On Thu, 20 Sep 2007 20:33:51 GMT The Medway Handyman wrote :
Remember the depth cubed factor though - 6" is more than 3 times
stiffer than 4"

I didn't know that - where could I find out more?

Strength is proportional to Z = BD^2/6 for a rectangular section

Stiffness is proportional to I = BD^3/12

Most timber beam (joists, rafters) sizes are governed by deflection
limits rather than strength - you could drop most floor joist sizes
by 25mm without any risk of the floor collapsing but it would be
unacceptably bouncy.

Compare (imperial units more manageable)

Strength: 8 x 2 Z = 21.3 7 x 2 Z = 16.3 - 23% less strength
(but probably still within limits for most floors)

Stiffness I = 85.3 I = 57.2 - 33% less stiff
(= 50% more deflection for same load)

--
Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk