Thrust on a Scissor Truss

[kmart30]

I have a scissor truss bearing on a 2nd Flr. wall that is being supported by a stout LVL(sketch attached). I know they typically run these trusses with a pin and a roller which will give you a horizontal reaction to satisfy. The supporting walls are both dbl. 2x6's at 16" o.c. (for brick veneer support). Tossing around ideas on how to take of this and not go too crazy if its even possible....anyone?

 

[JLNJ]

Pin-roller would give you no horizontal reaction, but the more flexible the truss the more it “squats” and deflects outward.

Yours looks pretty flexible. It seems like it’s going to push the walls outward unless you find some way to restrain it.

The assumption for the reactions also changes the force profile in the truss members significantly.

 

[Guest090822]

You could run it as the three hinged arch to make it determinate. This would give you the vertical and horizontal reactions.

 

[phamENG]

What does it look like in plan? You've already got a beam carrying half the load from the truss there on the left side - any chance you can put it up in the ridge?

 

[kmart30]

The beam is about 24' long but the upper ridge carries further toward the front and the ceiling profile changes. No chance the architect is going to allow a support in the middle of his great room. Design already changed from typical conventional framing, ridge beams, king posts, ect. where the thrust wasn't an issue but now because of costs they changed to scissor trusses.

I believe they are using a pine T&G ceiling which should help stiffen this truss but I need to ring to the truss guys and see how we can stiffen it up to limit the deflection that will ultimately get transferring into the top of these walls.

 

[phamENG]

How does the ceiling profile change? If it drops to a flat ceiling level with the top plate in the wall, you could do a girder truss at the transition that could support a ridge beam going back to the end wall. Then again, the deflection may be minor enough that the framing and finishes can handle it. I'll be interested to hear what options the truss designer gives you.

 

[KootK]

Short of changes to the profile, you'll mostly just have to let the wall spread occur. And that's usually only an issue as you approach the end walls where the top plates start to offer real resistance to heel spread. Your best friend in dealing with that is a stiff roof diaphragm well connected to an end wall shear wall that is itself prevented from spreading. This restrains the spread on the first few trusses an smooths the transition after that. This is where I'd focus your attention.

Your ceiling diaphragm may help some in this regard but will be unlikely to be detailed for shear transfer at the end walls unless you make it so.

 

[chicopee]

I looked at the diagram and I thinking that if something is going to go wrong then it will go wrong. Redundancy to your diagram is the key by having columns where the two roof shapes intersect.

 

[kmart30]

Horizontal deflection in that scissor truss is 0.8"(live load deflection plus creep component of dead)

 

[kmart30]

Talked to the truss guys....increasing the depth of the truss a bit and going with a 2x6 bottom chord gets us to <0.5" horizontal deflection. Not a truss guy but surprisingly just going with a 2x6 btm. chord doesn't help with the deflection due to the reduction of webbing when going with larger bottom chord. Still going to stiffen up the roof/ceiling diaphragm and detail a connection to the end shearwall but I believe this deflection is more tolerable....

 

[KootK]

- I actually thought that the original deflection was pretty good.

- I suspect that the deflections are always a fair bit underestimated because, to my knowledge, the software doesn't account for joint slip at that all important bottom chord peak joint. The steep scarf cuts on the incoming pieces make for a lot of gapping and, as you can imagine, it's quite difficult to place a rectangular plate on a joint of that configuration and still get a good bite on the pieces as you'd want to.

- It's probably not a reduction in webbing that limits the deflection benefit from the 2x6 bottom chords. Rather, what's usually happening is that the deeper chord makes the truss model mathematically shallower because the chord centroid shifts. For shallow trusses, that can have a noticeable effect. In part, that's why floor trusses are 4x2 instead of 2x4.

- A great way to improve these things is to flatten out the bottom chord some. 4:12 or 5:12 maybe. No doubt that's impossible though as your architect probably has the ceiling plan telegraphing out to the exterior wall someplace and the look would be different.

- Church?

 

[MotorCity]

I would place a ridge beam at the peak of the scissor truss, essentially turning the scissor truss into a beam and rafter system.

Unless you are brave enough to attempt a moment connection in wood, analyzing all of the joints as pins or rollers would make this arrangement seem inherently unstable.

 

[KootK]

You know how when you have a steel joist next to a stiff wall, some folks will tighten the joist spacing there or ask for the joist there to be stiffer to iron out differential deflection? I wonder if you could do something like that here by doubling up your first six trusses or something like that? I've never seen this done and, in all likelihood, probably never will. Folks don't much like spending money on imaginary problems.

 

[JLNJ]

Im surprised the original deflection isn’t quite a bit more than .8”. Less than an inch is great.

The truss guy did realize that you were looking for the horizonal, not vertical deflection, right?

 

[kmart30]

In a perfect world I could ask the architect for a bunch of items but I already know the answers...no chance on dropping the bottom chord. Rear wall has large windows and they are set on this ceiling pitch. The rear wall was a whole other back and forth with the archy which we won so going back and asking for more isn't worth it if we can make it work.

No church just a large custom home....crazy what people will spend on their personal residences..

 

[kmart30]

In a perfect world I could ask the architect for a bunch of items but I already know the answers...no chance on dropping the bottom chord. Rear wall has large windows and they are set on this ceiling pitch. The rear wall was a whole other back and forth with the archy which we won so going back and asking for more isn't worth it if we can make it work.

No church just a large custom home....crazy what people will spend on their personal residences..