> >>Also, the ribs tend to get ignored in these arch discussions. If >>the ribs are glued to the rim then they have an arch load path >>along the grain (their strong direction). The load will divide >>itself among the load paths according to their relative stiffness. >>If the rim is flexible and the cross grain stiffness of the panel >>is low and the compressive stiffness of the ribs is low then this >>path will be soft relative to the bending path. That doesn't mean >>it's nonexistent. Perhaps 10% of the load could be taken by the >>arch and 90% by bending. > >I can't believe it's anywhere near 10%, for all the reasons >discussed in the past when this came up. But yes, let's talk about >those ribs in the context of arches. ... >Now define point A and B at opposite ends of a flat bottomed (as >they usually are) crowned rib. If the panel didn't bend the rib when >they were glued together, the rib is essentially straight before the >string are installed. String load will then bend the ribs, bringing >points A and B closer together as the load increases. There's no >arch at all in crowned ribs. This rib doesn't produce any outward >thrust as crown is deflected. It pulls inward on the rim. It's like >a cable suspension subject to the same limitations as the arch >theory except it has more room. ... >Ron N Ron, One additional comment on this. This is an interesting idea. If you made the ribs deep enough in the middle, with the feathering at the ends, then the line of centroids of the cross sections actually would be defining a suspension cable sort of arc. Perhaps you could get some additional support or stiffness from this in addition to the bending stiffness of the deeper rib. Phil Ford > >_______________________________________________ >pianotech list info: https://www.moypiano.com/resources/#archives
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