Forcing Model Timber Frame Joints to Failure using a Hydraulic Press
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- Опубликовано: 5 окт 2024
- In both our in-person and online Purely Post and Beam Course, we spend time reviewing engineering theory and how it applies to timber framing. One of the most exciting parts of this engineering module is when we use a hydraulic press to demonstrate how timbers and joints withstand different levels of loads, and what happens when they fail.
In our new timber framing online course, we have worked hard to compile every last bit of content from our in-person course and put it in an easily digestible, video-by-video, online format. And not only that, but we’ve also tried to anticipate every question that you might ask as a student of our online course, since you don’t have the luxury of asking an instructor during class.
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As interesting as it is, isn't it a bit of a flawed example? Failures like the dovetailed timber wouldn't be the same if the tenons were in the mortised spaces preventing it folding in on itself. Would be more interested in seeing how the completed joints with both mortise and tenons compare to the solid timber.
Thank you for the question. Most failures in wood occur on the tension side due to the natural defects (knots, slope grain, etc) in wood, contrary to the theoretical idea that the maximum fiber stress value of a (clear) species of wood is the same on the tension and compression sides (top and bottom). Putting a wood plug (such as a tenon) on the compression side does not make an appreciable improvement in the performance of the dovetailed timber. From a structural engineering standpoint, we must treat the mortised timber as if it has an open mortise because the grain orientation of the inserted tenon is perpendicular to the grain orientation of the mortised memeber, and the perpendicular-to-grain compressive strength of the tenon is less than the parallel-to-grain compressive strength of the mortised member, to say nothing of the possibility of shrinkage of the tenon eventually eliminating any contact between the mortise and tenon. All of this is to say that the test being performed gives the best representation of the actual capacity and failure mode of the mortised member.
Great demonstration of the strength of wood. Even though the beams failed, they didn't break all the way thru and they gave plenty of warning they were going to fail with all the creaking and cracking. Metals do not behave like that at all.
What the heck are you talking about? They didn't load the beam past ultimate capacity of the wood, as they stopped loading it after the initial appearance of splitting/lamellar failure in the tension face of the sample. They specifically defined failure as splitting at the tension face which is why they stopped loading it; failure occurred suddenly, in a brittle manner, which is opposite to what steel does. Creaking doesn't mean failure as wood "loosens" under static load over time (a phenomenon called "creep"), just like concrete; creep isn't failure, but only reduced performance (think SAGGING over long term). Metals are insanely better for failure prediction and performance entirely; think about how many times you can bend a paper clip back and forth before it eventually breaks into two. By the way, the ultimate capacity of wood is amusingly varied between species and grade, and even inconsistent between batches which makes building large structures immensely risky, if you chose to build with it. Timber is good enough for what it is used for as well as architectural applications; metals are superior (though we use concrete everywhere due to its availability, but even that needs steel to "hang" with metals).
Hmm, increase the cross section where you have tension loads, keep knots out of loading paths. Of you can go to rounded corners the failure mode changes too. I expect if there was wood occupying the empty space to provide a block to compress into, the loading would change too. I wonder what the typical tension on the bottom to the compression on top ratio would be and if denser growth rings make for better tension or compression load resistencs?
This is fascinating stuff and the sort of thing I've always wondered about. Are there any books you'd recommend for a regular ol' hobby woodworker?
Cool video, gained : square more stress, round corners less stress.
Very interesting!
Not to be picky but it is a hydraulic press. It was informative to see it fail in real-time though. It would have been interesting to have measured the deflection at failure.
Very informative indeed. Isn't it hydraulic rather than pneumatic press?
All this test proves is that if you reduce the cross section of the test piece it will reduce it’s overall strength. Can’t see the point of the demo myself .
Not only that but the point load being spread across that plate on top simply moves the moment of inertia to the edges of the plate. More or less splitting the single point to two separate spots. I also love how no one is using safety glasses ....