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