Question about Floor Joists

[jeds]

...Only trouble is that at 8kg/m2, I make this 0.078kN/m2, where as you list this as 0.087kN. Could this be a typing error?
It was the chipboard that I did not understand John, bur after a lot of thought realised where I was going wrong. To the best of my knowledge MD chipboard weighs in at 625kg/m3, therefore 1metre square of 18mm = 0.111kN/m2. Slightly different to yours.. Perhaps I have not got the cube weight right?...

Just a couple of points to finish this off.

Actually, I made a slight error when I said BS648 gives plasterboard as 0.87. In fact that comes from manufacturers spec. so there might be small differences between manufacturers. The small difference in your calculations (i.e. 8kg = 0.078kN) comes about because you are using 9.82 as the factor for gravity whereas I normally just use 10. So Lafarge 12.5 at 8.7kg/m² becomes 0.087kN. And hotrod is right. The figures are given as a range ( I don't know why either?) and I just use the upper figure.

In reality it is daft to design joists on such tight tolerances and I don't defend it myself. I was really just pointing out to the OP that if he went back to his struct engineer to complain he would probably just say that his calcs are technically correct and the problem stems from the builder altering the design.

I would support your earlier post about 'I' joists. I've been using them for years now and although a bit more expensive the advantages significantly outweigh the extra cost.

If I do specify timber joists I err on the high side. Not necessarily for structural reasons but because the stiffer floor will be quieter. In my experience people are far more likely to complain about a creaky floor than anything else.

 

[treecott]

I promised I would come back to you all to let you know what the structural engineer has said about the floor joists.

Having done calculations again he has advised that the 47 x 195 joists ARE adequate for the span, and will pass any query from building control.
He has advised that "because timber is a natural meterial the design stresses used are very conservative and the joists will not act to the limits shown in the calculations". he has also said "the deflections shown are mainly dead load deflections, and will take place during construction, leaving only the live load to cause the springiness that we mention". He has advised "the floor is safe and I do not think any remedial work is necessary in view of the cost and disruption the work would cause".

The design of the floor joist - Span = 4250 (timber grade C16)
load per joist @ 400c/c = 1.90 x 0.40 = 0.76kN/m
Max bending moment 0.125 x 0.76 x 4.25 (2) = 1.71 kN/m
Bending Stress // To grain = 4.30 x 1.10 x 1.03 = 6.10 N/mm (2)
Z Required 171 x 10 (6) over 6.10 = 280 x 10(3) mm (3)
Try Timber Section 47 x 195 Z = 297 x 10(3) mm (3)
Deflection E Mean = 8800 N/mm (2) 1 = 29.4 x 10(6)

5x 3.20 x 4250 (3) x 10 (3)
384x8800 x 29.40 x 10 (6) = 12.36mm Span x 0.003 = 12.70mm

Deflection limit for domestic floors 14.0 mm

USE FLOOR JOISTS 47 x 195 @ 400 c/c

I hope that you can understand what has been shown above, unfortunately I cannot do mm squared & trippled etc, so they are shown in brackets beside the numbers.

I am happy that I have the calculations now for Building Control, and I hope that this will now solve all the problems.

Thanks to everyone who came back with help and advicel. I really do appreciate all the time you have taken to. I think this wraps this post up.