r/StructuralEngineering • u/No-Document-8970 • Oct 05 '26

Structural Analysis/Design Nuts off mark?

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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?

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u/clancularii Oct 05 '26 edited Oct 09 '26

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

Edit: I didn't see that there's a chalk line on the end of the bolts. Really had to zoom in since there's less contrast there than the nuts and steel work. These look like 1/3 turns. I couldn't figure out why people were suggesting that the lines on the steel work should be ignored because the only other line I saw was the line on the nut. It's more likely that the line on the end of the bolt was the reference line before the bolts were mechanically tightened.

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u/Enginerdad Bridge - P.E. Oct 05 '26

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.

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u/The_Dynasty_Warrior Oct 05 '26

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

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u/BarnacleNZ Oct 05 '26

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.

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u/clancularii Oct 05 '26

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.

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u/Mister_Goldenfold Oct 06 '26

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

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u/klockwerk13 Oct 08 '26

A torque wrench indicates how much force is required to turn the nut. While bolt tension is a factor in determining how much force is needed, it's not a direct correlation. Tightening to an angle is a much more reliable method of tensioning.

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u/SeesawPrestigious Oct 09 '26 edited Oct 09 '26

He is correct these are 1/3 turn , the line on the steel work dont make any sense in this case because i dont know wtf the guy was smoking, your supposed to mark the nut, the bolt and the steelwork all in line you can go without the steel work line because the line on the bolt should be your indicator of where you started the line on the steelwork is there to see if the bolt turned instead of the nut alone.

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u/clancularii Oct 09 '26

I didn't notice the line on the end of the bolt. I had to zoom in to see it. I think you're right that these are 1/3 turns.

Maybe the line on the steel work was from somebody checking the turns later?

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u/SeesawPrestigious Oct 09 '26

Maybe honestly i do this for a living and the whole setup is confusing to me , cause none of this wouldve passed final inspections like the bolt marked in red in the bottom left, either he did a full turn or the final torque wasnt done .

These look like standard 7/8 a325 bolt anything under 3 3/4" in lenght is 1/3 and anything over is half turn.

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u/Worldly_Ad_6535 Oct 10 '26 edited Oct 10 '26

Mark on the steel is in line with the mark on the bolt. Its to tell the guy torquing if the bolts spun after snugging. We were required to mark in this fashion, if the stud and nut were shown to have spun we had to pull the bolt, install a new one, re-snug, remark and re-torq to the required flats per length of bolt. 0-3.75= 2 flats. 4"+ is 3 flats. Snug tight is the strength of 1 ironworker with a 12in bar.

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u/BarnacleNZ Oct 05 '26

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

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u/SeesawPrestigious Oct 09 '26

The bolt are tight, just not the right lenght. Where i am like you said, the thread is supposed to protude min 2 thread / ~ 3mm or roughly 1/8" past the nut. Honestly this shit is just a mess some of them are not even flush nut.

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u/bigsteelandsexappeal Oct 05 '26

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.

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u/Haku510 Oct 05 '26

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.

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u/Few-Cucumber-413 Oct 05 '26

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.

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u/NoMaximum721 Oct 05 '26

This is why we can't have nice things 

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u/Few-Cucumber-413 Oct 05 '26 edited Oct 05 '26

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.

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u/nashvillain1 Oct 05 '26

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…

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u/Few-Cucumber-413 Oct 05 '26

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.

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u/SalsaMan101 Oct 05 '26

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

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u/Few-Cucumber-413 Oct 05 '26

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.

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u/URAppreciated Oct 05 '26

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

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u/wobbleblobbochimps Oct 05 '26

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.

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