Laminated Soundboard Ribs

[Phillip L Ford]

Terry,
In theory the bending stiffness of a beam (or in our case a rib) is dependent on
the length, moment of inertia of the cross section, and the modulus of elasticity
of the material.  Assuming two ribs of the same length we can eliminate that
variable.  The moment of inertia is determined by the geometry of the cross
section.  So, two sections with the same cross section (for example a 1 x 1 inch
square) will have the same moment of inertia, whether that cross section is solid
or built up of several layers.  The modulus of elasticity is a physical property of
the material.  So, pine will have one modulus, spruce another, maple another, etc.
Once again, the modulus of elasticity of the rib material shouldn't depend on
whether the cross section is solid or built up in strips, as long as all the strips
are of the same material as the solid cross section.  That's the theory.  In
practice your mileage may vary.

Here's a reference for some wood moduli:

http://www.woodbin.com/ref/wood/strength_table.htm

A couple of comments about this.  Dale commented that spruce will give a
stiffer rib than pine because it has higher stiffness.  As you see from the
chart, it depends on the type of spruce and the type of pine.  Sitka spruce
has a higher modulus than sugar pine, so in that case it's true.  A sitka
spruce rib of the same dimensions as a sugar pine rib will be stiffer.
But as you can see longleaf pine has a higher modulus than any other
softwood on the chart (however, it's also heavy).  So, a longleaf pine rib
of the same dimensions would be stiffer still.  I believe the more important
consideration for ribs, especially at the top end of the scale, is the stiffness
to density ratio (from the chart the modulus of elasticity divided by the 
specific gravity).
You want something that's stiff but light.  Sitka spruce is very good in this
regard, which is probably one of the reasons it has become a material of
choice.  However, as the chart shows, there are some others which are better.
Red spruce, for example.  Also, plain old doug fir (as long as you get a piece
of coastal fir) is better still.

The values given there are average values based on a large population of
samples.  Individual pieces of wood will of course vary.  I'm sure it's possible
to get a particular piece of sugar pine that is stiffer than a particular piece of
sitka spruce and would have superior stiffness to density ratio.  That's what
selection of material is about.  But the chart gives good general guidelines.

I think there is something to the idea that the grain of a laminated rib will follow
the line of the rib, rather than having the line of the rib cutting through the grain
as in a solid rib.  In theory it sounds better.  In practice I'm not sure how much
it matters.

Phil F
---
Phillip Ford
Piano Service & Restoration
1777 Yosemite Ave - 215
San Francisco, CA  94124


On Mon, 11 Feb 2002 21:53:15  
 Farrell wrote:
>I guess a lot of my thinking comes from laminating ribs on boat hulls. I do realize that a soundboard is not a hull. And the laminated ribs on the boat replace steam-bent oak ribs. The laminated ones are nice because they tend to hold their shape way better than the steam bent ones. You may well be right that laminated would not be any stiffer than a solid rib. Part of my thinking is also that you need to saw through the grain to make a crowned soundboard rib - but of course, you are only cutting into it 1/8-inch or so. Some boats have sawn ribs and you end up with much of the wood grain not in a preferred orientation. I guess not such a big deal on the piano. I was also thinking of making a good use for the old soundboard. If you had laminated ribs, you could just cut up the old soundboard and at least make nice laminated ribs from it.
>
>I'm also just posting because no-one will help me with my German F. Weber serial #19290!
>
>Terry Farrell
>  
>----- Original Message ----- 
>From: "Phillip L Ford" <fordpiano@lycos.com>
>To: <pianotech@ptg.org>
>Sent: Monday, February 11, 2002 9:52 PM
>Subject: Re: Laminated Soundboard Ribs
>
>
>> Terry,
>> What makes you think that the laminated rib would be stiffer than a solid rib of
>> the same dimensions (assuming the laminations are of the same material as
>> the solid rib)?
>> 
>> Phil F
>> ---
>> Phillip Ford
>> Piano Service & Restoration
>> 1777 Yosemite Ave - 215
>> San Francisco, CA  94124
>> 
>> 
>> On Mon, 11 Feb 2002 07:47:43  
>>  Farrell wrote:
>> >If ribs are supposed to add stiffness to a soundboard, I would think that a laminated rib might offer some advantages. It would be stiffer for the same dimension than a non-laminated rib, and I should think you could control the stiffness to a greater degree. Also, might the rib shape tend to last longer? Has this been explored previously?
>> >
>> >Terry Farrell
>> >  
>> >
>> >
>> 
>> 
>> Go Get It!
>> Send FREE Valentine eCards with Lycos Greetings
>> http://greetings.lycos.com
>
>


Go Get It!
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