[VAC] Re: Structural loading / aircraft flooring

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

On a three piece beam, with steel on top and bottom and a spacer in
between the bending load is carried by the top and bottom steel. The
strength comes from the spacing. Loosing spacing causes a rapid loss of
strength, as when a piece of tubing collapses in bending. Strength is
proportional to the fourth power of the spacing. The channel is pretty
well spaced by the web. In bending the flanges tend to bend in, so a
good wood filler would gain a little strength. There is a strong
tendency for the top and bottom to try to slip with respect to each
other under a bending load. The filler has to handle that shear load. As
flanges are strengthened that shear load on the filler gets greater.

Pine/fir weighs 32 pounds per cubic foot. A 4x8x 3/4" sheet of plywood
is 2 cubic feet, so about 64 +/- pounds a sheet. So the plywood floor
deck in a 20 x 8' floor would weigh about 320 pounds. Aluminum can be
light, but with two skins and spacers, the full skin probably weighs
more.

If you doubled the load carrying capacity of the frame by fitting more
frame elements side by side to the original, and of the same size, and
reduced the floor weight by going to honeycomb, you'd maybe come out
even because the doubled frame would probably weigh as much as the
improvement in floor. Then you still have to contend with axle, bearing,
and tire load carrying capacity. Water is very heavy and some water beds
have been known to crunch houses. Maybe there's the equivalent of an air
mattress in a THIN waterbed.

Gerald J.