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#33589 - 03/08/16 11:33 AM Re: Double-Wall Enclosure Concept [Re: Jay White Cloud]
TCB Offline
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Registered: 02/10/16
Posts: 26
Loc: Texas Hill Country
"As for "floor truss" systems, yes those have merit in some areas and application...though not typical for most domestic timber frame needs/requirements of load..."
Since I have a 'round' floorplan, and at present do not intend on having strategically-placed lower walls (just two columns near the center) I thought it might be desirous for strength reasons --granted I still need to run the numbers, but the 15-20ft span seemed a bit large unless giant joists are used, if I am to have a rigid floor surface. It is interesting that dual floors/truss floors aren't more common, though, since they do much of the same types of jobs as a roof.

TCB

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#33590 - 03/08/16 11:49 AM Re: Double-Wall Enclosure Concept [Re: D L Bahler]
TCB Offline
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Loc: Texas Hill Country
"I've been meaning to respond but I've been very busy."
Same here, I figure it's just that time of the year (I was getting a bit worried that there had only been like three posts across the entire site for about a week, though :))

"My first observation is, why worry about making the inner framework in your walls structural? This seems to me to be creating a lot of unnecessary joinery without a whole lot of benefit to the structure. My approach would be simply to make an inner curtain wall and let the outer wall do everything structurally."
When I went to look at designs for floor/wall/second floor joint layouts (each post spans only a single story to keep costs down), it struck me that the column assembly is significantly weakened by the presence of the cuts for the floor joists, as well as their off-axis loading. By splitting the column into two layers, the inner layer can be carved up to support the floor joists, leaving a simpler and more uniform outer spliced column element. Since the structure is only two floors, the second layer of (more weakly jointed) interior posts would support the less structural internal roof rafters, while the outer column would bear the brunt of the roof loading and deliver it straight down (stabilized by cross ties with the interior layer).

Basically, it seemed like a convenient way to split up the loads applied to vertical support posts in order to simplify the engineering side of things, if not assembly/construction. Since I plan on having a generous walkway cantilevered on the exterior opposite the interior floor joists (i.e. joists on both sides of the wall in some form) it may not be worth the trouble. I do still like the idea of no more than two or three timbers intersecting at any point, though.

If I can find a diagram of a wall joint layout where;
-Two columns are stacked/tied together along Z
-Two floor plates tie in along Y
-Two floor joists tie in along X
pretty much like an interior frame junction, only the result doesn't remove so much material from the members that another row of supports (i.e. an exterior wall frame) are not required to stabilize. Rather demanding task, I know.

"You are right that such a system could be more efficient from a materials standpoint. However, I would guess it's a lot more work to do and far less efficient from a labor standpoint."
I agree completely, and if it is not needed (see above comment about the spans involved) I'd just as soon not go this route. However, the complexity, I hope, is less of an issue than it might be, since my floor plan is basically a lofted arrangement, with only half the upper floor, floored (the remaining half of the octagon is an open cavity). So if it's twice the trouble, it's still only half the square footage. Or something.

TCB

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#33592 - 03/10/16 10:29 PM Re: Double-Wall Enclosure Concept [Re: TCB]
TCB Offline
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Registered: 02/10/16
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Loc: Texas Hill Country
Another question I had, about the Riegelbau framing concept; is there any particular rule about exactly where the inclined timbers should fall in a given wall/bent (center, edges), or which direction (inclined toward center, or toward ends)? It seems mostly important that they be balanced/braced against each other on each face of the building, but are there any off-axis loading considerations I should be careful of?

TCB

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#33594 - 03/16/16 07:06 PM Re: Double-Wall Enclosure Concept [Re: TCB]
D L Bahler Offline

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Posts: 946
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Regarding bracing,

The only 'Rule' is that braces be balanced, as in each brace leaning in one direction be counteracted by one leaning in the opposite direction.

