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]

That’s my feeling on it ax1e.

 

[1503-44]

Thought so, but I wasn't sure. As they say, "great minds think alike."
I'm not a believer of the jacked up from below theory. Don't see any reason to do that.
And damages done after construction are usually nearer to mid span, not against a wall.



“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

ON a fair number of similar examples I could see they all had a vertical member at the end.

That missing element, combined with some point loads on the lower element would seem to have pushed it beyond the limits, but it's very odd.

Did that extra load make both struts tensile and push the final strut into failure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

If it's uplift or overload, then where did the dent in the cross-piece angle (dark green side) lower flange come from? The part of the angle that looks to have no connection to any load path?

 

[HTURKAK]

Dear MIStructE_IRE ,

Do you have any picture when the roof installed ( before suspended ceiling and mech. installation ) ?. IMO, The buckling of the second last diagonal, ( the compression web member ) can be explained with overloading but will not explain the plastic bending of bottom chord from the same node and plastic bending of last diagonal (which is tension element).
These kind of deformations is likely due to impact loading which could be a forklift impact. IMO , The ceiling or mechanical erection group made this mistake and kept silence..

 

[phamENG]

By "significant copper bar services" I'm assuming you mean a large electrical distribution load center?

I'm having trouble with the overload from above theory. The location and nature of the failure suggests excessive shear - meaning it would have to be a point load or very confined distributed load on that joist near the wall. A load center could do that. BUT - if that were the case, wouldn't that severely degrade that joist's ability to resist the load? I would expect to see some sort of deformation in the deck and bending in the top chord of the joist now that the bottom chord and web members have failed. It doesn't seem to be there. That suggests to me that there was some sort of damage from below - whether it was jacking or hitting it with a lift or something else is impossible to tell - and the applied load from above has never exceeded the capacity of the damaged joist. It seems the most likely given the nearly pristine condition of everything else in the vicinity.

 

[rb1957]

I think someone wacked the short overhang ...

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

 

[1503-44]

3DDave
Presumably the "dent" is a local flange buckle due to compressive stress in the flange caused by latent bending.

The paint there and everywhere else appears intact, suggesting no impact force, or excessive care if there was any high loading there.

Little Inch
A vertical member at that joint, if installed, might have contributed somewhat to resisting excessive rotation of the joint, but of course it wasn't there.

Pham
High load anywhere on the truss could have overloaded the compressive diagonal. Once buckled, there is virtually nothing to prevent upward translation caused by the tensile load in the last diagonal. An evenly placed heavy uniform load from above might not tend to show any local deck deformation.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[oldestguy]

My bet this was done during initial construction. Damage in handling and they got away with that, unnoticed. No way with loads on top would,that end of bottom chord be bent up.

 

[1503-44]

MISTRUST
I think your initial post nailed it dead on.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

OldestGuy
Naw.. Too hard to believe nobody noticed that since Day 1.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[MIStructE_IRE]

Here’s a closeup of the connection. The diagonals are welded to the bottom chord, which may have provided just enough fixity to bend the bottom chord upward as the diagonal buckled.

I don’t think It was the top chord loading. I tHink it was the relatively new bottom chord point loads (which other trusses do not have)

 

[1503-44]

Thanks. Paint damage is noticeable now, showing nonuniform tnsion in that flange... from rotational bending? I think so.

Where, why and how do you get a point load from below there??? Especially the "why". Not buying that.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[r13]

 

[1503-44]

Looks like some separation of diag at the joint. That would tend to indicate the compression diag rotated the joint putting tension there, rather than a point load from the right hand side, which would have instead compressed that bit.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

I will only believe that if somebody explains the reason for the supposed upward loading.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

These diagonals are the highest loaded members in the group shown, so any buckle would have quickly released considerable energy. Over-rotation counter clockwise, followed by tension diagonal pulling it back a bit.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[oldestguy]

That upward loading was during handling when they dropped the overhanging end on the wall and it bent up the end of bottom chord and bent up that last diagonal. Explains that torn up end of last diagonal upward leg. Luckily the bending resistance of top chord still carried the load over that last few feet.

 

[1503-44]

Maybe. If it is new enough structure and we are seeing this for the first, or second time ever, yeah, maybe. But still hard to believe the deck was built on top with it like that without any flags being raised. OK, stranger things have happened.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[rb1957]

when was that bare metal structure installed ?

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