More peg info...

[Thehardway]

I have some questions about peg size and spacing.  I did a search on the forum for topics using the term "peg size" and came up with some good info as usual from Jim.

Qoute:

"There are some standard "Rules of thumb" on peg sizing and placement.

Usually the peg diameter is one half the thickness of the tenon. So a 2" thick tenon would use a 1" peg, and a 1 1/2" thick tenon would use a 3/4" diameter peg.

And next to determine the tenon size it should be 1/4 the width of the timber. So a 8x8 means a 2" tenon, and a 6x6 means a 1 1/2" tenon.

Usually pegs over 1" in diameter are only used when specified by an engineer.

The placement "Rules of thumb" are then based on this diameter, which is called "D" in the NDS books and the hole for the peg has to be so many "D's" from the end of the tenon and the side of the mortise, based on the type of wood being used as in the tenon, and mortise. There are different rules for hardwood frames vs. softwood frames.

Hope this helps.

Jim Rogers  "


This gives me a great starting point but then as usual I start thinking and trying to make rocket science out of it.  Here is my dilemma.

I am cutting the top of a kingpost. I am using White Oak. The kingpost follows the design given in Steve Chappells'  'A Timber Framers Workshop' on pg. 175.  In the book there is no placement or peg size mentioned however it does give sizes for the kingpost(7X10), and rafters (7X10) as well as the depth and location of the tenon (4" long 3" from top of rafter).  On page 108 a drawing of the kingpost is shown with 2 pegs per side.  I assume this is the same design however the shoulder depth is slightly different (1.5" vs. 1.25")    The width of the tenon is not stated however all tenons shown in the book on 7" timbers are 2" thick so I am cutting my tenon 2" thick.   Following Jim's rules of thumb above a 1" peg would be standard however a 1" peg does not allow for sufficient edge/end clearance according to my measurements but this is where I may be wrong.  NDS is calling for 3D from end 2D from edge and 2.5D spacing.

When measuring for end and edge distance is the measurement taken parallell to the grain or to the nearest point?

Is the NDS "edge" measurement taken on the mortice or the tenon?

Easiest solution appears to be to use 3/4" pegs rather than 1" pegs.  Would I loose too much shear strength by downsizing.  Also the friction joint runs at an angle and is not parallel with the end of the tenon, how does this affect placement.

There isn't a whole lot of meat or relish to play around with in this type of joint and it is the most critical joint in the truss so I want to make sure I have it right.

Am I over analyzing this?   What would you do?

[Don P]

A couple of things popped into mind as I quickly read. I'm sure Jim will have more practical info.
Determine the load on the peg. Look in the NDS for the steel dowel type connector that would be used for that load. Use the end and edge clearances that would be appropriate for that steel dowel (which is smaller D). They did the research around the more common steel connectors so this gets those clearances back to intended.

The kingpost top isn't usually the most critical joint, look at the stresses at the bottom chord to top chord connection carefully too.

[Thehardway]

Don,

I phrased myself poorly.   It is the most technically difficult joint to cut on this truss is what I meant.

However your criticism got me thinking last night. This is not the most critical joint in function as it is in compression rather than tension. Last night while laying awake and cutting the joint in my head, I recalled what I had read about the friction joint and kingpost top joint in the TFGuilds historic truss series on kingposts so I went back and re-read it today.  (pg. 19)



"if there is anything surprising that our examination of a great many historic trusses has
shown, it is that normal bearing or the lack of it at chord-to-kingpost
connections results in no truss deformation. The 1801
Windham Congregational Church in Windham, Vermont, with its
very heavily built kingpost trusses of 45-ft. span, is just one more
example of many whose rafters, both inner and outer, engage the
kingpost with no cut joggle of any sort, instead using a 2- or 3-in.
tenon with shoulders cut at the roof angle (Fig. 10 below). It may
be that the kingpost-to-tie joint is always weaker and that failure
will occur there rather than at the head. It may be also that the
weight and nailed-together matrix of roof boarding and shingles
keep the joint together at the very head of the post.
Another possibility is that when a truss initially bears its load,
the end grain at the upper end of principal rafters or braces compresses
itself into the side grain of the post, developing enough friction
that a smallish tenon with a pin is enough supplemental
restraint to provide a rigid joint with no slippage."

Based on this historical evidence that this joint will hold up over time without even normal bearing I think I can put aside my paranoid feelings and the 2- 3/4" pegs on both sides will be more than sufficient to keep it standing for at least the first 300 yrs. ;D

Just for good measure I am going to upsize the kingpost from 7X10 to 8X10 which will give me a little more wood around it.  I might even play with the idea of a couple joggles in it.