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.

Hi Paul,

For steady state or simple static conditions you are quite correct that there will be very little horizontal shear present at the king post / tie beam interface. However, when a transverse wind blows or there is an uneven snow load on the roof then things change. If the king post frame was as recently illustrated by Mike B in a parallel thread then both the braces and the raking struts would impart quite large forces onto the tie beam and this would in turn need to be resisted by the tie beam resulting in bending moments and horizontal shear being developed in same.

It is also quite easy to fall into the trap of thinking only in 2 dimensions and once again if the design were as per MB's recent illustration then if a longitudinal wind were blowing and some form of gable closure were present then the king posts would start to push out at the tie beam centre at 90 degrees to the tie beam due to ridge brace action and hence significant bending and shear would then be present in the tie beam but in a different plane.

The king post roof is not my favorite form of roof and is almost entirely absent in the southern home counties of England however it does make an appearance in traditional buildings in the north of England especialy in Yorkshire where stone rather than timber walls are more prevalent i.e. where cross braces do not need to be employed and thus bending and horizontal shear considerations are greatly reduced.

Ken Hume

In addition to what Ken mentions - there is also the problem of history... we obviously cannot envision what our buildings may be used for in 10 nor 100 years. 200 years from now, when things have settled out and its been wind loaded and snow loaded and a new owner added a loft... we may find our best intentions showing signs of their age.

That said - since we cannot anticipate - we should build with the best intentions for the loads / dynamics that we understand and think might come into play at the time we are building.

The first time I had to work out unbalanced snow load + the possibility of wind I wanted to... cry.

But, most structures can be resolved with simple best practices and simple calculations or spreadsheets - and the discussion here I think is valuable to myself and I would bet to Mo and others.


Mike 'dangerous enough with an FEA model having walked the path between the world of engineering, architecture, art, and craft' Beganyi

Ken,

I think I follow what you are saying. What about cross bracing between kp's?

Mike,

Makes sense to me. You should check this book out.
How Building Learn. Brand, Stewart That should say "How Buildings Learn"

Since there are so many well qualified people on this thread (yea you). Let me show you a picture. First, I think it would be very cool to build stages, outdoor amphitheaters, etc. That being said, and no wood chips flying right now, I've been messing around with this. The span is only 30' and I think this arrangement might be an overkill, maybe not. Maybe I could span further. Would like to hear what you all think.

Just a scheme right now with bare bones, but thinking about purlins up the principals (not shown), and then light common rafters housed into the purlins, tg decking, standing seam. The pitch would flare towards the eves. No snow down here. Do have wind though. Huge Footers?

Criticism welcome. Does anybody want to help build it?

I gotta cut something.

These might help a little. There.

Mo - Tried to find this for you in the Guild Wiki, but somebody seems to have messed with it, and all the entries after G are among the missing.

Panel Point - http://www.truss-frame.com/truss-glossary.html#p

That is a good book!

Can you not get 30' sticks?

Instead of bracing in the ridge direction have you considered bracing in the roof plane? Perhaps move that X to each side btwn the rafters? Or maybe even get some purlin to strut bracing going on? I love breaking the 'typical' planes we see in TF construction with bracing or unanticipated orientations of common components. You've already started to think outside the sliced bread of TFing. Take it a step further if it makes sense with the structure.

That all said - as a structure it is cool looking - overkill or not. As a stage I'd start thinking about sound transmission. That high ceiling with TG decking will wreak havoc with your acoustics. Might consider how the shape may swallow up the sound. But - as an outdoor pavilion that may sometimes be used for stage - it might work 'well enough'.

Nice design, mo

My first thought is that the princeposts are just adding cost, though. With that span, it would be more efficient to just let the struts support the prin rafters rather than an embedded truss.

The issue of wind load on the gable is a pain on these open buildings. Your cross bracing would tend to bend the tie beam if they got loaded. what if you ran a ridge from kp to kp near the top and also a lower horizontal beam from kp to kp near the foot and put the crossing braces into the ridge and the lower beam -- then as long as you can take the thrust in the heel connection, you don't load the tie up. Id also half lap them and do a cool hand forged clamping bolt with decorative washer and iron wedges.

If analasys showed you needed more stiffness in that direction, lateral stability, you could add a plumb purlin and a lower parralel beam running from tie to tie and do the same x bracing into them. you may have to add the princeposts back in in that scenario as they would vertically load the tie beam when loaded.

have to run, but this is fun

I have long bracing from principal rafter top to bottom in the roof plane, four in the roof, two on each end. It is a 30'x60' building, king post w/one piece tie.

