question on half dovetailed and wedged joinery

[Max sawdust]

In Steve Chappell's "A timber Framer's Workshop" page 90 shows a king post with the tie beams half dovetailed and wedged.  This joinery is in the Fox Maple School library. 

I am interested in this joinery because I am trying to find a way for a bent to span 18' without using a continuous 18' tie beam. 

My question is: I just do not understand how I would cut this joint.  Do the half dovetails butt each other or do they pass each other?



This half dovetail to king post seems to be a better tension joint than scarfing the tie beam over a center post.

(Yes I know I should just get an 18' tie beam.)  But it is more fun trying to find a way around it ;)

Any thoughts would be appreciated. 

Max

[Jim_Rogers]

I've been to the Fox Maple School and I have seen the Library from the inside.
Quite a neat building with it's thatched roof.

However the drawing you site on page 90 is not the king post and tie beam joint. It is again shown on page 96 and the text describes it as a tie beam to post connection, which allows it to have a through mortise and a long half dovetail tenon.

Further on page 108 it shows a king post tie beam connection using a half dovetail, with a continuous tie beam.

If you want to go with a two piece tie beam a better joint than two half dovetails should be used.

Several things have to be taken into consideration, such as the slope of the roof. At the library at Fox Maple, that is a 14/12 pitch roof, making it steep for thatching.
A steep roof pitch puts more load out at the wall posts from the rafters and less outward thrust than a low roof pitch.

All factors of your roof design should be considered. Such as, again roof pitch, snow load, dead load, span of building, spacing between bents, rafters, purlins and well as wind load.

Once all these facts are known then the proper joint can be designed.

If it was me, and you wanted a two piece tie, I'd use a one piece hardwood spline and have it go through the king post and be pegged properly (meaning correct spacing and size of pegs) to each half of the tie beam. Spacing and size of the pegs will be determined by the facts mentioned above.

This joint if it's not a continuous tie has to be done correctly.....

Jim Rogers

[Raphael]

  I have a hammer beam bent designed that spans 18ft., it's engineered for 9' bent spacing under a 6:12 pitch using EWP for a majority of the components (red oak for the central tie & Misc.HW* for braces).  It utilizes four wedged dovetailed tennons.  With a change of wood species and/or an increase in roof pitch the bent spacing could be widened.

  A hammer beam is overkill for an 18ft. span but it was designed to elevate the ceiling fan in my office above decapitation level; I wound up with an 18ft. beam there instead.   :(

  The Misc.HW is a list of tree's given to my engineer which includes:
White Oak : Birch, Beech, Hickory : Red Oak : Red Maple : Black Cherry : Mixed Maple

[Max sawdust]

Thanks guys.
Jim, is that just a shouldered mortise and tenon in the photograph on page 90. If it is not a tie beam what is it? I see the same connection to king post in the picture on page 86 (not the bent they are placing but the ones behind it.  Sure looks like a two piece tie beam.

The point on roof pitch is a GOOD one.  I will have quite a flat 4/12 roof to match the other miserable roofs on my house and garage :(
Max

[Jim_Rogers]

is that just a shouldered mortise and tenon in the photograph on page 90. If it is not a tie beam what is it?

Yes; it is a shoulder mortise and tenon joint, with a half dovetail tenon. It is called a shoulder with a diminished haunch as the shoulder starts at the shelf of the post at the bottom and decreases as it goes to the top edge of the beam (diminishes). And yes that is a tie beam as mentioned in the text referencing the drawing on page 96, which is the same drawing. The shelf helps support any load the tie beam is putting vertically down onto the post, so it doesn't all rely on the tenon lower surface. Because that lower surface is cut on an angle to make the half dovetail and to get these two surfaces (the one in the post, and the one on the bottom of the tenon) to match exactly so the load can be transferred is difficult to cut. If it is not cut correctly and the load is applied the fibers will crush until there is enough surface area to support the load. This crushing will or could damage a lot of the half dovetail tenon.


Yes, the bent they are placing (in the photo on page 86) and the bent behind it are king post trusses for his workshop and yes they are two piece tie beams.
But the joint used there (at the joint between the tie beams and the king post) is not two tenons with a half dovetails.
As you can see in the picture on page 86 that there are two queen posts going from the tie beams up to the rafters. These join the rafters about one third the distance up the rafter from the wall. And the struts (the pieces going from the king post up to the rafters) are joined to the rafter about one third down from the top. This one third up and one third down divides up the load and stresses so that there isn't that much thrust at the rafter to tie beam connection at the eave of the wall. And if there isn't much thrust there then there isn't much tension at the joint of the two piece tie beam at the king post.

About king posts you have to understand that the king post does not sit on the tie beam. The king post is hung down between two rafter tops, and it holds up the tie beam from sagging. The struts hold up the rafter from sagging. The queens hold up the rafter from sagging and also reduce the outward thrust at the rafter foot.

Your design needs to be carefully thought out and a low pitch of 4/12 will increase the thrust at the eave of the wall.

Jim Rogers

[Max sawdust]

Jim,
Thank you very much for the clarification.  What you are saying makes good sense too me.  You have given me much to ponder


Raphael,
A hammer beam seems like a practical solution.  It could very well be best solution for me.  (Lots of joinery practice too.)
Or I could just break down and cut an 18' oak tie beam for the tractor shed :D


Max

[Raphael]

A hammer beam would make it more of a tractor cathedral. ;)

  My thought's exactly on the practice joinery, the wedged dovetail is one I haven't cut before.  At 4:12 you'll definately want to make All the tension components hardwood.  The tension on my design's central tie is ~6000lbs at full load  (Panels +roofing +snow load) which is 45 to 55psf depending on what he used for the snow load.
  Of course the only unsound knot I recieved from the sawmill was dead center in the middle of that tie and I didn't see it until I started layout.  >:(