32' span

[Rjwassink67]

I'm in the planning stages of our home and want to span 32' with a king truss.  There will be an engineered scissor trusses above to carry the load.  So the timberframe will have no load on it (just the weight of itself).  Will a 8" x 12" Douglas fir span this?  Thank you

[Roger Nair]

Rj, your question needs supporting details and illustration, before anyone can offer any helpful advice.

[Rjwassink67]

 

 

Here is a rough drawing.  Do you need anymore information?  It will not have a load on it.  The load will be supported by the scissor trusses.

[Dave Shepard]

Why not make it load bearing and do away with the scissor trusses?

[Roger Nair]

One aspect I would stress is that the timber truss should not contact a conventional scissor truss, bearing against timber truss could stress the relatively light 2 x material and potentially break them. 

The other point I would stress is that the shape of the timber truss must remain constant thus full attention should focused on joint strength and especially tension joints and solid bearing at the post feet.

You will need unambiguous specs on the lumber truss and timber truss before going forward.

[Rjwassink67]

I don't want to only use the timber frame because I don't want the full cathedral ceiling.  I want the pitch for the roof (north west PA).  I also want the look, but don't want to lose the heat filling up all of that empty space.

The timber truss will be independent from the scissor trusses. 

[firefighter ontheside]

If it will be a 32' timber truss, then you don't need any member to span 32'.  The bottom chord will be in tension.  The middle will not sag since it will be supported by the King post.  The whole thing will be depend on the pitch of the ceiling under your scissor trusses.  The lower the pitch, the forces increase greatly on the truss members.

[Rjwassink67]

I didn't know about the pitch.  It will probably be around a 6/12 pitch.  I expect the scissor truss to be a12/12 roof pitch with a 6/12 ceiling.

[jason.weir]

I agree - make the kings post truss load bearing

This is my house 8x10 hemlock tie beam & rafters, 8x8 eastern white pine everything else, spanning 36 feet with a 9 pitch.



Jason

[Don P]

I'm in the planning stages of our home and want to span 32' with a king truss.  There will be an engineered scissor trusses above to carry the load.  So the timberframe will have no load on it (just the weight of itself).  Will a 8" x 12" Douglas fir span this?  Thank you

Sure it will carry its self weight. Many buildings also have overframing. Many paths to the top of the hill, nothing wrong with this one.

On this I'd also cost out just doing common rafters for the upper roof vs scissor trusses. The bottom chord of your timberframe trusses can be the necessary tie for that roof. purlins could carry the ceiling, that then carries the ceiling load..

As mentioned above you don't want to bear the scissor trusses on the timberframe trusses. one way is to drop the kingpost trusses a distance below the scissors that can accommodate deflection, let the truss designer know about the kingposts acting as ties for his numbers, apply the interior finish and then small trim strips to cover the remaining gap.

[Rjwassink67]

Thank you Don P!  Great suggestions and advice.

[Don P]

I was trying to think of some examples, this one isn't TF but is kind of cool, and we own this original sketch. aesthetics driven design, the old dome was not handsome on the outside.
The capitol dome is about 100' taller outside than inside, I believe a masonry dome inside, iron trusses, in a parabolic, efficient bearing shape to carry the lantern (dark black is the true load path) and then an iron outside shell with webbing back to that truss. The inner dome carries its self weight.


aside, Restoration work was completed and scaffolding was removed shortly before Trump's inauguration.

Many gothic churches feature masonry groined vaults visible on the inside. Above those, unattached, is the hidden timberframed roof quietly protecting the masonry.