Bolt Clamp Load Loss Due to Cyclical Loading 3

[edohma]

Hello,

I am trying to determine why bolts are losing clamp load on a steel car port structure. My best guess is that a semi-constant low speed wind is causing cyclical loading in the bolts and structure. My problem now is that I need to run the calculations to determine if this is actually happening. Does anyone know of a standard or document that has calculations for how many load cycles a bolted joint can to go through before losing clamp load (how many cycles for a given external load)? Any help is greatly appreciated.

 

[Tmoose]

For me, the reportedly out-of-control assembly, and resulting as-yet undefinable boundary conditions would put any analysis efforts on hold.

"First, build it stock." (advice to budding hot rod engine builders before spending big bucks on all the speed parts)
RTFI (the stated take away lesson by one of my College freshman professors after administering one of those tests with written instructions on the first page to "read thru the entire test". Yes, the test questions were brutal. The last "question" was to NOT solve any of the problems but pretend you were working so your fellow students would not catch on.)

Please post detailed pictures of the actual "structure".
Please provide pictures showing the locations on the structure where the fasteners have "loosened."
Please provide nice detailed pictures of the counterfaces of the nuts and bolts that have "loosened." and the surfaces they were attempting to clamp.
Please provide some nice detailed picture of the counterfaces and faying surfaces of the structure's components that were clamped by fasteners that loosened.
As other mentioned coatings etc on the faying surfaces etc can embed and throw away any fastener preload that may have existed on day one.

A lack of triangulation could turn most any outdoor stRuKtuRe into a never sleeping always watching twisty turny version of the much loved ( ) Junker test machine.

After welding sh*t together one of my least favorite former bosses would often say "that's not going anywhere ".
A significant percentage of the time he was flat out wrong because his "designs" were simply flawed. I never pointed that out to him.
Ever since, if I hear any fabricator, tech or millwright say "that's not going anywhere " I can not help but stop breathing for several seconds and bite my lip.

Keeping a nut locked to its bolt will not necessarily prevent slipping and wear of everything in the deficient joint's grip length.

https://www.boltscience.com/images/boltf6.jpg

https://brushdestructor.com/wp-content/uploads/2014/09/Bolts-badly-worn.jpg

 

[mfgenggear]

I am nobody but if that solar car port was at my place the 60-70 prevailing winds I get here would turn it into a pretzel. and stuff would get broken loose and fly off.
looking at the structure it appears weak to start with. so the strength and rigidity will have a big factor on those loose hardware. it would work in a very low wind protected area.
would it be possible to revamp or at least rework it strengthen it. put in a windy spot and run the test.

 

[dvd]

My suspicion is that poor quality-control on the hot-dip galvanized members and the (I think) very short grip length of the fasteners that are employed in the connections results in the zinc coating creeping under load and allowing the bolts to lose tension. The weld flash inside the tube from manufacturing of the tube may also, conveniently, be under at least one nut/washer.

 

[TugboatEng]

Tmoose, in the tugboat industry we're so disconnected we let the fitters design the piping systems. We only require the use the listed material. Obviously the engineers are lacking in material knowledge if they can't even steer a fitter. We have the stupidest problems.

 

[MintJulep]

Tmoose has written exactly what I would have written if I wanted to put in the effort.

 

[CWB1]

JMO but I would suggest following standard root-cause troubleshooting/failure mode analysis to logically understand and resolve the issue rather than guessing about extraneous causes or potential solutions as most in this thread seem to want to do. Work outward from the failure, slowly growing your scope until you resolve the issue. In this instance, a bolted joint relaxed and fasteners fell out so the first logical system to analyze is limited to the bolted joint itself. Given the original question, I would strongly suggest reviewing Bickford's Intro to Bolted Joints and other basic materials before performing a bolted joint analysis to confirm the original design is correct, then confirm with the customer every detail of how they are assembling it. Develop the ol' fishbone and use it to eliminate every potential root cause before increasing your scope.

A note on guessing - do NOT do it. I cannot stress this enough. Its unethical, makes you look EXTREMELY foolish, and ultimately reflects poorly on the rest of the profession. The customer's time and money is valuable, as is your own. If you dont use facts and standard process to draw logical conclusions then you open yourself and your employer to potential lawsuits if your guess is wrong, which it eventually will be. In many instances the best answer is the simplest and you said it yourself...

..who knows what the contractor actually did.

Until you rule out bolted joint design failure and joint assembly failures, any discussion of vortex shedding or advanced analysis is pointless bunk.

 

[mfgenggear]

I believe experience kicks in as well, what works and what doesn't