Real life truss failure! 8

[MIStructE_IRE]

I was called out to look at this truss which has failed. Its a Steel warren roof truss. The layout is shown below - but I’ve crossed out the vertical which doesn’t exist - which I think should have existed!

It looks to me as though the second last diagonal, the compression member buckled due to excessive compressive force, and as it buckled it pulled the last diagonal, which should be in tension, causing it to buckle also.

Any thoughts on the mechanism? There are hundreds of these trusses throughout the building and this one has failed.

Disclaimer - not my design!! I just got asked to review this failure!

 

[oldestguy]

Hell, could have been damaged any where between the fabrication and finally setting it in place. Rush job. Bet bottom of the bottom chord end has paint damage also.

 

[IRstuff]

While I can intuitively see the buckling as being caused by a point load from the overhang, it's hard to imagine the inherent stress causing that much buckling with very little ancillary damage.

One other possibility, given lots of people wandering in the plenum during installs and maintenance, is that someone fell against the diagonal, which. when added to the compression. cause the diagonal to buckle.

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[phamENG]

ax1e - I agree it should have been seen, but I've been on too many job sites with obvious damage that was never repaired that everyone else conveniently "didn't notice" until I pointed it out and then continued to "not notice" as I repeatedly pointed it out to them. It's rarely things as critical as this (in those cases I scream load enough to get the right person's attention), but the mentality is the same. When you've got the frame up and all the other trades lined up waiting to come in and get to work, the last thing anyone is going to do is hold up the whole project for a week or more while one replacement joist is manufactured and shipped to the site.

Based on the joist and the deck, I'd say this building is circa 1950s or 60s. Hard to say whether or not the people involved at the time were terribly concerned about this or not.

The new load on the bottom chord makes a lot of sense, especially since it looks to be localized near the end of the joist. Now that we know it's there, I agree that it's a likely culprit. That clamp connection looks terrifying. I see the unistrut markings - is it zinc coated? If not, and it's just bare steel, I'd be less likely to accept that as the reason as the joist bottom chord has rust where the paint has been damaged.

 

[Rabbit12]

I'm with oldestguy. I think this happened during construction. If this had been from an overload scenario, I'd expect to see some distress in the deck or adjacent joists.

 

[MIStructE_IRE]

This was built circa 1995. The trusses are 1000mm deep and the fiagonals are typically 50 x 50mm angles - so not something you’d damage by falling against.

It certainly could have been damaged by a forklift etc but I really don’t see it. This is a data hall. There’s no moving machinery etc. That’s not to say it didn’t happen during construction but again, I really find it hard to believe given its age and the fact that this is the only truss supporting heavy services and is the only one that’s failed.

Really interesting to get everyone’s view - so thank you all. Keep it coming!

 

[phamENG]

1995? Wow. I was way off on that one. Different countries, different trends and nuances in design, I suppose.

 

[MIStructE_IRE]

Yep - We’re miles behind you guys!!

 

[SteelPE]

Well, I don't know when the damage occurred. Is stand behind the point that if this type of failure happened due to overload you will see significant distortion in the top chord and the buckling and distortion of the web members will also come out in the top chord.

From your newest picture, it looks like the damage has been here for quite some time as the dust and dirt on the chord appears to be uniform. If this was something that was new, the dust would not be uniform on the members.

 

[r13]

Can be damaged by high-lift during roof inspection, pipe/cable installation...

 

[oldestguy]

This got hidden early on with that suspended ceiling. So who's to know?

 

[r13]

I guess this damage was uncovered by another ongoing/scheduled activity under the roof, don't think it has caused noticeable defect below the ceiling.

 

[BAretired]

The most likely culprit is the additional load hanging from the bottom chord near the end of joist. The second diagonal failed in compression, causing the end diagonal to pull the end of the bottom chord up. Rotation of the bottom chord caused the kink in the end diagonal.

BA

 

[gtaw]

I'll put my money on a manlift (scissor lift work platform).

The workers were going up with the manlift and not paying attention as they elevated the platform. The last two web members were "lifted" by a force coming up from below, i.e., the handrail around the manlift platform. The worker(s) involved aren't going to tell anyone if no one saw it happen. Their employer would have to pay to get it fixed the they would be out of a job.

There are sprinkler pipes that were installed after the structure was "completed". You have wall panels that had to be placed. You have the metal studs erected. Workers often use the manlift to hoist materials during construction. They are looking at something that has drawn their attention from watching the other end of the platform and bingo, they hit the underside of the node with the handrail around the platform.

 

[oldestguy]

One way or another that end of bottom chord got a whack going upward. Not load related at all.

 

[3DDave]

axle1 - the flange is in torsion, just like the other one. It's probably where the force was applied. A photo from the opposite side would have cleared that up.

gtaw - scissor lift makes the most sense. Probably when they were putting in the wires to hang the ceiling where they had to get to the underside of the roof. It's when the scissor mechanism has the greatest extension leverage and when all the structural people are off the site and/or not looking.

 

[r13]

Probable fix, if required. Add hanger if it is not there already.

 

[3DDave]

What's the bottom end of those extrusions look like? I wonder if they've been taking the load that should have transferred to the end of the truss. Probably best to leave them in place until a repair is made.

 

[BUGGAR]

How many jobs have I been on that the contractor put something like this on his punch list and promised to fix.

 

[Compositepro]

My money is on the workers installing the dry wall or drop ceiling did it with a scissors lift.

 

[r13]

Another situation might have led the owner to notice the damage - noise. As the connection is getting loose, when the joist deflects, some metal might rub against the other that produces a repeated squeaking noise.