Tuesday, October 27, 2009

French Connection 6

I thought it was time for an update of my work on the Mazerolle tréteau. Those readers who are new to this blog and haven't come across my previous posts on this topic, please refer to the label index to the right of the page and you should be able to track down the previous installments.

When I first posted up about this sawhorse on the blog, I was at a point where I was able to solve some of the drawing, but there were a few areas of it which eluded me, largely because it is fairly tough to visualize the connections in a piece like this, where each of the four legs is in a different rotational arrangement in relation to the top beam and all the braces. I am also coming from a background in Japanese carpentry drawing , so the French methods are a little different and more than a little head scratching goes on as I work to get my head wrapped around it. It gets easier with time - that, or my derangement grows, I'm not so sure!

Later on, with the help of SketchUp, I was able to develop the 2D into a 3D model, which allowed me to confirm the layout, and to finally get a good look at all sides of the piece. The book shows a perspective view from one corner only, in this is in fact mis-drawn in one area, so it was a struggle to imagine how the connections came together on the backside of the piece.

Once I had the drawing complete, I scaled it down to produce a sawhorse at the height I desired, which, in this case, is 24". That was how I left off the previous post in this thread, with the drawing complete and scaled.

The scaling-down had brought the piece to the desired height alright, however it left the sizes of the pieces of wood with some odd sizes. The interior X-braces that sandwich between the long main exterior brace sets were less that 3/4" thick, which was clearly a bit scant.

Of course, in studying this piece I initially followed the drawing in the book as faithfully as I could, at times with the aid of a magnifying glass and scale rule, and drew the initial versions in metric. I wanted to produce the 24" sawhorse with inch-scale components however. Then, when I set about re-drawing the horse with components at the desired sizes - the legs for example will be 2" square sections - I found I hadn't understood the drawing as fully as I had thought, as I ran into a few glitches. Trying to get those interior braces a bit thicker was not so simple as I had imagined.

So, another flurry of study ensued, and another 20 hours of drawing later, I think I have a somewhat better understanding - certainly enough to build the sawhorse at this point. Now I can specify the sizes of the components and vary the slopes of the lags and braces, and have some idea what the ramifications of these changes to slope will be. One thing about a horse like this is the utter inter-relation of all the parts, so a change to one dimension ripples down through the rest of the geometrical relationships, sometimes causing problems. A complete understanding of this piece will come after I've built a few of these, so it's years away at this point.

While I still have another 6~10 hours of drawing to resolve some of the remaining mortise and tenon connections - connections that vary on the sawhorse in detail depending upon location - I thought I would share with the reader some recently-completed drawings. I've illustrated the various pieces with different woods - Mahogany for the top and legs, Ash for the long braces and the short braces, and Purpleheart for the interior braces -I did this as I thought it makes it easier for the reader to see what it going on with so many different pieces involved. These are not necessarily the species I will use - I'll see what I can find later this week when I go trawling for some material. I want wood that will be medium weight, fine grained, decent workability, and good rot resistance - and not too expensive. I was hoping to use Black Locust, but I can't obtain any dry material, so that would appear to be out. That's a bummer, but I'm sure I'll find something. Uh, well, maybe not sure, but at least hopeful.

Here's some snapshots then.

Top View:

The illustration in the book shows the mortises in the top beam drawn incorrectly in three cases. That kind of thing doesn't help when you're trying to solve such a riddle. I'm imagining this drawing was either made by an apprentice who was copying from another drawing, or the mistakes are deliberate and meant to confuse. When this sawhorse is complete, the top view will be a little different as I will be fitting a sacrificial cap as I did on my other sawhorse (see the posts "Irregular situation", parts I through VI)), which will cover all the exposed tenons.

This perspective view is fairly similar to the one drawn in the book:

A view of one of the short ends of the horse, here the particulars are that the the left-side leg is rotated 45˚ to plan, while the right-side leg is aligned to the long axis of the plan:

One of the long braces extracted (part of the assembly termed the croix de Saint André dans les liens Mansards):

I'm going to do half-laps with mitered abutments as per usual, and in the above drawing the tenon at the lower end is not yet defined.

A view of the other short end of the horse, where the situation is different again - the left-side leg is at 45˚ to plan, while the right-side leg is aligned to the short axis of the plan, and I have removed one of the short side braces just to give a look:

I'll finish off with a view of the spaghetti junction - and not all the lines are yet on this drawing, As I haven't developed the rotational views of some of the braces yet:

Click on the picture and it will all become clear :^)

So, I hope to commence work on this sawhorse within the next week or so, and the next post in this series will be the commencement of the build thread for the piece. I anticipate that the upcoming tréteau build thread will not be so long as the previous build thread on the lantern. That's the plan anyhow, and hopefully there won't be too many surprises in that regard.

Thanks for visiting today. On to the build! <-- link

Wednesday, October 21, 2009

The 'Gambrel or Mansard?' Problem

This post has been revised umpteen times, as the topic to be covered is somewhat complicated...

English is a language, like all languages, that is in a continual process of change. Some of these changes come about by words being imported/borrowed (and often mangled) from other languages, some from new ideas/technologies that need new descriptors, and some, dare I say most, changes result from accident or pure ignorance in the part of the speakers involved, and the repetition of such mistakes made by others. I'll count myself among those who are ignorant in that regard, and admit I have a lot to learn yet about the English language.

The ways language can change is a fascinating topic in and of itself, and outside the scope of this blog for the most part. That said, I find I often have strong feelings about certain words and am attached to their meanings, and when I learn that I have been mispronouncing a word, or using it incorrectly, I am always happy to take such lessons forward with me. Some revisions are however largely futile, since other speakers of your language may no longer understand your speech if you make unconventional changes to it, and some changes happened so far in the past that it is all but pointless to hope to revert. Now, how far back is too far back is a question I'll leave to others.

One case in point is the word 'apron'. This word came into the language around between 1275 and 1325. The Middle English origin of this word is napron, which comes from the Middle French word naperon, a dimunitive form of the word nape, meaning 'tablecloth'. Nape stems from the Latin mappa, which means 'napkin'. In a curious process called juncture loss or rebracketing (<--a link) the 'n' can migrate back and forth between words beginning with vowels. Thus what was once 'a napron' became 'an apron'. That crafty little 'n' - ya gotta keep an eye on it like a three year old child or it can get into all sorts of trouble! Another English word that suffered from juncture loss is 'newt'. Originally the word was ewt, which etymologically relates to the word 'eft' (an eft means an immature newt), so 'an ewt' became 'a newt' and the shift stuck, this time the 'n' taking a sideways trip in the other direction. Same juncture loss process for an adder (the snake), which once was 'a nadder'.

