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!

 

[MIStructE_IRE]

Here’s the type of truss... can’t seem to upload 2 images for some reason..

 

[r13]

In the picture, looks like the top and bottom chords are angles. Do you have view with wider angle? Also, the actual shape of the members? What is the external force that seems pushed up from below.

 

[IRstuff]

Shouldn't be a problem with uploading more than one image; by high-res images take time to upload, so you might have thought it failed, but it was only taking a long time.



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[JLNJ]

Looks like a normal bar joist.

 

[MIStructE_IRE]

Yeah, all members are angles. Funnily there is no external force below pushing upward. My feeling is the bottom chord bent upward, was pulled upward, when the compression member buckled so badly.

 

[Ron247]

Seems like a lot of buckling and disfiguring in an isolated area with adjacent members undamaged. I wonder is someone bent the compression diagonal during construction or after so that it did not take much to make it fail. Is there a heavy point load above it that is new?

 

[MIStructE_IRE]

That one truss supports significant copper bar services, which the other trusses do not. No point loads above. You could be right Ron about just enough damage during construction to cause disproportionate buckling.

 

[r13]

The truss bottom chord is in tension with hinging weight (ceiling?). Both top and bottom chords seem straight in good shape, and the end support does not seem distressed, there is no good reason for the diagonal to buckle that much. Did the owner mention anything suspicious?

 

[BAretired]

"Significant copper bar services" would add considerably to the force in that web member. That could be all or part of the problem.

BA

 

[hokie66]

The single angle compression web failed. Whether due to lack of initial straightness or excess load, it is hard to tell. But single angle webs are tricky to analyze.

 

[JLNJ]

Uplift might cause such a failure. Was there an extreme wind event?

 

[3DDave]

That looks a lot like someone tried to jack it up at the end of the bottom chord; the last tension diagonal does not look as if it was in tension when the rest of the damage was done. Likely it happened after the roof was on and whoever did it was glad to finish the drop ceiling.

 

[r13]

I am more agree with 3DDave, an locally exerted external force from below.

 

[hokie66]

Yes, that could have done it.

 

[Eric C.]

Is there any indication that this failure is a recent event, chipped paint on either side of the bent area, etc., or could it have been hit by a lift some time ago during installation of the drop ceiling, sprinkler piping, electrical work, drywall, etc.?

 

[SteelPE]

What are “significant copper bar services”

Bottom chor appears to be deflected upwards. Top chord appears to remain straight. If this was a loading failure I would expect significant distorting the top chord. Very suspicious failure.

 

[human909]

Thanks for posting. But I figure this should be in the "Engineering Failures & Disasters" forum...

I agree, the failure is peculiar given the extent of damage to some members and the lack of damage to others. My initial thought are the top cord remained intact due to significant engagement of the roof diagram. The truss largely remain load supporting due to catenary action from the last tension angle and bottom cord. Also some load distribution to adjacent trusses is likely.

As to what load could have trigger the event. Hard to say without further information. Snow load, hanging loads, etc...

 

[MIStructE_IRE]

Retired 13, thanks, yes the member circled red in the left hand side of your markup is also buckled. This is a bottom chord restraint which is inclined back to the deck level wind girder.

I’m not ruling out wind uplift. However that would have put the badly buckled member in tension.

Its the only truss that has been loaded with these heavy copper services and is the only truss that has buckled in such a way. These services impose heavy point loads hanging from the bottom chord.

That said, its still a very unusual mechanism and I agree, if I didn’t know better I’d say someone tried to jack it from below. I just don’t see how that would ever have been the case. The client hasn’t mentioned anything unusual either.

 

[1503-44]

It could be from top loading. The second from last diagonal is normally a compressive member and I think it failed first by buckling. When that diagonal buckled it rotated the joint below counter-clockwise (in MIStruct's picture), causing a further upward bending deformation in the last diagonal, even though that member was and probably still is under some tension.

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