I = 1/12 x (W x T^3); W = rib width, T = rib height
I = A x D^2; A = breadth x thickness of skin, D = centerline of skin to neutral axis of box.
Itotal = I top skin + I bottom skin + I each core element
Y = (.021 x W x L^3) / (MOE x I), where;
Y = deflection (inches)
W = Weight/ load (pounds)
L = span
MOE = modulus of elasticity (1.5 E6 is avg for wood)
I = Itotal from above.
Torsion box pictured: 61" span, 3" x 3/8" web, 3/8" skins. Deflection: .018" (~1/64") @ 1000lb.
Sunday, February 10, 2008