On 2005-11-25, Ken Korona wrote:

The plans specify the use of 3x10s on 12" centers. These would be
23ft long with no support other than at the ends.

In this situation, the maximum deflection of a beam (joist) will grow
with the cube of the span, increase linearly with increased spacing,
decrease linearly with increased beam width, and decrease as the cube
of the beam depth. Knowing this, you can extrapolate from standard
span tables to your non-standard situation.

For example, according to my Code Check Building book, the 1997 UBC
allows 2x10s 12" o.c. carrying a 40 lbs/ft^2 live load to span 18'0".
The 3x10s are (2.5"/1.5" = 5/3) times as wide, so they can span (cube
root of 5/3 = 1.186) times as far, or 21'4".

By contrast, the 1997 UBC allows 2x12s 12" o.c. to span 21'11". To
get up to a 23' span, you need something ((23'/21'11")^3 = 1.156)
times as stiff, e.g. 2x12s at (12"/1.156 = 10.38") o.c. Or you could
use 3x12s and increase the spacing by a factor of 2.5/1.5 = 5/3 to
17.30".

At least, that's the theory as I understand it.

Cheers, Wayne