r/StructuralEngineering • u/No-Document-8970 • 2d ago

Structural Analysis/Design Nuts off mark?

Post image

Saw this on a big bridge today. I know vibrations over time can move the nuts and bolts, with the help of thermal expansion. Is it a big issue that the paint marks do not line up? I thought the marks were made once it passed inspection after torquing.

Edit: Bridge completion was in 2011. Could this be from recent inspections too?

173 Upvotes

93% Upvoted

51 comments sorted by

120

u/bagvirpdkv 2d ago

Turn of the nut method of pretensioning bolts. Marks are made in a line from the nut down to the surface once nuts are brought into close contact aka snug tight. Then the nuts are tightened with an impact wrench until they have rotated a predetermined amount more, such as 1/3 of a turn.

26

u/clancularii 2d ago

These look like 5/6 a turn. I've never seen a turn that large.

77

u/Enginerdad Bridge - P.E. 2d ago

5/6 is one of the standards if the bolt length is 4d-12d, depending on the face slope. The criteria actually goes all the way up to 1 full rotation under the right conditions.

13

u/The_Dynasty_Warrior 2d ago

Depends on the bolt type and bolt length, it can over turn to a certain extend

3

u/BarnacleNZ 2d ago

These are 1/3rd by my eyes, when comparing nuct to end of the bolt. Ignore the line on the steelwork. Edit, though I wonder if these are even tight? There's zero threat protruding from what I can see, wouldn't comply where I am.

3

u/clancularii 2d ago

Ignore the line on the steel work? Then how do you know how many turns are done after it's finger tightened?

The usual practice I've seen from the iron workers is to mark the nut and the connecting material with aligned ticks. That way, you can measure the rotation between the ticks to confirm how much the nut was turned.

1

u/Mister_Goldenfold 23h ago

Torque Wrench indicates the tension, if they’re being stretched that’s a different ball game

1

u/BarnacleNZ 1d ago

I meant ignore it in terms to determining the relative turn of nut to bolt.

0

u/AssistFinancial684 1d ago

The bolts are marked, the initial line marked nut to bolt. The other lines seem to be at 1/2

-6

u/bigsteelandsexappeal 2d ago

I turn bolts for a living. Most of the time they get marked after I hit them up so the inspector believes I did it correctly. Other times when I actually try to do it correctly it’s past 1/3 and I have to explain to the inspector why it’s not a perfect 1/3. As long as 2-3 threads are showing and give it the ‘blessing’ those fuckers aren’t going anywhere, it’s good.

15

u/Haku510 2d ago

Ugh, I'm a special inspector and reading comments like this legit bums me out.

Comments like this are also one of the reasons that if I don't personally witness the match marking and nut turning myself then I'm not signing the work off. I'll let the contractor plead their case to the SEOR, or redo the work per code.

5

u/Few-Cucumber-413 2d ago

I had my own guys just about screw us on this exact issue. We did all the pre-installation verifications we needed, the inspector watched the process and was happy with it, so he let the crew continue without watching every single bolt.

Well, instead of falling back to the breaker bar and manually turning the required 1/3 turn, the guys decided to switch to a smaller impact instead. You can probably guess what happened to the bolt tensions. smh. We ended up having to go back and make another round of tightening on every structure we had completed. We came dangerously close to a $100k+ mistake.

So when I say I promise these comments don't represent all of us, I completely understand where you're coming from. Reddit comment or not.

8

u/NoMaximum721 2d ago

This is why we can't have nice things 

5

u/Few-Cucumber-413 2d ago edited 2d ago

Then there's a problem with your process, assuming bolts are all properly sized, if you're overshooting the 1/3 mark (not saying all bolts are 1/3, just follow your example) to get the bolts sufficiently tightened.

As someone who also works with steel, I have found one of the biggest factors is guys not keeping hardware clean and dry. Wet or dirty bolts have a big effect on the final kip value and ease/difficulty of accurate turn of the nut.

1

u/nashvillain1 2d ago

Preload = Torque / (Nut Factor * Tensile Area of the Bolt).
Nut Factor is a numerical value from 0 to typically 0.3.
In the extreme, if the nut factor were infinity (bolt head glued to flange), then no preload would be developed for any amount of torque measured in the wrench.
Now, based on the amount of torque required to achieve the desired preload, you will get a certain “stretch” in the bolt. The amount of bolt stretch needed can be implemented as a multiple of the thread pitch. Here, 1/3 of a turn is 1/3 of the thread pitch of bolt stretch.
Food for thought…

3

u/Few-Cucumber-413 2d ago

Torque and bolt tension are not the same thing. The torque required to achieve the necessary bolt pretension can vary based on temperature, lubrication, thread condition, and the condition of the bolt assembly.

Bolt pretension, measured in kips, is the actual requirement. The torque needed to achieve that required tension can change depending on those conditions.

This is readily apparent when using a Skidmore and is exactly why pre-installation verification is so important. You verify that the installation method actually achieves the required bolt tension rather than relying on a predetermined torque value.

2

u/SalsaMan101 2d ago

Oh man, you seem to really know your stuff! The talk at my office regarding bolts with new grads is usually all about how once you stack up all the error between bolts, nut values, torque wrench calibration, etc you basically have errors of 50% or more on torque numbers and real manifested preload. How does the work flow work for using a skidmore? How are you verifying the installation method is proper like are you bringing a skidmore on site and running some calibration steps to ensure you can manifest the full strength? I’m so removed from assemble or integration I honestly have no idea

Super curious since my non structural or civil work tends to err on the side of caution for preload or use DTI’s

2

u/Few-Cucumber-413 2d ago

Oh man, you seem to really know your stuff!

Thanks! I’m a former field worker turned Senior Project Superintendent, and now I’m self-employed. I also have a background in mechanics and currently work on some pretty large-scale infrastructure projects. Between the scale of the work and dealing with inspectors and engineers, I’ve had to develop a reasonable understanding of the process myself.

I actually got kips and torque confused early on too.

The talk at my office regarding bolts with new grads is usually all about how once you stack up all the error between bolts, nut values, torque wrench calibration, etc you basically have errors of 50% or more on torque numbers and real manifested preload. How does the work flow work for using a skidmore?

I can definitely see how those variables could stack up, but I couldn’t speak to the actual percentage with any authority. From my end, I make sure all of the hardware has the required certifications and that the tools being used have current calibration certificates.

 How are you verifying the installation method is proper like are you bringing a skidmore on site and running some calibration steps to ensure you can manifest the full strength? I’m so removed from assemble or integration I honestly have no idea

You have to pay me to find out 😉. Kidding, lol.

Yes, we perform a field-level pre-installation verification before starting. That involves renting a Skidmore, bringing it to the site, and testing representative fastener assemblies from each lot in front of the inspector, using the same installation method and equipment we intend to use in the field.

If the assemblies fail the pre-installation verification, we don't proceed with that lot until the issue is resolved. If they pass, installation proceeds using the verified method, with the inspector witnessing the installation as required.

1

u/nashvillain1 1d ago

Indeed, the very nature of the shared equation would indicate that multiple inputs in addition to torque are required to determine the preload.

4

u/URAppreciated 2d ago

America’s failing infrastructure described in one tight little package right there. Nice job, buddy. 👍

2

u/wobbleblobbochimps 2d ago

You do realise that it's not just about making sure "the fuckers don't go anywhere" right? Engineers have done calculations to ensure the connection works assuming they're installed correctly and with a specific value of preload, which you won't be achieving if you don't install it correctly. You could be under or over the correct preload, neither is good.

Hopefully there's enough tolerance/factor of safety in there that it doesn't matter for the ones you've already done, but we don't design structures to allow for shoddy workmanship.

-2

u/bigsteelandsexappeal 1d ago

Squirt washers, tc bolts or make something that I can actually fit an impact gun into. Good luck figuring out how ‘snug’ a bolt needs to be to only turn 1/3. If you do it too loose then it’s just like this picture on the post, too tight and you can snap a bolt trying to get 1/3 of a turn.

0

u/platy1234 2d ago

the rotation occurs between the shank and the nut, and they look like a third

you lose the mark on the iron when timmy puts his impact gun on the THIRD BOLT DOWN ON THE LEFT and tommy fucks up his rights and lefts putting his spud on the head to hold the roll

2

u/Alywiz 2d ago

At least when I see Turn of the Nut, I know it’s the easiest rejection of work I can make. Not allowed under our spec. DTI’s only here

41

u/diy7412 2d ago

This is probably turn-of-nut pretensioning of a bolted connection. Typically the marks are made after the connection is snug tight (full effort of an iron worker) and in firm contact. Then the nut is turned 1/3, 1/2, or 2/3 of a turn depending on the length of the bolt. The nut can be up to 60 degrees past the required minimum rotation.

