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Author Topic: Swiss and German Roof Framing  (Read 842 times)

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Offline D L Bahler

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Re: Swiss and German Roof Framing
« Reply #20 on: March 24, 2020, 08:12:39 PM »
Taking some inspiration from another post,

There is a potential disadvantage to the Swiss/German methods of roof framing. Notably, the roof systems are typically entirely independent of the wall framing. 

When faced with wind loads, this can present potential problems. First, lets envision the structure with high lateral loads from winds. In Switzerland this is a serious concern, as they have the Fhn, which is a strong wind that comes down off the Alps, and will destroy underbuilt structures. This is one of the biggest reasons why Swiss roofs are so massively overbuilt, also combine this with this weather system's ability to dump several meters of snow at a time. (Why do you think the Swiss Alps are so famous for their skiing?) 





With a roof system just placed on top of the wall frames, the walls then bear the brunt of the wind load force. This is a pin connection, with no abilty to resist the lateral force at the pace where roof and walls meet. An exaggerated depiction of this dynamic looks like this:

 



But if we brace the roof to the walls in some way, or engineer some sort of rigid connection, the walls will better absorb the forces applied by these live loads.

 

 

So there are two types of failure we might encounter. With a pin connection, the high wind loads might just push the building over. Especially since traditional means of building have no connection between the frame and the foundation at all.
If there is a rigid or braced connection, the failure would come by bending the posts. 

In the case of Churches and Castles, the walls are typically a meter or more of solid stone. This thickness is there primarily (in the case of churches at least) the make the walls rigid so that live loads on the walls and roof don't break them. 

But what about wooden structures? Typically large Swiss buildings will have interior walls that brace the middle of the frames. 
But if they didn't, what might happen? Would the bracing in the rafter plane (pictured below) be sufficient to stiffen the structure and distribute the load forces to the braced gable walls? 


 

Offline D L Bahler

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Re: Swiss and German Roof Framing
« Reply #21 on: March 24, 2020, 08:56:22 PM »
One solution to this question is to include a horizontal network of bracing in the roof system, like below:


 

There are a number of different profiles this bracing might take. 
I can't say whether or not it's used before this, but this appears on buildings from the late 1700's and through the 1800's right up until modern times. This happens to be the point when the buildings constructed with this sort of roof system were the largest. Before this, this sort of bracing probably wasn't necessary.

Offline Don P

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Re: Swiss and German Roof Framing
« Reply #22 on: March 24, 2020, 09:16:11 PM »
Edit;
Ah, I was typing slow, there's the braced plane, way cool :)

Typically in modern conventional construction the roof and perhaps ceiling sheathing create a semi rigid diaphragm, a "plate". This fairly rigid plate then uniformly distributes the lateral load to the braced walls, typically no more than 25' apart, then to the floor diaphragm and into the continuously braced foundation, that's pretty much the current prescriptive lateral scenario. Engineered solutions can be more exotic.

In your sketch I see much bracing in both directions into heavier members that act as collectors. There is going to be a limit as to how far apart interior bracing wall lines can be safely placed, but the same thing is being accomplished here it looks like, show me what you are seeing.

Backing up and way aside. Someone made a comment about being surprised at the early use of iron fasteners, or, that was the journey I went on. I remembered nails and Romans from a couple of thousand years ago and had the day yesterday to read a book I've meant to for some years. DL had mentioned at some point that the Swiss had not "lost" roman information in the early Middle Ages the way much of western Europe had. This particular book was safely stored in a Swiss monastery, "we" rediscovered it at the dawn of the renaissance, so it was lost to western Europeans for about a thousand years. It inspired many men of that time who's names we all know. It was written by the Roman architect Vitruvius. I'd like to point to something in there, a reference to bolting together a trispast, a hoisting machine. Second sentence under "hoisting machines" here;
https://www.gutenberg.org/files/20239/20239-h/20239-h.htm#Page_285
This was a reference to structurally bolting something together about a hundred years before Christ. They have found a nut from around this time which is in a museum in Germany. Just like at my house they can't find the bolt. Many references to nailing various things together. Sorry for the side track, back to where we were.

