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Fink Truss failure

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PSUengineer1

PSUengineer1

Structural
Jun 6, 2012
143
Hello.
I have a wood-framed fink truss. A rough sketch is attached. As you can see in the sketch, the top/bottom chord interface at the left hand side is not at a support. This is where failure occured, which I attached a photo of (note that a temporary wall is now in place below the failure point and the wall you see in photo is not by design). The wall support at the right hand side is 1 foot higher than wall support at left hand side, so yes, this failure occurs away from the left hand side wall support.

I am looking for the simplest way to describe this failure to a layman. Here is what I am thinking....."The tensile capacity in the bottom truss chord(at the point of failure) was not adequately designed to resist the horizontal component of the top chord compression. Looking for suggestions or possible improvements to my statement. Thank you.

Regards,
Jim
 
 https://files.engineering.com/getfile.aspx?folder=09da877f-6a2f-40f3-a9fe-27888846640a&file=Fink_Truss.pdf
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That's a bending failure of the top chord. Which honestly is not shocking given that configuration. I can't imagine that's how the truss was originally designed to perform. I'd bet there was a change on site where someone asked if they can gain some extra space by moving that wall outwards.
 
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1) Describe it as a tail bearing truss and indicate that this is a notoriously fragile configuration which passes the entire weight of the truss and any loads applied to it through the overhang in bending and shear (non-truss action).

2) I'm guessing that the particular failure that you're looking at is really a version of the notch effect in wood. It introduces tension perpendicular to grain along the axis of the top chord.

3) I don't feel that this has anything to do with the bottom chord of the truss so I'd not mention that.

4) You simply cannot beat a good sketch for this kind of thing, particularly when communication with a lay person.
 
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A much better version of something like this. You know, assuming that this is actually true.

C01_wnm1s1.png
 
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This is better if it reflects reality. An interior side bearing overhang on a tail bearing truss is kind of a nightmare condition from a fracture mechanics perspective.

C01_cogztr.png
 
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I agree with KootK 4 points and jayrod12's speculation concerning the change in bearing location. The knot doesn't help, either. I assume there are other trusses. Are they behaving the same way?
 
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@dylansdad: yes, there are several others all behaving in the same way. Thank you.
 
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I'd suggest avoiding large words. Just tell them the truss was installed incorrectly, and the bearing wall should have been under the BC.
 
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