Another example of a word usage that has stuck, for completely different reasons, and the subject for my post today, is the word gambrel. I'm not sure how well known this word might be to readers outside the US, Canada, Australia, and to some extent England, but it is a word typically used to describe a building with a roof that looks like this:

While to many readers, that might well seem to end the discussion on the topic, I am not so inclined to stop, personally, because from the research I have done so far it would appear that the word gambrel has become, due to some accident of history, mis-applied - and after all I've got a blog to write this seems like a worthy topic to investigate!

'Gambrel' is a Norman English word, sometimes spelled gamerel, gamrel, gambril and gameral and having the meaning a crooked or hooked stick. A 'Gambrel' defined in a modern dictionary as a stick or piece of timber used to spread open and hang a slaughtered animal by its hind legs. Here's a picture of a modern form of that device:

Here's a picture of an old patent application for an adjustable gambrel:

As you can see, the form of the gambrel is that of a bar with hooks on the end, and in concert with the suspension chains, takes the form of a triangulated arrangement.

In action, the gambrel serves to suspend the slaughtered animal and facilitates the removal of the hide and meat from the leg areas, among other tasks:

Notice how, in the above example at least, the triangular gambrel angles outward in use. I may be reading something into nothing however.

The word gambrel also gets applied to the frame which an animal's carcass is hung off of, as in this example:

The word 'gambrel' has another definition; it is a term for the joint in the upper part of a horse’s hind leg, the hock:

As an anatomic structure, the gambrel of the horse's leg, when you peel the skin and surrounding tissues back, is actually a projecting spur of bone, and looks like this:

If one considers only the definition of 'gambrel' as a horse's hock, and ignores the other definition of hooked stick, and ignores the fact that a 'gambrel' is a horizontal speading bar, then one could simply see the bent portion of the horse's leg, the hock, and associate it to the roof that looks like this:

If you're going to do that though, then one has to take a somewhat peculiar view of the horse's leg arrangement, since the form of the above roof is only clearly seen at the gable ends. By that I mean: if that roof shape were to be akin to a horse's back leg, then taking both roof sides together would give a view of a horse with an impossible akimbo stance. The stance, if were to be anything, would be that of the bow-legged cowboy who rides the horse! And there is no horizontal bar evident, nor does the roof lean outwards like the slaughterhouse gambrel shown in use above. So, I thought the association of the older meanings of 'gambrel' with the modern didn't quite accord somehow, at least not to me.

Digging into the etymology of the word 'gambrel' a little further, I find that it comes from the Old Norman french gamberel, which is akin to the modern French jambier (meaning legging) and jambe, meaning 'leg'. Tracing further back, we have Middle French gambade, or gambolde, which means a leap or a spring. In modern English, we have the perhaps seldom-used word 'gambado', which means the spring or leap of a horse. These words, along with 'gambit' and 'gambol', trace back to the Latin gamba meaning 'leg'. Gamba in turn comes from the Greek, kampe, meaning 'bend'.

While knowing the etymology back to the ancient Greeks is interesting in its own way, it must be said that the meanings of the words that derived from kampe and gamba are only of relevance in terms of what they meant at the time the epithet 'gambrel' was applied to a roof form. Since the slaughterhouse equipment/hooked stick/springing leg meanings of 'gambrel' were certainly in place before the use of the word as a roof name, we can presume, I think, that the roof shape called 'gambrel' was so-named for either its perceived physical similarity to, or connection semantically with, the slaughterhouse gear or the springing leg or spreading bar motif.

Returning to those word roots and their derivatives again, one comes to the sense of the word 'gambrel' as having a meaning of a leg bent back, a leg ready to spring. The gambrel stick bends the legs of the animal back and apart. The horse's hock shows a bone that looks bent back, forming a crooked line. Again, although the gambrel roof as commonly described does indeed have a bend in it, there is nothing to suggest leaping or springing in the form. Nothing juts out, or bends back, in the roof now called 'gambrel' in N. America.

The intrigue of this question as to the origins of the word 'gambrel' was furthered for me personally when I took a class in French carpentry drawing a few years back, taught by French Compagnon Boris Noel. He was showing some architectural slides, and at one point I asked him what the French call the following roof form:

He replied, "Mansard". No surprise there. Then I asked him what the French called the same roof form, but in a two-sided version, with gable ends. His answer: "Mansard"(!).

That was an 'a-ha!' moment for me.

So, while the French seem quite happy to call both roof forms 'Mansard', in North American usage (I would include Australia as well) the term 'Mansard' is applied only to roofs in which the doubled roof planes extend around all four sides of the building.

The French idea of a Mansard roof, as either 2- or 4-sided, would appear to be the definition used in other parts of continental Europe, as this clipping from a Turkish site indicates - both types of roof at the top of the picture are termed 'Mansard':

I also checked various German sites (one link), and found nothing for 'gambrel', but under 'Mansard-dach' one finds illustrations of both 2- and 4-sided roofs with double pitches. While the French and Germans often have trouble agreeing on a wide variety of matters they seem to concur with their roof terms at least.

The word 'gambrel' in application to the roof form then came really from American vernacular usage - and likely confounding of terms. I think the English borrowed it later from this side of the pond to describe that bent-plane roof form roof form. I can't prove that as of this moment, but it's the theory I'm moving forward with.

Here's another outrageous theory to consider: prior to permanent European settlement in America, Dutch mariners and traders had visited or settled the area of south east Asia now called Indonesia. It was there that they saw dwellings with a roof style where the end of a roof started as a hip and finished as a gable end at the ridge. The gable end was in fact an opening in the roof to allow smoke to dissipate from the cooking fires. This design of roof was brought back to Europe, where it saw minimal adoption - however, in the American Colonies the situation for building was a little less restricted by traditional forms, and the Indonesian roof form was adapted to local conditions in some places. This likely happened in the Dutch colonies along the Hudson, who also built roofs in the Mansard form, as it is a very practical kind of roof.

The roof style where the end of the roof starts in a hip and finishes in a gable - the hipped gable - is still in existence today in Indonesian rural communities:

A view from the side of an Indonesian hipped gable roof:

Notice in the side view the strong resemblance to the horse's leg, the bend at the gablet akin to the hock, and how the gable at the top also seems to spring forward from the line of the hip. Notice if you took both sides of the hipped gable together, the lines of the hip ridges through the gable to the peak, the lines would not be so different than that of a horse's rear legs as it was crouching to spring. The line of the gablet jutting out is similar to the line of the bone in the horse's leg which produces the hock form. Also notice how, like the gambrel stick and chain assembly used to slaughter an animal, pictured earlier, the triangular gable pediment leans forward from the end hipped roof pitch. Also, considering the gambrel stick again, the bottom of the gablet on a hipped gable is defined by a horizontal bar, and forms a triangle. The hipped gable seems to tie far more clearly to the word 'gambrel' in it's other senses. I think that somewhere along the line, the terms for the different Dutch roof forms got confused in the New World.

