Issues with 1 1/8" dia holes for 1" dia bolts 1

[Nitesh Sadashiva]

Hello all,

AISC 15th Ed. manual specifies 1 1/8" dia holes for 1" dia bolts. Accordingly, we used 1 1/8" dia holes for 1 " dia bolts for truss connections. Now EOR has commented that the additional 1 1/8" hole dia may cause bolts to slip and results in additional truss deflection. So either we have to reduce the hole diameter to 1 1/16 or we need to use SC bolts to prevent the slip. I am of not sure what EOR commented is correct. Does providing 1 1/8" hole for 1" dia bolts really create additional deflection? Then how AISC specified it for bearing bolts? Can anyone help me on this? Thanks in advance.

 

[dik]

If the connections weren't spec'd as slip critical, I would think that if the AISC specs 1-1/8" holes for 1" bolts as standard, the engineer messed up. I'm not sure what AISC stipulates, or how the contract ties AISC into steel supply and fabrication. If AISC specifically states that holes may be 1-1/8" then the engineer should have stipulated SC. If there is an added cost for SC fasteners then it should be an 'Extra to Contract'. That's how it would be handled in our neighbourhood.

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

-Dik

 

[Agent666]

So would additional deflection even matter, presumably your chords are continuous for example so you're only getting the web members contributing to any additional 'shear' deflection.

You could assess the deflection and see if you want to just go back and say 'so what' to the reviewer. Modify the axial stiffness to account for the additional worst case movement and see. Remember as well not all bolts will slip the full amount as some will be installed at the extremes of the holes due to tolerances.

Offset if required (even if you don't believe it) with some preset or precamber and move on.

https://engineervsheep.com

 

[human909]

I agree with Agent666.

I was curious about it myself and modified a model of a 21m long truss from a previous project. Deflection at ultimate limit state went from 1/395 to 1/360. So pretty much negligible difference. This was with web member 2.5m long. Longer web members would mean less effect of tolerance effects.

 

[DrZoidberWoop]

The EOR might be technically correct in saying there will be more slop in a 1.125" diam hole, but that doesn't matter until it is written into the contract documents. Currently, per code, if you have STD holes in both plies, you are allowed to designate the brace/truss-member bolts as "bearing" rather than "slip critical". It would be absurd to specify slip critical bolts at every location on a job (cough cough LERA cough cough), but it may be reasonable to use SC bolts anyway. Your erector/shop will not be pleased if you are using STD on STD holes at both ends of the truss members.

We have adopted the new hole size, per AISC 15th, but have not had any pushback from EORs about this issue... yet.

 

[Nitesh Sadashiva]

Agent 666 and human909,

Thanks for your responses. As a connection engineer, I was designing the end connections for the provided end forces. If my connection necessitates increased bolt dia, I will use the required bolt size with AISC prescribed hole dia. I am curious how I can ensure the slight increase in deflection due to the hole dia is still within the limit. Do EOR need to verify that deflection is within the limit when STD holes are used per AISC regardless of bolt dia (1/8" extra in hole is only for 1" and larger bolt dia, For bolts with lesser dia, hole size will be bolt dia + 1/16)? Otherwise shouldn't it be specified in design drawings to use SC bolts?

 

[Tmoose]

"Otherwise shouldn't it be specified in design drawings to use SC bolts?"

I think a few posters said exactly that.

How experienced is the EOR ? Second rodeo?
What are the descriptions and tech references and applicable Standards for the truss design?
Omission of "useful" specifics and details from drawings or specifications sounds like a rookie mistake.
Unless there is something in the 5 pages of contract boiler plate that covers it.
Still, I think the drawings and specifications are the place for technical details.