A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline D L Bahler

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Re: Swiss and German Roof Framing
« Reply #23 on: March 24, 2020, 11:19:12 PM »
Looking at the roof system without the braced deck, we have this:
First a side view:


 
And a top view:


 
In German this is "Windrispen" which is more or less, Wind Bracing. It is designed with resisting lateral loads in mind, in both horizontal planes.
The "windrispen" in a "Liegender Stuhl" roof in Switzerland generally has this large X-shaped profile as seen here, and the braces themselves are designed to work in both compression and tension (so, basically, the braces are bigger than they need to be, with a strong enough dovetail lap to act well in tension, and still have enough of a shoulder to act in compression) 
In Germany you might instead see "Kopfband" braces -basically knee braces- that are set in plane with the supports. These are only useful in bracing against wind loading on the gables. 

The Library of Sankt Gallen is a pretty fascinating place. If I recall it's the world's largest collection of ancient and medieval books. Also more or less the birthplace of harmony in Western music, presumably under the influence of the Alpine yodel. It has German-language Bibles that predate Martin Luther by centuries. 

We've talked about old trusses before. It's possible the oldest truss in existence is actually in Rome, the support framing for the roof of the portico on the Pantheon. It could be a queen post truss, but it's not entirely clear. 
From what I've been able to learn through my attempt to study it, this support structure is made of hollow beams of hammered bronze with heavy bronze reinforcing straps. 

It's certainly true that Roman truss designs were retained in parts of Europe. Italian craftsmen definitely held on to the technology, and of course we shouldn't forget the Byzantines. I think you will find that some knowledge of the truss was retained in Italy, Spain, Greece, Switzerland, Austria, and even into parts of France (like Provence and Burgundy) and southern Germany. I think the real issue is that two distinctly different fields of carpentry departed from each other rapidly in the Middle Ages, before the "high" architecture of the (mostly Italian) masters eventually merged into the vernacular traditions near the end of the Middle Ages, and the English and Germans then replacing all of it with mathematically designed trusses in the 19th century. Anything post Bismark is heavily industrialized, and it might be fair to say that his modernization campaign dealt the death blow to traditional German carpentry. 

The Burgundian architecture in what's now Switzerland was definitely built under the direction of Lombard craftsmen -the Burgundian Kings looked to Italy instead of France as the center of their cultural and religious life, with Lombardy being their immediate neighbor and Milan being the most important city of the day. The French kings were their rivals, and when the Burgundian kings died out they ceded their lands to the Holy Roman Empire rather than to France. (which is why I don't speak French today, my family being from the part of Switzerland that was formerly Upper Burgunday) A lot of fairly low-sloped kings post roofs that were relatively narrow -the Lombard style church has a high nave, and two side aisles with separate shed roofs, 3 apses. Something like the Spiez church is a stereotypical Lombard design. This is in stark contrast to the more German style, which typically has a single roof, that you see in eastern Switzerland. I think it's fair to say that these more German style church roofs don't display the same connection to Roman trusses as their Lombard counterparts. The Fraumnster and Grossmnster churches in Zurich are both good examples of Romanesque German style architecture from just a little later. 

Offline Tom King

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Re: Swiss and German Roof Framing
« Reply #24 on: March 25, 2020, 04:31:28 PM »
Thanks so much for this thread.   It goes on my One of the Best Ever list.

Offline D L Bahler

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Re: Swiss and German Roof Framing
« Reply #25 on: March 25, 2020, 11:19:58 PM »
I thought I'd put some pictures of real world examples to illustrate some of these concepts.

First, here is a structure with a very large roof with a large unsupported space on the inside, so the joist plane is braced to resist wind loads perpendicular to the eaves. In this particular example, the "joists" are stub joists and don't go much into the interior, so as a result the bracing is as we see here, entirely on the cantilevered exterior (this wide soffit is typical)


 

I've mentioned a "verzahnte balken" a few times. Here's an example, in this case used to make an arch for a free-span bridge, but the same principle can be used for wide span beams as well.



 

Next, for some perspective here are some of the farmhouses like we've talked about, where a "liegender stuhl" would be the typical roof support:


 


 
Since we've also talked about Swiss/German bridge trusses, here are some:


 
This first bridge is the sort of complex polygonal design that's fairly typical. Next, the massive kings posts of the Neubrgg north of Bern, built in the 1500's


 

 

 

 
And finally, a few church buildings so that we can get an idea of the sort of structures we're talking about:
First, the Bern Mnster, with a high nave and side aisles with shed roofs:


 
Next, the late Romanesque Zrich Grussmnster, with a large single roof:


 
and finally, a small retangular church more typical of the German style, in contrast to the "basilica" style of the Lombard churches that we've reviewed:


 


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