Uncommon Truss Profile

[TRAK.Structural]

Did an evaluation at a townhouse property earlier this week and came across a truss profile I haven't seen before. It's a gable roof, but the ceiling has vaulted rooms at each end of the truss and a flat ceiling in the center portion. My guess is that the perpendicular wall below the transition between flat and vaulted ceilings is carrying the load from the truss but I'm not totally sure. The truss bottom chord is basically just toe-nailed to the top of the wall (see pictures). Think that interior wall is load bearing??

 

[phamENG]

In an attic truss, it should come as a huge surprise.

If it's a 12' open space, then yes. But it looks like about 6' based on the proportions in the photo. And around here I see quite a few 'truss within a truss' profiles for attic floors using 2x4 chords where I could very easily take the same pictures shown in the OP. Not saying it would be a good design choice, but it wouldn't be out of the ordinary either.

But a poorly coordinated, un-engineered truss layout coupled with a prescriptive design could certainly result in a partition wall being used incorrectly as a bearing point in the analysis. I've rejected a few truss designs where they tried to pull that on me.

 

[KootK]

And around here I see quite a few 'truss within a truss' profiles

Tell me more about this truss within a truss business. The attic panel here looks like a trapezoid approaching a square to me. Vierendeel for shear between those interior walls.

@TRAK.Structural said 8' middle span so 6' wide open is probably about right.

 

[phamENG]

Rather than a 2x10 or 2x12 for the attic floor, they use a 2x4 all the way across. Then they put another 2x4 parallel to the bottom chord about 10" up, and add diagonals in between. So you end up with a floor truss inside an attic truss. Here's a shop drawing of one of similar size, but a wider attic room (top chords in OP don't look to be spanning very far without webs).

I realize the sketch in the OP doesn't show the doubled bottom chord, so it's probably not like this, but if the floor is only spanning 6 to 8 feet, they may have been okay with a 2x4. Visually graded No.2 2x4 can span over 7 or 8 feet as a ceiling joist using prescriptive tables.

This is all speculation, of course. Whether it was intended to be a bearing wall or not, it is clearly doing (or trying to do) that job now.

 

[jerseyshore]

Rather than a 2x10 or 2x12 for the attic floor, they use a 2x4 all the way across. Then they put another 2x4 parallel to the bottom chord about 10" up, and add diagonals in between. So you end up with a floor truss inside an attic truss. Here's a shop drawing of one of similar size, but a wider attic room (top chords in OP don't look to be spanning very far without webs).

I realize the sketch in the OP doesn't show the doubled bottom chord, so it's probably not like this, but if the floor is only spanning 6 to 8 feet, they may have been okay with a 2x4. Visually graded No.2 2x4 can span over 7 or 8 feet as a ceiling joist using prescriptive tables.

This is all speculation, of course. Whether it was intended to be a bearing wall or not, it is clearly doing (or trying to do) that job now.

View attachment 5624

Cool truss. Seems too good to be true, but don't worry their engineer signed off on it.

 

[LittleInch]

The truss us not exactly duplicated as the RHS only has two vertical spars not three as shown on the LHS.

 

[gte447f]

pham, I am with KootK on this one. Without the trussed top and bottom chord sections at the central attic area (like what is shown in your example truss drawing), then this truss as a whole has negligible shear capacity at the central attic panel. Not to mention, as a simply supported, single span truss, it is not a stable structure without triangulation of the central attic panel.

Nevertheless, I think that the matter of the cracked gyp above that 2nd story door has little to do with the roof truss and whether that wall is load bearing and more to do with deflection of the 2nd floor framing. That wall probably was likely to crack whether it was load bearing or not, although granted, if it is load bearing that does result in more load on the 2nd floor framing which of course results in more deflection of the 2nd floor framing.