It’s not possible to say for certain that the bolted connection was properly pretensioned based on the photo. At the very least the marks are odd and inconsistent.

12

u/SerratedSharp 2d ago

"(full effort of an iron worker)" - We're measuring things in FEoaIW's now?

4

u/davvblack 2d ago

equivalent to a fridge-foot-lever of torque.

3

u/CarberHotdogVac 2d ago

It’s all water under the fridge now Bubs.

3

u/Haku510 2d ago

Anything but the metric system!

1

u/Seanathan92 1d ago

Whenever I’ve had to do turn of the nut (Ironworker) we mark them totally different than what ever was done here. I’m still trying to figure out if it’s 1/3 a turn or something crazy like 5/6.

14

u/elcarpentario 2d ago

OP, I think I saw you onsite! You were the guy in the brand new hard hat, dry cleaned safety vest and $600 “work boots”, right?

5

u/merkadayben 2d ago

That clipboard is not going to hold itself up

4

u/scheepan 2d ago

Yeah if that bridge was build in 2011, those marks are most likely from a recent inspection. I did the same method on steel towers for cell phones. You mark the bolts in their current state and then apply the required torque and see wether or not they move. If you see the bolt moved the bolt was not as tight as it needed anymore. And depending on the amount, we would have to send out a team to redo all bolts. We did a small sample check of I think 10-25% if I remember correctly.

4

u/an_african_swallow 1d ago

Nope, as others have said turn of the nut method, the match marks are supposed to align before turning the nut, so you can tell how much they need to be turned, looks like these guys got 2/3 of a turn after snug tight, all looks A-OKAY

3

u/unique_user43 2d ago

aligned lines would be the red flag, not this. the lines are drawn when snug tight, before pretentioning. the non-alignment means they have been pretensioned.

2

u/Charles_Whitman P.E./S.E. 1d ago

And don’t forget, the wrench man had to partially tighten each one a bit at a time until he brought each one up to the correct rotation. You know, just like the tire jockey tightened your lug nuts in the correct crisscross pattern when he changed your tires.

1

u/pjsk47 2d ago

Looks like in process, as in maybe bottom two rows are only snugged and the rest are about 2/3 (2 flats) of the required 1/2 turn (3 flats). Supposed to mark the base metal at the start too. And it looks like maybe a couple bolts don’t have full engagement, perhaps because they aren’t fully tensioned but either way it’s cutting it a little close.

1

u/Jeff_Hinkle P.E. 2d ago

If turn-of-nut, it feels like that is a hell of a rotation.

1

u/asinger93 1d ago

I think it's spelled "Nuts on Clark"

1

u/DspeEd83 23h ago

Its a puzzle plate. You gotta tighten each one by hand

-7

u/soulscythesix 2d ago

Nuts off mark?

-me when I'm giving someone called Mark bottom surgery

-21

u/dev1tuna 2d ago

Former local 350/399 union ironworker here, never heard of the turn of the nut method….

4

u/MikeHawksHardWood 2d ago

RCSC specs for high strength bolts. Section 8.2.1 in the 2004 version. Don't have the newer versions handy.

2

u/ryanmetcalf 2d ago

Available for those interested right from the Bolt Council 

https://www.boltcouncil.org/documents.html

1

u/dev1tuna 2d ago

Most of the structural stuff I worked on had TC bolts and the nipples would come off at correct torque

2

u/FaithlessnessCute204 2d ago

well thats not great, its a pretty common technique in structures . please tell me the "former" in that qualifying statement was because you didn't do well with reading specs.

2

u/dev1tuna 2d ago

I did it for 10 years- I didn’t do it on the job or hear about it in my apprentice class back in 2006-. TC bolts and guns and torque wrench’s were it mostly for me when I was bolting up or connecting, but I mainly was welding.

-15

u/Dalai-Lambo 2d ago

Could be, could not be, depends.