Hi,

The span thread has me thinking. Has anyone ever placed a scarf under a kingpost in a truss? It seems if you could resolve the tension in the bottom chord with maybe a through tenon and some iron banding, then the scarf might be feasible. Sticks are getting any longer.

Anybody have any thought on this?

here is an image for clarity


then if this could work, I guess ill have to figure out how to scarf the top chords too.

Hi Mo,

You are struggling with one of the major challenges facing the timber framer - how to join two timbers together to make an effective longer timber that has the same structural strength properties as the constituent parts.

The added iron bands will help hold the joint together and help prevent distortion of the joint but since these are all positioned within the joint region they will not do much to prevent horizontal shear splitting the main beam either side of the scarf in the plane of the scarf. How would you install the bands and what would hold them in position when the main timbers shrink due to drying ?

The beam now has an effective depth of only one half of each of the full depth beams. The pegs will encourage the two scarf faces to move apart as the timber dries.

There is no easy answer.

Regards

Ken Hume

Not saying I have all the answer...

But is not part of it, to go with edge-halved scarfs not face-halved ? To maybe not peg the bottom chord / king post connection but to wedge the through tenon below the tie and to have straps also be wedge tightenable to deal with shrinkage as it happens ?

Or mayhaps the scarf should not be at the panel point at all ?

Yup, more questions than answers.

Mo,

I'd consider wedging, not pegging and also changing to a edge halved scarf like Will T said. But I'd be comfortable taking that to our engineer -- looks like it would work.

Mo -

I seem to remember that there is a scarf in the loft @ Heartwood - not sure if it is part of a truss though...

Per your example above - a few concerns - and this is totally theoretical as I've never done this type of joinery... and would have to run it by an engineer if it was mission critical.

1. The hole in your through tenon limits how effective the KP can be as it goes into tension. You'd have to be sure you have enough relish on the tenon before you get to decoration.

2. The iron bands might want to be 2 u-shaped segments that are wedged together to allow for tightening (or use RF dried or reclaimed)

3. I'd move the struts up away from the tie... while I like the look of struts coming into the corner of a truss - if the rafters bend and the struts go to compression they push down on the tie - further stressing the KP. By moving them up 12" or so (dependent on joinery and how much relish you need between the through tenon and the mortises for the struts) you eliminate this variable. If the struts are not mortised but only deeply housed you can probably get away with 3-6" between bearing on strut and top of tie (depending on loads).

4. You might consider doing a splayed scarf with undersquinted ends - this will help key each end into the other.

5. Pegs could be a problem as Ken described - maybe wedges under the tie through the tenon to pull it up to the bottom of the KP? But this affects relish, etc...

6. In tension - longer is probably better.

There is information here on a 64' span that was scarfed for 'heavy tension loads": http://www.tfguild.org/joinery/part6.pdf

Look for the 'stop splayed with multiple tables'... although I see no example of a KP coming through the scarf. This may be due to wanting the scarf to act in as few directions as possible - as in a scarfed plate placing the scarf in the sweet spot away from the top of the post and high shear area and where the bending curve is not yet too great.

Ken, if I understand the issue, a big if, then the chord is in tension and if designed and analyzed properly not in bending therefore horizontal shear due to bending would not occur. I haven't been following the span thread so I may be missing the point. But I think if this is indeed a true king post truss then there is no force being applied downward into the chord, however it is under significant tension. Therefore as drawn its only resistance is in the pegs, however a splayed & tabled scarf might be just the ticket. Wouldn't it be slick if the stop were the kingpost tenon?

By the way, I think the proper way to design a kingpost truss would be to have the struts installed into the kingpost and not as drawn as this would indeed impart a downward force on the chord. As a matter of fact, the only need for struts other than the aesthetic is to reduce the span on the principal rafters, if they are sized big enough the struts are redundant, and in reality in a properly designed truss so is the kingpost! It should be in zero tension or compression (unless the struts are working struts and attached to the kingpost).

I agree that this could be made to work. Other than it's own weight, you're not asking the bottom chord (tie beam) for anything but tensile strength. The thru tenon on the king would have to shear before the scarf would fail. Add some pegs or shear blocks along with your straps, and it should all add up to enough tensile strength. BUT, when I say it should "all add up", I mean that quite literally. Do the math. CB.

Hey Everyone,

Thank you for the great insight.

I have tried to reconfigure to you all's advice.



















I think maybe those struts have to move further away for relish.

Is this what you all were saying? Some of the terminology I need help with.

Will T, panel point?

Paul, when you say stop of you referring to the kingpost creating a stop for the tables like this?

Mo -

Look at the mortise for the struts. Now look at your tenon. Drop that king post on the tenon and you are likely to blow the relish between the tenon and bottom of the strut mortise. Of course in practice this is dependent on if a tension load develops in the KP... but I'd look at using a 1" housing on the struts and forgoing the tenons (or moving them further up the KP).

I think you are getting at what Paul suggested - essentially using the KP tenon as a key block in the scarf. The issue I have with this is differential shrinkage in the tenon vs. the length of the ties. This could work if you wedged it... but using the tenon as a key block will allow the scarf to slide open as the KP dries and the tension load develops. A wedge from the top could work if properly designed - but you'd need someone to check back in 10, 20, and 50 years to tap it down and make sure it stays put.

I think you'd want to have the long grain push against itself and not rely on the tenon to lock it together. Flipping the joint back to horizontal will allow you to do this a bit easier, I think as you'll have more relish / continuous grain on either side of the tenon to work with.