The word gambrill was part of the Dutch language in 1601 according to one account I read, though I have not been able to confirm this as of yet as it was no citation for the comment. Perhaps any Dutch readers of this blog who have access to libraries with old dictionaries might be able to assist me in this matter?

Various references are found in the original colonies in America about gambrel roofs including: 1737 Old Times, New England “One Tenement two stories upright with a gambering roof.”; 1765 Massachusetts Gazette “A large building with two upright stories and a Gambrel Roof. Sometimes with the long sloping roof of Massachusetts oftener with the quaint gambrel of Rhode Island”; 1779 “The gambrel ruft house”. 1824 “In a Gambrel roof’d home”; 1858 “a small farm with a modest gambrel roofed one story cottage”.

Those are references from New England mind you, and it is said that the 'gambrel' originated in The Hudson River Valley with the Dutch settlements.

In the Dictionary of Americanisms by John Russel Bartlett, 1848, pg. 153:

This dictionary is available online from Google books and has many interesting descriptions of all sorts of terms, many of which I had no idea originated in the US.

The question at this point is what John Bartlett meant by 'hipped'? If he meant it in the sense we consider 'hipped roof' today, then this roof is not anything like the one we call 'gambrel' today.

The French or Mansard roof is attributed to the French Architect Nicolas Francois Mansart, 1598 – 1666, probably not something he invented but but certainly a roof style used by him. In its basic form it consisted of a King post truss (more correctly: King Piece Truss) on top of a Queen Post Truss. This provided usable roof space as additional accommodation, and possibly was intended, depending upon which account one reads, to take advantage of tax regulations at the time, or for use in areas where bricks and stone were expensive (as bricks and stone formed the walls).

The basic Mansard roof with gable ends could be called a single or 2-sided Mansard roof with the roof having two different pitches, the lower (or pitch from the eave) being steeper than the upper pitch connecting with the ridge. A Mansard Roof which has hip ends is also called a curb roof (except by the French and the rest of continental Europe, who stick to their guns and call it a 'Mansard') where the upper pole plates become a curb. The word 'curb' in this sense means to control or restrain, or perhaps enclosing framework or border. A synonym for the curb of the roof would be purlin plate. On some roofs, the curb is exposed and covered with fascia, marking a clear change in roof planes:

The roof shape of the Mansard was varied and pitch proportions were modified over time to accommodate dormer windows, and sometimes curved ends were formed at the eave to reduce snow slippage. Some roofs kept the double roof planes but made one or both planes curvilinear. Looking in old carpentry texts reveals numerous proportioning methods for the Mansard roof.

Now, as a counterpoint, one can imagine that a double pitch roof in which the lower edge of the lower plane were curled upward somewhat - hardly common but a form the roof can take - would have a strong resemblance to the hooked stick meaning of gambrel, so that is one possibility as to how the roof form names may have got confounded:

In the 1919 publication The Colonial Architecture of Salem, by Frank Cousins and Phil Riley, they devote an entire chapter to 'Gambrel Roofed Houses'. There, they describe a gambrel roof as "an evolution of the seventeenth century Mansard roof". Later, they describe how the roof form was adopted:

I wonder who these well-traveled 'American Builders' were who visited Paris and were impressed by the exposed cross-sections of Mansard? It's an unattributed passage without footnote or reference. I also wonder about the account in regards to many of the buildings described in that chapter, as they have build dates in the late 1600's and early 1700's, while the word 'gambrel' didn't come into use to describe a roof until 1735 at the earliest according to my dictionary. The locals and the builders had to be calling that roof something prior to 1735, and I wonder what the name was?

While an online dictionary search for the term 'gambrel' will lead to the current usual definition in most cases, in some architectural Dictionaries, like Dictionary of Architectural and Building Technology 4th Edition (by Henry J.Cowan and Peter R.Smith, published in the US by Spon Press 2004. Library of Congress ISBN 0-415-31234-5) you will find a different definition, to wit: "gambrel roof: A sloping roof similar to a HIP ROOF, but with the addition of small gables part-way up the end sloping portions".

Another book I have, The Complete dictionary of Wood, by Thomas Corkhill (1982, Scarborough Books, ISBN 0-8128-6142-6) also shows the gambrel roof illustrated, on pg. 211:

With the picture, comes the following definition:

"Gambrel or Gambril roof. The end of a roof partly hipped and partly gabled."

Or consider this English site, looking at buildings, and their depiction of roof forms:

It seems not all the English are in agreement about the word 'gambrel'. Notice that the gambrel they depict matches that of Corkhill. Their written definition for 'gambrel'?:

"A hipped roof which turns to a gablet at the ridge."

Notice at the right of the above picture the various forms of gables depicted. Curiously, if you use the term 'Dutch Gable' in Australia or the US, the image normally ascribed to that term is this:

It would appear from what I came across while searching, that in Australia, what I call the hipped gable roof is a very popular form for detached open air car garages.

Frankly, the term 'Dutch gable' is only helpful in regards to the fact that it associates the hipped gable form to the Dutch. In truth, as mentioned earlier, the form is not actually Dutch, but Indonesian. As far as what a REAL Dutch gable looks like, in terms of buildings located in Holland, well, the form is a wee bit different. Of course, the Dutch don't call it a 'Dutch gable' - one form of real Dutch gable, for example, is termed a klokgevel ('bell-shaped gable'):

The 'Dutch gable', gevel, in fact is generally a type of facade on the short end of a building. Here's an excellent link to view some of the various types of real Dutch gables (<-- link), and here's a look at the structure of one of those classic Dutch buildings:
Speaking of the Dutch, thanks to the wonders of translation software and several hours of searching, I have a had a good look around several Dutch architectural websites trying to research the gambrel/Mansard conundrum. The term 'gambrel' is nowhere to be found (nor can I find a translation for in in a Dutch dictionary), however on one site which details a wide variety of roof shapes, we find the following entry for Mansard-dak ('dak' like the German 'dach', meaning 'roof'):

"A Mansard is a gable or hipped roof, each of which was nodded. The under surfaces are steeper than the upper surfaces which creates more room on the top floor."

Note: -A gabled OR a hipped roof-

So, again it would appear that most of continental Europe has the term 'Mansard' to describe 2 or 4 sided roofs of double pitch, and it would appear that they can live with that situation in a most untroubled manner, while we here in North America, and to an extent in Britain seem to prefer to have separate names for roofs with double roof planes (that are bowed out), even though one of the names may once have referred to a hipped gable roof. It's quite a tangled nest of information to wade through and get to the bottom of, but that is what I intend to do as I continue my inquiry into this topic. I will have a follow-up post in this thread at some point in the future. I am in contact with a few different people who may be able to point the way, and I intend to do some research in the next month or so at the Boston Athaneum and then the Library of Congress in Washington D.C., in terms of looking at architectural and builders dictionaries from the 1750~1850 to see what they might have to say.

