laminated ribs

[David Love]

The problem, I believe, is that "strain" is not an engineering term that I'm
familiar with and its specific meaning in the description below is vague.

David Love
davidlovepianos at comcast.net 

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ric Brekne
Sent: Monday, April 03, 2006 2:04 PM
To: pianotech at ptg.org
Subject: laminated ribs

Hi Dale

This argument about ribs not supporting crown has bothered me from the 
first moment I heard it. And after reading Nossamans well written 
article in the latest Journal  I think I know why.  Ok, nobody questions 
that in a CC board ribs do not provide beam support for the load. But 
thats not quite the same thing as saying they dont support load in a 
different fashion.  The same thing goes for the crown arguement. And 
thats where Rons article comes in.

About 2 years ago I posted a couple threads with some drawings trying to 
explain why I thought the ribs in a CC board had similiarities to a 
cable in the sense that they attempt to constrain the board from 
expanding... so the panel has to bend instead. That very resistance to 
the panels expansion is every bit as much a load support but in an 
entirely different way. What the kicker back then was, was that I tried 
to argue that the ribs strain (note the word usage) against the 
expansion forces from the panel.  I was told then that no.. the ribs 
dont strain.... they simply bend against their better nature.  Enter 
Rons article disclaiming the buttress arch.  In that article he shows by 
experiment that the top half of the ribs not only bend, but they expand 
lengthwise. That expansion is critical to his whole arguementation 
(which by the way made perfect sense to me).

But that same rib expansion shows conclusively that the ribs do strain, 
and significantly so against the expanding panel.  If you stop to think 
about it this only makes sense.  If the ribs can not strain lengthwise 
at all, then neither could the panel crown, yet if they strained equally 
through their height then they would not constrain the panel at all.  It 
is because they DO strain ... more on top and increasingly less towards 
the bottom combined with the panels compression that crown and crown 
strength occur. And it doesnt really seem to me to be so much a stretch 
of the mind to imagine mathematical explainations for all this that 
would fit very nicely into design thinking.  The height and width of 
rims dont add up to combine in a kind of beam strength / mass 
relationship... but rather a kind of strain strength / mass one.  

One thing is clear about load support in CC boards. The more you push on 
it, the more it resists... until its overloaded of course.  But until 
that point there is definately load support and the ribs are definatly 
part of that... just not in the sense of beams.

Cheers
RicB






------------------

Dale,
Those ribs were originally built into a CC board. How can  a
compression crowned board get mechanical support from the rib
scale,  however "good" the scale looks? The ribs in CC boards
resist the crown  that panel compression is trying to form and
maintain, and just put more  compression load on the panel.  -----Ron
 
  No, I get all that Ron, but if the ribs are built significantly  taller &
of stiffer material by design  then more panel  compression can be taken 
out of
the equation.  Another thought is, &  I've witnessed this quite a few times
is, that Stwy A's (1 & 2"s) in  general can produce a pretty wonderful 
sound
even with a  flat or  flattish board providing there is some small but
consistent bearing load  still intact so in this case it would seem that 
there are
enough impedance  factors about the rib scale to make the system work 
rather well.
Ok maybe a  freak of nature but it happens fairly frequently.  
   About 5 years back I had such an long A I was going to  resell it. It 
had
Steinway hammers which were quite soft &  made  it sound really good.  
It was
hard to imagine that a new board would make  it sound much better. It truly
sounded glorious but it was a spec job  & I don't sell old boards very 
often. I
didn't do any thing to the  action until later for a really good A b
comparison.
   So  I built a board with the same number of ribs  making them crowned at
about 60 ft. Made em taller but not much. Used sugar  pine in the bottom &
yellow pine in the top.  The sound was  cleaner and the sustain was 
about the same
which was awesome.  It just  had it!!
  I attribute much of this to the original basic rib scale  design. 
Something
was working or several things were.  Do you see what  I'm saying?
  I'll crunch some numbers & see what I got.  I greatly  Appreciate the
design sharing & information swap.
  Thanks
   Dale


It's  an entirely different system. Do a bearing load analysis
on the ribs as  load carrying beams and see what the numbers
say. They'll say that the rib  scale isn't adequate to support
bearing without substantial panel  compression support.