24mm Joists

[Gizzydash]

Doing a bit of decorating and had a sneaky look under the floorboards to see what the story was, floor bit bouncy (to very bouncy) in places.
Much to my surprise the joists are 24mm thick (by about 100mm).
House was built early 60s, would that have been common? Seems massively undersized, closer to fenceboards!

 

[jacko555]

What's the span and centers?

 

[Gizzydash]

Roughly 1.5m spans at 400 centres. I suppose how narrow they are was a surprise, wouldn't have expected anything less than 40mm, 24mm seems super thin

 

[^woody^]

Modern engineered joists are thin ply.

Are there any noggins? That's usually the remedy for bouncy floors.

 

[Dereekoo]

Modern engineered joists are thin ply.

Are there any noggins? That's usually the remedy for bouncy floors.

Don't know why noggins make a floor less bouncy, structurally they add no strength in the vertical plane?

Re the actual joists they are just on the cusp of acceptability for strength and deflection assuming c16 grade

	DATA INPUT
Imposed Load /m2 =	1.5​	kN	=
Spacing of joists =	400​	mm
Mean E value	8800​	N/mm2
b=	24​	mm
h=	100​	mm
Span=	1.50​	m
Dead Load/m2	0.5​	kN	=
Density of joist	4.2​	kN/m3	=
Bending Stress (C18 or C24)	5.3​	N/mm2
Load sharing value	1.1​	1.375​	temp snow loading enhancement value
Strength class	c16	c24 or c16
Shear stress	0.67​	N/mm2
Compression stress perp to plane	2.2​	N/mm2
Compression stress // to plane	6.8​	N/mm2

Outputs
Actual deflection (5/384(w x l^4)/EI + 12/5(wL^2/Ebh))	3.24​	mm
Allowable deflection (1/360 and 0.003)	4.50​	4.17​	mm	Max 14mm
Actual BM for UDL beam(wl^2/8)	0.23​	kNm
Allowable bending moment	0.26​	kNm
Applied shear force (W/2)	0.61​	kN
Actual shear stress (3V/2A)	0.380​	N/mm2
Permissible stress	0.737​	N/mm2
Bearing stress on 100 wide wall plate(W/100b)	0.253​	N/mm2
Permissible bearing stress	2.42​	N/mm2
Bearing length required if permissible bearing stress used	10​	mm

 

[^woody^]

Don't know why noggins make a floor less bouncy

Two possibilities

1 component binding and load sharing
2 Voodoo

 

[Gizzydash]

Definitely not c16 grade, been there since 1960s, and no noggins. Here's hoping they last another 30 odd years!

 

[Dereekoo]

Two possibilities

1 component binding and load sharing
2 Voodoo

my money is on voodoo, load sharing already done via flooring

 

[Dereekoo]

my money is on voodoo, load sharing already done via flooring

Thinking about it 24mm is super slim and the joists could be considered slender with quite a lot of flex in them at the bottom end, maybe woody has a point (on this occasion) and central noggins may help??

 

[tony1851]

Thinking about it 24mm is super slim and the joists could be considered slender with quite a lot of flex in them at the bottom end, maybe woody has a point (on this occasion) and central noggins may help??

But would it matter in this case? The bottom of the joist will be in tension. Its the top which may potentially buckle by being in compression, though the top will be held by the boarding.
(Similar to the case of steel beams, where its the top (compression) flange which needs to be restrained to prevent lateral buckling).
Although inspectors often ask for noggins, and the old taables recommend them, BS 5268 says you don't need them for a depth/width ratio of less than 5 - which it is in this case.
The original joiners must have been very careful in fixing the boarding, as the joists are so thin, nails could easily have split the grain and come out through the sides of the joists?

 

[Bouy]

But would it matter in this case? The bottom of the joist will be in tension. Its the top which may potentially buckle by being in compression, though the top will be held by the boarding.
(Similar to the case of steel beams, where its the top (compression) flange which needs to be restrained to prevent lateral buckling).
Although inspectors often ask for noggins, and the old taables recommend them, BS 5268 says you don't need them for a depth/width ratio of less than 5 - which it is in this case.
The original joiners must have been very careful in fixing the boarding, as the joists are so thin, nails could easily have split the grain and come out through the sides of the joists?

Good point, I guess I'm just grasping at straws for a reason for the bounce as opposed to deducing it from a structural perspective