On Tue, 12 Aug 2003 22:22:33 GMT, "Michael Daly"
wrote:

On 12-Aug-2003, Tom Watson wrote:

D = 0.1563wl^4
_____________

Ebh^3

Whe

D = deflection (in inches)
w = load per lineal inch of span
l = span (length)
E = modulus of elasticity
b = base (width)
h = depth (thickness)

Actually, this is likely to be a lower bound to the deflection. Since the
sheet is significantly wide and fairly flexible, there will be some sagging
resulting from bending in both directions (plus or minus anticlastic curvature
effects). The deflection of a plate is a lot more complex than that of a beam.

You'll never get better than a rough estimate from basic elastic theory, since
wood in general (including plywood) is not a nice isotropic material.

You really want to reinforce this plywood sheet, as so many have pointed out.
Wood will be more weight efficient that steel angles. Tables are made like
tables for a good reason. The torsion box is a nice alternative.

Mike

I should have referenced this formula in my original post. It comes
from the AWI spec book and they got the formula and the field testing
from the University of West Virginia wood sciences people.

My own estimates of the deflecting strength of a table are more rough
and tumble.

If I've a question about a table's strength, I stand in the middle of
it. If it adequately resists my two hundred pounds, distributed
through two size eleven shoes, I figure it'll hold up the grits.

I miss the old days of American engineering, where three times theory
was the norm.



Regards, Tom
Tom Watson - Woodworker
Gulph Mills, Pennsylvania
http://users.snip.net/~tjwatson