Bridge design

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Erwinspiano@AOL.COM=20
  To: pianotech@ptg.org=20
  Sent: July 12, 2002 12:29 PM
  Subject: Re: Bridge design


  In a message dated 7/12/2002 10:31:34 AM Pacific Daylight Time, =
pianobuilders@olynet.com writes:



      Del
       Thanks=20
      Pre-molded rib radius. That is slick. Makes sense. Any  =
observations  to tonal differences? /superiority of Lam. ribs verses =
crown cut ribs? I suspect not and any  tonal enhancement difference is =
in the design. Still on the learning curve.
      What about number of laminations? Have you found  an optimum? My =
thought is long deep ribs will require more thinner lams. or less =
thicker lams. and shorter ribs the same thing depending on some decision =
as to what's enough for adequate stiffness, consistency etc.
      Economically more lams. means more machining
      Dale Erwin

    1)    Tonal superiority. All things being equal I doubt there is any =
though I've done any specific testing to prove this. I'm not sure how I =
could. There are too many other variations job-to-job to be able to =
isolate just the one factor. In other words, if there is a difference, =
it's not dramatic. The laminated ribs are more consistent rib-to-rib =
since the variations of wood characteristics are evened out. For the =
same reason I believe crown stability to be better and longer lasting.

    2)    Number of laminations. I know the lay-up schedule we use =
--it's different for each height of rib -- whether that is optimum or =
not I don't know. It works and works well.

    It's not really a matter of 'adequate stiffness.' The stiffness of =
the finished rib is going to be a function of the wood characteristic -- =
its stiffness, or its MOE -- and the overall height and width =
cross-section. It's more a matter of how much spring-back is allowable. =
When bent laminates are made there is always some springback when the =
laminated is taken from the press. This is true whether a glulam beam is =
being made for your church roof or a grand rim is being taken from the =
rim press. A rib with more, thinner laminations has less spring back =
than does one fewer, thicker laminations.

    3)    I did not start developing and using laminated ribs as an =
economy move, though they probably would be on a much larger scale of =
manufacturing. They certainly make better use of excellent quality wood =
that would otherwise go to waste. Nor do we continue using them for =
economy reasons now. Indeed, they are some more expensive for us to make =
on our very small scale of manufacturing -- whacking out a few solid =
spruce ribs would be some cheaper. We continue using laminated ribs for =
the various reasons mentioned in earlier posts.

Del

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