Scissor Truss Thrust 1

[TRAK.Structural]

I've seen some threads on scissor trusses before but wanted to get a fresh take on this as it relates to my specific scenario below.

I'm working on a small 1-story garage where the designer has drawn up the building section using scissor trusses to accommodate a car lift. Its a rectangular plan with dimensions of 14 ft x 28 ft.

1. How are engineers estimating the truss thrust that gets applied to the top of the supporting walls? Is this something that is easily obtained from a truss supplier?
2. I'm stumped on how I am going to provide something to resist the above mentioned thrust considering the car lift space requirements, any ideas??
3. At the gable ends, is it common to specify a flat bottom chord or would the wall top plate be built to match the typical scissor bottom chord profile?
4. The gable end trusses need to be able to transfer the diaphragm shear into the walls, on a residential project is it reasonable to believe that if this loading criteria is specified that it will actually be accounted for?

 

[RWW0002]

As long as the truss supplier runs the truss as pinned @ 1 end and a roller at the other (as it probably should be run)and there is adequate truss depth,there should be not trust reaction from gravity loading.

Lateral deflection of the truss would have to be investigated to make sure that the truss pushing the walls "out" a bit does not cause a problem. There are other ways of handling the thrust if this movement is not acceptable.

 

[phamENG]

1) It doesn't. Those trusses will be designed as pin-roller supported and will have 0 thrust at the reactions. They will, however, have a lateral deflection value. You need to make sure your walls can handle that much out of plane movement without becoming unstable or damaging finishes.

2) No thrust to resist.

3) You could do a gable end truss, but then how do you brace the hinge? Usually best to "balloon frame" gable end walls with scissor trusses. Though, at only 14ft, you might be able to get a girt to work. The wall is likely to be 2x8s anyway.

4) That load wouldn't go through the truss, per-say. It would go into the truss top chord and right back out again into the sheathing that laps with the shear wall below.

 

[TRAK.Structural]

RWW and Pham - Not sure how to prove/justify the stability of the walls against lateral deflection at the top. Walls being pin-pin, seems like the diaphragm is what has to hold this all together? Any common practice or references on putting numbers behind this?

Pham - The top plate of the gable end walls will have to be braced regardless of balloon/platform frame (I don't like to use the ceiling for bracing). In either case I will probably specify some kickers. Maybe another option could be bottom chord bridging for the trusses, but I still think that load has to be resolved by transfer into the roof diaphragm.

 

[RWW0002]

If the truss is a pin roller it should be stable under gravity load without a lateral restraint from the top of wall. The comments above were in regards to making sure that the lateral deflection from the truss (available via truss designer output) does not cause an issue with finishes or "out-of-plumbness" of the wall. Common approaches I have seen generally involve taking the lateral deflection from the truss system, applying it @ the top of walls, and allowing the exterior walls to rotate to accommodate (generally trying to hold H/300 or so - something similar to serviceability limits for lateral drift).

 

[phamENG]

Stability - as RWW0002 mentioned, apply the lateral displacement to the top of the wall. It's going to induce an eccentricity and a bending moment in the stud. Does it still work?

Gable end wall - if it's "balloon framed" (I do quotes because it's not true balloon framing in the pre-1930s sense), the top plate will be at the roof diaphragm and it will be braced - I'm talking about full height studs here. Tying the trusses into it at the bottom chord by running all required bracing to the wall is certainly necessary as well.

 

[cliff234]

I’ve always considered scissors trusses to have pin / roller supports. (Be sure the truss supplier is aware of this support condition and your assumptions. Based on your diagram the walls won’t be stiff enough to resist horizontal thrusts. Calculate the allowable horizontal movement at the truss supports, define which end will see the horizontal deflection, identify the allowable vertical deflection (and horizontal movement) on the drawings (so that the truss supplier can design the trusses accordingly), and put this info on the contract documents along with connection details at each end of the trusses.

Also, I would put the Lamborghini on top in order to prevent oil from dripping on it.

 

[beav-eng]

I have specified scissor truss clips previously, which can accommodate around 1-1/4" of truss deflection.

 

[XR250]

I have specified scissor truss clips previously, which can accommodate around 1-1/4" of truss deflection.

Nobody should do this. It is another Simpson product that answers a question nobody asked. Do you think the framer is actually going to come back and set the nails in the clip after the dead load is applied? If not, then the wall is flapping in the breeze. Just toenail the trusses down, add a hurricane clip and call it a day. The diaphragm will do the rest. This topic has been beaten to death on this forum.

 

[phamENG]

Geeze, XR...tell us how you really feel...

 

[XR250]

Geeze, XR...tell us how you really feel...hammer

Hey, I just quit coffee so cut me some slack

 

[TRAK.Structural]

Pham - A little confused on your earlier comment about sheathing overlapping the trusses over the end walls. If the studs go full height to the roof diaphragm (sloping top plate) than the wall sheathing wouldn't be attached to a truss at all?? The last truss would be 2 feet-ish from the end walls and the diaphragm would transfer the lateral loads to the shear wall directly.

Cliff - Good point on the Lambo

XR - Appreciate your candid response; And condolences on the loss of your coffee consumption

 

[dik]

I've seen it done with 4' spacing to the gable and 2x4 or 2x6 cantilevered outlooks to provide a roof overhang of 18". This provides a 4''strip' to help accommodate the lateral deflection of the scissor truss. I thought it was 'neat'; I'm not big into housing. I suspect there will not be much snow/load on that slope.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

I was addressing the idea of a gable end truss. I wouldn't use one, but IF you did my comment 4 above describes how I see the load path working for those lateral loads.

 

[dauwerda]

I'm 100% with XR on this.

 

[StrEng007]

For fear of beating this subject even further, XR250, may I get your opinion on a previous comment?

Nobody should do this. It is another Simpson product that answers a question nobody asked.

I understand the reluctance to rely on a contractor back checking nails on a job. Totally understood.

What about jobs that are located in jurisdictions with very a very detailed special inspection program? That is, if I could guarantee that the Simpson scissor truss clip was installed as intended in the manufacturer specs, would you still have the same feelings towards use of that particular product?

Some things that I speculate is whether there is enough fiction at the truss to plate to prevent the clip from doing it's intended job. As in, the clip is useless and the wall will naturally want to deflect through friction alone?

 

[XR250]

That is, if I could guarantee that the Simpson scissor truss clip was installed as intended in the manufacturer specs, would you still have the same feelings towards use of that particular product?

You cannot.

 

[jayrod12]

And honestly regarding the Simpson clips, what's the benefit?

Now all the lateral deflection happens at one side?

Or not I guess. But how does the sheathing that likely extends past the top plate accommodate the movement. How do the exterior finishes accommodate it?

 

[1503-44]

https://seblog.strongtie.com/2015/10/accommodating-truss-movement-besides-vertical-deflection/

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[RontheRedneck]

On a 14' scissor you won't get too much horizontal movement. A truss supplier will gladly run a truss for you and give you a drawing.

Actually, I wish more architects and engineers would do that. It would save some headaches in the long run.


I'm in agreement with those that say the Simpson scissor truss clips are a joke. At a trade show I asked some of the reps how they were supposed to be used. They didn't know.