Thanks for visiting! I must confess my post was long and rambling today, and by no means cohesive or ever proving anything - yet! - so my apologies. Your learned comments or insights are most welcome.

Monday, October 19, 2009

Practical, Sensible, Frugal

Last week I picked up and read a recent publication on green building practice, a work by Stephen and Rebekah Hren: The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit (Chelsea Green, 2008). Compared to the previous book I reviewed on this blog on 'green building', Green from the Ground Up, in a post titled Greenwash a few months back, this work by the Hren's is a real breath of fresh air. The work is remarkably free from trademarks and brand endorsements, and while they do look at and assess various commercial products, they do so with an eye less to promotion, but more to their personal experience. The book is fairly well researched, and one of the authors (Rebekah) is engaged in her own solar retrofit business so they do walk the talk.

When I initially thumbed through this book in the local non-profit cooperative bookstore, the passage that caught my eye came early on as the authors related their experiences coming to grasps with the horrors of the world's energy dependence upon fossil fuels, the fact of oil companies often weaseling their way out of paying for oil spills, and growing threats to all of us in terms of climate change: as many have done before them, they pulled up stakes and bought a chunk of land in the countryside, in an effort to become 'independent':

"We built our own passive-solar house out of cob (a traditional mix of clay, sand, and straw), went off-grid, and tried to grow our own food and raise poultry."

This story doesn't go quite where you might expect however,

"Meanwhile we still had jobs back in town and often wanted to socialize there, which meant frequent 40-minute drives. Not long after the elation of successfully installing our photovoltaic panels, getting our cob home past its final inspection and legally moving into our new home, we learned that an automobile uses as much energy while its running as 350 100-watt bulbs! Driving an hour to town and back used the equivalent amount of energy as running our home's electrical needs for a month.

How ironic! We had been been criticizing our friends who refused to replace their incandescent bulbs with compact fluorescents, yet here we are, driving all the time, the worst offenders of all! On top of that, the idea that we could live self-sufficiently out in the woods turned out to be a cruel joke. although neither of us had much of a green thumb, we had assumed it would be easy to grow most of our food organically and with minimal petroleum inputs. Instead we spent a great deal of time doing a halfway job and getting meager returns. The garden became our enemy, and we were shackled to it, every evening, picking bugs and pulling weeds in the blistering heat.

It became obvious we had made a mistake. We'd tried to start from scratch and throw everything out, including the established communities and the infrastructure of towns and cities, and do it all ourselves. Running away to live in the woods isn't an option for most people, and if everyone tried to do it, it would be become even more futile. We were and are convinced that as day turns to dusk for fossil fuels, we must take a good look at our surroundings and learn to live with what we have already built, what we've spent our free fossil currency on: the infrastructure, especially the housing, that already exists in our towns and cities. For us, it was a time to learn from our mistakes and move back to the city...it was time to do things in a way that others could see and emulate. It was time to do it right.

Given my own knack for making all sorts of mistakes, and moving forward from them as best I can, even learning to welcome them (on a good day!), the story of two people who had followed a path similar to my own (see Mud and Sticks, part I and II) and had found the 'good life' not quite what they expected or wanted, who had failed at their attempt to live simply and then managed to maintain their determination, stuck to their beliefs, now tempered by experience, and found another path which works for them. These are compelling themes to me and intrigued me so much I bought the book on the spot.

This book is less about marketing 'green' building as yet another form of consumption, and more about how the average person can take what they have and make small changes, going beyond simple light bulb swaps, to lessen their impact on the planet in terms of carbon emissions. Chapter 1 starts out with a basic analysis of home energy use on the basis of Btu consumption and then moves on to assessing one's own home to see whether it is worth retrofitting/converting to be become less of an energy hog. In itself, the Hrens are engaging in an uphill battle against the dominant paradigm in modern North American culture, the idea sold to us that the route to happiness, the 'American Dream' is one of increasing one's consumption, not cutting back. I recall a comment by George Carlin that, "it's called the 'American Dream' because you'd have to be asleep to believe it", but setting that point aside, the Hrens do realize that one way to sell green building is through an appeal to the green$$ that can be sometimes found in one's wallet:

"...if a two-adult household could eliminate one of [their] cars and instead bike and use public transportation the annual savings would be over $6200!"

"Solar hot waters have higher up-front costs, but once you install one, you see a $20 to $40 monthly savings on your utility bills."

What I like about the way the authors present the information, is that instead of simply preaching what we all should do, they relate how they themselves have made those changes and the combination of their efforts has led to an $11,000 annual savings in their household. Even if the 'small is beautiful', 'less is more' approach remains a hard sell in this advertising-saturated culture where new and shiny is the sexy thing we're all supposed to trample our neighbors down to obtain, the Hren's put forth a message that is sensible, conserving, and thoughtful, and gives most readers, I suspect, the sense that hey, I can do this too!

In Chapter 2 the authors delve into renewable electric systems such as photovoltaics, wind turbines, and microhydro. They convincingly show that the payback and benefit of installing solar hot water systems greatly exceeds that of photovoltaics, and yet still advocate for photovoltaics (PV), if not on a simple cost-benefit analysis, then on a 'moral' one. I'm not so sure myself about the PV option, as from what I've read the amount of energy that goes into producing them, mining and transporting some of the raw materials, exceeds the electrical energy they will produce over their lifetime. Plus there are the recyclability issues and disposal of toxic wastes which associate to many types of PV set ups, and their lifespan appears to be 25 years at best. We already deal poorly with the recycling of computers, so I don't see a massive increase in PV consumption/production as such a great thing.

In terms of the usual 'return on investment' argument about PV's, the Hren's make a good point:

"Tangentially, what else do you demand a payback on that you purchase? Cars, furniture, clothes, food, even fossil electricity? Of course not! You pay and pay and never expect a cent back...why should energy efficiency alone require a payback? Perhaps because capitalist society has so devalued efficiency itself in the drive for growth. Each step you take to efficiency limits the growth for fuel providers, limits the expense of environmental cleanup, and so on. What impetus is there for efficiency besides morality and frugality?"

