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mangled floor trusses - how to repair?

mangled floor trusses - how to repair?

(OP)
ok folks - I have a homeowner client that wants to shorten some existing floor trusses at a balcony. That part I can deal with easily. However, in the investigation, I discovered that trusses next to the balcony have been badly mangled at the steel support beam. The house was built in the late 80s. Nothing appears to have failed yet, but now that I've seen it, I'm required to fix it. I'm considering cutting out the mangled vertical, installing a new vertical at the shortened end, and adding plywood gussets. I'd then hang the repaired trusses off a new wood beam and let the existing steel beam just take the load from the wall above. Any thoughts are welcome!





(and yes - all the trusses are like this!)

Please remember: we're not all guys!

RE: mangled floor trusses - how to repair?

Do you know the truss manufacturer so that you could contact them for a remedy?

The warranty is void, obviously with the changes.

If the truss manufacturer cannot help, even though the trusses are top hung, I would design a bottom hung connection that is bolted to the web of the steel beam on each side.

Mike McCann, PE, SE (WA)


RE: mangled floor trusses - how to repair?

(OP)
There is absolutely no truss information. I sure wish there were. Thanks for the idea, Mike.

Please remember: we're not all guys!

RE: mangled floor trusses - how to repair?

No expertise claimed here, but...

It looks like the existing verticals aren't doing much, and couldn't because of the coping job,
but the diagonal and the top chord and the splice between them look ok.

Could you sister the existing verticals,
and apply plywood gussets secured with screws,
right over the existing splice plates,
without cutting anything?



Mike Halloran
Pembroke Pines, FL, USA

RE: mangled floor trusses - how to repair?

That is a very strange condition, but I’m not so sure, as yet, that’s it’s in that bad a condition….

Assuming they didn’t cut the truss in half etcetera, looks like they may have manufactured the truss ½” too long or misplaced the beam by ½”. They notched the bottom chord to vert connection which has almost no load in it. At the top chord connection all the real load is in the diagonal web and the connection plate is only notched ¾” into the vert but not into the diagonal. Also at the top chord, the left plate with the nail is a separate plate just holding the double top plate together. The load carrying plate looks pretty good, less the notch at the vert.

Not saying nothing should be done, but I’d check the force in that 1st diagonal web and see the magnitude of the situation. Could be minor.

RE: mangled floor trusses - how to repair?

The first vertical member is likely a zero force member with the shear taken by the tension diagonal Same with the bottom chord extension from the first bottom panel point... also a zero force member. It is not likely an issue if they have been mangled... be concerned about the condition of the top chord and the first diagonal as well as the attachment of the TPIC plates... there should be no gap between the plate and the wood member or if there is, it should be less than the thickness of a business card.

Dik

RE: mangled floor trusses - how to repair?

Looks like the whole truss is supported by the top chord resting on the steel beam. I would attempt to create some sort of custom joist hanger arrangement such that you pick up the bottom of the truss. Or maybe install a seat or haunch attached to the steel beam for the truss vertical to sit on.

RE: mangled floor trusses - how to repair?

Look at the next truss over, and the third one. I don't see press plates at the next one (could be covered I suppose)- and the third one has the webs running the other way. Does the bearing condition change?

RE: mangled floor trusses - how to repair?

(OP)
hawkaz, that's the end of this truss run. All of the trusses that have this bearing condition have the same mangled ends.

thanks, everyone.

Please remember: we're not all guys!

RE: mangled floor trusses - how to repair?

Next option - remove the trusses and beat the contractor with them.

Mike McCann, PE, SE (WA)


RE: mangled floor trusses - how to repair?

Hard to answer completely without knowing the actual forces.
The end vertical probably has very little load and sometimes is not even installed for top chord bearing trusses.
The member force in the diagonal web is typically the highest of all web members, but
because the diagonal is not doubled, the force is not as high as a double top chord would normally dictate.
Draw force lines along the centerlines of the wood members to determine how much moment & shear exists in the top chord extension.
The lumber probably still works because the reaction is usually centered in the first 1.5" and not the 4" of extension.
Calculate the polar moment of inertia of remaining MPC contact area to determine that eccentric force on tooth withdrawal is not an issue.
Use the manufacturers die-stamp embossed on the plate to identify which approval report to use in calculating
the net-section and shear forces allowed in the steel, and verify that the remaining steel is adequate.

There's a good chance all these calculations were done at delivery time and that the trusses work as is with no repair expense.

Attach a copy of your final engineering design directly to the truss so
that future inspections don't generate more engineering expense.

Some engineers would design a plywood gusset repair or alternate support because
it is within their design expertise and comfort level. The client deserves not only 100% structural safety,
but the most economical design too, which sometimes means verifying what's there works.

RE: mangled floor trusses - how to repair?

For the reasons mentioned above, I'd be comfortable with the assumption that the mangling hasn't materially affected the truss capacity and, therefore, does not require your attention.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: mangled floor trusses - how to repair?

I did not review all of the responses, but as some others have said, this is probably not a significant concern. The double top chord near the end is common with top chord bearing trusses. The first vertical web and the bottom chord up to the junction with the first diagonal web do not serve a structural purpose. If repairs were needed plywood or OSB gusset plates are the typical fix.

RE: mangled floor trusses - how to repair?

As noted above, check to see that the plate hasn't separated from the dimensioned lumber... max gap should be the thickness of a business card.

Dik

RE: mangled floor trusses - how to repair?

That's a pretty typical top chord bearing detail for a floor truss.

The plate was not seriously compromised. Neither were the diagonal web or double top chord.

While it doesn't look good, I wouldn't lose any sleep over it.

(I designed trusses for almost 30 years, so I have some experience to back my opinion up with)

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