Stud Wall Stability With Scissor Truss

[JoshH726]

Considering a standard scissor truss bearing on a 2x wood stud wall, how is stability of a pinned-base wall maintain if using a sliding clip, such as Simpson TC, to relieve lateral thrust from the truss under load? Reading through the previous threads, I have not read a consensus. If a diaphragm is present in terms of roof sheathing, does this provide the necessary stability? If so, how? Or is there anything published or is it engineering judgement?

Case in point, I’m looking at a 48’ clear span (8/12-4/12) scissor truss 24” c/c. Ground snow is 50 psf. Preliminary numbers provided by a truss manufacturer are 2,600# thrust with 1/2” of lateral movement. So my spidey sense doesn’t see a wood wall working on its own. I’m considering some kind of “bond” beam between dedicated posts (say 10’ c/c) and infilling with stud walls.

Constructive thoughts are always appreciated. Thanks in advance.

 

[JAE]

If the stud wall is a pinned element, free to rotate/lean, then there will be 0 thrust. The truss will simply deform, the walls will lean outward a bit, and the key is to verify that the wall lean amount is within acceptable limits.

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[XR250]

The wall will lean until it reaches the limits of the slide clip. It can then transfer its out-plane loads into the truss and subsequently into the diaphragm and out the gable end.
I have never used these slide clips and have chosen instead to let the walls splay outwards- which historically seems to work fine. Seems like they would also require special detailing at the ceiling/roof sheetrock junction.

 

[kipfoot]

If you model this truss with pinned connections on both sides, you'll get a horizontal reaction. If you model it with a roller on one side, you will get a horizontal displacement.

I suggest that you specify that the truss be designed with a roller with a maximum displacement. This gives you no thrust and an acceptable deformation at the top of both walls. (be ready to discuss a change in truss geometry)

The installation instructions on the Simpson connector are a bit wishy-washy - suggesting that there are a number of ways to use the clip. I agree with your spidey sense...I don't think it would function as a sliding connection at the top of a wood wall.

 

[JAE]

Now that I look at the Simpson TC clip - I would suggest that the clip is an attempt to release the lateral connectivity between the wall and truss.

This really only works if:
1. The friction between the truss and top plate on the wall is low (never really is), and
2. There is not concern over relative displacement between truss bottom chord and wall interior face - i.e. you don't have gypsum sheathing on the wall and ceiling as the relative movement will distress the joint between the two planes.

Otherwise, by using the TC clip, there is literally NO lateral brace at the top of the wall so the wall would have to be its own stabilizing element - i.e. the wall is otherwise laterally supported by cantilever action, other walls, or other braces of some kind independent of the truss.

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[LonnieP]

Suggest you specify the maximum "spread" you are willing to accept and let the truss designer work it out.

 

[XR250]

Otherwise, by using the TC clip, there is literally NO lateral brace at the top of the wall so the wall would have to be its own stabilizing element - i.e. the wall is otherwise laterally supported by cantilever action, other walls, or other braces of some kind independent of the truss.

There is no bracing, other than sheetrock tape and friction, until the wall reaches the limit of the clip. I feel this clip is an answer to a problem that really does not exist in practice. It attempts to solves one problem but creates another.