Laminated ribs.

[Delwin D Fandrich]

----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: May 09, 2003 12:07 AM
Subject: Re: Laminated ribs.


>
>
> Delwin D Fandrich wrote:
> >
> >
> > Except for the laminated ribs being somewhat more consistent I've not
> > noticed much of any difference in their strength or stiffness
> > characteristic. Why should they be? They are made of the same wood.
>
> Well, you are the expert here on this subject matter so if you say so I
> buy it. I just was under the impression that when pressing and glueing
> laminants into a desired curviature for some useage where the laminants
> are parallel to whatever stresses were to be applied, that they are
> stronger then solid wood if the stress is in the same direction of the
> curve, and weaker if the stress goes against the curve.

Ah, I see what you mean. In this case the curve radius is so large that any
difference in strength is going to be negligible.


> >
> > What about compression issues? Why should there be any more compression
> > stress on the wood fibers in a laminated rib than there are in a solid
rib?
> > We've tried stressing the things every which way and have found
invariably
> > the solid wood rib cracks or breaks before the laminate does.
>
> I can believe this for sure, laminants dont usually crack or break at
> all, they splinter when they fail. As far as the compression bit... dont
> any two wood surfaces glued in such a way that they contrain each other
> cross grain create compression issues when stressed ? Or are these
> laminants glued with grain all going the same way ? Forgive me for still
> being rather unitiated with all this :)

Hmmm. I'll have to go break one and see how it fails.

There is no cross-grain here. All of the laminae run the same
direction--lengthwise with the rib.


> >
> > Laminates have been used in
> > everything from automobile bridge members to church beams to piano
rims.
>
> True, but its the direction of the laminants against whatever stresses
> are applied I was asking about.

In the case of the examples I mentioned all of the laminae run the same
direction--there are no cross-plies. (Well, in the case of the piano rims
the laminae usually run in the same direction although more an more
builders are beginning to use some cross-plies for improved moisture
stability. And, probably, ease of bending.)


>
> > If
> > I were making the rocking chair in question I'd want to know why the
thing
> > was breaking.
>
> If you saw the demonstration you'd see it right away. But I suppose you
> can do this yourself. Glue and press say a 5 plies that are about 2
> meters long into a U shape. When dry stress the U by pressing the ends
> together, then apart.

Next time I make a rocking chair I'll try it. I suspect something is
happening as the bend radius becomes smaller. Still, the alternative would
be steam bending and that would weaken the wood still more.

Del