Bridge design

[Delwin D Fandrich]

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Re: Bridge design
  ----- Original Message -----=20
  From: Keith Roberts=20
  To: pianotech@ptg.org=20
  Sent: July 08, 2002 9:04 AM
  Subject: Re: Bridge design


      A laminate structure is far more resistant to flex.=20
Not so. According to the USDA Wood Handbood: Wood as an Engineering =
Material, the stiffness characteristic of a glulam timber is the same as =
a solid timber of comparable size and grade. That is assuming that the =
grain density, freedom from defects, etc., are the same -- and this is =
the key. Given today's lumber supply it is very difficult to get beams =
of any size that are of clear, uniform stock. Laminating smaller boards =
of better grade together overcomes this limitation and the glulam of =
higher (average) grade stock is stronger than a solid beam of overall =
poorer (average) grade stock.


  Bend too straight pieces of wood and glue them together. They want to =
stay where you glued them and will stay that way longer and with less =
warpage than a piece of solid wood cut that way. (presuming the glue =
joint....etc.) I've been in the construction trades 30 years and a =
laminate beam is far more desirable structurally than even pattern =
nailing a bunch of boards together.
All this is true, but things like stability, freedom from warpage, etc., =
have nothing to do with ultimate strength. It is these things -- =
stability, consistency, etc., that make glulams desirable. Not greater =
strength.=20

Our own experience bears this out. We've been using laminated Sitka =
spruce ribs for many years and our direct comparative tests have not =
indicated that they are any stiffer than comparable solid Sitka spruce =
ribs. They do have many characteristics we find to be desirable, but =
greater stiffness is not one of them.


      Now here's where I may be presuming two much. It seems to me, as =
the bridge is loaded and the soundboard/bridge structure approaches the =
point of failure, the impedance would change exponentially.=20
I can't imagine how a bridge/soundboard assembly could possibly be =
loaded to anything close to its point of failure. Relative to their =
ultimate load-carrying capacity these systems are very lightly loaded.


  Small initial changes can become major. In my limited experience I =
haven't heard of any real model test of 2 identical boards and bridges =
glued up and run through a battery of tests. It was my feeling (my 2 1/2 =
years of engineering at UCSB is now a jigsaw puzzle with a bunch of =
missing pieces), that the glue joint as a variable might not have been =
correctly assessed.
I'm unconvinced that the glue joint has much of anything to do with the =
ultimate performance of the bridge/soundboard system.


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