Bolts Loosening and Coming Off

[StructSEZ]

Hi guys,
I've got a situation where an aircraft hangar (trusses on concrete columns) is experiencing bolts falling out.

Some of the guys reckon the builder/rigger did a poor job of tightening them. However, I find that a bit hard to believe.
In your experience, what would cause snug-tight (not fully tensioned) bolts to come loose to the point of falling off? Three of them have actually dropped to the ground and when I went up there to inspect, about 40% of them were loose to the point of being able to twist the nut by hand with very little physical strength required.

I should also mention that it's an aircraft hangar, and the guys over there reckon when the wind blows it shakes a fair bit. From that, I'm thinking there's a harmonic being setup which is pushing the nut off along the threading.
That does sound a bit fantastic though, so if I could get some ideas thrown in maybe I could have more angles to work to finding the cause and remedying it.

Thanks in advance,

 

[ishvaaag]

Harmonic or not in the presence of movement and absence of cleavage, this thing can happen. Sometimes once tightened a weld point is placed to avoid such rotation and may be the easier solution in your case without having to substitute the entire set of fasteners by some with any device of cleavage, something that can be difficult or inconvenient for a working structure. Since your structure is already showing the kind of failure, start by 2 points of weld instead of 1, and then be alert to if fatigue is enough to break the new weld points.

 

[csd72]

Going from memory here but I am sure the AISC codes now recommend using pretensioned bolts when the loading is from reversible wind loads. This is probably why.

I have seen loosened bolts on a portal frame once, no way to determine the extent except the hard way one bolt at a time.

How old is the hangar? Where are the bolts located?

 

[StructSEZ]

Ishvaag,
Firstly, let's just point out, this structure is not 'mine'. I was not the designer for this thing. I'm consulting on how to fix it.

Secondly, the whole tack weld this is a bit of a band aid solution and as you say yourself, it is prone to fatigue. Clearly, this thing is cycling/moving a lot. More than a tack will take.

Thanks for your input, although I'm trying to figure out a more holistic proposal as to what's going on. If you will, find the root cause...

 

[StructSEZ]

csd,

The hangar was handed over about 3 months ago only. (brand new!)

Most of the bolts that are loose are on the fly-braces and in splice plates on the primary trusses. Also, some of the bolts on the roller door hangers as well.

So, pretensioned bolts might be the way to go then...
I'm not familiar with the American codes, but I'll take a look see.

 

[ishvaaag]

I agree entirely that if feasible replacement by pretensioned bolts is a far better solution if feasible. Many times it is even if the structure is loaded since replacement can be made on a per bolt basis. Even more if the structure is as new as this. Yet the band aid solution may be the convenient sometimes, maybe old structures quite loaded with scarce redundancy at the bolts.

A relative of mine that heads the maintenance of national roads here says that on signs on the roads they are continuously repairing fatigued boled joints, particularly at baseplates, either the plates break, the nuts get loose, and things alike. That I remember, for bolts fatigued by wind, pretensioned bolts are also mandatory.

It is worth to mention also that: the bolts you are mentioning may have not been considered part of the lateral resisting system, nor contributing to the resistance to wind, nor have been (if just for bracing) been included in the structural model. This is another reason why I am a fan of including bracing members in the structural analysis, you start to see what they take and not. Then the structure may have been dimensioned too flexible for lateral forces, but economy asks so many times.

 

[ishvaaag]

fatigued bolted joints, of course; then later, the bracing members not been included in the structural model (not the "bolts")

 

[Ron]

Why do you have such a hard time believing that the bolts would not have been tightened by the contractor? I see that routinely. If someone doesn't make them do it, they won't, particularly with certain structure types and erectors.

I've seen a lot of "finger tight" bolts. You can tell from the paint marks if they've ever been tightened before. This more often occurs in lighter structures (PEMB's in particular), but happens in structures where the erector does mostly lighter framing, not heavy beams/moment connections.

Yes, bolts can loosen, but that isn't common and usually occurs under high cycle fatigue and/or at relatively high bolt stresses.

