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Heavy timber truss.

Started by Dave Shepard, February 15, 2013, 08:11:08 PM

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Dave Shepard

I have been pondering heavy timber trusses lately. Wondering what the limits of a king post truss with two struts in 12"x12" white pine or hemlock would be. Thinking of building a workshop with these trusses and perhaps a common purlin roof system, or short "ridge" beams between the king posts and common rafters between the trusses. Thoughts?
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Rooster

Dave,

What kind of width and roof pitch would you like to shoot for?  What does the 12 x 12 parameter represent? How long is your longest available beam?

Got my thinkin' cap on too tight!!!   smiley_mad_crazy

Rooster
"We talk about creating millions of "shovel ready" jobs, for a society that doesn't really encourage anybody to pick up a shovel." 
Mike Rowe

"Old barns are a reminder of when I was young,
       and new barns are a reminder that I am not so young."
                          Rooster

Rooster

Dave,

How about a 32 ft x 57 ft?

Here I have it drawn with rafters...



 



 

And with out rafters (to show the truss, purlin and ridge framing).



 

The 8x8 purlin is supported by a "stub" purlin post.  The purlin post has a tenon on the bottom that passes through the top of truss and passes by the tenon on top of the truss strut.

Well...at least I think it does.   ::)

Rooster

 

"We talk about creating millions of "shovel ready" jobs, for a society that doesn't really encourage anybody to pick up a shovel." 
Mike Rowe

"Old barns are a reminder of when I was young,
       and new barns are a reminder that I am not so young."
                          Rooster

Jay C. White Cloud

Hello Dave S.,

What Rooster just showed you is...for lack of better words and details...the "boiler plate," that you would design from for a "king post truss,"  assembly.  The king posts are usually pass through with wedge for greater spans, the purlin posts are normally a bracket type assembly, not an oblique post configuration.  However for a boiler plate design I would have drawn it pretty much the same way Rooster did. 

You have some pretty serious engineering to account for, and some fun joints to cut, but depending on your budget, (time and/or money,) you could span 40' or more.

Good luck,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

Dave Shepard

This is sort of a theoretical exercise at this point, but it is something I've always wondered about. Let's stay within the 40' span for now, although I do wonder about longer spans. I have access to timber and a mill capable of cutting over 40'. I have been thinking about a 40'x80 building, half woodworking/timber shop and half mechanical shop for the farm. Roof pitch is not critical, whatever makes the most sense engineering wise. Walls would not have to be timbered, they could just be rough 2"x8" for a big insulation cavity.
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jueston

I am no timber framer, but it seems to me if the limitation is the load on the bottom of the truss, then you could turn the bottom truss into a truss of 2 beams tied together and accomplish a huge span....

anyways, I'm sure an expert will either say I'm way off or confirm that its possible, or that it might just be worth making the beam bigger.



I realize this is not the traditional king post truss you were looking for, but just what popped into my head when you said you wanted a long plan.

Dave Shepard

I've seen doubled up tie-beams similar to that, but not in a truss. Jim_Rogers has photos of a doubled beam with bracing in it for a long span on a sawmill shed that I like.

I am not partial to any particular design, I just thought that a King post was a good place to start the discussion. We can discuss any kind of timber trusses here. It's a good winter topic to talk about while we're all waiting for the snow to go away. (At least up here anyway).

The 12"x12" dimension again was just a starting place. We could go with bigger parts, or thicken that bottom chord up a bit if we needed to. A 12"x16"x40' timber wouldn't be out of the question here.
Wood-Mizer LT40HDD51-WR Wireless, Kubota L48, Honda Rincon 650, TJ208 G-S, and a 60"LogRite!

Rooster

 

  


  

  

 

Am I getting warmer?   ;)
"We talk about creating millions of "shovel ready" jobs, for a society that doesn't really encourage anybody to pick up a shovel." 
Mike Rowe

"Old barns are a reminder of when I was young,
       and new barns are a reminder that I am not so young."
                          Rooster

jueston

i have seen the keyed beams, but somehow I've always imagined a truss being stronger then keyed beams built from the same size beams. any one with some engineering know how want to confirm or deny that?

and since the model was open when I returned from my jog around the lake waiting for me, I also finished the model...