I'm with them on the morality/frugality argument, though I think one of the aspects to improved efficiency which they do not take up is that of Jevon's Paradox: the proposition first put forward by William Stanley Jevons, in his 1865 book The Coal Question, that technological progress which increases the efficiency with which a resource is used, tends to increase (rather than decrease) the rate of consumption of that resource. It's an interesting point to consider in regards to the holy grail of improved efficiency, and those that have considered the matter in some detail often advocate for legislated limits to consumption as part and parcel of moves to improved technical efficiency - again, enforced limits to consumption is an idea which does not sell too well at this time. Without those limits however, it would appear to be the case, as one writer put it, that "human consumption habits seem to be ruled by a principle of "waste homeostasis," where the energy savings we get from better technology is used to fund better toys". Still, with some items, like home fridges and washing machines, improvements to efficiency yield positive results, since the amount of use associated to these products tends to remain a constant in a house, regardless of how much power they are using. Also, it ought to be noted that Jevon's Paradox applies primarily to technological improvements which lead to fuel efficiency, not to regulations and policies, corporate or government, which impose higher efficiency standards, as these often increase the cost of use of a given resource.

Anyway, I digress slightly...

Later Chapters of The Carbon-Free Home deal with appliances, lighting , heating, refrigeration, domestic hot water, cooling, rainwater collection, etc, and in each case the format is much the same: an introduction of the topic, an account of the Hren's personal experiences in that area, and then a detailed look at the various alternatives which confront the homeowner. In many cases, simple changes in behavior, such as detailing which washing machine or dishwasher settings are energy or water hogs and best avoided. Sometimes the solution is to buy a more energy efficient appliance, as the savings pay for themselves - sometimes it s a matter of adding extra insulation to your fridge or constructing a simple low-cost evaporative cooling box. The 36 projects detailed in the book are well within the reach of the average person, in terms of difficulty and the specialized tools required. I like the empowering, DIY approach the Hren's advocate.

In the section of the book which deals with heating and cooling, systems which, on average, consume 47% of the homeowners annual energy budget, the authors make the point that vinyl framed double-glazed argon gas filled windows are not all they are hyped to be:

"While double-paned windows are more insulative than existing single-paned windows, the difference is quite small, raising R-values [from] R1 to R2 generally. Oftentimes they are very expensive and degrade the historic integrity of the homes where they are installed. Many are made of vinyl, a product that lasts only about 20 years before severely cracking when exposed to ultraviolet light, compared to wood or metal windows that can last for 200 years or more when properly cared for. In addition, the seal around the double-pane is made of synthetic rubber that also lasts only about 20 years before it cracks and all the argon between the panes dissipates, resulting in condensation between panes and loss of visibility. In our opinion, replacing a functioning window that could potentially last many decades if not several centuries with a window that will be defunct in 20 years is planned obsolescence designed to sell as much product as possible. It is inherently energy inefficient and not viable in the long run."

I think they are right, though I would hardly consider a single pane glass window for new construction. The solution they put forward for dealing with leaky windows is to seal up windows which are 'openers yet rarely opened', and then to retrofit other windows in the house with storm windows, along with insulated curtain and shutters. Windows with sealing insulated curtains achieve an R-value of around 7 or 8, and provide excellent sound-deadening quality as well, the authors point out.

In regards to insulation the Hrens point out that it is far and away the most important thing homeowners can invest in, and note that worrying about which types of insulation are 'green' or 'sustainable' is a bit moot considering that despite the energy intensive or environmentally-polluting nature of most insulation product manufacture, the energy-savings realized by improved insulation more than offsets the downside. While I agree with that, I still have reservations about large scale production of certain types of insulation, both in their environmental impacts and their health impacts for the installer, and don't see them all as essentially equivalent in that regard.

Refreshingly, the Hrens devote a section to making light clay straw as a cheap and natural, locally-sourced and low impact material for alternative insulation. Hooray!

The later section of the book tackles issues in rainwater collection and water use, as well as a truly critical issue - dealing with our shit. Literally I mean. The book ends with a consideration of plants and landscaping in relation of improving energy performance of the home, and how we might make different transportation choices to reduce our carbon footprint even further.

All in all, The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit is a work I really enjoyed and I have added it to the 'Worth a Read' list at the lower right of this page. The reader may also wish to petition their local library, as I did, to have the book added to the local public collection so that the information might become more widely available. It's not an expensive purchase - following only one of the solutions the Hrens propose in their book would easily pay for the book, if I might have a little fun with the 'return on investment' point.

Thumbs up from me.

Thursday, October 15, 2009

Poly Gone?

This particular adventure started at the grocery store, when I bought a package of egg noodles. Not the most auspicious of beginnings I suppose, and it's not exactly a thrill-a-minute type of adventure, but what the heck, a start is a start. The journey has been good so far at least.

Later, when I went to make dinner, I tore the package of noodles open and what did I find inside, besides the dried noodles of course, but this little gem:

Not that a cardboard tray is especially valuable, but the shape - an octagonal hopper with unequal sides - now that was beautiful to me! I also realize there here lay a geometrical drawing challenge. So I set about drawing such a hopper, thinking that a wooden box in that form would look pretty sweet, and the faceted corners might even make for an object that would suffer less damage at the corners when knocked around. Here's what I came up with:

And a close up of the corner, exploded view:

While SketchUp can provide, by measuring directly off the drawing, the angles I need to make such a piece, simply to rely on SketchUp would be the wrong way to go in my opinion. A carpenter needs to be able to solve geometrical problems, on the job, with as few crutches as possible and with simple tools like a straightedge, compass, and framing square. I would say a pocket calculator would be a good inclusion as well. Still, having the model in 3D was handy because it enabled me to confirm that the results I could produce with 2D developed drawings for the various angular measures needed were correct.

With a through-tenoned hopper (or a finger-jointed one for that matter), there are 4 angles that need to be solved for:

1) face cut
2) edge cut (miter)
3) edge cut (butt miter)
4) top/bottom lines for mortise

With a regular plan, regular slope hopper, in which the corner in plan turns 90˚ and the boards, if plumb would meet at a 45˚ miter - the conventional hopper in other words - the kō-ko-gen method can be used to solve for all the angles fairly readily. For other polygons however, like this octagonal example I show above, the kō-ko-gen method does not apply. Too bad.

I had in the past constructed a pentagonal stool, the study of which had left me with all the tools to solve the developed drawing problems which apply to polygonal hoppers:

Still, when re-visiting a particular geometrical issue, in a new application, I like to refer to various books I have to see what methods they might offer, and as I mull over various ways of drawing the same thing, I can often come to new insights. Thinking that an octagonal hopper was a fairly ordinary enough form, it seemed likely that one of my western carpentry texts would deal with the layout issues for such a creature in some depth. Well, wrong.

I was very surprised as I looked through resource after resource and came upon almost nothing on this topic. Books on the steel square were pored through, Ellis's books on joinery, Walmsley's "Construction Geometry", Monckton's work from the mid 1800's. etc, etc.. Then I looked in my French and German layout books. And then I looked through the dozen or so Japanese layout books on my shelf. After all that searching, I had found very little material on polygonal hoppers. It wasn't as if they weren't covered at all - I found several books that had short sections on octagonal and hexagonal hoppers, but they only detailed the drawing methods to produce the face and miter joints. I was very surprised to see none of the books made any mention of the butt joint in this application, and, well, not so surprised perhaps that the 4th angle mentioned above, for tenoned or finger jointed boxes, was also not mentioned. Maybe they never existed and I am the first person to think of putting together a polygonal hopper with mortise and tenon joinery? Hah! I very much doubt it.

