Timber Span Question

[padstar]

My SE & Architect have specified 8x2 timbers for the ground floor suspended timber floor. Reading the guidance notes, standard floors should be designed to carry 0.25 - 0.50KN/M2. The spans are generally just under 4m (which just about works on the tables) but a few are just over 4m (which don't).

I plan to install dwarf walls mid span to all areas. Does this mean that the lengths all work (do i then work on 2m spans etc). I am not looking to reduce the timber sizes but don't really want to increase to 9x2's or more.

 

[tony1851]

Why not put sleeper walls in and reduce the span to 2m? - in which case you could use 6 x 2s.

(Unless the ground is poor, in which case its adviseable to span the full width unless the sleeper wall itself is on a good foundation. But where it is a little more than 4m span, you could bring the joists in a little - to - say 350mm centres).

 

[padstar]

Sorry by dwarf walls i meant sleeper walls mid span. If you mean brickwork with a wall plate taking the timber mid span?

 

[tony1851]

Sorry by dwarf walls i meant sleeper walls mid span. If you mean brickwork with a wall plate taking the timber mid span?

Yes (honeycombed brickwork to let the air circulate).

 

[padstar]

So by using mid span sleeper walls and sticking with 8x2's over a circa 4m overall span and plenty of noggins i will end up with a very solid floor structure?

 

[oldbutnotdead]

If you're careful setting your noggins you can reduce the wastage on PIR insulation (if that's what you are using)- mind, with 8 x 2 you'd have room for rockwool or similar instead (which would be my preferred option)
EDIT And yes using support walls reduces the effective span. 8 x 2 on 2m span nogged will be very sturdy.

 

[Bouy]

So by using mid span sleeper walls and sticking with 8x2's over a circa 4m overall span and plenty of noggins i will end up with a very solid floor structure?

Why noggins?

 

[oldbutnotdead]

They stiffen the structure up massively.

 

[Bouy]

They stiffen the structure up massively.

The span will be 2m, TRADA tables suggest noggins on spans greater than 2.5m, noggins are there to stop long slender joists twisting and losing their rigidity/deflection in the x- x plane so would offer little benefit in this case.

 

[oldbutnotdead]

The effective span for loading purposes is indeed 2000, but the joists are still free to twist and flex (the only anchor on the top of the centre point will be the floor covering).
Joists that size on that span will be pretty stiff, nogging will improve that stiffness.
TRADA tables (like many other tables) specify the minimum- have a look at the permitted deflection for floor joists that meet those minima- think at 2 metres it's about 6mm- and decide whether you can live with that amount of movement (it'll be safe, it won't fall down but it would p*ss me right off)

 

[Bouy]

The effective span for loading purposes is indeed 2000, but the joists are still free to twist and flex (the only anchor on the top of the centre point will be the floor covering).
Joists that size on that span will be pretty stiff, nogging will improve that stiffness.
TRADA tables (like many other tables) specify the minimum- have a look at the permitted deflection for floor joists that meet those minima- think at 2 metres it's about 6mm- and decide whether you can live with that amount of movement (it'll be safe, it won't fall down but it would p*ss me right off)

Yes allowable deflection 6mm but actual deflection 0.6mm (assuming C16 at 400 c/c), actual bending moment 1/6 of allowable bending moment