Thread: Wood Movement

Originally Posted by John Samuel

Below is a day bed made recently. I now see that the 325 mm deep side and back panels are unable to expand where they are glued to the legs. I could, in the future, make the side panels (say) three separate pieces of timber with space between them, or I could make it with the frame and panel method. Any other ideas/suggestions?

Originally Posted by ian

the "proper" or traditional way is to form divided tenons on the ends of the panels. For the moment I can't recall the proper term, but think of a tenon in the shape of an E with very wide horizontal strokes -- the corresponding mortice would have three pockets, two of which would be oversize. Only the centre portion of the tenon is glued, the other two can them expand and contract in their "pockets"

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 Publishing

Originally Posted by John Samuel

Here is a wild untested and unsupported theory developed in my never-ending pursuit of over-engineering.

About 30 years ago I built a timber double carport. I wanted a clear span, and because I was as poor as a church mouse, needed to be able to make it myself at low cost. I needed two beams to span the 6.5 M carport, the rear beam was supported by a post, but I wanted no posts in the middle or front of the carport.

I decided to make laminated beams out of 3X2 inch oregon pine. The beams were curved. I made them by putting a 60 mm block in the middle, bending the first member over this and nailing it to the carport floor at the ends. Then I built it up to about 10-12 inches thick using 308 glue and screws. Then I put some bolts through the lot (told you I was good at over-engineering). I made the first beam and was delighted. You could park a battleship on that beam and it would barely flex.

Sadly, I used all the 308 glue on one beam, and it was as dear as poison ... so much so I could not afford to buy any more. I stumbled around the hardware store until the owner decided to give me some advice. He recommended another glue he assured me would do the job admirably, but at a fraction of the cost.

I made the beam ... with an early version of what we now call construction adhesive ... and discovered that this glue moves ... a lot! Under a very light roof, that beam flattened ... lost it's 60 mm curve entirely ... stopped only when the bolts hit the sides of the holes and locked everything into place. Drove past it recently. It's still there and still flat.

So, here is the question. If instead of leaving some joints without any glue at all we were to use construction adhesive on these joints, would the glue be flexible enough to move with the humidity? Would it move enough? Would it harden with time and lose its flexibility? I note that some manufacturers claim their product remains flexible ... but how flexible?

OK guys ... be honest ... but be gentle. I'm feeling a little sensitive after having all my wood movement sins exposed to the entire universe and I am just bouncing ideas around at this stage.

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"

which of course is the cue to insert the youTube clip of "Galloping Gertie"

The pity is that the lesson of Galloping Gertie was forgotten when London's Millennium Bridge was designed. Fortunately the results weren't so catastrophic.

Originally Posted by ian

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

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,