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Thread: Wood Movement
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7th November 2013, 04:38 PM #31
John - using flexible glue is fine, and would work in some situations. The problem I would have is figuring out how flexible would be required for given situations. Too flexible & you get problems like your carport beam, not flexible enough, and you are back to square one. One of the things I don't like about PVA glues is their tendency to creep under load, but I reckon it helps it to hang on in some of the pieces I've seen where too much cross-grain glueing has been done. In the end, it's far easier, I reckon, to just design around woods' foibles. Think of it as adding to the fun(well, sometimes).
Cheers,IW
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7th November 2013, 11:49 PM #32
OK I've looked up the correct terminology for this -- it's called a double (or triple) haunched divided tenon.
the rule of thumb is wherever the width of a single large tenon is more than 10 times its thickness, the tenon should be divided into thirds — tenon - haunch - tenon
so for a 325mm wide board, the approx 300 x 12mm tenon will be approximately 25 times its thickness so divide into fifths — tenon - haunch - tenon - tenon - haunch - tenon, each about 60mm wide
John, a useful reference for furniture building joints and wood movement is:
Illustrated Cabinetmaking by Bill Hylton, Fox Chapel Publishingregards from Alberta, Canada
ian
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8th November 2013, 12:07 AM #33
STOP IT
STOP IT
sounds of hair being rent
from what you describe, 30 years ago you made two different types of beam
the one with the 308 was a glue lamination which relied on the initial curve and the glue to obtain its strength. The screws just acted as clamps to hold the glue surfaces together
the second (with the construction adhesive) was a nail (or if you insist bolt) laminated beam -- for Sydneysiders, the old low level bridge across the Nepean River at Yarramundi (replaced in the late 80s if I recall correctly) was nail laminated construction so the method is plenty strong enough for a garage roof beam.
please stop thinking like "any fool builder" and start thinking like an "engineer" — as in any fool can build something that will stand up, it takes a real engineer to build something that uses the absolute minimum of material to stand up.
which of course is the cue to insert the youTube clip of "Galloping Gertie"regards from Alberta, Canada
ian
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8th November 2013, 08:12 AM #34which of course is the cue to insert the youTube clip of "Galloping Gertie"
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8th November 2013, 10:29 AM #35... and this too shall pass away ...
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Ian,
You are absolutely correct. That much was understood 45 years ago, long before those beams were constructed. My dad taught me about laminated sections when we were building boats and replacing bearers in an old house. Had I known that the glue I was using on the second beam remained flexible, I would never have used it. However, my post was not about engineering, which was mentioned only to illustrate how much that adhesive moved, it was me whimsically musing about whether we could use that characteristic to our advantage in furniture construction and any limits that might apply.
Nevertheless, you are correct to suggest a good engineer designs to use minimum material/cost to achieve the desired strengths. You doubtless recall the significant safety factors built into the formulae used in Australia to design structural members for buildings and bridges. When compared with those used in places like some South American countries (in the early seventies), our member sizes were quite a bit larger. Not much "plastic engineering" happened in Australia at the time.
What I did not mention in the original post was that the design was checked by a mate who was a civil engineer. We did some calcs and then because the block sloped and the roofline was high (higher than any surrounding houses) and exposed to wind, we increased the beam depth accordingly and connected everything from the rafters to the foundations with steel to make sure it would not disassemble/fly away in one of Brisbane's wild storms. Funny ... I still recall calculating the section modulus for the beams and then using the Lysaght tables to get rafter sizes. Of course, you are correct to suggest that any fool builder can do as much. It is very simple.
I know I have a tendency to over-engineer, but don't care at all if we are discussing something being done for myself as opposed to a bridge beam that will cost the taxpayer a small fortune. Perhaps in part this tendency is a consequence of my army days. I can recall being given 48 hours to design and build a bridge (with timber we felled locally) over a creek that would support armoured vehicles for a brigade. You can safely assume it was over-engineered. I measured the deflection in the main beams with a large towed roller before the brigade's vehicles went over it. It was 5/8 of 4/5 of not a great deal. If I had not demolished it with explosives some time later, (much faster and a lot more fun) it would likely have been there until mother nature reclaimed it with rot and insects. Compare this with a young engineer I once visited who spent about a year designing some beams for a large bridge to ensure he met his design criteria at minimum cost.
Ian, please don't rend your hair. I doubt I was as naughty as it may have seemed. Because it was not central to the point I was trying to discuss I failed to mention that the bolts I put through the beam went also through the rafters. I know ... over-engineered ... but it was a cheap and effective way of ensuring the entire roof structure was held together with steel in an exposed area visited by cyclonic winds. My real crime was using an adhesive recommended by a store owner who did not know what he was talking about. If this situation had been one where the myopic led the blind we might have done OK, but it turned out to be the blind leading the myopic. Lesson learned. We should be very careful about from whom we accept advice. The good news for me is that there are many skilled and competent cabinetmakers at this forum, like you, the other Ian and Wongo (for instance) who are a very valuable resource.
Thanks,
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