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Reinforcing a Fink Truss

Reinforcing a Fink Truss

Reinforcing a Fink Truss

(OP)
Working on a single level 24' wide home built in 1974 with 2"x4" pre-engineered fink trusses at 24" OC (3/12 slope). The trusses span the 24'. There is a 2"x4" interior wall that runs the length of the home and is located at the midpoint of the truss...this wall is not load bearing. There is a wall in the basement directly below this wall that is load bearing (it supports the floor joists).

OK, so this winter on the east coast of Canada we have received a record amount of snow. With the large amount of snow on the roof the trusses deflected but the "non-load bearing" wall suddenly became load bearing. The problem happened at the one opening in the wall (a 3' arched doorway, has no structural header as it was not to be load bearing). With the load of the trusses now bearing on the center wall the wall above the opening failed causing the 2 trusses directly above the opening to sag 2-3" at the center severely cracking the gyproc around the opening.

Also, if the trusses were bearing on the non load bearing wall the bearing point would be located at the midpoint of the truss, in this case the bottom chord only with no web support near it. My concern is if we get more significant snowfall in the next winters and if the truss continues to bear on the interior wall at this point the bottom chord could be damaged or break considering it wasn't designed to bear at that point.

Would reinforcing the connections with plywood be sufficient? One fellow involved asked about adding a kingpost to the trusses to allow bearing on the wall below. If this is to work the opening would have to have a structural header installed. But, since the wall wasn't built to be load bearing it wouldn't have a double top plate.

Any advice would be appreciated.

RE: Reinforcing a Fink Truss

Interesting situation. Would I be right in thinking that all of the trusses over the length of that non-load bearing wall are similar in construction and loading and that, hence, every truss wanted to deflect similarly thus effectively bearing along the entire wall, but that only this doorway was weak enough to reveal this condition by exhibiting signs of stress?

RE: Reinforcing a Fink Truss

(OP)
Triangled,

You are correct. All trusses are the same and the door was the weak point.

RE: Reinforcing a Fink Truss

The nice thing about that weak spot doorway is that it served as the owner alerting mechanism which got an engineer involved.... kind of the canary in the coal mine. Once that's strengthened so that it won't crack when the truss chord presses upon it....

The two possibilities that occur to me for the local truss situation are, 1) adding a truss panel point in the bottom chord above the wall as described above or, 2) strengthening the truss panel for bending between panel points, maybe something like a 2x10 functioning like a simple span beam from the panel point left of the nonbearing wall to the panel point right of the non bearing wall, and supporting a point load pushing upward at the non bearing wall.

Then of course, I'm sure you'll be looking at the truss globally.

RE: Reinforcing a Fink Truss

Quote (Woodman1967)

Would reinforcing the connections with plywood be sufficient?

IMHO, no. The connections of the two trusses that sagged above the doorway have certainly been compromised. Even the remaining trusses have likely had their connection weakened by being heavily loaded. However, reinforcing all the connections with plywood can play a part in a multistep solution.

A 3:12 truss is so shallow that effective truss action under heavy loads is limited... this can be used to your advantage:
Install the kingpost that was suggested to you, I'll get to how to make the interior wall load bearing in a little while. Consider the 12' sections on each side of the kingpost to be individual trusses that just happen to share on common member... the king post. This is a reasonable assumption since the 12' sections are more rigid than the 24' length. Ignore the 24' truss action since it is minimal compared to what the 12' sections can do.

Install plywood gusset plates on all joints, using construction adhesive and screws (or nails). The adhesive is most important - see the Forest Products Lab paper "Longtime Performance of Trussed Rafters with Different Connection Systems" for the reason:
http://128.104.77.228/documnts/fplrp/fplrp444.pdf
BTW, the plywood gusset plates will simplify installation of the king post.

Converting the interior wall, including the door header, to load bearing will take some work, but is not unreasonable. Remove all the sheetrock on both sides of the wall. Replace the sheetrock, on both sides, with 1/2" plywood, using construction adhesive and nails or screws. Be sure to connect the plywood to studs and the single top plate. This will tend to make the wall framing / top plate into box beams that can carry load. Be sure to install the plywood with the face grain horizontal to maximize its shear and bending capacity. Install sheetrock on top of the plywood for appearance and fire resistance.

Minor fringe benefits for the Owner is that they can now easily hang pictures, mirrors, etc. anywhere they want on this wall and the wall's sound transmission will be reduced. smile

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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