Plywood Section Modulus

[jtrenam]

We have a Florida job that needs to be sealed by another engineer.
They have used a formula to calculate the section modulus for plywood.
The formula is:

Save= {(# of plies) Su + (# of plies) Sperp]
--------------------------------------
(Total # of Plies)

Reslutant S = Save ( total # of plies )2

Here is an example using 5/8" plywood:

Save= {(1) 0.361 + (2) 0.238] = 0.279 in3 / ft
-----------------------
(3)

Resultant S = 0.279( 3 )2 = 2.511 in3 / ft

The formula is used to calculate the bending stress fb = M/S

Does anyone know how these formulas were derived? Why is an average calculated then it is multiplied by the total number of plies squared? Is this an engineering mechanics problem?

 

[boo1]

Recommend KS be used in bending strength calculations, instead of Section modulus.

See:
Plywood Design Specification Form D510,

http://www.apawood.org/