Jay C. White Cloud

Jueston,

Son, you have a good head on your shoulders!!  When the Creator was hand'n out common sense, it appears you got in line twice.  I do like you natural abilities to see timber wrighting solutions...good job.  Little tweaking here and there, and we will make a timber wright out of you yet.  ;)

Rooster,

What can I say...with you and Jueston beside me, I could span a 100', and it would look good too!

Nothing else for me to do, but read, watch and enjoy the conversation!

Regards,

jay
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

swampfox

I love heavy timber trusses. 

Another option instead of terminating your bottom chord at the post with a joint is instead to run it over the post. This can solve two potential problems:

1.  You create more beam relish at the top chord (principal rafter) bottom chord joint.  Lots of force at that joint.

2.  You will have significantly more bearing area with the bottom chord running over the post.  With the bottom chord housing into the post, that housing is carrying a significant load.  Say you house it 1.5" that housing bearing area would have to carry the entire load of the truss, ridge, and a good portion of the overall roof square footage.


D L Bahler

Are you determined to use a king-post truss? One of the best truss systems I have ever seen is the Central European 'Liegender Binder' or Liegender Dachstuhl, in one specific variation found across the Aare River valley on the Swiss Plateau.

Some speculate this as a derivative from some central European form of the Cruck.

This roof is used all over the Canton of Bern to support enormous roofs over huge spans without any mid-span support, and even without any bottom chord such as a tie beam in some cases. These are enormous roofs, with heavy slopes (12/12 and greater) and snow catchers in order to retain large amounts of snow on the roof (read: tremendous weight). In typical Swiss fashion, they are engineered beyond what they will ever have to take.

I can upload some pictures some time if you are interested.

Dave Shepard

No, I'm not set on a Kingpost, that was just a jumping off point for the discussion. I'd like to see some photos of the Swiss trusses. :)
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S.Hyland

Jeuston,
   I like the double bottom chord king-post truss! That is what I would call an elegant solution. Simple yet effective. I wonder what kind of spans one could accomplish with a design like that?
I second the motion for pics of Swiss trusses! This promises to be a good thread...
"It may be that when we no longer know which way to go that we have come to our real journey. The mind that is not baffled is not employed. The impeded stream is the one that sings."
― Wendell Berry

jueston

I'm no engineer, just a guy who reads some books and plays around in sketchup, so I too am curious what kind of span could be accomplished, I imagine it would be substantial. I don't know how to run the numbers on a truss like that to know how much weight it could take or what kind of span you could get.

of the top of my head the longest span on a roof truss I have read about was 60 feet by Vermont timber works and that was a scissor truss, but I'm sure if I did some research looking into bridges I might find longer.

jueston

I know jay will enjoy this --

while doing research on long span timber framed bridges I found some architectural papers [which is like trying to read a phone book for me] about Chinese rainbow bridge structures. while it's not currently standing there is a historical account of one that spanned 40m (120feet)....

after the large timbers in china started to dry up around 1900 most of the skill that was required to make these incredible bridges was lost. now some architects are studying the remaining ones trying to fully understand the methods used to span such distances.

and a link for those who want to read about it -- http://www.pinehillpark.org/php/Arch_Bridge_files/Timber%20Arch%20Bridges%20in%20China.pdf

even if your not up for the read, there are some pretty pictures :)

Rooster

 

 
I want to use jueston's drawing to propose some questions and comments, even though the diagonals don't intersect the lowercord or top cord of the multiple kingpost truss :

I my opinion, a true kingpost is not supported by the lower-cord, but is suspended from the diagonals and helps carry some of the mid-span load of the lower-cord which is in tension between the points of contact (joints) of the diagonals as they intersect the lower-cord. Jueston's drawing makes it seem that the lower-cord truss (multiple king-post) is supporting the king post which in turn is supporting the top of the diagonal/principle rafters. So, did he draw a king-post truss on top of a multiple king-post truss?...or do we have just a post resting on top of a multiple king-post truss that is also supporting rafters/diagonals?
At certain pitch levels, we should be able to remove the jueston's "king-post", because the multiple king post truss will span the distance and carry it's own weight, and the rafters can rest or lean against each other as long as they don't slip out of their joists at the top cord of the multiple king-post truss.