 

[phamENG]

I think the most important point is that the EOR wants the holes to be 1-1/16. It's ultimately his/her design and he/she is responsible for the overall performance of the structure, so if that's what is required that's what is required. Should it have been clearly stated on the contract drawings? Absolutely. Are you, DrZ, and others justified in your frustration with EORs who produce inadequate steel contract drawings? You bet. But it's still the EOR's baby and it should come out the way they want it. If it means the fabricator and/or erector get a change order because now they have to set up a test rig and go through calibrating DTIs...so be it. That's up to the engineer to work out with the owner (though it would be nice to let them know: your drawings state X and Y, but you're asking for Z, and it'll cost $$$ extra).

But I am curious...where is the project located? Did the drawings indicate the 15th edition of the Manual, or did they indicate the 14th? Not everyone has switched over to the 2018 IBC or later, and so some engineers may not have the 15th edition on their desk and may not know about the change.

I know guys who are still using the old green book. There's more than a few engineers out there who would be blind sided by the change in diameter.

 

[Enable]

Just wait until the EOR did their calculations assuming drilled holes and finds out you punched them...

BTW in case you haven't seen it AISC / RCSC have jointly published quite a nice commentary (for free) on latest measures for joints using high strength bolts. Can be found here.

Here's their commentary on the change from 1/16" tolerance to 1/8"

 

[jorton]

IMO, using an industry standard size hole does not mean that there will be more deflection. I think a lot of the statements above assume that all plies of the connection have perfectly matching holes and that the bolts are perfectly straight, perfectly round, and exactly 1" in diameter. There are tolerances for all these different components. Galvanizing the bolts changes things too. In reality, some bolts will initially be in bearing and others may not, but detailing per industry standards helps ensure that the pieces will bolt up and perform as expected. Seems to me the EOR is specifying something that may not bolt up.

With that said, I'm no expert in how all these tolerances work together. I do know that a lot of time and deliberation goes into the developing these industry standards. The committees consist of producers, fabricators, detailers, erectors, engineers, academics, etc. I am hesitant to spec something more stringent without first understanding why the industry standard is what it is. Chesterton's fence comes to mind...

https://fs.blog/chestertons-fence/

 

[phamENG]

jorton - I love that. Good read. I was unaware of it before now, but I think it cuts both ways and sums up my point nicely as well. The EOR decided they want 1-1/16" holes or slip critical connections. They may not be right, but it's their call and the result is theirs to own - and maybe the framing in question was already designed to L/361 deflection and the added slip (if it occurs) will bring them down to L/350 and it'll violate serviceability requirements for the space. We don't know what the EOR's reasoning is, so we shouldn't insist on changing it without first understanding it.

If the fabricator and/or erector have serious concerns about fit-up and conflicts between the bolt manufacturing tolerances and the holes, then the right answer is probably to go with slip critical and tell the engineer they will do it but it will be a change order for reasons a, b, and c. (Namely, drawings called for standard holes and fabrication per the 15th edition of the AISC manual, and standard holes are 1-1/8" per the 15th edition.)

 

[canwesteng]

Easy fix - send an email or RFI referring to the AISC or RCSC spec, and watch the EOR accept the +1/8" dia holes.

 

[XR250]

I know guys who are still using the old green book. There's more than a few engineers out there who would be blind sided by the change in diameter.

Hey, I resemble that remark!

 

[human909]

IMO, using an industry standard size hole does not mean that there will be more deflection.

I don't know how you come to that conclusion.

In reality, some bolts will initially be in bearing and others may not, but detailing per industry standards helps ensure that the pieces will bolt up and perform as expected.

That is an incorrect statement as far as tolerances go. If you are modelling assuming zero slop and by building by following industry standard tolerances then you won't magically get performance just like your model simply because it is industry standard.

The reality is that hole tolerance can add to the slop and the deflection. This movement based on the slop can readily be calculated as I did so and stated above. It would normally be negligible. I doubt the EOR has calculated the increased deflection.

 

[dik]

I've always thought that was a silly part of the code...

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

-Dik

 

[dik]

Another comment to my note above... what version of AISC is tied into the code of the jurisdiction. If an earlier one, then the 1/16" oversize may be applicable.

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

-Dik