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Opinions Wanted -What system should I use?

Started by D L Bahler, July 08, 2013, 03:32:58 PM

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D L Bahler

UPDATE
18 July 2013.
The Barn to be built will be a classic Bernese style timber frame with a fairly complex form of roof framing. It will also be the focus of instructional courses.

Anyway,

I'm building a barn. And I need some feedback. This barn will be built in a traditional Bernese Swiss style -but I'm looking for feedback on which one. Not because I can't figure it out myself, but because I hope to involve others in this project, and am trying to cultivate interest, knowledge, and admiration for these traditions.

Also, while this project is ongoing, I hope to host a number of workshops, hold demonstrations, and use it as an example for teaching -as well as a project for a book on one of these methods.

This barn is going to be either Blockbau, or log building in the tradition of the Bernese Alps, or timber framed according the the Lowland Bernese -or Berner Mitteland- customs.

One involves some impressive and heavy logwork, with straightforward structural design and simple roof geometry, with a gently sloping roof. The other involves timber framing with a fairly basic wall system, but some rather complex roof geometry involving the Liegender Dachstuhl roof system, and the 'Viertelwalmdach' or the half-hipped roof (yes, the German term literally means 'Quarter-hip roof'), all executed on a rather steep roof (12/12 or greater pitch)

So I'd like to hear from you -where is the most interest. What project would you all like to see?

Looking forward to responses.

D L Bahler

OK here are some pics to show the options...

First is a framed structure, as in the Berner Mitteland tradition. The drawing shows the roof structure. This is a fairly complicated system involving many joints designed to be in tension. As others have pointed out elsewhere, this type of frame can be compared to the Cruck, the scissor truss, and the hammerbeam, though it is not really analogous to any of these. The roof pitch of the example is very steep, around 14/12, which is typical of older examples of these buildings.

The second picture is an Emmental farm, any of the buildings shown can be seen as fine examples of this style of roof framing, aside from the 2 smallest outbuildings to the left, which are log built.

 




The NExt pictures are the blockbau, or log built, barn. The first pic is a conceptual drawing of the front elevation. The second pic is an example of the style.
 

 

Jim_Rogers

I'm not sure if you want to, but you could do one of each.....

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

D L Bahler

I could

But not right now.

So something has to be FIRST.

I'm not worried about building one and not the other, because both styles WILL get built, sooner or later.


Jim_Rogers

Do the easy one first. Then you'll have a good handle on how it will all work to do the hard one.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

D L Bahler

Thanks for the tip

I've done workshops before, I've also got experience with these styles of building.

As for overall difficulty, on a building of this size -22x30- there is not a whole lot of difference. On a larger building, the frame would certainly be easier to execute and raise.

Time wise, the frame wins again.



I've also been examining the truss sytem, trying to understand the load dynamics and figure out how it works. I know it does work, I've seen it work. But I want to understand the system to know WHY. Here's what I think is going on.

In this particular example, loads are born into the walls at 2 locations. First, there is the load at the top of the supports which, along with the weight of the support system itself, exerts force downward and, due to the fact that the supports are leaning, slightly outward at the plates. Now, the purlins bearing on these supports are fairly high up on the roof, and only bear a small part of the roof loads.
The second support is down level with the top of the wall, but cantilevered out away from the wall itself. Here there is another purlin, and the rafters rest on the purlin and pass over them, they are not joined or fastened. As a consequence, the rafters at this point actually generate an inward force. To understand this, hold a pencil at one end, let it rest against your finger at the bottom end and push it so that it is at an angle. Remove your finger, and observe where the pencil goes. It returns to the vertical, meaning it exerts an inward force against the purlin supporting it -in this case your finger.

The reason this 'truss' works is because of the interplay between the opposing forces. The tying members bear forces in opposing directions. In addition to this, the braces at the top of the support structure help to resist some spreading (in reality, their primary function is to brace against live and uneven loads from wind, snow, etc.) The lower bracing, which is upright, acts in compression, resisting the inward thrust b transferring it to an upward force up toward the top of the roof, where it is met and negated by the downward forces already present. This is how we can get away with not having a member tying both walls together across the width of the structure.

The braces on the outside of the structure, it can be observed, brace against 2 forces. First there is the obvious fact that the rafters want to push the cantilevered purlins down. Second, the inward force from the rafters pushes up on the posts on the inside of the roof structure, which has a resulting force that wants to drive the outside of the horizontal ties down once again.

