Quote:
Originally posted by utabintarbo
And given the fact that a solid top was deemed to be most appropriate, your point is...?
Bob
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His point is that the top may not be required for structural purposes. What's described is the method for calculating a moment of inertia, which largely determines bending "strength". While the relationships are correct, they only apply to a solid member. In short, a fin crossing the base doesn't make it 40X stronger. It's a little more involved than that.
In summary, you must first determine the neutral axis, which will correspond to the center of gravity. Since the fin is "thin" compared to the width of the base, the neutral axis will be very close to the center of the base. Then you calculate each member's area moment relative to its own center (this is the 1/12*base*height^3 portion). Finally, you add the member's area multiplied by its distance from the neutral axis^2. This is the composite area moment. It's basic strength of materials stuff, but pretty much greek if you haven't studied it before. The Roark book I mentioned earlier goes into just a tad more detail and tabulates copious loading conditions for plates, beams, cylinders, etc., etc.