18mm ply glued and screwed to the top of the joists (instead of unglued
chipboard) , forming a T beam was a solution proposed by a structural
engineer and adopted by me when I did a loft conversion some years ago. We
didn't put a piano up there, but the purchasers of the house did install a
water bed and there have been no adverse reports from neighbours whom I have
remained in contact with. The increase in strength over the unglued floor
was impressive, confirming in practice what the Moment of Inertia
calculations indicated. We were able to jump up and down on the floor and
detect very little spring.

Our joists were 6x2 inches, at 16inch spacing and spanned 12 feet, to give
you an idea of our configuration. The resulting stiffness was, if I remember
correctly better than what would have been achieved with recommended joist
depth for that span. Sorry I can't remember if 8 inch joists would have been
sufficient for that span. I remember that the addition of the ply to form a
T beam gave lots more improvement over doubling up the beams. Where our span
increased to 14 ft . the solution was to double up on the 6x2 beams

I'll try to find my own calculations which I did to "prove to myself" the
efficacy of the scheme, and perhaps substitute your 4x2 beams.

regards
Ian




"Steve" wrote in message
.uk...
John Rumm wrote:
Steve wrote:

Thanks for that Tony. I will download the demo and have a play with
various options.

I just did a quick calc in SB with a 1.3m 4x2" beam. Assuming a uniform
floor load of 0.8kN per joist, plus a worst case point load in the middle
of the span of 1.6kN (i.e. one piano leg), your current floor fails to
meet the spec in bending and deflection - but not by a huge margin.
Doubling up the exiting joists with another the same size sat beside it
(and nailed too it would probably be simplest at that size), seems to
make it stiff enough.


Thanks very much for that John.

I have downloaded the software from Tony's website and had a quick play.
But it is clear that I need to follow the advice on there and print out
the instructions! It does seem a very nice piece of software that has the
potential to teach me a lot more apart from this question... but I need to
learn how to drive it! Also I was very impressed that the only feature
'nobbled' in the demo was the ability to print out the results - fair
enough. Nice not to have a time limited demo with lots of features
disabled as so many are.

So thanks also to Tony for generously making the software available to non
professionals like myself.

Coming back to the joists, I had a gut feeling that deflection would be
more than desired and your results back this up although you have quite
rightly taken the worst case with the load in the centre of the span. In
our case most of the load will be near to a sleeper wall for each leg. But
I would rather be conservative and use worst case!

One idea was to double the joists up i.e. putting extra joists in between
but in the areas of high load to have double joists joined, but I had
thought of glueing them together and bolting through the neutral access.
Any problems with glueing and bolting?

I also had an extension to this idea...I thought of cutting 8" strips of
18mm WBP, glueing two together and then sandwiching that between two
joists, all glued and bolted. In effect a 'T' beam. Obviously the ply
can't extend the total length because of the sleeper walls but that would
probably not matter as shear is unlikely to be a problem, I think. Any
comments on this idea?

Cheers

Steve