On 20 Sep, 16:30, Steve wrote:
Time soon to put new floor down in lounge, but it is a bit more involved.

This floor has to support a substantial dead weight or two - A Steinway
Concert Grand Piano that weighs in at 480kg, or about half a tonne. The
weight is, according to Steinway, evenly distributed over the three legs
i.e. 160kg per leg.

The other dead load is a lighter but still heavy - Boston upright piano
that is probably half the weight or less and with weight on four feet,
although centred on a much smaller overall area. .

The remaining "live" load will be furniture etc. - normal things :-)

The existing joists are 4" by 2" and are at present supported by sleeper
walls at 5 foot centres, the spacing between joists is 10" with no noggins.

I plan to increase the number of joists by about two, depending on
arranging things to suit the Caberboards that I think will do the job.
Running double joists where the dead weight will be centred.

This will, obviously, make it harder for noggins if they are needed to
be fitted, but the close spacing between joists should make them
unnecessary I think?

Am I on the right track here? Any ideas or information would be most
welcome before I do it and find that our beloved piano has crashed
through the floor!

Thanks

Steve

Dear Steve
I suspect I am probably the man to help you here as I have a degree in
timber engineering (ICST 1975 Civ Eng Dept) and have spent the last 30
plus years working on such floors in a practical fashion.
No one in the group has taken into account a factor in the design of
timber floors called "Duration of Load". (in essence were you to load
a beam say for a fraction of a second it would take 130% of its (so
called) maximum stress (derived from 5 min tests to destruction) of
100% by definition and conversely IF you were to load it to as little
as 60% of its "maximum" it may well fail though it may take some years
so to do.
This is particularly true if the timber is green and goes through the
fibre saturation point under load and vast proportional creeps are
manifest...
So you need to take that into account and the Codes of Practice do so.

On a practical basis you need to do the following

carefully identify and lift up the floorboards (marking with pencil so
you know where they come from) the board in the areas of the pianos
and beyond the next wall plates - staggering the cuts (if needed) to
alternate joists and cutting on the joists. Use a right angle square
pencil and ~Fien multi master to get neat cuts or lift all the board
Once you have access to the sub floor (check ventitlation whilst you
are at it) check the structure of the sleeper walls and that they are
sound enought and founded well - improve if needed
Consider at this point if it is worth it putting in an addtional
sleeper wall - the closer to you point loads the better
consider putting in INDIVIDUAL supports to the concrete as suggested
by others
whilst on about the concrete consider a trial pit remote from the
load to see how thick it is and if it is ok fine
If not spread the load on the point supports with 4" of new concrete
with 142 mesh so it is over the old concrete
I guess about 1 m square for each corner would be more than ample but
if in doubt calculate (engineer)

Next simply run more 4" joists alongside the old ones on the existing
and new wall plates such as to cover the 6" of your spreader cicles -
that is only likely to be 3 or at most 4 new joists between the two
wall plates for each of the legs and with a bit of luck two will
coincide!

4 x 2" does not cost much and it is not likely that you will need
much more than 8' for each set of 3

If you are fussy screw or nail them together so you get a shared
effect and avoid individual variations in strenth becoming manifest

lay floor boards back down - I would use brass screws but I am fussy

Any problems come back to me
If you can scan and send me a plan locating point loads and existing
wps I will work out a rough idea of where to go should you so want on
our pro bono terms - ie FOC - you indemnify us!
Chris