Some people apply rules about there being a brace every X amount of meters, things like that, but there is no hard set universal rule about this.

Most 'rules' about bracing are regional characteristics and local architectural preference more than anything else. Some regions, for example, like to place the braces in the first cavity in from a corner or wall junction. Some like to put them in the second cavity. Some like to first lay out where all the windows and doors will be, and then put the braces wherever they will fit.
Sometimes braces are completely absent from exterior walls due to windows and doors taking up too much space, and the structure is stabilized by bracing in the interior walls and the stiffness of all the cross timbers involved in framing windows.
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#33596 - 03/17/16 01:09 AM Re: Double-Wall Enclosure Concept [Re: TCB]
Jay White Cloud Offline
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Registered: 11/06/08
Posts: 479
Loc: Vermont
If following strict a Riegelbau framing modalities, or other historic formats, then each region (as David has suggested) has its individual predilections. If being fully historically accurate, then one could follow the given regional modality as a guide.

Originally Posted By: TCB
It seems mostly important that they be balanced/braced against each other on each face of the building, but are there any off-axis loading considerations I should be careful of?


Yes..."compression resistance" in "oblique bracing" works better in a balanced format. Further, "Oblique Bracing" tends to "work" more effectively when larger in size of the triangle it forms, there by not acting as a fulcrum against the corresponding joinery. Also, "oblique bracing" modalities also function better in the "buttressing context" (i.e. at the base of a post) and can also work very well in the "horizontal" configuration within floor and roof systems as well. (i.e. "strong back" and "dragon beam" systems)

If this structures is not an example of historic replication, then "bracing" can take on many different formats that may be not only applicable but more easy to facilitate.

"Bracing" does not have to be "oblique" in nature to be functional and/or enduring. I seldom use "oblique bracing" systems in structures I design or facilitate. Asian (and some European) modalities also rely heavily on "horizontal bracing systems" and/or "infill systems" to give the structure the requisite "resistance" against racking that it needs to be enduring...Horizontal systems typically are less difficult to facilitate/build. Additionally, horizontal systems of bracing tend to never interfere with fenestrations, ingress/egress logistics within the frame and other consideration that oblique bracing can often obstruct and incomber.
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#33597 - 03/17/16 02:50 PM Re: Double-Wall Enclosure Concept [Re: TCB]
D L Bahler Offline

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Jay, the system we are discussing in regards to Riegelbau is not oblique or triangular bracing. It is s different concept that functions on different (although related) principles

The theory behind the long slanted bracing in German building styles is that a brace to a post compromises its integrity, so the braces here join to the sill and the top plate. This results in a lack of any triangles, so rather than relying on the immutability of the angles of a triangle to maintain squareness of the walls, we are relying on the fact that the brace has to stand upright for the walls to shift.

This is why we have to balance braces in this system: A brace slanting one direction has to stand upright for the walls to shift against its lean. The weight of the structure alone is enough to prevent it. In contrast, the brace simply has to fall over to shift the walls in the other direction, which the brace itself is actually actively encouraging to a small degree.

Contrast this to standard oblique bracing, where we balance bracing due to the nature of the joinery: a standard mortise and tenon only works under compression, so the brace is only effective in one direction. Contrast this further to older styles of oblique bracing using dovetail joints, which don't have to be balanced because the joint works well in either direction. (In most cases in southern Germany and Switzerland, slanted uprights directly replaced dovetail oblique bracing, without ever seeing the use of mortise and tenon oblique bracing like we are accustomed to in the US)

Setting history aside, and going strictly toward practicality I recommend using the large agricultural buildings of the central part of the Swiss Plateau as a good model to follow (since this is pretty much what they are designed to be: pure practicality without any fancy and unnecessary architectural elements like you might see in old cities or most of Germany)