The whole key to this, it must be noted, is the very steep angle of the roof. Were it a more gently sloping roof, this would not work at all.

Thehardway

D.L.

Without being presumptuous I would first say the only way to know  how it works, would be to do an extensive dynamic load analysis on each member in varied real world conditions.  I think the style of framing you show is very intriguing indeed and could be very deceiving. Likewise, it could prove dangerous to design and build something of which the mechanics and dynamics are not fully understood.  The Tacoma narrows bridge comes to mind.  I would like to see more detail of the un-joined rafter/purlin connections and the tension joints as well as some dimensions to give perspective.

I would propose that this design can be looked at like a giant balance or cantilever system.  The Swiss are well known for working with balance and counterbalance systems and it can be seen in their clockworks and their buildings.

This can be both a blessing and a curse.   Here is my analysis and it is merely a hypothesis. The horizontal beam at the plate which resembles a hammerbeam  for lack of the proper term, serves as a lever and the wall as a fulcrum.  Downward force of the rafters (weight) on this lever outside of the wall,  force the inside end upward and exert an inward force on the wall at the same time, through the bracket, preventing outward thrust on the wall.  As such, the joints you have marked as tension joints may be in either compression or tension depending upon roof loading from snow, wind, or other sources.  They collectively form an inward arch which prevents deflection of the long rafter spans but are not functioning as a direct load path for the roof.

How many of these have you designed and built and did you copy dimensions?  Is there a design rule used for placement of the hammerbeam on the wall and how long it must be? 

This reminds me of the Scottish Caber toss.  If the Caber is balanced, thrust is negated and weight is the only factor while running. At the same time the grasp must use both leverage and thrust to get the caber in motion and the hold changes from compression during the carry to tension and then thrust at the moment of release.  It is very dynamic and requires strength, skill and balance to properly execute.  If not, the caber falls flat.  Lets hope nothing falls flat ;D

This is much more interesting to me than the log building although others may feel differently.  Love those pictures.  Make me want to go on European tour.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

D L Bahler

Yes you are right, the whole design revolves around a system of balanced forces. If you try and isolate a part of the system to examine its function, you will just get confused and frustrated. The whole system works as a single unit, intricately redirecting and opposing its own forces.

The observation of the interior structure as an arch is good. Structurally, the Alemannic carpenters devised a system that is, basically, a wooden arch.

The proportions of this arch are the central aspect of roof design. Other dimensions are as a consequence of these proportions. Generally, the cross beam which we call the 'Zange' -i don't know the English term- is located vertically at a point either 1/3 or 1/4 the height of the roof. The former being an older way, the latter a bit more modern.

So, then, the locations of the 'Büge' or braces are determined as a result of the framework.

In most cases, the whole structure is simplified by having a tie beam for this truss, which is rarely a single piece of wood. Often it is 2 or more pieces, both pulling in opposite direction against a plate on a centrally located wall or line of posts. When this is done, the upright bracket is unnecessary, and is excluded from the design.

On these larger buildings, the truss as drawn is used where the framework exists beyond the gable wall, cantilevered out past the structure and, as a consequence, unable to support a tie beam mid-span.

I have a reworked design, which shows the truss with interrupted tie beams.

Add to the complexity the fact that, at the gable the truss is not used, instead the purlins are supported directly by posts. The truss design is intended for use on the interior of a structure where a large open space is needful. This is the case of the Bernese farmhouse, where the uppermost section under the roof is used as a driving floor and hay loft.

D L Bahler

After listening to your comments, I've decided to go with the frame for the first project.
Construction will begin as soon as the Board of Animal Health gives me the go ahead.
In Cass County Indiana, there are no permits or inspection for agricultural buildings  8)

The biggest factor was the point made that, this framing is a lot more technically interesting than the blockbau. Furthermore, I believe it is a lot more applicable and mass-consumable than a blockbau structure can be. That is, more people are likely to go with an Alemannic framed house than an Alpine log house.

Also add to that, I've already done a blockbau workshop, and have never done a workshop on this type of framing.

So now that that's settled, I'm working up the more complicated technical drawings...

D L Bahler

Here is a picture of a building very similar to what I am designing. The principal difference being, this is a 2 story and mine will be a single story building. Also, this building is designed to serve a slightly different function.



