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Rusted plates on MPC truss, how to analyze

Rusted plates on MPC truss, how to analyze

Rusted plates on MPC truss, how to analyze

(OP)
My client had me inspect his warehouse roof system consisting of metal late connected trusses.  The 3.5" thick foil backed insulation seems to have caught the moisture in such a way as to induce rust in the top chord plates.

Is there a method to verify the existing strength of these metal plates?

RE: Rusted plates on MPC truss, how to analyze

Other than load testing the truss, I don't know how you can verify them with any level of confidence.  

You can inspect the plates up close (magnifying glass) to see if the tongues have been pulled out of the wood surface, and also to see what sort of section loss on any one plate might have occurred.  That would be my first steps if I was forced to evaluate them.  

 

RE: Rusted plates on MPC truss, how to analyze

Wood is weaker than steel. Is the wood (in general and at the plate teeth) in good condition? If it is, than the holding values for the plates should be okay (if not, than you need to repair the joint). You can then do a free moment analysis of the forces at the joints to check the shear and tension applied to the plates. Get the plates values from the manufactor. Be careful  as you cannot use the width of the plate only the width pressed into wood of the truss, not blocks. Plus a little but you would need the TPI book to get the equation. Now it depends on how much rust you have and what safety factor you feel is needed.

Garth Dreger PE
AZ Phoenix area

RE: Rusted plates on MPC truss, how to analyze

(OP)
woodman,
can you give me a link to find the TPI book?
Is there a software would you recommend in general for analyzing MPC trusses.  I am running across them more often it seems.

Thanks.
 

RE: Rusted plates on MPC truss, how to analyze

The Truss Plate Institute at www.tpinst.org writes the TPI-1 that is referenced in the IBC and IRC. The truss plate manufactors have the best software but you would need to find a truss manufactor (or a truss designer at one) to design the trusses for you. There are no cheap programs to analysis the plates at the joints that I am aware of.

Garth Dreger PE
AZ Phoenix area

RE: Rusted plates on MPC truss, how to analyze

If there is enough corrosion to result in section loss of the plates, I would think that corrosion of the teeth would be applying unacceptable splitting forces to the wood.

RE: Rusted plates on MPC truss, how to analyze

(OP)
hokie...interesting...i'll have to think on that one...and get some closer photos

thanks everyone
 

RE: Rusted plates on MPC truss, how to analyze

On the other hand, it's the rusty nails which are always hardest to extract.
(not that we are dealing with direct pullout here, but I can imagine pullout of the teeth as a failure mechanism)

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