I use the theory behind the multiple king-post truss as an example:
The truss load is transferred from the mid-span of the truss from the top of the center king-post, down the strut/diagonal to the lower-cord or bottom of the next king-post away from the center of the truss...and so on.  The posts are not holding up the struts/diagonals, but the struts/diagonals are holding up the posts, which in turn are each connected to the lower-cord.

Does anyone have any thoughts on this debate?

Thanks,

Rooster
"We talk about creating millions of "shovel ready" jobs, for a society that doesn't really encourage anybody to pick up a shovel." 
Mike Rowe

"Old barns are a reminder of when I was young,
       and new barns are a reminder that I am not so young."
                          Rooster

D L Bahler

Ok, here are some pictures I took this past summer during a trip to Switzerland.

First, a farmhouse from the Emmental region of the Canton of Bern.

With a disclaimer, this is considered to be a small example of this particular building type, believe it or not.



  

  

  

  

  

 

The first picture is the exterior of the farmhouse, This is the house of the Heiniger family near Sumiswald, where I stayed a couple of days while travelling in the Emmental. It was originally the servant's house on a large, typical Emmental farm. Hence the smallness of it.

The following pictures are details of the roof structure as seen in the top level of the structure which serves as a hay mow and machinery shed. The space below is divided between a cattle stall and house. Typically, these are dairy farms.

The next photo is from a farm somewhere near Sumiswald. This is a very typical Emmental and Bernese farm in General. Keep in mind that the large house shown has no intermediate support within the top level to support the roof, only the large, open truss system like shown above.



 

And the following pictures are from a small Stöckli at the cheese museum in the village of Affoltern. These are exterior pictures, but should help to show how the roof structure fits in with everything.



  

  

 

And the last set of pictures is from the old Medieval bridges in the city of Thun. These bridges also double as floodgates to control the level of Lake Thun and to hold off the floods which used to ravage the Swiss Plateau to the north.



  

  

  

  

  

  

 


If there is any interest in some real massive heavy timber truss systems, I should dig up the unbelievable Framing of the Castle of Thun, some of which dates back as far as the 13th century, the incredible roof framing dating from the 15th century.

D L Bahler

I will explain also one more thing,

One particular of the Emmental style is the double roof pitch. How this works is that the top portion of the roof follows the pitch of the truss, which is very steep, but the bottom portion is much flatter. However, in other Regions this is not the style. For example, in the Gürbetal region where my own family is from, the roof is a single pitch, steep but not as steep as the angle of the truss.

The principal behind this is that the trusses support purlins that are well above center. Often time there will be no framing above the cross member which sets just under the purlins in many cases. However, the Emmental houses shown do have framing above the cross member, which supports a ridge beam. This is not typical across the Canton, but is a regional variation.

This truss type is almost universal across the larger region, but the specifics of how it is done are incredibly variable depending on locality.

Also, these buildings were developed with steep roofs because they were originally thatched. The tile roofing that today dominates is a fairly recent development.

jueston

Quote from: Rooster on February 17, 2013, 11:11:22 PM


 
I want to use jueston's drawing to propose some questions and comments, even though the diagonals don't intersect the lowercord or top cord of the multiple kingpost truss :

I my opinion, a true kingpost is not supported by the lower-cord, but is suspended from the diagonals and helps carry some of the mid-span load of the lower-cord which is in tension between the points of contact (joints) of the diagonals as they intersect the lower-cord. Jueston's drawing makes it seem that the lower-cord truss (multiple king-post) is supporting the king post which in turn is supporting the top of the diagonal/principle rafters. So, did he draw a king-post truss on top of a multiple king-post truss?...or do we have just a post resting on top of a multiple king-post truss that is also supporting rafters/diagonals?
At certain pitch levels, we should be able to remove the jueston's "king-post", because the multiple king post truss will span the distance and carry it's own weight, and the rafters can rest or lean against each other as long as they don't slip out of their joists at the top cord of the multiple king-post truss.

