The problem, I believe, is that "strain" is not an engineering term that I'm familiar with and its specific meaning in the description below is vague. David Love davidlovepianos at comcast.net -----Original Message----- From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf Of Ric Brekne Sent: Monday, April 03, 2006 2:04 PM To: pianotech at ptg.org Subject: laminated ribs Hi Dale This argument about ribs not supporting crown has bothered me from the first moment I heard it. And after reading Nossamans well written article in the latest Journal I think I know why. Ok, nobody questions that in a CC board ribs do not provide beam support for the load. But thats not quite the same thing as saying they dont support load in a different fashion. The same thing goes for the crown arguement. And thats where Rons article comes in. About 2 years ago I posted a couple threads with some drawings trying to explain why I thought the ribs in a CC board had similiarities to a cable in the sense that they attempt to constrain the board from expanding... so the panel has to bend instead. That very resistance to the panels expansion is every bit as much a load support but in an entirely different way. What the kicker back then was, was that I tried to argue that the ribs strain (note the word usage) against the expansion forces from the panel. I was told then that no.. the ribs dont strain.... they simply bend against their better nature. Enter Rons article disclaiming the buttress arch. In that article he shows by experiment that the top half of the ribs not only bend, but they expand lengthwise. That expansion is critical to his whole arguementation (which by the way made perfect sense to me). But that same rib expansion shows conclusively that the ribs do strain, and significantly so against the expanding panel. If you stop to think about it this only makes sense. If the ribs can not strain lengthwise at all, then neither could the panel crown, yet if they strained equally through their height then they would not constrain the panel at all. It is because they DO strain ... more on top and increasingly less towards the bottom combined with the panels compression that crown and crown strength occur. And it doesnt really seem to me to be so much a stretch of the mind to imagine mathematical explainations for all this that would fit very nicely into design thinking. The height and width of rims dont add up to combine in a kind of beam strength / mass relationship... but rather a kind of strain strength / mass one. One thing is clear about load support in CC boards. The more you push on it, the more it resists... until its overloaded of course. But until that point there is definately load support and the ribs are definatly part of that... just not in the sense of beams. Cheers RicB ------------------ Dale, Those ribs were originally built into a CC board. How can a compression crowned board get mechanical support from the rib scale, however "good" the scale looks? The ribs in CC boards resist the crown that panel compression is trying to form and maintain, and just put more compression load on the panel. -----Ron No, I get all that Ron, but if the ribs are built significantly taller & of stiffer material by design then more panel compression can be taken out of the equation. Another thought is, & I've witnessed this quite a few times is, that Stwy A's (1 & 2"s) in general can produce a pretty wonderful sound even with a flat or flattish board providing there is some small but consistent bearing load still intact so in this case it would seem that there are enough impedance factors about the rib scale to make the system work rather well. Ok maybe a freak of nature but it happens fairly frequently. About 5 years back I had such an long A I was going to resell it. It had Steinway hammers which were quite soft & made it sound really good. It was hard to imagine that a new board would make it sound much better. It truly sounded glorious but it was a spec job & I don't sell old boards very often. I didn't do any thing to the action until later for a really good A b comparison. So I built a board with the same number of ribs making them crowned at about 60 ft. Made em taller but not much. Used sugar pine in the bottom & yellow pine in the top. The sound was cleaner and the sustain was about the same which was awesome. It just had it!! I attribute much of this to the original basic rib scale design. Something was working or several things were. Do you see what I'm saying? I'll crunch some numbers & see what I got. I greatly Appreciate the design sharing & information swap. Thanks Dale It's an entirely different system. Do a bearing load analysis on the ribs as load carrying beams and see what the numbers say. They'll say that the rib scale isn't adequate to support bearing without substantial panel compression support.
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