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Real life truss failure! 8

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MIStructE_IRE

Structural
Sep 23, 2018
816
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!

6A910EB4-629F-4B8E-A59F-D54C56E7A309_bp0jqn.jpg
 
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All, please do me the courtesy of giving me 20 min to make one more contribution before we proceed. Some new information has been brought to my attention. I can weave it in, it's going to be killer, but I need 20 min to do the Blue Beaming. And, you know, a little work stuff...

3DDave said:
My concern with this as an explanation is this - how much energy was absorbed in that amount of distortion and where did that energy come from? In typical buckling the energy comes from a change in vertical height of some amount of weight, but here no other part of the structure appears to have moved at all.

You're right, and I'll answer for that shortly.
 
I agree Koot. The last diagonal tension member is bent, not technically buckled.

I fully agree with your theory above and from my initial calcs, this particular member is in theory overstressed - assuming the roof is even subject to its design imposed load, which it may never have been. However having gotten a closer look today - look at the localised damage to the bottom chord restraint. Its just at the tip of the angle flange (right hand side of the photo) and I don’t see how anything other than impact would cause this. Anyone disagree?

That said, the member is overstressed regardless and i think the slightest tip of accidental damage was enough to cause this failure!

69A1FE1A-0C8C-40D2-9174-9C7B7BDEDAB1_fkajcu.jpg
 
So a friend has approached me offline and challenged me to explain the presence of the kink at the upper end of the last diagonal, as shown in the first sketch below. And I'm grateful for that because, while it represents an oversight on my part, I believe that piece of the puzzle can be also be woven into the story that I told earlier and, in the weaving, make it a richer and more complete tale.

Updated sketches below and attached. The difference:

1) Initially, I'd assumed that it was natural causes that created the perturbation in the second web that led to its eventual overload buckling. Misalignment, eccentricity etc.

2) Now, I proposse that there was some impact event delivered to the first web that created the perturbation in the second web that led to its eventual overload buckling.

So still genuine buckling but initiated and/or exacerbated by whatever impact event happened up near the seat. It's pretty common that real world failures are the result of a combination of things rather than by any one thing. So, in this story, most everybody ends up being correct to some degree. Princesses, dragons, and wizards as it were.

C01_lvty6k.jpg


c02_sazgcs.jpg
 
IRE,

Can you red-mark/pointing out the broken part on the overall view? I couldn't make the connection and get good picture of it. Thanks.
 
MIStructE_IRE said:
Its just at the tip of the angle flange (right hand side of the photo) and I don’t see how anything other than impact would cause this. Anyone disagree?

No, I agree, that's pretty damning..

Also the twist that you see between the connection plate and the angle. The bottom chord tried to fight back.

C01_i0bipk.jpg
 
I agree Koot. I think the already high stress combined with the impact created the perfect storm here. So everyone was right in a sense... gold stars all round!
 
I hate to say it given the amount of effort that I've already put in going the other way but I'm pretty solidly in camp impact now. The last web may well have compression buckled.
 
I do believe the overload contributed to the failure by impact from below. It needs the forces from both the top and bottom to cause damage like this. Note that this is a localized failure, the neighboring members and chords are conspicuously remained straight.

image_r4wf30.png


I was joking to pledge for star, but appreciate for getting it anyway. Thanks.
 
I think the impact explains why the tension tie is bulging upward. Also, the truss has rotated that caused the separation of the bridging.

image_jkm3sq.png
 
did anyone see how the drywall was installed? Maybe the scissor lift operator hit the wrong lever.
 
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