More on soundboard crown

[Phillip Ford]

>>The structural arch works by virtue of being made of a non-compressible
>>material such as stone, concrete, etc. It is buttressed by an immovable
>>support on either end. When loaded, it supports the load because of the
>>non-compressible nature of the arch.

There are numerous examples of arches made from wood and steel (both 
compressible materials) in service as bridges, some of them for 
centuries.  Here is ye example of an olde bridge from days of yore:

http://www.quns.cam.ac.uk/Queens/Images/old-walton-bridge.html

Most covered bridges are wooden arches or arch-trusses.  Here's one 
in which the arch structure is apparent:

http://www.calgoldrush.com/travel/new.html

The covering on the bridge was basically there to protect the wooden 
load bearing structure from the elements.

Here's an interesting article about a steel arch:

http://www.railroadextra.com/brhellg.Html

On a local level (to me) San Francisco has the Golden Gate Bridge 
which is renowned as a long span suspension bridge.  However, the 
bridge approach from the San Francisco side is actually a massive 
steel arch which spans over Fort Point, which was a working army base 
at the time the bridge was built and presumably didn't want to be 
turned into a huge block of concrete for anchoring the suspension 
cables.

http://sfphototour.tripod.com/ggbridge_ftpoint.html

Stone is not selected for arches.  Arches are selected for stone.  If 
stone is the material that you have to work with and you have an 
unsupported span you don't have much choice but to use an arch. 
Stone is very good in compression but notably bad in tension (even 
more so the mortar).  The arch allows the load to be carried in 
compression rather than in bending, which would result in tension. 
This doesn't mean that arches can't be built with other materials.

>>The soundboard panel is made of wood
>>which is far from non-compressible. Even longitudinally. M&H makes much of
>>their little demo showing how their spider is supposed to support crown.
>>What they don't mention is that it takes only light finger pressure to
>>flatten the piece out. Nor do they mention that if they leave the piece in
>>place for any period of time, say a month or two, the crown disappears.
>>Wood, at least given the crown radii typical to the piano, is not rigid
>>enough to function as a structural arch.
>>
>>Del


I agree that if I was designing a panel to work efficiently as an 
arch I would use much smaller radii.  However, if I was designing a 
rib to work efficiently as a beam I wouldn't make it as wide as it is 
tall and I wouldn't have the same cross section over most of its 
length.

Phil Ford