Natural Frequency Reccomendations

[dmaughan25]

Are there any recommendations for natural frequencies of a box truss? The natural frequency of my truss and structure is 1.52 Hz in the lateral direction and 1.78 Hz in the vertical deflection. The truss is 180 ft long and 8 ft deep. (see attached image)

Per ASCE 7-05 if a structure has a natural frequency of 1 Hz or higher it can be considered a rigid structure. This means I can use a wind gust factor of .85, but does it also mean I don't have to worry about harmonic frequency problems.

I am worried about the truss hitting its harmonic frequency in the wind. I am worried about the variations in wind speed (gusting) and vortex shedding may cause the truss to start vibrating.

Is there any literature on this?

AASHTO recommends a natural frequency of 3 Hz or higher for pedestrian bridges, are there any other recommendations out there?

Any help would be much appreciated.

Dallas Maughan
Structural Project Engineer
ES2

 

[dmaughan25]

I found this article that has shed some light on it for me

"The Nature of Wind Loads and Dynamic Response" by Daryl Boggs and Jeff Dragovich.


It states the following:

"The building’s fundamental natural frequency of vibration, f0, is the most widely accepted property of a structure used to determine whether dynamic response will be significant under wind loading. It has the added feature of being concise and commonplace in the vocabulary of a structural engineer involved in seismic design. The ASCE Standard [ASCE 7-05] classifies a structure as dynamically sensitive, or “flexible” if f0 < 1 Hz, otherwise it is considered to be “rigid.” The classification used by the ASCE Standard is widely accepted as a reasonable boundary between dynamic and rigid behavior"

So based on this I do not need to worry about the harmonic effects of wind.

Does anyone see any issues with using the logic here?

Dallas Maughan
Structural Project Engineer
ES2