On 26/10/2012 23:45, John Rumm wrote:
On 26/10/2012 19:11, GMM wrote:
On 24/10/2012 16:55, John Rumm wrote:
On 24/10/2012 09:55, GMM wrote:
On 23/10/2012 01:43, John Rumm wrote:


Probably not as "industry standard" or trustworthy, but there
are a number of smart phone apps about that will do calcs at least on a
single beam.
Might be worth trying one of those if you have platform for that.


It's interesting that there seem to be more apps available than
web-based calculators. Unfortunately most of these seem to be in
American and I'm not sure how C16 compares with their specs but,
putting
a trial dimension of 4m in, using Redwood or Southern Pine (which seem
to come out the same and probably equate broadly) gives a deflection of
69mm for a 6x2 for a light floor loading, which seems an awful lot.
For
some reason, the one I was using seems not to accept 2" beams after the
first use so I can't go back and check but the deflection for 8x2 was
about 20-something mm so about a 2" difference.

Give me the specs of a beam if you like and I will see what numbers I
get...


Thanks John, that would be very useful. All lengths are 4m (a little
variation but no more than 100mm) and the TRADA tables say a 47 x 195
joist is good for that at 400 centres but of course give no further
information. (A number of apps etc simply give the same data in their
calculators - you put in the size and it tells you the max span.)

Well if I model that with a uniformly distributed load of 0.8kN/m (long
term load), and treat them as a load sharing system, you get a
deflection of just under 11mm. Go down to 175mm and the deflection goes
up to just under 15mm. However its worth noting that it is also flagged
as failing in maximum permitted bending stress at that point. 150mm and
deflection is nearly 23mm. 100mm and deflection is approaching 80mm!

That is a full floor load though - unless storing a large magazine or
record collection its unlikely you would reach that.

Clearly very small ones would be inadequate at this length, but it would
be interesting to know what the performance of 7" (ie 47 x 170) and 6"
(47 x 145) would be at 4m for C16 timber. From what the tables
indicate, the advantage of C24 is pretty minimal, so hardly worth the
trouble of sourcing etc except in very marginal situations).

C24 at 175 for example would be a "pass" on 175mm (deflection 7.61mm)
and on 150mm (11.98). (125mm and you are back to failing in bending stress)

Note also you can push it firther with short term loads - so you don't
necessarily need to make adjustments to factor in someone clod hoppering
around up there shifting the boxes.

I'll most likely cover with chipboard (8x2 t/g sheets), which comes in
18 or 22mm. I'm assuming that there's little difference here for 400mm
joist centres (and would go for 18 as it's lighter) and the more
important factor is to screw it down to every joist but I could easily
be corrected on this (!)

It interlocks, so only really needs enough screwing in this application
to stop it sliding about.

It's also pretty clear that a span of 4m will benefit from strutting at
mid span, so that's on the agenda too.

It will make it bounce less. How much that matters for storage space is
debatable.

Friday night - must be time for a cold one (!)

Sounds like a good plan ;-)

Thanks for doing that John, it's very interesting: It looks like 150 at
C24 is pretty much comparable to 195 at C16 in terms of deflection: I
didn't think the difference between the two would be anywhere near as
much, from the other information I've seen.
Of course, the C24 may be more expensive even in the smaller size (I'll
have to get a quote) but if it's not too much more, the extra 2" might
be a useful space saving: It would certainly reduce the step up onto
this floor from the new access door, which might make it more user friendly.

(It took me a while to get back - I've been away for a few days and it
seems Firefox can't connect to the server from other networks)

All the best