 

[gtaw]

Missing bolts, loose bolts, bolts tack welded in place without nuts or washers, I've seen it all.

Poor or no inspection can inspire the erector to take all kinds of short cuts.

I had an erector tell the owner and me, "at any one time he is working on ten different projects. You can't inspect all of them. The one or two jobs that you do inspect, I can afford the rework."

The owner fired the erector on the spot, but it was telling of the mindset of the erector. The cost of getting caught and having to do the rework was simply a cost of doing business. If he got away with substandard work on 9 out of every 10 jobs, he made money.


Best regards - Al

 

[StructSEZ]

Thanks for the input guys.

I suppose I'll have to really take a look at the option of poor worksmanship.

Detective work's afoot!


Thanks again,

 

[connectegr]

Snug-tight bolts structural bolts should not loosen to the point of "falling out" with normal loading. Even if the bolt elongates or nut loosens, normal loading would not allow a bolt in shear to fall out. Is there a significant vibration or cyclical loading on the connection? Wind load is not considered cyclical enough to fatigue the bolts or require pretensioning. You mentioned concrete columns. Are these bolts or mechanical anchors. If mechanical anchors are falling out, you definitely have a workmanship issue. If there is significant vibration or fatigue loading, mechanical anchors and/or snug-tight bolts should not have been used.

If these are structural bolts, A325 or A490, I do not recommend damaging threads by welding. Quenched and tempered material should not be welded.

http://www.FerrellEngineering.com

 

[MiketheEngineer]

Call the contractor and have him come out and FIX it to your satisfaction. It is only 3 months old!!

 

[StructSEZ]

Mike,

Yeah, we've done that. The issue is the Contractor is in Africa and we're a tropical island in the indian ocean.
They were flown in and the steel was shipped in.

The argument to get them back here is ongoing, however, in the mean time we're going up there ourselves to tighten the things up on the short term.

Of course their standpoint is 'It's not our fault' hence why I'm trying to consider any plausible explanation for the phenomenon.

 

[connectegr]

Another thought... The "shaking" when the wind blows, is a symptom of the loose connections, and can cause the additional vibration necessary to loosen other locations.

http://www.FerrellEngineering.com

 

[msquared48]

I see your problem and can feel your pain....

If you can, I would retighten all the loose bolts, and check with a wrench the others.

I would moniter the situation by putting a small stripe of paint over the bolt to the bearing surface and see if any move over time. Would not have to do this on all the bolts, but certainly the ones that were the loosest. Periodically check the selected bolts for any rotation.

Primitive, but it works.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[racookpe1978]

Nothing in your coments shows any indication that the bolts were ever tightened in the first place: Small island off Indian Ocean on a off-the-shelf building, with a non-professional/temporary/untrained/un-inspected workforce (?) working for a contractor that has already moved off-island?

I doubt they even had a properly sized torque-wrench available on-site, much less used it for every bolt.

Get the bolts re-tightened firmly fast - before they rust out and can't be turned at all. Then TORQUE them. And charge the contractor.

 

[dik]

connectengr has it ... they shouldn't loosen if they were properly snugged... any connections that are critical should be tightened to slip critical conditions... even if not designed for it.

What kind of bolts are used? A307, A325? makes a difference... and, if A325 were spec'd, are they the ones that were used? May have substituted something else.

Dik

 

[ishvaaag]

Being an hangar the turbines of the motors at some point will cause resonance for ample ranges of natural frequencies. This may be the cause as much as wind, but who knows. Like the rattle when the bus passes around.

 

[gte447f]

I certainly wouldn't bend over backwards searching for an explanation beyond what is staring you in the face, which is the bolts were never tightened in the first place. For the reasons stated by racookpe1978, I would be willing to bet this was simply shoddy construction.

As for what to do about it. Follow msquared48's advice and check/tighten every bolt in the building, then monitor the situation. If the problem reappears, you have a vibration problem (I doubt this will happen). If not, you have a claim against the contractor.

 

[delagina]

it is started practice in my industry to use double nut for vibrating equipment support.
seems like you have vibration/movement from what you are saying.