It is wonderful to be able to come up with solutions for such things. It reminds me of Raiders of the Lost Ark when Harrison F. is in a fight with this really good sword fighter, he is all most done in when he stands up, straight and pulls out the revolver and plugs the fellow holding the sword. Find a long stick of timber and get 'er done.

On those strut mortices, I would move them up the king a bit more, also. A further question with same, if the mortice is in line with the tenon, is there still a relish problem? I can see it if the strut is not in line with the tenon, this is more desirable with the struts moved up the king. What I struggle with is the king post through tenon has very little fiber left in the whole of the timber, the opposite of relish.

Tim

Panel point, see also, node.
If you haven't played with it Mo, this is a fun simple truss program to analyze the statics.
http://www.jhu.edu/~virtlab/bridge/truss.htm

I'd be appreciative if Mike made me cry

Hi Mo,

At the risk of banging on the same old drum I think that your draft design appears to be a very big roof balancing on legs with very little infil to help stiffen the "posts" and provide alternative load path shedding for the roof which is currently (in my opinion) potentially vulnerable to disproportionate collapse.

The recent barn frame raising videos posted on the website demonstrate for example how a frame without sills behaves during a raising. The post legs take on a life of their own appearing to be hinged at their only connection point to the tie beam.

I well realise that I am probably wasting my time in making these comments but I would be remiss if I did not bring to your attention that there is a world of difference between making a little 12ft wide frame and one which is 30ft wide.

I have recently been examining a barn frame that has a span of just under 19ft and this too is a fairly open structure below the tie beam. This building has developed a pronounced transverse lean following the failure of the lower cross brace pegs and tear out of the top of a jowl post. To right this building will require substantial de weighting of the roof to help enable the building to be pulled back up to plumb thus a heavy price might well need to be paid later on for scrimping on the wall and lower cross frame design.

Try doing a timber count above and below the tie beam to see if what you are proposing is a reasonably balanced structure.

Regards

Ken Hume

I see I'm getting a bad rap here on the forums!
No crying or whining!

Don - that is a cool web based link. Have to spend some time with it a bit - but the description looks like it would be fun to play around with.

I have access to MultiFrame 2D - so I usually just export my 3D model to stick lines and use that program to study the loading, wind, piece sizes, etc.

Again - I'm no engineer - but can get far enough along...

Will T, thanks for the reference to panel point. Gotcha.

Mike B, Timbeal,

yea I guess I could find one, it would actually be a 36' considering the length past the post.

Gabel, Thanks for the compliment. I like the idea of plumb purlins and possibly a beam in the same vertical plane to Xbrace.
I took your advice about the X bracing between kp's. Wrought iron, ofcourse!

Timbeal, I don't like the strut mortises above the tenon either. I think Mike B suggestion of housing with no tenon might make sense as those are in compression. Put a screw in there, or maybe some more banding.

Ken, I like the idea of the timber count, that makes sense. I have added some wings that I think would stabilize those posts some more. They are braced at the bottom 3rd. Does this help the issue?

Mike, what file extension is your analysis program compatible with? Would you be willing to plug it in?

Built these wings, for concealed space for band and engineers, electrical area, etc. Talked to my brother (sound engineer) about the acoustics.

"There should be no problem as far as slap(echo) because the open back(upstage) would minimize a lot of that. As far as monitor world is concerned it would be best to try and build a wing off of either side of the stage, space and money allowing. I'd line it up with the front of the stage(downstage). Amplifiers for the house PA are going to need somewhere to live and the wing would allow for that as well."

Mo - PM sent.

2d DXF file for import into MultiFrame. I can work with a DWG as well. The program is fairly particular - I draw lines on axis of the timbers, flatten it all down to be sure it is in 2d in the world UCS, then have hand work to break and sort and trim and extend lines to get the members to work to the program's logic. Doesn't take much, just some thought when setting it up. In HSB I have a function that spits out the axis lines for me - then I clean up the intersections.

Need to know your roof snow load and rough self weight of whatever you want for the decking and the roofing materials. Roof snow load can be taken as reductions for pitch, materials, and category of use... or we can ballpark it just to see how it comes out.

I can play around with it. No promises on when I can get it back to you... and the results... well they will be as good as any other free advice you get on an internet forum.

You will more than likely need a significant moment connection to the FND to make it all work with any sizeable wind loading. The wings certainly stiffen things up a bit... but this would make a great TF kite under the right conditions.

Hi Mo,

An article has just been published in The Royal Academy of Engineering's magazine - "Ingenia" re sound engineering for public places (e.g. concert buildings). This article can be seen and downloladed from :-

http://www.ingenia.org.uk/ingenia/articles.aspx?index=562

Regards

Ken Hume