Now, as far as polygons go, it is not too uncommon to see hexagons and octagons, but other shapes are comparatively rare. Maybe all but non-existent. Further, once you get much beyond 6 sides in a splayed box, the butt-mitered and through-tenoned option begins to make less and less sense due to the angle of the butt joint becoming quite acute. Once you move beyond 10 sides, you are well on your way to coopering for that matter. So I can see why the most common cuts that might be employed would be the face cut and the miter cut, and I must conclude from all the materials that I have looked at so far, that anything otherwise was quite uncommon. It's a bit weird - a polygonal hopper does not seem like a particularly exotic proposition.

I drew another box to explore the topic a little further - a pentagonal hopper with through tenons:

A close up of the corner intersection reveals the appearance of a butt-jointed connection instead of a miter:

The corner of the board sticks up a bit from it's neighbor. I like the look actually - the circular array of points reminds me of a cutter in form, and I could imagine placing the hopper on a spindle and drilling enormous holes with it (I guess some carbide tips would help with that fantasy, and it probably wouldn't get far given the tapered shape, but 'oh well'):

The through-tenons on this box also incorporate a tongue and groove connection along the inside face, which, cleanly cut and accurately dimensioned, should make for a hopper which is watertight. Or, I could do a version of the pentagonal hopper, perhaps, which uses wedged sliding dovetails to seal the corners, as I did on this waterstone pond I made a few years back:

The edge on the bottom side of the pentagonal hopper I have drawn is mitered. I think it would be fun to make these two polygonal boxes, and I'll probably endeavor to do so soon enough. I'm calling the eight-sided one "Octabox".

At this point I have solved the 2D drawing issues to produce the necessary cut angles, and that is the main thing. If you can draw it, you can make it.

As for the more conventional hoppers, I did detail in four previous posts (titled "kō-ko-gen", parts I through IV) most of the geometry needed to solve for such beasts. I have however removed those posts from my blog and am currently compiling and re-editing that material, adding some new bits in, and forming it into a comprehensive article on regular plan, regular slope hoppers. This is amounting to 65~70 pages at this point, though I hope to condense it a bit. It will be thorough. I will make this material available for purchase soon enough, as part of a self-study series on Japanese layout I am putting together. I'm selling it because I have a lot of hours into the study and drawing work, and I should receive some compensation for that. The market can decide, as the pundits tell us.

This carpentry drawing series will be sold through a link I will put on the blog main page at some point soon. The self-study series will start with the hoppers, move to splayed-leg construction, and then on to roof study, with hip roofs and so forth. There will be an exam component for each module - those people who feel that they already have the material down say, for the hopper, can opt to not buy the study material and can simply challenge the exam. Passing the exam involves not simply pen and paper work, but completing the project in question, be it a hopper, or sawhorse, etc..

My hope, to be upfront about it, is that a series of study materials, like proverbial dangling carrots will draw more people into this rewarding area of carpentry material. Also, those that move along in their study of this material will, concomitantly, encourage me to forge on with completing further material for the comprehensive book on Japanese Carpentry drawing and joinery that i have been working on. And hey, I might make millions from such an endeavor and finally be able to afford that Bugatti I've been wanting. People would see me and would say, "oh hey, that guy's the hopper millionaire..." I'd need to hire bodyguards, and perhaps a helicopter would be a good idea. Yah, right!

Those readers who are interested in the first article, please contact me directly and I'll put you on the mailing list. I haven't set the prices yet, and they will vary with subject matter, but the comprehensive hopper essay will probably be about $20, and the exam around $5.00. I'll work out the details soon enough.

Thanks for dropping by today.

Wednesday, October 14, 2009

Bracing Situation IV: Design for Compression

In the previous post I outlined the reasons why designing a brace for a hinged door or gate that relies upon a down brace (a brace in tension), is a poor idea and one that it bound to result in a sagging gate over time. It's a shame really that the weakness of wood in shear parallel to grain precludes designing around the tremendous strength of wood in tension parallel to grain. The next best thing however, is designing around the strength of wood in terms of compression parallel to grain. For those readers new to this thread who might be perplexed by the lingo I'm using, I might suggest taking a look back at the previous installments in this thread so as to get, er, on the same page. I hope that last phrase was not too murky for those readers who are not native speakers of English. It's a funny language.

As noted in the first post, the strength of wood in tension parallel to grain is 2-, 3- or even as much as 5-times greater than the strength of the material in compression parallel to grain. However, despite that, the strength of wood in compression parallel to grain still far exceeds, by a wide margin, its strength in shear or in tension/compression perpendicular to grain. So, designing around the use of compression is the next best thing, in terms of a descending list of wood's 'greatest strength qualities', and more importantly, is the very best thing overall in terms of the practical carpentry matters inherent in making a strong structure that resists creep deformation or shear problems without extensive recourse to metal fasteners.

When I took a look at the shortcomings of designing around tension, I first considered the simplest case in which the brace is simply cut and nailed/screwed bolted to its adjacent pieces (i.e., a rail and/or a stile). That form results in the strength of the connection being entirely dependent upon the fasteners (which are vulnerable to rot), and shear parallel to grain issues. With a brace designed the opposite way, namely for compression, and simply cut at the end to meet the adjoining pieces, we have an entirely different situation. In the compression situation, instead of fasteners doing the work, the entire end of the brace will bear the load, and transfer that load against the adjoining pieces. Any fasteners in this case will serve more to locate the connection than to actually fix it. Any down loads on the gate by gravity are immediately met by the end grain of the brace. If loads go up, say a heavy person decides to sit at the free-hanging end of the gate, then some deformation will take place - the brace will want to deflect and if the load is heavy enough and of sufficient duration, then one might expect the side grain surfaces of the rail/stile to crush to some degree. The main thing however, is that the up brace does its job as intended, and does so employing all of its fiber effectively, unlike a piece in tension which, as I noted yesterday, must be over-sized so as to have adequate material at the end to resist shear parallel to grain.

Borrowing again from the work of James Newlands, let's have a look at the up-braced form in its elemental simplicity:

Again, 'a' is the hanging stile, 'b' the rail, 'c' the brace, and 'W' denotes the load on the end of the gate. By the way,the term hanging stile refers to the vertical side of the gate on which the hinges are mounted - the opposite stile on the gate (if it has one) is termed the falling stile.

