Sunday, July 11, 2010

New Hand on an old Farm (II)

Please take a look at the archive to the right of the page if you missed part one of this thread and want to get up to speed with the topic.

Today I want to give some attention to one building on the Avon Old Farms school site - the Chapel. From a distance, this structure with exposed timber work, brick nogging, and Jerkin head roof with gablet is quite charming:


Moving in a little closer one can see the exposed pegging and rational use of longer opposed wall braces at the corners instead of knee braces:


A slightly different angle takes in more of that front wall:


Inside one sees a frame fairly typical of an English barn, employing a form visually similar to king piece trusses except with intermediate purlins abutting the struts:


So, they aren't really working as trusses despite the superficial resemblance to a truss seen in the structure.

The knee braces along this wall, and elsewhere, are also typical of barn construction:


Now, let's zoom in a little closer shall we, to the place where the right side knee brace in the above picture meets the post:


Whoa! What's going on here? I surmise it wasn't a bad initial fit we are viewing, but rather a situation of significant subsequent moisture loss from the members after the frame went together.

Another typical cluster of connections in the chapel frame:


And zooming in a little closer to where the tie beam in the above picture joins the post, what do we see?:


Are these gaps the exception? Um, no. I wonder what's left of the tenon inside the post?

Here's another:


And another:


The horizontal member in the above picture, because it is not housed into the diagonal to which it joins, is relying only upon the tenon to bear loads, which in reality means the brick nogging below it in this case as the tenon is looking vulnerable at this point to failure.

And another example:


Looks like complete failure of the tenon's relish on this one:


I could go on with many more examples, but why don't we look around the outside some more?:


Structural integrity of the above connection looks questionable at this point. Structural integrity, when you get down to it in joined wooden structures, is invariably negatively affected by significant moisture loss in the wood members, along with the cracking and checking that accompany that shrinkage.

The sills have seen better days, too:


The frame is a curious mix of post on sill and post on ground with infilled sills.

Dare I zoom in a little closer to the base of one post (of many in similar condition)?:


On this portion, the 'sill' is little more than a fake, a covering board for the concrete foundation:


That was disappointing - I hate fake stuff, and these pseudo-sills were thankfully the only part of the frame that were so-configured.

Yes, the building needs some work. That's not the point of my posting these pictures though. Hey, what can we learn here?

My point relates to the pitfalls of using unseasoned (ie., 'green') material for timber framing and how all those laboriously cut joints tend to look and, more significantly, function, after a little time has passed by. This building is around 90 years old, so it has likely outlasted a modern stick built house by a factor of two, but one can see that the promise of a timber frame lasting 150~200 years, as many framers seem to hope, is looking rather unlikely in this case.

One can fully expect rot and severe degrade with any timber in close proximity to the ground. This is not a new lesson in the building sciences, but one that goes frequently unheeded, as in the case of this chapel. The mudsills also suffer a similar fate to the post bottoms, however the brick nogging makes replacement of the mudsills are fairly involved job, to put it mildly. One is unlikely going to be able to jack this building up in the air and just slide in new parts, and dis-assembly is going to take quite a while otherwise. Because access is difficult for repair, the parts tend to not get repaired. It would have been vastly better upon initial construction to have made the foundation come up 18" or so off the ground and then placed the frame on top. It would have been better to have a deeper eave, although in comparison to many East coast buildings, this chapel does at least have some eave.

The interior framing is protected from the weather, however because green timber was used, and was of a material (oak) which shrank a lot as it dried, the gaps rendered at the connections have in turn left the frame with minimal structural integrity. Don't let anyone tell you that green frames somehow shrink and "tighten themselves together". Such might occur with the unrelated example of chair construction using a green seat with dry rungs, but not in a timber frame.

Angled braces suffer always from having minimal relish on their tenons, and if careful attention is not paid to the details of peg size and distance of peg from the tenon shoulder, there is not much meat there to resist tension of any sort. Some of the tenons make have blown during assembly of course, which is not an altogether uncommon thing to happen. Braces of course act as struts, which means they are designed to handle compressive loads, so the pegging is only useful on brace tenons insofar as holding the part in place during frame assembly. In most cases, if the frame can be assembled without the recourse of pegs to keep the braces in a particular position during assembly, then the pegging is best eliminated from braces altogether. There are a few exceptions to that, but in summary, it is best to design timber frame structures around compression forces and not tension, insofar as possible. Braces work best in compression.

If you consider the lifespan of the building, and the say condition of the frame due to the use of unseasoned material, one can see that a few months of extra time taken at the beginning to prepare that material properly would have paid major dividends down the line. Why, in our society, it has come to be seen as 'un-economic' to do that is an interesting question. People would seem to want cheap up from costs, while ignoring the costs that come later in maintenance, alteration, and repair, which generally exceed the initial fabrication cost several times over.

Looking at old buildings is a great way to learn about the ravages of time and other factors upon a building. I suspect that the chapel at Avon Old Farms has been well maintained during the years, yet it suffers from several design flaws which shorten it's ultimate lifespan and make it difficult to repair. At this point, the structure could benefit from a complete dis-assembly and rebuild, though I imagine that won't happen. More likely it will be demo'ed and replaced by some sort of modern day clumsy clone, since that is a lot cheaper to do. That will be a sad day if it comes.

Despite the chapels core shortcomings in structural and design terms, it remains full of charm and I'm sure is well-loved by those that use it. I found it a pleasant space to explore and from which to learn.

Thanks for coming by the Carpentry Way on your travels today. Comments always welcome.

2 comments:

  1. I guess we now know the real reason why the wolf was able to blow the little pigs house down.

    thanks chris that is very interesting to see, i would not have imagined quite so much shrinkage.

    ReplyDelete
  2. Gregore',

    yes, a lot of shrinkage. And consider too that the building lacks a central heating system, which would otherwise exacerbate the problem.

    ~Chris

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