On 21/10/2012 17:47, GMM wrote:
On Sunday, October 21, 2012 4:25:44 PM UTC+1, John Rumm wrote:
On 21/10/2012 13:09, GMM wrote:
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?)
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).
A timber merchant ought to be able to get 6x2 in 5.3m lengths at least.
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 just checked, there is still a demo available. It has printing
knobbled - but that won't be a handicap for your needs.
http://www.superbeam.co.uk/sbwdemo.htm
(the usual caveats about it letting you design unsafe structures faster
apply, if you don't stick in sensible values!)
It will show you the calculated deflection for whatever load you apply,
and also tell you when you are exceeding the safe working limits on the
timber.
If you model your longest timber that should let you get a feel for the
changes. The loadings to apply for a normal floor appear further up the
thread (if you know what you are storing etc you may be able to use
lower figures)
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.
Might be worth working out what you could "get away with" and then
comparing the cost difference to doing it to full spec.
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.
I would not expect them to change in substance at all really. They may
grow to include more on composite joists (i.e. man made beams with
struts top and bottom and some sheet material webbing)
(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!)
Its a combination of not wrapping and all the lines being double spaced.
The former is easy to fix in a reply with a quick CTRL + R in
thunderbird. That latter takes slightly more editing!
--
Cheers,
John.
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