Timber · Bracing & floors

Timber floor vibration & long-term deflection calculator

Joist fundamental frequency, deflection under a 1 kN point load, and long-term creep deflection with j2.

Standard: AS 1720.1 Cl 2.4.1 (j2) · fundamental frequency check

Inputs in the worked example

The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.

Result	Value	Unit
I = bd³/12	51,840,000	mm⁴
Mass per metre (G only)	22.94	kg/m
Natural frequency (π/2L²)√(EI/m)	13.39	Hz
1 kN point load deflection PL³/(48EI)	2.98	mm
Long-term deflection j2·5(G + ψl Q)sL⁴/(384EI)	7.74	mm
L/300	14.00	mm

Diagram for the worked example

I = bd³/12 = 45*240^3/12 = 5.184e+7 mm⁴
Mass per metre (G only) = 0.5*0.45*1000/9.81 = 22.936 kg/m
Natural frequency (π/2L²)√(EI/m) = PI()/(2*4.2^2)*SQRT(10000*1E6*5.184e+7*1E-12/22.936) = 13.387 Hz
1 kN point load deflection PL³/(48EI) = 1000*(4.2*1000)^3/(48*10000*5.184e+7) = 2.9774 mm
Long-term deflection j2·5(G + ψl Q)sL⁴/(384EI) = 2*5*(0.5+0.4*1.5)*0.45*(4.2*1000)^4/(384*10000*5.184e+7) = 7.7376 mm
L/300 = 4.2*1000/300 = 14 mm

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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.