Joist strength

[Steve]

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

[Robert]

Would you be worrying if six people (each weighing 80kg) stood on your floor in three groups of two? That would give the same weight distribution as the piano but you probably woul dnot even give it a thought. Maybe nothing needs to be done.

Robert

[Steve]

Thanks and that was one of the scenarios we considered!

This is to do with protecting a very valuable item of ours and as it has to be standing in the same place for years (because it cannot easily be moved), I was asking whether the idea's I had put forward had any merit or not.

If not, and while the floor is open, I would prefer to do the best thing.

As you say, maybe nothing needs done.

Steve

[tony sayer]

In article , Steve scribeth thus

Dunno about the stress .. but will it affect the sound;-?....

[Bob Eager]

Depends if one uses gold plated, oxygen free cut nails! :-)

[dom]

You probably have a very good idea where each foot of your grand will be positioned.

You could consider independent masonry pillar supports topped with timber plates pressing up against the underside of the floor boards at these locations.

[Cicero]

================================== Google pictures suggest that it's on castors which in turn suggests very localised loading. It would be worth considering some kind of load spreader made from material other than wood which can't be easily penetrated or indented by pressure.

Cic.

[Steve]

Indeed, the load is on castors and that would amount to considerable point load as you suggest. The piano is presently standing on castor cups recommended by Steinway that spread out the load to 6" circles. They are, I believe, made of some sort of plastic.

Thanks

Steve

[Steve]

LOL Which are best...?

Steve

[Steve]

A very good idea! We have a triangle made from lengths of wood that show us where the the piano will sit more or less exactly.

This is something that I had considered. The existing sleeper walls seem to be built on top of the rough floor concrete so extra support should be easy to add as you suggest. I had it in mind to provide to provide that additional support under the joists in these areas if necessary.

Thanks

Steve

>

[Owain]

I'm slightly more concerned about the point loading too. I think I'd be inclined towards thickening the floor rather than the joists.

If belowfloor supports are used, it might be worth recessing polished metal plates into the finished floor to (a) spread the load, and reduce indentation into a soft floor, and (b) indicate the correct location of the piano feet.

Owain

[The Medway Handyman]

Couldn't you play the mouth organ instead? :-)

I built a deck for 2000kg hot tub last year. 6 x 2 joists at 12" centres supported every 4'. Not a millimetre of movement!

[dom]

Remember the depth cubed factor though - 6" is more than 3 times stiffer than 4"

[George]

Metal 1/4" x 3" plate both sides of the intended joist

[The Medway Handyman]

I didn't know that - where could I find out more?

[The Natural Philosopher]

Take an engineering degree.?

[The Medway Handyman]

Bit OTT for a handyman with a thirst for knowledge?

[Tony Bryer]

Strength is proportional to Z = BD^2/6 for a rectangular section

Stiffness is proportional to I = BD^3/12

Most timber beam (joists, rafters) sizes are governed by deflection limits rather than strength - you could drop most floor joist sizes by 25mm without any risk of the floor collapsing but it would be unacceptably bouncy.

Compare (imperial units more manageable)

Strength: 8 x 2 Z = 21.3 7 x 2 Z = 16.3 - 23% less strength (but probably still within limits for most floors)

Stiffness I = 85.3 I = 57.2 - 33% less stiff (= 50% more deflection for same load)

[The Medway Handyman]

Ok great, so what do Z, B, D & I stand for?

[John Rumm]

Download the demo of superbeam from Tony's site, and have a play with some different joist sizes and loads.