Bridge design

[Ron Nossaman]

>     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.

A piano bridge isn't like a load bearing rafter or joist in a building. The 
panel and rib assembly support the load, the bridge distributes it. The 
loading is quite different too. Unlike a roof, as the soundboard "fails" 
over a period of time from compression set and creep, the crown lessens, 
the bridge sinks, the string bearing angles decrease, and the load 
decreases. The decreasing load explains why we see more failed roofs lying 
on the floor than failed piano soundboards.

>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 coorectly 
>assessed.
>Keith R

I don't know of any definitive testing, but from my practical shade tree 
engineering experience, I'd say that a system that depends on the presence 
or absence of a glue joint to function (as opposed to making it possible or 
convenient to build in the first place) is way too close to the edge of 
functionality, and practically impossible to reliably duplicate. In an 
actual piano, the bridge needs to be at least stiff enough, plus enough 
stiffer to provide a margin allowing for the inconsistencies of wood, and a 
little more for Murphy. Beyond that, you're adding mass. There is 
considerably more aurally detectable difference between stiff enough and 
too flexible, than there is between stiff enough and stiffer than minimally 
necessary. If the glue joints make a difference at all, that difference 
should be buried in the minor variations well into the designed-in safety 
margin. At least that's the way I see it.

Ron N