"Springy" floor

[ahorne2]

I have an extension over my garage where the floor is "springy".

The floor structure appears to meet Building Regs regarding timber size (8"x2" at 450mm spacing over a 3.8m span)

I`m thinking that the simplest and least disruptive way to reduce the "springy" feel is to add a steel H structure under the floor in the garage.

As the steel is to aid what is already there I`m not sure what size to use - I`m looking for the smallest suitable.

76x51mm Joist looks ideal, but using the 1/20th x span rule of thumb gives 190mm so a 203x190 Joist looks suitable, but that`s the steel alone to do the supporting and not factoring the timber contribution !

Any suggestions greatfully accepted

 

[Chukka63]

you could go for 2 timbers bolted together at 600mm centres.easier than a steel and more readily available.

although your joists may be up to spec size wise they could be under spec timber grade wise.just 1 possibility from a few.

 

[ahorne2]

Thanks Chukka63.

Timber grade is correct also, BC also signed it off ! Reasonable thought though.

My main driver for Steel is that I`m hoping to do it with a lower "profile" than timber, as I use the garage for working on cars and have a hydraylic lift that needs the height.

Cheers

 

[Chukka63]

i wouldnt trust it just cos BC signed it off.

 

[ahorne2]

Okay, but that doesn`t resolve the "springy" situation !

 

[ajrobb]

Okay, but that doesn`t resolve the "springy" situation !

I assume you have a fire ceiling in your garage. It is attached to the extension's floor joists?

I'm trying to get a handle on how much stiffness a ceiling can impart to joists.

 

[ahorne2]

Hi, yes there is a double layer of plasterboard (9.5mm each) on the underside of the joists.

On the top side there is 18mm chipboard flooring, with one mid position strut.

For info, the joists (200*50mm SC4) are on hangers that are fitted over trimmers that are fixed to the walls at 450mm spacing with 150mm bolts.

Per the Building Regs these should be okay for 0.25-0.5 kN/m^2 loading, but not sure how that equates to a 2.0kN/m^2 Distributed load also referred to in the Regs for floor joists

 

[noseall]

On the top side there is 18mm chipboard flooring, with one mid position strut.

Does this mean you have solid mid-span bracing?

 

[ahorne2]

No, a "noggin" I guess might be another term for it

 

[noseall]

Herringbone struts or full depth? What do you mean by a noggin?

Having correct bracing/bridging stiffens the floor up no end.

 

[ahorne2]

A full height timber brace at the mid point with two 6" nails holding each side to the joist. These are between all joists.

 

[noseall]

That rules out the bracing then.

 

[ahorne2]

Agreed.

Any thoughts on supplementary steel size ?