  

 

This building is a 'Küherstock' or Cowherd's cottage (i dialekt, Chüjerstöckli) date 1741, today in Afoltern im Emmental, formerly located in Waldhaus, Lützflüh.
The building is used as a cheese house -one room is where the cheese is made, the cellar is where it is aged, and the remaining structure is where the cheesemaker lived.

D L Bahler

Then there is this approach:



 

Look at that for a while, and tell my how and why it works.

Specifically, what prevents the cross beam under the ridge post from deflecting or failing under load. This is NOT a kind-post truss. It is a posted ridge beam

Satamax

Well, just a guess.

There's no other purlin than the ridge and plate. So all the weight of and on the rafters is pushing the plate downwards. Plate is pushing down and out the end of the tie beam, which is cantilevered. Then the tie beam being bent downwards outside the pivot point, pushes upwards in the center counteracting the downwards force of the center post. Equilibrium state of a triangle. It could even be done without the centerpost. Providing the timbers are stiff enough.
French CD4 sawmill. Latil TL 73. Self moving hydraulic crane. Iveco daily 4x4 lwb dead as of 06/2020. Replaced by a Brimont TL80 CSA.

grweldon

If you look very carefully, I believe you can actually see deflection in the crossbeam, so to answer your question, nothing keeps it from deflecting.  I don't have much experience in load/stress calculations so I have nothing to back up my opinion, but I think that is a very poor design for a truss.
My three favorite documents: The Holy Bible, The Declaration of Independence and The Constitution of the United States.

D L Bahler

I think Satamax has the right idea here, another example of the cantilever and balance approach of causing beams to do tings they wouldn't be able to do otherwise.

As for the deflection of the beam, I'll say 2 things.

First this structure is likely very old. It's located in the Ballenberg Museum, which houses old buildings taken down from all over Switzerland and reassembled to preserve structures in their past states. It's a wonderful place, it's like the kind of thing a timber framer would have dreams about, or the place you'd go to in a timberframer's fairy tale. Especially since it's nestled on a little hill in the forest between 2 huge mountains...so anyway, back on topic, I'm pretty sure this building is rather old, it's a pavilion from an old guesthouse/inn that was put up at the Museum, and may be a couple centuries old. With that in mind, yes slight deflection can be expected to to extreme age and it has also proven itself as a viable truss design, due to it's long service life.

second, the apparent deflection may well be a photo illusion, or just a regular old optical illusion. visually, the design of the system is going to create the appearance in the brain that the beam arches downward, due to the angle of the rafters and the vertical post coming at it's mid point. The only way we can know for sure is to go there and run a string along the beam.

What is interesting in this example is that the truss design on the front is a true truss in this example, it is a single rafter pair stiffened by the trussing members, not a rigid support for purlins

Thehardway

Another brain teaser.  I'm with Satamax  my guess is a variant on a cantilever design. It balances the load to eliminate thrust and then centers it on the plate.

In most design and architecture, especially in the agricultural sector, "form follows function".  I'm a little confused in this regard because I don't know how this design element enhances function.

First glance would say the truss portion gives you a clear span and eliminates central posts. That goes out the window though when you see whats behind it with the tie-beam and ridge post.  This is a mystery. knowing more about how the building functioned and the daily activities would give better insite on the how and why of the design.

It's pretty to look at but for loading hay, equipment, or storage it functionally appears to be a bit messed up.   
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

D L Bahler

This is reminiscent of the shelters built to cover ovens in the Berner Mitteland region. A structure of this size would likely have covered a large oven built for the drying of fruits or cereal grains. This was once an important economic activity in the region. Fruits would be dried up so they could be stored for the next year.

So then, we can start to see how form follows function. The oven shelter is open underneath, where once a very large stone structure might once have stood.

D L Bahler

Ok as promised, here are some drawings, detailing the construction of the planned building.

While I wait for approval from the BOAH, i might slap together a small model frame for demonstration purposes.



  

  

  

  

  

 

Unfortunately, these were all too large for my scanner, and had to be photographed. So there might be some camera-related distortion if you try to measure things.

