Roof without flat ceiling - how to resist thrust? 2

[buleeek2]

Hello,

I am wondering if there is a way to utilize wood trusses like in the picture below. The owners do not want ceiling joists and for the sake of the discussion, let's say that a ridge beam is not an option (the span would be over 35 feet). I am worried about the thrust, as once I tie the trusses down to the walls it will create a horizontal pin at each wall. Does anyone have a suggestion on what to do in such a situation? Maybe a bracket that allows for lateral movement?

 

[XR250]

Typically we do a pin on one end and a slip, with an actual slip connection clip - something like the Simpson TC, the opposite and alternate every other truss. This way you do have a pin for the wall/diaphragm connection and you have a slip the opposite end.

The roof diaphragm will prevent this from actually working.

 

[buleeek2]

Thanks for the responses. I think I agree with the statement that it is safer to require a certain acceptable truss horizontal deflection from the manufacturer (1/2" max.)

Regarding a detail for properly transferring the force from the wall onto the scissor truss, do you have any tips?

 

[gte447f]

Thanks for the responses. I think I agree with the statement that it is safer to require a certain acceptable truss horizontal deflection from the manufacturer (1/2" max.)

Regarding a detail for properly transferring the force from the wall onto the scissor truss, do you have any tips?

No special detail is needed. I think you are misunderstanding the solution to this problem. If not, forgive me for thinking so. The scissor trusses are treated just like typical gable trusses and are connected to the wall top plates just like typical gable trusses. The truss heel will be pinned to the wall top plate (by nails, hurricane clips, etc.), but the wall will act like a roller connection by leaning outward because the wall has no out-of-plane stability on its own without being pinned to the roof diaphragm. You simply accept that the wall acts like a roller and you limit the horizontal displacement of the roller (i.e., the top of the wall) to something reasonable and tolerable. You do this by designing the scissor trusses to have limited horizontal deflection under design loads.

 

[RontheRedneck]

I think I agree with the statement that it is safer to require a certain acceptable truss horizontal deflection from the manufacturer (1/2" max.)

Before you require it, I'd suggest asking a truss manufacturer to see if it's possible. Better to know ahead of time than to have to change the plans later.

 

[buleeek2]

gte447f,

This is exactly what I thought and there is no confusion here. I was asking about any special requirements to address in the area where wall sheathing, roof sheathing and scissor truss meet.

Thank you all for your input.

 

[BAretired]

Typically we do a pin on one end and a slip, with an actual slip connection clip - something like the Simpson TC, the opposite and alternate every other truss. This way you do have a pin for the wall/diaphragm connection and you have a slip the opposite end.

Not really; every second truss is pinned at one end only, but the roof deck prevents adjacent trusses moving in opposite directions.

 

[buleeek2]

The architect says there is no room for a header between openings and the bottom of trusses. They propose installing a continuous header and attaching scissor trusses to its side (with a hanger perhaps). Do you see any issues with such a solution? I can only think of properly attaching the continuous header to the top of the wall. See below.
c

 

[ANE91]

The architect says there is no room for a header between openings and the bottom of trusses. They propose installing a continuous header and attaching scissor trusses to its side (with a hanger perhaps). Do you see any issues with such a solution? I can only think of properly attaching the continuous header to the top of the wall. See below.
View attachment 7137c View attachment 7138

Not uncommon to use rim board as header for full-height openings. I can’t weigh in on the truss bit, as I’ve never dealt with this situation, which is basically a rim board at the roof instead of at the floor.

 

[jhnblgr]

How much deflection occurs in the weak axis from the truss thrust?

 

[gte447f]

I'm sure an architect probably drew that window header detail, but they don't even allow the top plate to be continuous. I would say this architect needs some education.

Assuming you go with some version of this rim board header concept, the truss to header connection is simple. Just use hangers.

Even though we have already discussed how the wall, although a pinned support, acts like a roller support by deflecting horizontally, I would still focus some attention on how the header is connected to the wall at each side of the opening so that the header to wall connection has some horizontal (out-of-plane) load transfer mechanism/capacity.

 

[ANE91]

I could see a clash where the rim-board-as-header is strapped for continuity. Give thought to how much space you’ll need on the inboard face of the rim board to dedicate to straps relative to the truss hangers.

 

[Flotsam7018]

Run the header long and fasten as many SDWC screws up from the top plate as your heart desires. Depending on the size of the opening and width of the header I have called for straps on the underside of the beam to the bottom chord to resolve the torsion.