Joist weight/mass per m3

[Casterbridge]

I want to try to calculate the the appropriate size of joists for various loads from first principles without reference to the standard tables (as these tables exclude joist weight). I know there is a lot to this calculation, but my first issue is trying to calculate the weight in kN/m3 of the joists (which you need if doing it from first principles). I am trying to set up a spreadsheet so I can simply enter data, and it can calculate the size of joists I need.

Ok - so I can set up the spreadsheet correctly, I have based my initial calculation on C24 timber , which I believe has a mean mass of 420 kg/m3 and I have assumed centres a distance of 0.4m. I believe that when you start off on this, you need to make an assumption of the size of the joist (as it will contribute to the dead weight, and you need to have this), and then iterate if you need to increase the size of the joist (if the total weight including joist weight means the initial assumpotion about the joist weight is too small).

Can anyone confirm my calculation for the weight per m3 of a C24 joist, at 0.4m centres, with a depth of 100mm and a width of 50mm

For a 1m length of this joist, I think I have a mass of

(420 x 50/1000 x 100/1000) = 2.1 kg, so at 0.4 centres I must have a mass per m2 of the following :

2.1/0.4 = 5.25 kg/m2

To convert between mass and weight per m2 I need to multiply by 0.0101972, so I get that the a 100mm x 50mm joist has a weight per m2 of 0.054 kN/m2.

Does anyone know whether this calculation is correct and is the 420 kg/m3 mean meass for c24 graded timber correct?

 

[jeds]

Use 480kg for softwood. (BS648) 0.06kN/m² (rounding) for 50x100 at 400 centres.

 

[Casterbridge]

Thanks John, so at 480 I get 0.058 =(0.6) that's right?

 

[Static]

Or just take 0.2kN/m2 self weight like everyone else..

 

[maltaron]

It always amuses me when I read of fancy calculations of joist sizes. We used to use the simple term Span in feet divided by 2 + 2 is the size of joist in inches. We did not have C16 or C24 just 7x2 or 8 x 2 sawn softwood and I have yet to see a floor fall thro'. The matchwood that is used today for roofs worries me, yet the powers that be will not allow a cut roof with timbers sizes as used for donkey's years with a proven record without masses of calculations.