queen truss

[Alexis]

Hello

I would like to know if there is any structural difference between these two trusses. I've seen the the right one in the chappell book and the left one in a few photos...

sorry, couldn't succeed in posting the picture... my image is in the sketchup 3d warehouse under : queen truss

thanks

Alexis

[Don P]

 

[Thehardway]

In strict definition a truss is self supporting and has no mid-span posts.  Both drawings are showing support posts at midspan intervals.

As a rule of thumb the formation of right triangles in a truss adds a great deal to its strength.  If  they were true trusses I would choose the one on the truss form on the right as I feel it would be stronger.  Since they are not self supporting trusses however, the picture on the left  (with proper wind bracing) would probably be stronger and hold up longer without signs of sagging due to the vertical alignment of its posts.

As for virtues, the right one has more headroom and would make better use of attic space.  The left one is probably stronger for static loading purposes but may not do as well in a wind shear situation.

[moonhill]

How wide is the building, bent spacing and other pertinent info?   Tim

[Alexis]

The building is 29 feet wide, 42 feet long, three bay15-12-15. The headroom factor isn't important for me.

I tought that by definition a queen truss had supporting post...

Alexis

[Thehardway]

Alexis,

The post in a "queen post" truss is confined to the perimeter of the truss.  It does not extend into the lower portion of the frame.  The drawing shows the posts extendiing all the way to the ground.

Trusses are typically used when an area needs to be "spanned". 

[Don P]

This queenpost truss is comprised of 8 wood members and one diagonal wire tension member across the "room".


A truss is made up of triangles, shapes with more than 3 sides can be deformed, a triangle is fixed. You have drawn what would be correctly called frames I think, which can be fine too  .

[Thehardway]

Thanks Don.  Your picture is worth a thousand words and describes exactly what I was trying to say.  You illustrate a true truss. 

I think the correct term for the design shown might be a "tall posted cape" rather than a Queenpost Truss.  Much of the weight of the roof actually being shared and transferred to the ground through the midspan posts rather than through a truss which would transfer all weight downward at the eave.

Many of the timberframe elements look similar but structurally function much differently from each other.

[Alexis]

thanks for your reply, since it's not a truss, let me reformulate:

If you had to choose betwoon the two bent illustrated, wich one would you choose and why?

[sawmilllawyer]

For my purposes, I would choose the bent on the right. Your mileage may vary.

[Thehardway]

The one on the right looks best to me also. :D

[witterbound]

Seems to me that the structural difference between the two is that the attic floor in the first one may need a deeper beam than the second frame, and the third floor posts do not line up with the first and second floor posts.  On the other hand, if you are going to mortise and tennon in the post tops and bottoms, the first frame will require mortices from the top and bottom of the beam, which will remove more of the wood, which might require larger beams . . .  If I were only an engineer I could figure this out . .....

[matt eddy]

the truss on the right would be my choice, however both truss drawings seem to use the rafter to post connection.  which in my view makes it a high posted cape.  i also assume its a perlin roof system.  if you are going common with principle rafters i would put my queens into coller and the tie.  so you raise the the H and put the plate on.  than drop the principle rafters onto the plate and and the queens.  I personally don;t like the rafter to post connection.  but alot of companys do it. good luck
matt

[Alexis]

thanks for the reply,

sorry the picture is small but there is a plate that goes the lenght of the building... I'm not a fan of rafter to post either, can't see to understand why most timberframe company does this!

You were right in your asumption that it is a principal rafter with common purlin roof.