Vibration check for timber floor 1

[m_struct]

When checking a timber floor for vibrations, 1170.0 Table C.1 suggests 1kN point load at midspan with deflection less than 1 to 2mm.

For a timber floor, with say 20mm strandfloor on 240x45 @ 400 spanning 4m, how much load sharing would be reasonable? It is there a rule of thumb for this? Also, it is reasonable to take the 2mm deflection limit (not the 1mm)?

 

[Settingsun]

I would say spread it over three boards and consider them all fully effective. [Edit: misread your floor structure description]

Whether to use the looser deflection limit depends on how much a problem vibration is. Note that the check you're proposing to use isn't bulletproof. I'll try to find the reference that discusses it.

 

[Settingsun]

See the 1991 paper here: Thomas M. Murray "Building Floor Vibrations"

https://www.aisc.org/awards-and-competitions/t.r.-higgins/TR-higgins-past-winners/#27786

Apparently it was originally the 1mm limit, though the Murray paper suggests tightening that by a factor of four for commercial floors. Although that reads as a 'deemed to comply' sort of limit and a detailed check might give a less strict result.

BRANZ SR57 suggests 1.5mm for highly damped floors: see Table 1A

https://www.branz.co.nz/pubs/research-reports/sr57/

 

[m_struct]

Thank you for that. I will have a read through those. Vibration papers also seem to be lengthy.

The condition in question is a residential floor in a single family unit. The member spans from perimeter wall to wall (4.3m) and has a 600mm that supports the first floor wall and roof above. The (dead load) point load at the end of cantilever, takes out the a lot of deflection, but is not considered in a vibration case, it is suggested to use just ψ_sQ.

Distributing the 1 kN over 3 members gives a 1.3mm midspan deflection. Again this is not considering any G, notably at the cantilever, which would act like a precamber. Can that G load (exterior wall and roof above) be accounted for - it is permanent dead load?

 

[rowingengineer]

The Nash standard has the formula you want, there is an equivalent in an old timber standard as well but cannot remember the number.

 

[m_struct]

In regard to load sharing, 1720.1 and 3603 address load sharing for a centrated loads in a grid system.

1720.1
G₄₁

3603
k₉

My numbers showing both g41 and k9 are about 60%.

 

[Agent666]

The "1mm limit" is an extremely poor indicator of a low frequency vibration issue (i.e. floor system resonance under footfall), a 1mm (or other) deflection limit is a better indicator of a high frequency vibration issue (i.e. caused by high frequency vibration of plant or similar operating).

Unfortunately, because a deflection limit is typically easy to apply, as opposed to trying to understand and apply other more rigorous vibration assessment methods, it is wrongly abused/employed as a general vibration limit to identify both high and low frequency issues.

In addition to the load-sharing effect, if you're after a procedure suitable for timber floors, the recent BRANZ multistorey timber building design guide addressed the problem with a design method. It is freely available here, but you have to register

https://engineervsheep.com