular to the load path.

nutte, No, I'm just using it as supporting evidence that smaller bolt spacings are ok in addition to the commentary.

 

[Archie264]

Yeah, I think I'm envisioning the same thing. Let's exaggerate it to emphasize it. What if the spacing was so decreased that there was only 1 mm of material between the bolt holes? Wouldn't the sum of of those 1 mm slivers of metal be the only thing holding the connection together? Or am I still misunderstanding it? Perhaps a sketch would help.

 

[dik]

little holes held together with slivers... how clever. Sounds really strong.

Dik

 

[Lomarandil]

Archie -- I don't know.. would you be opposed to a oval-shaped 1"x4" cross section "bolt"? It almost becomes more of a shear key than a traditional steel fastener...

In the right circumstances (loading), I could buy off on that.

----
The name is a long story -- just call me Lo.

 

[JoshPlumSE]

Maybe I'm thinking about this differently. But, I think the question (when taken to the extreme) would be like multiple bolts in a long slotted hole. In on direction they still resist some shear through bearing on the material. But, it has to be a decreased value, right?

 

[Lomarandil]

I'm not sure the value decreases that much... AISC J3.10 gives the same bearing allowable for load perpendicular to standard, oversized & short slotted holes. Even in the lower bound limit of long-slotted holes, you only take a 16% hit.

Anecdotally, when we pull rivets out of bridges, they occasionally show a shear plane between the plies they were connecting -- but you don't see a significant flattening at the bearing point between the rivet and the base material hole. So I (similarly) suspect bolt shear strength doesn't require too much assistance from "confinement" around the periphery of the base material hole, and it happens mostly in local hertzian stresses.

(Yes, those ones are a bugger to get out).

----
The name is a long story -- just call me Lo.

 

[ahypek]

Here's the best example I can offer to you guys:

The minimum bolt spacing for standard, oversized, short-slotted and long-slotted holes is the same. Let's keep this discussion without the bounds of bearing and say I have 2 columns of 3/4" bolts with short-slots perpendicular to the load path. My minimum spacing, as per AISC is 2". My short-slots are 1" long leaving me with 1" of steel between my bolts with absolutely no effect on the strength of my connection besides reducing my tensile rupture plane. Therefore, by deduction we can assume that a spacing of less than 2" with standard sized holes is in line with the AISC but yet we are still given this minimum requirement in Chapter J of the Specifications.

Archie264, how about this for another example: For an L5 angle, a workable gage of 1.75 is provided between bolt columns. As per AISC, the minimum for a 3/4" bolt would be 2". Therefore is this workable gage even compliant with AISC360 specifications? I know that structurally it would be ok, I'm just wondering if there anything in the AISC that would allow me to bypass that minimum requirement.

dik, your adjacent net shear plane is still smaller, there is enough net tension area to resist block shear and there's no direct bearing pressures. Still confused?

JoshPlum, in my case I'm not trying to eliminate the material completely from between the bolts but you're on the same page as I am as far as the question goes. If the connection and reduced member passes all strength checks, the only concern I would imagine is moving closer to a theoretical roller with snug-tightened bolts and possible stability issues if the opposite connection was the same.

Lomarandil, exactly! That's why I'm wondering if there's anything in the text to protect me from confused special inspectors or the buildings department. I have zero concern with this from a structural engineering perspective... only legal.

 

[dik]

dik, your adjacent net shear plane is still smaller, there is enough net tension area to resist block shear and there's no direct bearing pressures. Still confused?

Not yet, but, thanks...

Dik

 

[ahypek]

No problem, your response didn't seem well thought out so I figured I'd ask. Feel free to contribute, I'm all ears.

 

[Archie264]

>>>I think the question (when taken to the extreme) would be like multiple bolts in a long slotted hole. In on direction they still resist some shear through bearing on the material.<<<

Josh, yes, but surely only due to the material at the edges, right?

ahypek,

I should have prefaced all of this by saying that I'm not any form of steel expert while there are plenty here who are. As such I perhaps shouldn't have weighed in. But I am interested in learning and will at least be following this discussion. Perhaps what's throwing me is the "perpendicular to load path" portion of it. To me that implies tension, where the fracture plane is a critical component of the analysis per Sections D3 and B4.3b. It seems to me that reducing the spacing would reduce the net area, A[sub]n[/sub], which is what resists the tension. Maybe not by enough to matter and I suppose that's really the question at hand. I will try to dig a bit more deeply into it on my end, as time permits, but that's perspective I'm coming from.

 

[Archie264]

PS: I missed this part, "I have zero concern with this from a structural engineering perspective... only legal."

Just anecdotally, it would be interesting to know if the minimum spacing was based on in-depth research or simply established practices. I.e., are the any interferences among the stress fields, or something like that? I almost hope for the latter, given what the in-depth research has done to concrete anchorage requirements. (Tongue-in-cheek intended.)

 

[Archie264]

PPS: My understanding is that AISC's technical staff are very responsive to questions such as this; you might want to ask them. Certainly a response from them, in writing, should carry quite a bit of weight with the local building department.

 

[ahypek]

Sent over an e-mail, will update this thread with their response. Thank you

 

[ahypek]

Update from AISC:

Without being entirely explicit, they seem to be in agreement with my train of thought and it could be done provided constructability demands are met. They reference Chapter A1 of the Specifications as reference text to protect myself.

As far as the 1.75" gage goes for the 5" angle leg, the 15th Edition of AISC360 has already corrected this with 2".

Hope this information is as valuable to you folks as it is for me.