Timber Framers Guild

scissor truss & racking

Posted By: tburt

scissor truss & racking - 05/03/17 11:36 AM

Hi all,
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 a sound 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.

Posted By: Cecile en Don Wa

Re: scissor truss & racking - 05/05/17 09:27 AM

I go with the simplest answer in this instance.
Posted By: Jay White Cloud

Re: scissor truss & racking - 05/05/17 12:28 PM

Hi Tburt,

This frame does look a bit on the..."light and airy side"...but that is perhaps the goal with a Pavilion style frame. I could be incorrect but this seems to be a Veranda or related wing of a house perhaps..??...or maybe a freestanding Pavilion completely, it's hard to tell.

You have very valid concerns for thinking..."...this is not a sound design..."...but it is really hard with just one photo to tell whether those concerns are true or not.

Here is why I think this:

1.) Was this frame actually designed by someone with timber frame design experience...??

2.) If so, they most likely had it back check by a PE with the same level of experience (we always do...90% of the time at least if not more.)

So, what may not be seen here is a very robust Moment Connection at the foundation, and hidden hardware...even things like Dyneema strapping that is further hidden from view...or...the PE said for the location the loading of the frame will meet local environmental stresses...

All in all, this is a difficult thing to critique from one photo, and not seeing all the mitigating info that could (or may not) be present...I think it would be impossible to say either way.

If you like something like this..??..build a model and explore its strengths...Maybe for what you want and where you want build it, it may be...strong enough.

Good luck and enjoy your timber framing!!

Posted By: Paul Freeman

Re: scissor truss & racking - 05/05/17 05:05 PM

I think it will greatly outperform a beach umbrella.
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