Truss Natural Frequency

[kelty71]

Can anyone provide guidance on a minimum natural frequency, or a range of natural frequencies to avoid, when designing a relatively long-span steel box truss? I'm designing a two-span box truss supported on trussed towers with truss spans of 118 feet. The trusses are supporting process piping and cable tray approximately 40 feet above grade. The trusses need to be about 10 feet tall for geometric reasons and at least 4 feet wide. With my current design, deflections are reasonable and the natural frequency of the system in the horizontal axis is about 2.0 Hz. Is this a high enough frequency to avoid resonance with wind loading? I have heard suggested lower limits of 1.0Hz and up to 3.0Hz. I would have to widen the truss to almost 6 feet to get to 3.0 Hz which I don't want to do unless I have to. These trusses will never see foot traffic and are located in a low seismic risk area.

 

[thaidavid40]

I'm definitely NOT an expert in this area, but if you haven't yet looked there, AASHTO might have some good guidelines for you, as these types of trusses are so often used for overhead sign supports exposed to wind loads.
Happy hunting.

Thaidavid

 

[BUGGAR]

Are you including the mass of the supported piping?

 

[kelty71]

I checked the natural frequency with and without the mass of the supported pipes and the difference was not significant. I'll take a look at the AASHTO. Thank you for the suggestions!

 

[BUGGAR]

I did some pipe bridges and remember following a code recommendation on this in either IBC or ASCE, so there is a code number (or recommendation) to follow. I don't remember the numbers but it's best that you look up the latest.