Scissor Truss & Racking

[vtburt]

Hi all,
First, I just posted this over on the TFGuild forum, so I apologize to anyone who finds this as a duplicate.

Ok, I am am considering designing something vaguely similar to this:


I am having a hard time determining how a design like this resists racking in the direction of the trusses, without any knee braces in that direction.

The simplest answer is that this is not good design...is that true?

Otherwise, obviously the truss will resist thrust, but what is going on at the plate interface to stop the whole thing from being sent sideways with a broadside gust of wind?

Thanks again to everyone for sharing their wealth of knowledge and insight.

Cheers!
Taylor

[ChugiakTinkerer]

Welcome to the forum Taylor.  I agree with your assessment, it doesn't look like the best design.  There doesn't appear to be anything keeping the trusses from rolling, and the whole roof could move away from the camera view with not much trouble at all.

[Ljohnsaw]

Perhaps something is missing from this picture?  What is the intended use of this frame?  Could it be a porch/patio cover?  If so, maybe behind the camera is the house that it is attached to and would keep it from wanting to roll out away from or toward the camera.

I was thinking the OP was asking about a sideways stability of this structure.  It looks to me that there is a lot of faith in the ground attachment of the posts to resist a sideways force since there is no bracing in that direction as well.

[ChugiakTinkerer]

Yep, that too.  I wasn't even looking at that.  The problem would be the same with a king truss or similar, but at least then you could bring in a knee brace to connect the the posts to a tie beam.

To sum up, there doesn't appear to be anything in this picture preventing the trusses racking North-South (assume camera view is north) and the posts from racking East-West.  Does that capture it?

[btulloh]

I'd like to hear what an engineer has to say about that frame.