First Harmonic in Bridge calculations

[PMcGG]

I am designing a steel truss pedestrian bridge and I have run across a problem.

My company uses the AASHTO first harmonic criteria of 3.0 hertz or an alternate weight criteria of W>180e(-0.35f). The customer wants me to explain why the alternate weight criteria can be used, but I do not know where it came from. I have found several mentions of it in varying DOT standards but no document expalaining it.

Does anyone know whre I could find an explanation of the alternate weight spec for harmonic frequency in bridges?

Any help would be greatly appreciated.

 

[Qshake]

The dynamic problem can be simplified into two components that will characterize the dynamics of the bridge: mass and stiffness. The fundamental frequency is a function of mass and stiffness. Therefore if additional mass(can be related to weight by acceleration due to gravity) is used then the frequency will be lower and less likely to be excited by pedestrian or vehicle traffic.

Regards,
Qshake

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[graybeach]

I believe the second formula in AASHTO is based on the work of Professor Thomas Murray of Virginia Tech. I have also read, but I cannot recall where, that Professor Murray did not intend for his work to justify pedestrian bridges with natural frequencies less than 3 hz.