laminated ribs

[Ric Brekne]

Hi David

No proplem here. You have stress and strain on the one hand, and tension and compression 
on the other hand. Tension and compression are opposite types of strain resulting from 
applied stress. Stress and strain are neutral with regard to direction. Stress is applied
force, and strain refers to whether or not the the thing being stressed actually becomes
either compressed or tensioned as a result or not. Used in engineering all the time 
and referes to the deformation per unit of the body being stressed. With that in mind
I think you will find its specific meaning below quite clear.

Cheers
RicB



-----------
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 <https://www.moypiano.com/resources/#archives> 

-----Original Message-----
From: pianotech-bounces at ptg.org <https://www.moypiano.com/resources/#archives> [mailto:pianotech-bounces at ptg.org <https://www.moypiano.com/resources/#archives>] On Behalf
Of Ric Brekne
Sent: Monday, April 03, 2006 2:04 PM
To: pianotech at ptg.org <https://www.moypiano.com/resources/#archives>
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