On Sat, 18 Mar 2006 01:42:20 -0500, "Proctologically Violated©®"
wrote:

How did you determine what an "acceptable" deflection would be? Building
codes?

I Googled for building codes (this is in the UK and there are some fairly
clear and simple national regulations). I also looked for more general-purpose
architectural stuff relating to things like "acceptable beam deflection".

IIRC the number that popped out was a deflection of 1/400th of the span was
acceptable for a roof beam. This worked out at 0.81" for my beam. I designed
for 0.4" when supporting the entire roof load. Effectively this was a 4:1 over
design. I made no separate allowance for snow loading, since the worst snow
loading in this area is two or three inches a couple of times in the last 50
years.

When the walls were built and the roof beam was up, but without the roof on
it, the beam was noticeably springy if you jumped up and down on it. Once the
roof was fixed on to the walls and beam, the whole lot became pretty well
rigid in feel when walking over it. More so than a typical wooden floor in a
house. Roof and walls are both 6" thick SIP's.


Regards
Mark Rand
RTFM