On 27 Sep, 14:00, Tony Bryer wrote:
On Thu, 27 Sep 2007 04:45:59 -0700 Martin Bonner wrote :

Does anybody (eg Tony) know if Superbeam will model such a system?

I'm still working on my shed, and Superbeam tells me that the 2x3
rafters at 2' centres aren't good enough to hold a heavy green roof.

Without checking out the calcs (am drowning under work ATM), most
timber calculations result in the member being sized to keep the
deflection within limits, and the standard limit (0.003 x span) is
really to do with preventing damage to plaster finishes and stopping
floors feeling uncomfortably springy (whilst being safe). If the
calculation shows the applied bending stress as being less than the
permissible then all should be OK.

Actually the stress was a problem with 45x70mm beams (the actual
size), but was OK with 45x88mm (assuming the OSB just adds 18mm to the
thickness of the beam). I /think/ I can get away with that.
a) I know that OSB will not be as strong in compression as C16
timber.
b) I am relying on the screw + glue transferring the entire load
-but-
c) I am not really adding a 45mm strip of OSB - I am adding a 590mm of
the stuff (no, I don't think that will be 13 times as strong as a 45mm
strip; but it ought to bring it up to C16 levels).

What I was hoping was that SuperBeam would be able to model such a
composite structure - but I suppose it isn't a fully blown finite
element modelling package (It certainly isn't priced like one ...
though you wouldn't know that from the support service :-)