With weight, 'W' applied the the gate structure, the various parts and their connections are subject to loads - in the above case, the rail 'b' is put under tension, and the brace 'c' in in compression. As for the connection points in relation to the brace, the joint at each end will be in compression. The rail 'b' will have tension joints at both ends. On the surface, it may appear as if this arrangement is scarcely better than the one described yesterday, in a similar manner, for a brace acting as a tie.

However, one needs to consider the way in which a gate is going to be framed and mounted. The problematic piece in the puzzle is the rail 'b' which is in tension. The hinges of the gatepost typically - I should say ideally - have long straps to attach them to the gate itself- thus the upper hinge strap, which can be made to overlap a good portion of the rail, will act as a means to effect tension support at that location. As for the other end of the upper rail, the weakness at that location is that of shear parallel to grain. Like the photo from the previous post showing an example of a shear failure in a bridge post, the brace in this situation has the potential to shear off material from the end of the rail. The solution to this issue is to connect the brace to the rail at a good distance back from the end of the rail, so as to provide a sufficient amount of relish to resist shear loads.

Here's a drawing of a well-designed gate embodying the principles mentioned above:

Notice at the right of the picture the long upper hinge strap, and the set back of the brace connection at the upper rail. A distance of 10~12" would be sufficient set back for the brace in most cases. A falling stile, 'b' in the drawing, has been added, along with horizontal bars, e, f, g and h. The gate posts have been well-buried and bolstered with packed stone. If the bottom of a direct-buried post is well-drained and kept away from soil, it will last longer. Charring the post to make it less appetizing to insects and fungi is also a wise move. Typically, in such a construction the hanging stile is a larger section of wood than the falling stile.

Here's a cross section detail of the same ideal gate form, looking at the hinged end and in cross-section:

There are numerous subtleties to the design of this gate which may not be immediately apparent to the reader. Given that the hanging stile is a bigger section than the falling stile, usually about 0.5" fatter, the top rail is tapered along it's length so as to cleanly attach at each end. At the ends it will be tenoned into the stiles. This tapering is additional work to be sure, but the purpose is to lighten the gate at it's extremities. Similarly, the brace itself would taper - from say 4.5" tall in section at the lower end to perhaps 3" at the upper end. The bars e, f, g and h also taper as they move from the hanging stile to the falling stile, say 1~1.5" over their length. The upper rail is over-sized, as it is in tension, and the greater width of the piece compared to the brace, lower stile, and intermediate bars below it, and thus serves to provide a small amount of weather protection to the lower parts of the gate. The upper tie would further be beveled or rounded on top so as to drain water more easily - a process known in western carpentry as saddle-backing. The joint connecting the brace to the upper stile has some vulnerability to the weather, so the abutment in that joint would be ideally sloped so as to throw water from the joint, or, even better in my view, caulked on the long grain sides.

Note that the lower hinge does not need to be as long as the upper hinge, for the hinge, and the gate frame in that area, are subject to compression loading. The cross bars are horizontally oriented so as to reinforce the gate structure a small amount - vertical bars would only add weight and provide no resistance to sagging over time, as is evidenced on a great number of picket fences one sees out there.

The brace should be properly let into the tie so as to bear against it with a good portion of its end grain. Here's an example of that from a State Park gate I photographed up in southern Vermont recently - look closely and you will see the outline of the joint:

A slightly-improved version of the joint in the picture would take a little less meat out of the underside of the tie and reduce the stress riser effect, by tapering the top abutment of the brace. In fact, there are several better versions of that sort of connection than the one shown above, like these as three examples out of perhaps a dozen or more variations:

I certainly wasn't expecting to see that sort of joinery on the park's gate - it is soundly made but not a highly-refined sort of item.

Here's a look at the entire gate, note the lengthened hinge straps (unnecessary on the lower end):

I have looked all over the place for examples of well-made and soundly designed braced gates to take pictures of, and so far this is the best example I have come across in New England. That's a bit sad to say I guess, given the commonness of the braced gate form. I'll keep looking.

And what about 'X'-braced gates? At first glace it might seem like a certain minor amount of support might be gained from the portion of the 'X' that is in tension, however, as mentioned in the previous post, the tension tie component is really pretty much useless at 'bracing' anything. There's more fault to find in such a construction however.

The 'X' brace might be composed of either 2 or 4 pieces. If it is of 4 pieces, that is, a continuous brace, giving one leg of the 'X', with two smaller pieces forming the other leg, and the continuous brace is oriented in the up-brace manner, then this is the best possible case for 'X' braced hinged gates. In the best case, the non-effective brace components have become simply decorative, which is another way of saying they are useless work. They are something trying to look structural but actually aren't, and in fact the extra weight of the parts serves only to add more load on to the hinges.

If, on the other hand, the 'X' brace is of two pieces, which in order to be in a common plane need to be half-lapped into one another, then not only has useless work been competed, but the strength of the only useful member of the 'X', the up-brace part, has been weakened by half. Thus it is a inferior way to brace a gate or door. I think that such is the case in this picture, a close up of the one shown in the first post of this thread:

Here's another example of an 'X' braced gate/door, however this time there's nothing wrong with the construction from a structural standpoint:

The difference here, despite the similarity of the 'X' bracing in that above door to other gates and doors, is that this door is not hinged, rather it is track-suspended. The loads on the door frame structure are completely different in such a case. I wanted to show this as a good example where unthinking imitations of a form, as evidenced by hinged gates with 'X' bracing, can lead to unsatisfactory outcomes.

This concludes a look at hinged braced gates and doors - I hope the reader found it worth the read.

Next up: polygonal hoppers.

Tuesday, October 13, 2009

Bracing Situation III: Tension Design Shortcomings

When I left off the the previous post in this series, it was with the note that, while wood is strongest in tension parallel to grain, what holds for wood as a material quality does not always translate into something useful when it comes to building structures with wood. The reason I say that is because it is generally not possible to connect wood pieces together so as to take any real advantage of the strength wood has in tension parallel to grain. How so?

When joining pieces of wood together, especially when it comes to outdoor structures exposed to regular trashings from the sun and rain, freezing and thawing, etc., the choices come down to all-wood connections, such as mortise and tenon, bridle joints, scarf joints, etc, and simpler, usually cheaper connections which are reinforced with metal fasteners, be they nails, screws, threaded rods, metal straps or brackets.

In the simplest form, the pieces of wood to be connected are left intact at the joined locations, and are bolted, screwed and/or nailed together side by side. This was the case with the down brace depicted in the previous post. With a fastener passing crosswise through the sticks of wood, though tension or compression is indeed resisted by the piece generally as loads are applied, at the connection zones the mode of loading changes - from tension parallel to grain, it becomes shear parallel to grain. This is clearly illustrated in Hoadley's fine book "Understanding Wood" (pg. 119):

The load that precipitates shear on the piece may be a pushing or pulling one along the longitudinal axis of the stick - shear parallel to grain is shear, regardless of push or pull. Here's an example of a covered bridge post that has failed in shear parallel to grain:

Notice how the brace has pushed up and sheared off a section of the post.