The photos are:
1 The support structure located in the middle of the stall area. Note that there is no post corresponding directly to this 'slice' on the right hand side, there is an open bay at this point

2 The middle partitions, one dividing the stall and hallway, the other dividing the hallway from the 3 rooms seen on the right of the floor plan (6)

3 The front (north, in this case) Gable. Note the open bay at the right, and the largely open loft space.

4 Side view, showing the west wall, and the wall of the bay.

5 Floor diagram, showing all of the floor supporting timbers, all timbers in this diagram are in a single plane.

6 Floor plan.

The front (north) facade will be half timbered, all other sides will be board clad.

D L Bahler

The project is now officially opened up for workshops beginning in August. Let me know if interested.

https://forestryforum.com/board/index.php/topic,67786.new.html#new

D L Bahler


Satamax

French CD4 sawmill. Latil TL 73. Self moving hydraulic crane. Iveco daily 4x4 lwb dead as of 06/2020. Replaced by a Brimont TL80 CSA.

D L Bahler

Zweifach Liegender Binder mit Kniestock und Hängepfosten

This is what we would call this truss, that you have shown in your post. Don't ask me to translate it to English though! ;)

I have a picture very similar of a 'modern' type of truss system used in the Bernese Alps.



 

It is interesting the similarities across the greater Alpine region. No doubt, we Alpine people are all distant kin, in one way or another  ;D
But, you still do it different in some ways.

Some time, you should make a trip over to Canton Bern and see the old farmhouses. I see in your post, you are interested in making a 'fake cruck' Well some have said that is precisely what the Bernese roof frame is, although its origin is not quite the same as the cruck. It never was made of curved 'blades' in Bern, the idea here rather was to take the king post used to support a high ridge beam, and replace it with 2 reclining posts so that the attic space could be completely open, because these are farm houses and they need place to store a lot of hay and equipment like wagons and carts and tools and such.

In Bern, they developed this so they could drive the hay cart right into the building and throw the hay out into the loft. Easy way to do it.


In Bern, they took what you call the 'Blochet' or maybe a stub tie, in German it would be Bundbalke, and use it similar. The difference is, the 'Jambe de force' or Binderstrebe as we call it rests above the wall directly, instead of passing through the stub tie. Then the rafters sit on a Pfette or purlin out several feet past the wall, applying leverage against the stub tie, which goes through the tie and up into a post that ties the Binderstrebe and stub tie together, which then balances the forces.

Where the Bundbalken goes all the way from one wall to the other, it serves as a true tie beam, and the system is simpler. This is what the last picture shows.

Here is how it is without a continuous tie:



 

What they did here was to find a way to balance the system, so that instead of tying the two walls together to prevent thrust they just deal with the thrust in each wall independently. A continuous tie works, because you have two opposing forces in it from opposite sides of the roof. The stub tie in the Bernese truss works because you have two opposing forces at each wall.

But even so, you still will almost always tie the sides together, because you need to have a floor in there somewhere.


D L Bahler

Another update on the workshop.

https://forestryforum.com/board/index.php/topic,67786.new.html#new

Also edited the OP of this thread to keep up with current status of things...

mesquite buckeye

Dumb question from someone who never built a timber frame. How do wind loads in Switzerland compare to the American midwest? I'm thinking they are less, but have no experience to confirm this.
Manage 80 acre tree farm in central Missouri and Mesquite timber and about a gozillion saguaros in Arizona.

D L Bahler

Have you ever been in the mountains?

Do you know what the wind can do in the mountains?

The only thing we have that is worse is a Tornado.

These roofs actually have a pretty complex system of wind bracing tying all of the rafters together. Long braces stretch from peak to sill, this is called Windrispe

Then all of the purlin connections are reinforced with angled braces, known as Klauenbüge. These and the 'Jadgbüge' that ties the truss system together are often designed to act in both tension and compression, in order to act against dynamic and unpredictable wind loads.

Then on the front of the Bernese house, you have what is called the 'Ründi' here is a wikipedia article on it:

http://de.wikipedia.org/wiki/R%C3%BCnde

this is a round shape under the roof. The purpose of it is to prevent a strong wind from coming under the roof and tearing it off, and at the same time to close out drafts in the living area during the long Alpine winter. So you see, protecting from the wind is a very important concern.


D L Bahler

Also in the midlands to the north of the Alps, we have a number of long, narrow valleys with fairly flat bottoms. These run mostly north to south, and can become like giant wind tunnels during certain times of the year. So you see then why wind bracing and protection from a heavy wind is important.

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