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Scantlings #7519 03/08/99 10:30 PM
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Doug Ford Offline OP
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It was suggested that this forum was a better place to ask my question. Our boat restoration group is investigating a 60'or80'x140' building. For informal estimating purposes my question is: What kind of spans could be achieved at this size and what size of timbers might we need. The buiding itself is a simple "cannery" style place.

Re: Scantlings #7520 03/09/99 03:08 PM
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Grigg Mullen Offline
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A 60' span is probably beyond the reach of a conventionally joined timberframe structure, particularly given today's building codes. However, large trusses can be made with the addition of steel shear rings, plates, etc to carry the stresses in the joints. I have some experience with spans up to about about 30'. Not quite what you need.
However, Ben Brungraber has done some research on the old building methods, and has a number of references on the old metal plated, wooden trusses. I expect he would be a better source on this one.

Re: Scantlings #7521 03/09/99 03:12 PM
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Grigg Mullen Offline
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It appears that Ben doesn't list his e-mail in the Guild directory. His work number is 603-352-0395.

Re: Scantlings #7522 03/09/99 04:21 PM
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Joel McCarty Offline
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1) Brungraber can be found at ben@bensonwood.com
2) Doug Ford is in Vancouver, BC, and as such should be accessable to TFG members on the way to Whistler.
3) Shelburne Farms breeding barn in Shelburne Vermont is a solid standing example of such a truss, beyond 60'in span with all manner of iron rod and plate work visible to the visitor. It's quite attractive, really, though no one would confuse it with 'pure' timberframing.

Re: Scantlings #7523 03/10/99 08:17 AM
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Ed Levin Offline
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17th and 18th Century New England meetinghouses were regularly built with forty to fifty foot spans and wider, using king and queen posts trusses. Can't lay my hands on proof at the moment, but I think if you check in a book like Kelly's Early Connecticut Meetinghouses (see below), you will find multiple records of spans in the sixties and at least one seventy footer. So, depending on where you live, the technology for long timber frame spans may be as close as the attic of a nearby old church.

There are certainly numerous examples of long span timber trusses with steel rods and fixtures to be found in 19th Century builders guides by authors like Peter Nicholson, Thomas Tredgold, Frederick Hodgson and James Newlands. No implication intended here that this is easy, or that it should be undertaken casually, simply that it has been and can be done (This from a framer who is currently working on the design a 60' span truss).

Author: Kelly, John Frederick, 1888-1947.
Title: Early Connecticut meetinghouses; being an account of the church edifices built before 1830, based chiefly upon town and parish records.
Collation: 2 v. illus., map, plans. 32 cm.
Imprint: New York, Columbia Univ. Press, 1948.
Language: eng
Subjects: Churches -- Connecticut.
LCCN: 48007952

Re: Scantlings #7524 03/10/99 06:41 PM
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Rudy R Christian Offline
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The fact that long span trusses are complex from an engineering perspective, and difficult to fabricate correctly, cannot be overstated. As with all principal member framed structures, whether wood, steel or concrete, the loads developed in supporting large areas of loading (big floor or roof areas) are tremendous. Because of this concentrated load condition, joinery, materials, design and fabrication skills have to be the best available. These factors combined begin to explain the lack of economy that is inherent to long span trusses.

It can be very rewarding to consider the need for a long span truss as suspect to begin with. Often the goal is to emulate a picture in a magazine (normally residential) of a great room with a grand sprawl of clear span timbers. Other cases are much more real. Like an airplane hanger.

Unfortunately Ben, Ed, Joel, and Grigg can support the fact that many of the whimsical creations, seen in the coffee table magazines, designed to fill these requirements, are in trouble before they ever leave the shop. Sometimes it's a case of empowerment by observation (yeah, I can do that) but more often it's a lack of understanding of the real value of professional engineering services.

It is important to look at the real needs in your construction project, before assuming gigantic trusses are required. For instance: Will 30' trusses (much more economical and obtainable) in a central bay, flanked by to 15' bays work as well? If so, not only is the difficulty in engineering reduced, but the bay spacing (center to center location of trusses) is as well. This can often lead to a significant decrease in cost.

One other aspect to consider is allowable deflection. That's how far things can sag and still meet code. In a roof structure l/240 is often acceptable (in a simply loded condition) this means that a timber spanning 20' can sag a full inch and still be within the design parameters (for deflection). Push it to 30', and it's 1 1/2" etc. As deflection (sag) increases, so do the problems associated with it. Common practice says "let's hold this total to 1/3 inch or less". This can be a very real challenge in long spans.