I use the theory behind the multiple king-post truss as an example:
The truss load is transferred from the mid-span of the truss from the top of the center king-post, down the strut/diagonal to the lower-cord or bottom of the next king-post away from the center of the truss...and so on.  The posts are not holding up the struts/diagonals, but the struts/diagonals are holding up the posts, which in turn are each connected to the lower-cord.

Does anyone have any thoughts on this debate?

Thanks,

Rooster

I had never looked at a king post like that, but after reading your post, I see it in a whole new way.
in my truss I had thought of the load being primarily carried by the lower truss, and in many ways defeating the point of the king post design, since the struts do not [by my intent] act to support the king post, but only to avoid deflection in the rafters, which would only imagine happening in a principle rafter and purlin design, not a design like the sketchup drawings you posted at the start of this adventure.

you have opened my eyes to the engineering complexity of the king post truss rooster.... and just when I thought I had it figured out.  ;)


jueston

and if the upper king post truss is not bearing a load, you could dispatch with it and go with a very asian/european hybrid design like this



if you imagine the truss being a log in its natural form you have a traditional japanesse roof system.

Jay C. White Cloud

Gosh, I'm just all giddy, what a great conversation!!

Jueston,

I loved that paper when it came out, I think in 2005 or 2007,  it was a great read.  I have been reading and following there work as well as Dr. R. Knapp's for about 20 years.  Great stuff.

Rooster,

It's past mid night, I read it fast, but...from what I read you are seeing the way a King post assembly works perfectly.  The tie does not hold up the king post, the king post holds up the tie, which is in tension from the two obliques and that is the way of it in most King, Queen, and related truss assemblies...I start to blabber...

Hey D.L. Bahler,

Wow, thanks for sharing so much.  I haven't forgotten our other conversation, just want to make it worthy of your attention. (not to mention I keep editing it to make it shorter...way too longer for a post, but I'm trying.)  If I had to make an observation on true regional specialties it would be the "Truss," work of Europe.  They are the true masters of "oblique," member application.  Particularly your ancestries applications and advancement of them.

Regards,

jay

"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

swampfox

Rooster,

That's how I understand it to.  In the battle of terminology a king post truss needs to have all the forces neutralized to a certain extent where what you are left with is just a downward load (excluding wind) with where the truss is supported.

The Kingpost is in tension
The Principal Rafters (top chord) are in compression
The Bottom chord is in tension resulting from the rafters feet thrusting on each end
The struts are in compression by constraining the deflection of the rafters and thereby putting more tension on the kingpost as well.

You can illlustrate it with something as simple as two playing cards leaning against each other on a table on top of a piece of paper.  Push down on the convergence at the apex and they crash.  Now glue the bottom side of the cards to the piece of paper with the same set up and push down on the apex.  Much stronger.  The piece of paper is your bottom chord and goes into tension.  The triangle is the strongest geometric shape.

Here is one that I will be building soon.  This trussing is really the four primary members, the secondary members inside are doing a little work from the mid-run purlin loads.


Jay C. White Cloud

Hey S.Fox,

You got it, and from there it only can get better!  That is why a Covered Bridge can span so far.  Excellent discussion!! 8)
"To posses an open mind, is to hold a key to many doors, and the ability to created doors where there were none before."

"When it is all said and done, they will have said they did it themselves."-teams response under a good leader.

D L Bahler

In the spirit of heavy timber construction and trusses, here is the heaviest truss system I have ever laid eyes on, the incredible framing of Schloss Thun, a castle originally built by the Normans (hence the norman look) when the Burgundians controlled this part of Switzerland.



 



   

   

 

 

 





 



   

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