On Sunday, October 21, 2012 4:25:44 PM UTC+1, John Rumm wrote:
On 21/10/2012 13:09, GMM wrote:
On Sunday, October 21, 2012 2:16:39 AM UTC+1, Tony Bryer wrote:
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).
So for a 14ft span (as here), the 8" joists I originally proposed
would be about right? I wonder if the tables are constructed from
that rule of thumb or from a complex calculation that gives the same
result? I guess most of the discussion (now) is about what you might
be able to get away with, rather than what should be done, but I'd
prefer to over-engineer than under, for the sake of a couple inches.
Of course, I'm equally concerned that they are mounted securely, as
Mr R outlined......
If you can rule out that the space will ever be converted to habitable,
then you can undersize a tad from the tabulated values. As Tony
mentioned above, its normally the deflection limits that dictate the
size rather than the shear or bending limits. (i.e the floor would be
likely to damage decorative finishes, feel to bouncy, and upset
inhabitants of rooms below, long before the timber is in danger of
actually failing)
For your application (i.e. with the new beams some distance above the
existing ceiling, and not ceiling to be mounted on the underside of the
new joists), deflection beyond normal limits is a non issue. So it
reduces to a problem of what is adequate in terms of bending and shear
loading on the timber (assuming you don't mind it feeling a little more
bouncy than "normal" given that you know it is still structurally sound).
Perhaps a play Tony's excellent bit of software might be in order
(assuming there is still a demo version available for download?)
--
Cheers,
John.
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Yes John - it may potentially be that 2 x 6 joists (which are significantly cheaper per metre) could do an
adequate job in this application, if they are available (my local timber yard couldn't supply that length
for the living room when I was costing it: There, the lower spec was due to the wall running along the
middle, effectively halving the span).
TBH, I didn't spot Tony's software - I thought he just mentioned the rule of thumb(!). If I did go below
spec (say 4m of 2 x 6), it would be great to have some idea of how wobbly such a floor would be. The
joist tables just give maximum length for size, as far as I can see.
I'd still feel a little uncomfortable that it would deny the option of making the space habitable in the
future though, even though the rest of the house is big enough that it shouldn't be an issue.
Although I get the point that BR specs change over time, they surely can't ask for joist that are much
deeper than they require now, so I would have thought the current specs won't change much.
(Apologies if my posts are hard to read. Someone told me a while ago they weren't wrapping, whilst in
a recent thread someone else told me they had a lot of empty lines!)