All this is not meant to say "Don't do it!", but rather to say "Do you really need to do do it?" and if so, make sure you understand what it takes to do it right. It's kind of like driving in Montana. It's legal to drive 100 MPH, but is it a good idea?

Re: Scantlings #7525 03/11/99 04:49 AM
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Chris Hoppe Offline
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Though not true timber frames, I have worked on a number of buildings with wood roof trusses in the 70-foot range. They were bowstring trusses built in the 1950's with split ring connectors. The designs were elegant for their simplicity and lightness. Analysis indicated that they were slightly under-designed by today's codes, but new trusses could easily be engineered to meet current code loads. Also, deflection was not much of a problem because the height to span ratio's were in the 8 to 10 range and stress goverened the design. The most recent one I looked at spanned 70' and the largest single truss member was a 3x12x14'!
Chris Hoppe, P.E.

Re: Scantlings #7526 03/11/99 09:32 AM
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Bill Keir Offline
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I'm still new enough to this form of communication to be getting a big kick out of it.
As with the linseed oil issue, I'm glad to see all the usual suspects have added their 2 penny's worth, and very illuminating it is too.

Instead of rushing straight off home to deal with howling babies smelly nappies (diapers?)etc. I can visit here and pick up some stimulus for the ride home. Thank you.

I do however hope that there are more than we happy few visiting this site, and would ask that all visitors feel free to add to the debate, and/or simply say 'hello'.

As to the big truss debate:

All our big areas to date have been aisled barns, these plainly have the clear spaces interrupted by posts coming to ground. I have seen French markets where there are huge open sided covered spaces, fantastic mediaeval timberframes. Its hard to describe, but it seems that sometimes, they only bring to ground every other post , the intermediate posts, being picked up on beams spanning the ones that do come to ground.

Plainly there are mediaeval roof trusses that have large spans, (like the 50 foot span of our Scottish Hammerbeam roof at Stirling castle, but as we all know these need a considerable mass of masonry wall to buttress them

There are aircraft / balloon hangers, with monumental spans, (hundreds of feet), Mills too, and mostly in North America?

Moving closer to Timberframe, the Late Georgians, & Victorians were past masters of building large spans, using some metalwork (mainly straps). I had always thought (in this country anyway), that their designs were driven by a need to conserve / reduce the timber. This would also reflect the reducing cost of metal, (and the fabrication there off) as against the cost of big timber. We, in this overcrowded small island have always imported softwood, and the change to tension joinery, based on the Kingpost Truss, with slender lower chord, held up by a Kingpost in tension, was matched by a move towards building in cheaper softwood.

When these trusses became very wide with lots of queen posts as well, you could start to consider them as girder trusses.

Once when dealing with an Enquirer for a sports hall I designed a clearspanning truss 60 feet wide, all Oak - No metal, the project didn't get of the ground, so we didn't build it, but I did get a provisional nod from our Engineer, who thought it could be made to work. Although the span was 60 feet there were no timbers longer than 26 feet, and all the joints were proposed as wedged / pegged.

As the price per cube, (we buy by[?] the cubic foot not the board foot 12 Bd. Ft = 1 CuFt), rises exponentially as the length increases, it is economic to keep maximum lengths in the low twenties.

Is there the means to post visuals into this forum, and could someone as techo-challenged as I be trained to do this?

buy by for now, talk later


Bill

The pessimist sees difficulty in every opportunity

While

The Optimist sees Opportunity in every difficulty
Re: Scantlings #7527 03/11/99 12:44 PM
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Brian Wormington Offline
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I am aware of the bowstring truss type roof that Chris mentions. There was (emphasis on WAS) a fine example in one of our neighboring towns. The building served a variety of purposes over the years from meat packing to it's final guise as a hardwood supplier and carpentry shop. One could see the evidence of years of roof leaks and I suspect marginal repairs due to the steep height to width ratio.

Several winters ago a series of heavy snow storms took a spectacular toll. The roof collapsed with such force as to explode the masonary walls and make the result look like a bomb had gone off. Sadly, an undetected electrical short fueled by an ample supply of kiln-dried hardword produced a grand over night fire that melted most of the machinery.

The lesson to me was the one in all truss design of whether any redundancy exists. When a single member fails all the others can go with it. For many more entertaining examples of this sort, I recommend the reading of "Why Buildings Fall Down" by Matthys Levy and Mario Salvadori, ISBN 0-393-31152-x (1992 Norton paperback)

Re: Scantlings #7528 03/11/99 01:41 PM
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Bill Keir Offline
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I shall buy this book at once, perhaps i will at last be able to start learning from my mistakes


Bill

The pessimist sees difficulty in every opportunity

While

The Optimist sees Opportunity in every difficulty
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