Strengthening Tension Chord in Truss?
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Question

Strengthening Tension Chord in Truss?

PYDC402040 (Structural)

(OP)

27 Sep 23 18:55

Hello,
I have a client that is knocking out what was likely intended to be a non-load bearing internal wall. One story home, the wall runs perpendicular to the truss system (open W style, 24 ft span, with webs meeting the tension chords at 8'ft spacings, 2x4 members), an analysis shows that the truss was designed to span from exterior wall to exterior wall. The problem:

(1) Is determining if the wall is still non-load bearing? It has been in place for 80 plus years, possibly taking load and changing the load path of the system.

(2) The design load is higher now that it was when built in the early 40's - the snow load is higher (even accounting for new LFRD factors in ASCE -7 22) and they are adding solar panels. It won't exceed the capacity much -by about 12% or so. Compression is not an issue in any of the other chords/webs.

The bottom chord member is at about 104% capacity using the old snow load and no solar panels for the full exterior wall to exterior wall span.

The least expensive solution would be best, so if that is a beam replacing the wall, that would be easiest, however, I don't see that as the most conservative because then you are changing the load path (if it has not already been changed).

Increasing the section of the bottom chords comes to mind as a better fix, but I have not been able to find a resource for altering or strengthening trusses, at least not the tension elements through AWC.

I'm looking for resources on how to increase the capacity of the tension chords (including connections) or any information on if long standing non-load bearing walls affect the load path and if they should be replaced with a beam if removed?

The framing of the wall is currently visible as well as the truss from above, the ceiling is in place, so unless I open it up, I cannot see if the trusses are bearing on the top plate or if there is space between. If there is a good chance construction method then left space between the truss chords and the wall, I could do that to verify that it is still non-load bearing?

jayrod12 (Structural) · Answer 1 · 2023-09-27 20:13:24

I'd bet there wasn't a slip joint provided originally. If the trusses were truly designed to span between exterior walls for the original loading, and the wall in question isn't at a panel point, I'd be allowing removal regardless with the caveat that due to potential stress changes caused by the wall removal that cracking may appear in brittle ceiling finishes in other locations.

PYDC402040 (Structural) · Answer 2 · 2023-09-27 20:38:42

The wall being removed is at 12' from each exterior wall, center of the truss. The web truss members attach to the bottom chord at 8' from each end. The wall was not located at a point to buttress the truss correctly, if the wall is supporting the truss, it is inducing bending in the center 2x4. Thank you jayrod12!

Yeah, I don't think they would have intentionally left a space for deflection of the truss back in the 1940s either.

Any suggestions for adding capacity for the tension member? Or design guides to go to for the extra 12% capacity?

jayrod12 (Structural) · Answer 3 · 2023-09-27 21:08:09

If you must increase the capacity, I would assume it's only over the centre 8 feet? If so, I'd be looking at adding a second bottom chord with a series of nails that extend far enough beyond the panel point that the nails required for full tension transfer are provided beyond the truss plates. At a quick guess, you'd be looking at a 14-16 foot long member.

PYDC402040 (Structural) · Answer 4 · 2023-09-28 17:53:41

RE: Strengthening Tension Chord in Truss?

PYDC402040 (Structural)

(OP)

28 Sep 23 17:53

Thank you Jayrod12!

EngGen (Structural) · Answer 5 · 2023-10-20 14:22:58

I concur with @jayrod. I think scabbing on another bottom chord with an appropriate nailing pattern should be everything you need to be sure of sufficient tension transfer.

JAE (Structural) · Answer 6 · 2023-10-20 16:17:04

I agree with jarod12 as well but with a concern: You may have analyzed the framing members of the truss but did you consider that the gang-nail plates connecting them have limits to their capacity as well?

I'm not concerned about removing that wall as it probably hurt the truss more than helped by adding bending to the tension bottom chord.

But the addition of the solar panels and, what you report, as higher snow loads would lead me to be more careful here.
The IBC typically only allows 5% overage on gravity loads on existing structures. You're reporting 12%.

PYDC402040 (Structural) · Answer 7 · 2023-10-20 17:05:45

Regarding the metal connector plates, the existing axial compression capacity of the truss chords is higher than the improved tension capacity, so I assumed that if the plates were designed to take the higher compression reactions, they could also transfer the increased tension loads. But no, I just included a note that said to visually inspect the existing connectors and if there was any sign of tear-out or damage to let me know and I would add capacity with OSB gussets.

I probably should look at that in retrospect to see if the assumption was right and to get a calc crunched out for my own peace of mind before the ceiling is closed up/ or a heavy snowstorm rolls along. Nailing patterns I am comfortable with, but I haven't yet tried to find resources on those toothed plates for capacity. Any resources/starting points appreciated!

phamENG (Structural) · Answer 8 · 2023-10-20 19:48:42

Those plates are usually not intended to take compression. Compression is handled in bearing between truss members. Besides, the direction of the loading and the resulting edge distances are very different.

If you're doing anything with MPC trusses, TPI1 is a must. The 2014 version used to be on their website for free. Don't see it now.

https://files.engineering.com/getfile.aspx?folder=4050c2ff-5a9e-46ea-ac23-8

JAE (Structural) · Answer 9 · 2023-10-21 02:25:30

Those plates take tension and I'd expect the tension in some of the diagonal members would increase over what they originally were designed for.

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Strengthening Tension Chord in Truss?
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