> > 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...................... > > > > If I were making the rocking chair in question I'd want to know why > the thing > > was breaking. > > Del > >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. > > >-- >Richard Brekne Ric, On this demonstration with the U shaped beam, did you also see them perform the same trick with a solid piece of wood? If so, then I would expect the same result - it will break faster bending it one way than the other. That 's because it's a curved beam, not because it's laminated. I agree with Del - I see no reason why the laminated beam should be any weaker than a solid beam, since they're both made from the same material (allowing of course for variations among individual pieces of wood). Curved beams do not behave in the same way that straight beams do. For a straight beam, assuming a rectangular cross section, the neutral axis will be at the center of the cross section. The compression stress on one extreme fiber is the same magnitude as the tension stress on the opposite extreme fiber. Bending it one way or the other causes the sign of the stress to change but not the magnitude. In other words, bending it one way will give you tension on top and bending it the other way will give you tension on the bottom, and the magnitude of these stresses will be the same. I would expect the load at which this beam breaks to be the same regardless of the direction of the load. This is not the case for a curved beam. With a curved beam the neutral axis is displaced toward the center of curvature. For a rectangular section you will have a different magnitude of stress on the outer fiber than on the inner fiber. This is dependent on the radius of curvature. For a severe bend like you're talking about here, this effect will be very pronounced. Bending it one way gives one extreme fiber compression stress, bending it the other way gives a different extreme fiber compression stress. Same with the extreme fiber tension stresses. I would expect the beam to fail at a lower load bending it one way than bending it the other. A Google search on bending of curved beams should give you more info if you're interested. For the radii of curvature and the cross sectional dimensions of typical piano ribs, I would expect this effect to be fairly slight. Phil F Phillip Ford Piano Service & Restoration 1777 Yosemite Ave - 130 San Francisco, CA 94124
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