Standard oblique bracing (triangles) can be scaled up to a point, once you get too large then they begin to act as levers and pry the posts out of their sockets. I suspect it is this fact that led to the invention of the Riegelbau style of oblique bracing, as carpenters realized that longer braces are more effective (study Swiss roof framing, where enormously long braces are frequently used) but have inherent issues in certain scenarios. (The English and French appear to have arrived at a different solution to this problem, tying their extra long oblique braces across multiple posts to negate the prying effect)

There is no strict Riegelbau framing tradition. Riegelbau is a general classification that includes a great deal of closely related framing styles. The only necessary element to classify something as Reigelbau is that a light framing method with an interruption between stories is used (thus the 'Riegel' or 'Rail') How the stories are joined together, how the structure is braced, etc. are all flexible. 'Riegelbau' is only slightly more specific than 'North American Timber Framing'


Edited by D L Bahler (03/17/16 02:53 PM)
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#33598 - 03/17/16 07:26 PM Re: Double-Wall Enclosure Concept [Re: D L Bahler]
Jay White Cloud Offline
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Registered: 11/06/08
Posts: 479
Loc: Vermont
hi David,

I don't want to get too far away from TCB's queries and topic line of his "concepts" of a double wall system... I will cover some points of clarification then leave it be for TCB continued questions...


Originally Posted By: D L Bahler
...the system we are discussing in regards to Riegelbau is not oblique or triangular bracing...


Actually it very much is in the "oblique bracing" category of systems. Agreed it is perhaps one of the best, oldest and most functional of these systems, however any time there is the formation of a "canted system" that does most of its work..."in compressive"...loading, then it is considered and "oblique system" from an engineering viewpoint, no matter how slight the angulation may be or the relative size of the bracing modality itself.


Even the the broad range of "Riegelbau" bracing systems are variable...they are "slanted" and do work in a compressive loading condition for most of their work in the architecture. They do this marvelously because of their larger size, and "sill to plate" formating. They still are in the "oblique system" as they are a...

Originally Posted By: D L Bahler
...long slanted bracing...building styles...braces here join to the sill and the top plate....


And this does indeed...from an engineering perspective (having discussed this numerous time with my own PE I work with)... form a "triangulated system" of compressive resistance, which is the primary function of most of the working systems within the "oblique bracing" modalities...Again, this is one of the best because of the large "slanted" (aka bracing) format, its "buttressing effect" and "sill to plate orientation."

I would further add in support that, indeed the older styles (and larger) "oblique bracing" systems did have some limited work capacity in "tension" bearing capacity. This does not change the primary "work force" of these systems to do most of there function in a "compression resisting" format, as "let in" bracing..."strengthening"...in the tension format was not the primary function, nor has this been bore out in any of the historic research of merit that studies these vintage systems and the application to the architecture...

I more than agree that most contemporary or "standard oblique bracing" primarily "only works under compression" and offers virtually no "tension effect" within the frame...I would have to challenge that the "dovetail joints" of any of the let in systems "works well in either direction." Some yes...but this system is still primarily a compressive system not a tensioning one, though agreed it offers a limited capacity in that regard much more so than the shorter "stub tenon" methods of a standard brace system we find today...as these "stub tenon" oblique bracing systems historically in some region do not even have pegs at all..Some are simply "well fitted" and/or "wedged in place" after raising further facilitating "tightening" at a later date by more pressure added to the wedged mortise. The ones that do have pegs in their small tenons do little work beyond holding the brace itself during raising...Unlike the "dove tail" which does indeed do "more work" when well executed and better pegged/wedged...

I would offer that of all the systems found in Europe the longer "pass through" (or by) bracing of the Swiss and French systems do the "most work" of all the "oblique systems" of bracing we find in European designs. I would not disagree in either of these recommended modalities, though the are not the only "simple systems" that exist historically in timber architecture...

I would not go beyond this level of discussion detail further so as not to distract from TCB's post topic, and perhaps take this offline if there is a need to explore any details further about "bracing systems."

