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.

 

[MintJulep]

The cycles may be cause something to happen.

But you need to identify possible somethings to see if they are actually happening.

A properly designed joint will not lose preload because of cycling loads (within the design envelope).

 

[rb1957]

"why" is nice to know, better to utterly prevent it ... anti-rotation device on the nut, wire-tie the nut, cotter pin, ...

a constant wind in unlikely ... more likely may be a buffet ? or a thermal cycle.

if some outside cause is making the joint relax, then you'd need to physically lock the head and the nut.

another day in paradise, or is paradise one day closer ?

 

[EdStainless]

Sounds like you need a lock, this is most likely either thermal or vibration (high freq) related.
Make sure that you are really getting this tight enough and then pin it.

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[dvd]

It seems appropriate to show a photo of the connections. Was this a hot-dipped galvanized structure? Are there countersinks involved? Are you connecting tube? How were the bolts tensioned - per AISC/RCSC? I don't think that properly tensioned fasteners will suffer nut rotation - I would suspect that there has been some sort of relaxation due to your materials moving around.

 

[IRstuff]

Unless the wind is causing flutter, it should stay put.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[hydtools]

The clamp load is not high enough.

Ted

 

[dauwerda]

Most car port structures that I see are made of steel tube, is that the case for yours? If you are using through bolts with hollow sections I suspect it is due to the walls of the hollow sections yielding. If that is the case, you need to rework the connections so that you aren't using through bolts, or add sleeves that are welded to the walls of the tube that the bolts pass through.

 

[edohma]

rb1957, IR, Ed-Yes, I wasn't exactly clear in the OP, but I think the structure design is having some vortex shedding/buffeting. To give some visualization, the roof is basically an inclined flat plate at 7 degrees. So there are possibly vortexes created on the leading and trailing edges, also maybe some downwash or uneven loading at the structure ends. Lock washer/nuts are clearly the next step, but I need to get some numbers to present to my boss and client.

dvd-I haven't mentioned it yet, but I did not design the structure. The only info I have is from engineering drawings, images from Google, and second hand information from my manager. I have attached an image showing one example of the structure. There are different options for mounting the roof beam (cantilever/center mount)... Everything is galvanized steel, no countersinks, and the main members are made from boxed cee's welded together. The drawing shows multiple methods for tightening (torque nut, load indicating washer, and tension controlled bolt), but who knows what the contractor actually did.

Based on the feedback from everyone, I think my new solution is going to be to try and calculate the vortex shedding frequency first. If I have this I can maybe run a modal analysis (and maybe aeroelastic analysis) on the structure to see if there is some correlation. This will probably be the best way to show there is vibration in the structure and then I can just recommend locking hardware.

 

[rb1957]

if vortex shedding is the issue, how about adding vortex generators ?

ok, that's not really a suggestion, as the previous physical restraints should work against vibration.

if you're saying your need Proof of vibration before doing a simple re-design, then place accelerometers on the roof near (and far) from the fasteners.

but i'd've thought that there's a reasonably straight line of logic from fasteners loosen > vibration loads > physical restraints

another day in paradise, or is paradise one day closer ?

 

[edohma]

rb-I will hopefully get to the point where I recommend locking hardware, and then tell them to do the accelerometers if they need more proof. Per your last statement, I completely agree. I am assuming someone found the loose hardware and from there the contractors or managers assumed this to be a massive design flaw without giving this an insightful look... Anyways who knows, thanks for the help

 

[dauwerda]

I expect its not a vibration issue at all (at least not fast cycles that are associated with vibration and vortex shedding), just the cyclical nature of the load. Your picture shows a cantilever column with a beam cantilevering each direction off the top of the column. I'm sure this joint sees/resists all sorts of back and forth rotation in any wind loading. The bolts can not be properly preloaded because of the flexible walls of the HSS. Locking type nuts or loctite will prevent the nut from loosening to the point of falling off, but they won't help with the clamp loss, as you still have the flexible wall of the HSS problem. As long as your joint performs adequately with a snug tight bearing type connection your locking solution will work. If you need a more rigid connection that requires pre-tensioned bolts the locking devices won't fix that. If this structure is supposed to be designed per IBC/AISC this joint would require pre-tensioned bolts.

 

[edohma]

dauwerda-I'm not exactly sure if this is what you are saying, but the bolts do not go all the way through the member. There are access holes to get to the interior beams as needed

 

[dauwerda]

Ok, my assumption of a through bolt was incorrect. So, flexibility of the HSS sidewall is not a problem. Then yes, I see no issues with providing locking devices to prevent the loosening as you suggest.

 

[IFRs]

Unless it is widespread, it could just be sloppy work / inspection.
If on hollow shapes and they will always work loose, your failure mechanizm is losing the nut and then the bolt - could you just paint the heck out of them?

 

[edohma]

IFRs-I was told this is an issue at multiple sites, but I don't know how true this is. I have limited info from the client... I am assuming this is a design specific issue at a specific site with a unique wind flow path. Also, no painting allowed since everything is galvanized that removes the need for rust protection and it would add cost. Thanks for the input

 

[EdStainless]

I would usually go to Nylok types first, but for outdoor where you have a lot of temp cycle I would use castellated nuts with cotter pins or bend up lock tabs.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

Slipping in a spacer sleeve and running a bolt that passes completely through the beam will do a lot to improve the joint.

 

[r13]

I think through bolt and spacer sleeve is a good idea. The wall is too flexible to handle the stress due to cyclic wobbling of the cantilevers. If you have program available, A FEM model analysis can provide better picture on the local stresses.

 

[hydtools]

Might be more like variable wing wind loads than vortices. If you cannot sufficiently increase the clamp load with existing bolts, add more bolts.

Ted