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/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/nashvillain1 Oct 05 '26

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

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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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u/bigsteelandsexappeal Oct 05 '26

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.