Regards,

j
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#33599 - 03/17/16 09:00 PM Re: Double-Wall Enclosure Concept [Re: D L Bahler]
TCB Offline
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Registered: 02/10/16
Posts: 26
Loc: Texas Hill Country
"The theory behind the long slanted bracing in German building styles is that a brace to a post compromises its integrity, so the braces here join to the sill and the top plate. This results in a lack of any triangles, so rather than relying on the immutability of the angles of a triangle to maintain squareness of the walls, we are relying on the fact that the brace has to stand upright for the walls to shift."
I think I understand the concept, now, which makes it a lot easier to visualize how my design should proceed. The purpose of this system is to reduce the vertical/diagonal structures composing the wall into purely tensile/compression load paths, which resolve into point loads & moments along the horizontal plates (which from what I can tell are themselves among the more beefy structural elements, often composed of an upper & lower beam sandwiching the floor joists).

Makes a lot of sense from a load-efficiency standpoint, since vertical wall elements are typically long & lanky, therefore ill suited to carrying moments specifically (though not so long as to buckle under compression loads with even modest stabilizing braces along their length). Likewise, the horizontal plates only span between posts, which is far shorter a distance than the height of each story, and are very rigidly attached at these points, the result being far less deflection for a given moment. Seems inherently stronger for a given timber size than shorter (stiffer) braces at 45deg angles that terminate in the middle of the posts. I have noticed that a lot of the traditional Riegelbau frames seem to use similar-looking sharp angled diagonals, but break the wall into a tight grid with more frequent horizontal members --I presume this is to break up a daub-style infill into more manageable chunks?

So I suppose the tactic is to design the walls as though the columns are masonry (i.e. no bending, no tension), the diagonal braces steel rod (tension/compression only), and treat the floor as 'rigid' for all intents & purposes (reinforcing it as needed to get its loads to acceptable levels later on in the design)

" Also, "oblique bracing" modalities also function better in the "buttressing context" (i.e. at the base of a post) and can also work very well in the "horizontal" configuration within floor and roof systems as well. (i.e. "strong back" and "dragon beam" systems)"
Dragon beam as a floor support? Color me interested, I thought it was a roof-only thing (and Google search for 'Dragon Beam' helpfully yields mostly fantasy-art featuring reptiles). Since I have an octagonal floorplan, the typical rectangular joist/support systems don't seem like they will fit nicely, but a dragon-looking angled junction coming in to cover triangular areas seems feasible; any good examples of a floor structure done this way?


This is a rough sketch of how I'm laying out my walls; this is a single 20ftx10ft side of one floor of the structure
-4X's are pictured, but I'm mostly just laying out locations here
-33" centers*
-Top plate rests between the inner/outer posts on the tie-bars
-Diagonal members sit between the posts, joined to both plates (they would be entirely hidden by the plank-based infill I've proposed)
-Dashed lines at the ends are where the wall panels meet; horizontal members will be lap-jointed for attachment
-The short horizontal intercostals are for attaching (non-structural?) interior walls where needed --aircraft-style)
-The long horizontal member is that intermediate sill which forms the internal routing path for utilities

*Since such a simple sketch doesn't convey much information on its own, I wanted to explain my approach a bit more. My goal is to create a 'wall plan' that is adaptable to the other fifteen walls (down to 'interchangeable' beams), and can be easily tailored for the various doors/windows/walls that lie on it. I figured that if I get a good solid structural box at the sides, I'll be able to more freely remove, shift, and replace the structure between the second posts from each end as desired --being octagonal, each of those boxes will tie to an adjacent wall to form a ~5ft wide shallow-V-shaped column at the corners. So for the north-end of the structure with a second floor to support, there will be more posts to carry floor joists, whereas the south-facing side with only a narrow cantilevered walkway inside & outside can be pierced by good-sized windows along with the second floor.

I plan to analyze the structure with none of the intermediate framework between the second to last posts present; wherever loads show deficient, I'll go back and verify whether or not the posts in the middle pick up the slack, or if the posts/plates need to be beefed up to get below acceptable limits. Think of the building like two Gazebos stacked on eachother (hey, that's a cool idea)

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#33600 - 03/17/16 09:08 PM Re: Double-Wall Enclosure Concept [Re: TCB]
TCB Offline
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Registered: 02/10/16
Posts: 26
Loc: Texas Hill Country
"And this does indeed...from an engineering perspective (having discussed this numerous time with my own PE I work with)... form a "triangulated system" of compressive resistance, which is the primary function of most of the working systems within the "oblique bracing" modalities"
I agree it is effectively a triangle, but there are ultimately four members involved instead of merely three, so I guess a trapezoid with one really short leg? One aspect of the design that has me thinking is the low buckling limit of such a long support on the side of the frame in compression --the longest leg of the structure is always in compression with this system. Granted, it only takes an intermediate support or two to mitigate the risk of crippling, and I suppose the compression load certainly does serve to strengthen the truss over a joint scheme that ideally only carries tension (whatever that may be)

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#33601 - 03/20/16 02:47 PM Re: Double-Wall Enclosure Concept [Re: TCB]
Jay White Cloud Offline
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Registered: 11/06/08
Posts: 479
Loc: Vermont
NOTE: Text in "Blue" have links attached.

Originally Posted By: TCB
Dragon beam as a floor support? Color me interested, I thought it was a roof-only thing...any good examples of a floor structure done this way?


Only a few photo links...here and there under... "Dragon Beam" ....when doing searches in "English."

"Quotes" around search topic often yield better results, and "like this" searchres within context searches expand the information further...

Originally Posted By: TCB
I agree it is effectively a triangle, but there are ultimately four members involved instead of merely three, so I guess a trapezoid with one really short leg?


As I wrote earlier, these large wall oblique bracing systems are absolutely the strongest and best application of any oblique system one could employ within the...roof, wall or floor diaphragms...of a timber frame structure...

They are the primary "triangulated resistance forms" found in Middle Eastern and Asian design modalites...WHEN...they do employ an oblique bracing modality...Which they do employ in key areas more often than many "students of architectural history" believe to be done. Because they are so often in roofs and floors...and/or employed outside the typical European context...they are not generally noticed or understood by casual viewing..

I tend to find and see Asian application of "oblique systems" to be the best applied and understood forms found for resistance to extreme tectonic loading. Since the "building environments" these timber framing styles evolved in are some of the most seismically active zones in the world...naturally the understanding of "strengthening and resistance to racking"...modalities would evolve to be some of the most advanced in the world...

This is further borne out, and considered reasoning why we find a significant paradigm shift in some of Southern and Eastern European timber framing systems, and why the significant regional differences and uniqueness. As such...we find radiating from the connective tectonic plates emanating from Jura Mountains...down into the end/edges of the Iberian peninsula, through all of the Apennine peninsula and into regions of the Anatolian peninsula, very different timber framing styles to what is found in Western Europe.

Middle Eastern throughout Asian...Timberwrights have learned to employ both..."flexure and Locking"...racking systems to work in concert with one another...The "horizontal bracing modalities" of Japan (without expanding the disucssion into Korean, Chinese and Hymilian systems) reflect strongly this architectureal developement style...Moya, Tsumabari, Koyabari, nokigeta,....and most critical...the Nuki...all work extremely well to resist racking while still providing flexability within the structure. Some systems employ these almost exclusively in very seismically active zones, some also having oblique systems within the floors, roof, and perhaps wall corner intersections as well. Most being very large in triangulation, many in a "horizontal configuration, and most connecting within the context of "sill and plate" configuration.

Ketayuki sujikai, Koya sujikai, and very importantly the Hiuchibari found within floors and ceiling assembly...are all very critical "oblique systems" found in these timber framing modalities.
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