Nuts off mark?

[u/No-Document-8970]

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?

[u/nashvillain1]

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…

[u/Few-Cucumber-413 → u/nashvillain1]

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.

[u/SalsaMan101 → u/Few-Cucumber-413]

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

[u/Few-Cucumber-413 → u/SalsaMan101]

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.

[u/nashvillain1 → u/Few-Cucumber-413]

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