pedestrian bridge natural frequency

does anybody can help me? I need to perform a dynamic analysis of a pedestrian bridge: 50 m single span. It is a steel structure with a concrete deck...anyone knows a formula to calculate the natural frequency? thanks a lot

 

[GregLocock]

Analytical solutions only exist for a few simple structures. Failing that you'll get in the right ball-park for the first vertical bending mode by calculating the vertical stiffness at mid span, and using a mass equal to half the bridge's mass, in the SDOF frequency equation. This could give errors as large as 200% quite easily, but usually works.

These days you are probably better off using an FE model, which are occasionally quite accurate.


Cheers

Greg Locock

 

[RCHeli]

Check out a publication from AISC Engineering Journal, an article written by Thomas M. Murray, PhD, from Virginia Tech. I believe it was circa 1991. Dr. Murray's expertise is in floor vibrations subject to human occupancy. If you can't find the publication, he can be reached at the Civil Engineering Dept. at Virginia Tech, Blacksurg, VA. I'm sure he can provide the appropriate technical guidance to you.

 

[ishvaaag]

If of constant section you can find formula (and example, I think to remember) in Table 11-12 p.564

Stress, Strain and Structural Matrices
Pilkey
Wiley Interscience 1994

For pinned-pinned support and uniform beam

fi=[landai^2*sqrt(EI/ro)]/[2*pi*L^2]

fi natural frequency for i mode, result is in Hz

landai=i*pi
pi=3.14
L=span
ro= mass per unit length

 

[RHK]

The simple formula for natural frequency is
f = 16/sqrt(d)
where f = frequency in Hertz
d = maximum self-weight deflection (at 1.0 g)

If you want the frequency for lateral vibration, then simply analyse the structure for gravity in a different direction.

Cheers

RHK Russell Keays