On Sat, 20 Oct 2012 22:33:14 +0100 John Rumm wrote :
Out of interest I had an experiment with superbeam to see what you can
get away with on a 3x2 (well 72x47mm) and a typical floor load
(uniformly distributed 0.8kN/m on each joist). 1.3m seems to be about
the limit - so you could probably still do a landing with it and comply
with modern building regs. (having said that, its generally simpler to
use one depth all over to save having to buy lots of timber sizes)

Our old rule of thumb when I was a BCO which matched the tables pretty
well was that for floor joists double the depth in inches and subtract two
to get the permissible span in feet; flat roof joists, subtract one (2"
joists).

As you say, in most cases practicality requires all joists to be the same
depth (you need tops of joists to be level and want them all to bear on
wall at one level) so except for the largest span they are generally
oversized. There's also more in reserve in that for virtually all joists,
deflection governs the size, not bending stress and few floors are loaded
to BR design loads (1.5kN/m2 30lb/ft2).

--
Tony Bryer, Greentram: 'Software to build on',
Melbourne, Australia www.greentram.com