So, how do shear parallel to grain strength values compare to tension parallel to grain values? I'll post the chart from yesterday again - click on it to enlarge if you find yourself squinting:

It is readily apparent that shear parallel to grain values are significantly lower than the values for tension - or compression - parallel to grain. Though tension parallel to grain values are double (or more) those for compression parallel to grain, the compression parallel to grain strength is still some 5 times greater (on average, dependent upon species and moisture content) than shear parallel to grain. Clearly, the limiting factor for tension connections is the shear parallel to grain strength of the material.

Here's another telling drawing from Understanding Wood, showing the ASTM testing procedure for tension parallel to grain:

The notable thing is the despite only testing the midsection of the piece of wood, a mere 3/8" x 3/16" in cross section, the ends of the piece have to be enlarged considerably - to a 1" x 1" section- so as to preclude shear parallel to grain problems from affecting the test. The ramification of the above fact is that when a structural member is designed to be large enough so that its attachment fastenings can safely transmit the required loads, the cross section of the piece turns out to be far greater than needed along its entire length to carry tensile loads. Put another way, if the piece of wood to be placed in tension is not sized up to compensate for the weakness of the material in shear parallel to grain, the connection will invariably be prone to failure. if the piece of wood is up-sized to compensate, you are adding unnecessary weight to the structure, further exacerbating the loading problem at the joints.

Wood structures do not often fail from tension loads, but it does happen, as in this example from a warehouse truss lower chord failure:

Here's another example, again from a bottom chord, this time in a covered bridge:

Both above-pictured failures of timber in tension seem to associate to the presence of bolts passing through the member, thus decreasing the amount of fiber in that area. It's hard to comment from those pictures as to the mechanism of failure.

Compounding the problem in outdoor structures employing metal fasteners is that temperature variations between night and day are considerable at most times of the year, and thus dew forms. The dew point for metal, fine conductor that it is, is much lower than that of wood, so moisture condenses around metal fasteners more readily than it does on wood. Moisture around metal then is in direct contact with the surrounding wood, which gets moist, and thus rot tends to precipitate at such locations. Look at any old nailed fence and you will see the points of failure and greatest degrade are invariably at the fastener locations. The same goes for a down-braced gate - the lower end of the brace in particular also suffers from being closer to the ground, thus tending to pick up soil bacteria from rain splash which further accelerates the rot out. Topping that all off is the fact that moisture cycling and transfer in a piece of wood is most manifest at the ends (end grain) of the stick, and thus the degrade in a stick exposed to moisture cycling tends to also be most pronounced at the ends, where the rot-inducing metal fasteners are often located.

Further compounding the problem is that when wood is at a higher moisture content, as it would be at the humid times of the year, it will be weaker in strength. And finally, wood might resist a certain shear load if it is but intermittent or temporary, but with a braced gate, the loads on the parts from gravity are continuous. As Hoadley puts it, in a description of static bending strength (this also applies to other forms of wood strength like shear parallel to grain, etc.),

"...wood loaded to failure [within] one second (rather than the standard 5- to 10-minute duration)...would show about 25% higher strength. If held under sustained load for 10 years, it would show only 60% of the static bending strength. To put it another way, if a beam must support a load for 10 years, it can carry only 60% as much load as it carries in the static bending test. This reveals that in addition to the immediate elastic response which is apparent upon loading, there is an additional time-dependent deformation called creep."

If one moves away from metal fastenings at the connection points, and opts for all wood joinery, some problems remain. While the durability is likely improved, the connection of one piece of wood to another with joinery, be it a mortise and tenon, bridle joint or half lap, involves a reduction in the size of the piece at the connection. The piece is only as strong as its smallest section.

In a full depth half lap joint for example the maximum amount of wood that could remain after cut out is 50% for each half of the joint. Then, say, if that half-lap were pegged, the actual mechanical strength if the joint is going to be reduced to the pegs and their mortises for the most part, and the remaining wood will be under 50% of total cross section per side:

Now, given the mechanism of loading in this connection is again shear parallel to grain, a peg could be more or less substituted for the bolt in the first drawing in terms of its mechanical performance in the receiving stick (albeit, a peg is not nearly as stiff as a bolt). A mortise and tenon joint, of typical proportions would leave a tenon at 1/3 the thickness of the receiving piece, and then there is the peg, which again brings us to a shear parallel to grain issue. The above picture depicts a longitudinal connection or splice; in a down-braced gate, the brace might suffer from the same shear, while the receiving pieces for the lap, the stile and/or lower rail, would be loaded more or less somewhere between tension perpendicular to grain, and shear parallel to grain - and the strength values for tension perpendicular to grain are even poorer than for shear parallel to grain in most (if not all) species.

In fact, when it comes to designing pegged wood connections, the timber framing practice is now to follow the established engineering standards for bolted timber connections: compression joints require a minimum 3x peg diameter measured in length of material which remains in the tenon beyond the peg - the relais, or 'relish' as it is termed. For tension joints however, 7x peg diameter is the minimum. All things being equal, a tenoned tension connection is more more than twice as weak a connection than one in in compression, and must be compensated for by more than doubling the relish in the tenon if the tension joint is to perform adequately.

Let's look at a simple diagram depicting the loading issues with a down-braced gate:

This illustration is from Newlands "The Carpenters Assistant", p. 176. In the drawing above, 'a' is the stile from which the gate hangs, 'b' is the lower rail, and 'c' the down brace. The circle marked 'W' indicates the loading the gate experiences. With this loading, it is obvious that component 'c', the down brace is in the role of a tie, while 'b' is in the role of a strut. Further, the connections where the brace joins the hanging stile, 'a' and where it joints the lower rail 'b' will both be tension connections. Only the joint between the hanging stile and the rail will be a compression connection.

As Newlands succinctly puts it,

"...if timber be used for the brace 'c', it is evident that the strength of the brace has very little to do with the stability of the framing; that, in point of fact, the stability is due entirely to the strength of the nails, or to the slight resistance to tearing that the fibers of the timber between where the nails are driven and the end of the brace offer; or it must be insured by adding and iron strap to each end of the brace. But this extra iron is expensive...".

Not only were blacksmith-produced iron straps more expensive, the same premature rot issues detailed above in this post would associate to such construction, and more so on the lower end of the gate nearer the soil. The 'slight resistance to tearing' that Newlands mentions is of course shear parallel to grain.

In the final post in this thread, I'll take a look at why an up-braced solution to supporting a hinged gate or door is best, more so even than 'X-braced', and show what good construction detailing for such a gate or door might comprise.

Thanks for swinging by today. Please close the gate behind you when you leave :^)

Go to part IV (<-- a link)