4x3 joists

[geraldthehamster]

Hello

Does anyone know how I could work out the span for 75x97mm joists, at 600 centres? That size in not given in the old span tables.

https://www.labc.co.uk/sites/default/files/approved_document_a_1992_superseded.pdf

Cheers
Richard

 

[John D v2.0]

I'd be tempted to just extrapolate from the other sizes of you're just working it out roughly, as it'll be deflection that's the issue

Which is proportional to the cube of the length and inversely proportion to the width. And proportional to the cube of the depth. The self weight of the joist will have an effect, but less significant on relatively shallow sections.

 

[geraldthehamster]

Thanks John. I did try comparing the spans for 5x2 and 5x3, and applying the same ratio, but the ratio varied quite a lot depending which side of the table I was on... That might be the best guide though.

 

[tony1851]

What loading are you looking at? - is it a normal domestic floor?

 

[vinn]

No extravagant calculations are needed if full information is supplied?
Whats the span?
Ground floor or upper floors?
Why the 600mm centres?
And the questions tony 1851 asked?

 

[geraldthehamster]

What loading are you looking at? - is it a normal domestic floor?

It's a flat roof, OSB either side of 100mm Celotex, with EPDM. There's not a table for flat roofs as such, so I'm referring to Table A1.

For other reasons bigger joists are now being used anyway. Although I'm still interested if there's an answer to the question.

Cheers
Richard