Modal Frequency/ Harmonic Loading

[Lutchmanen]

Hello,
We are designing a stainless steel feature consisting of CHS arched over a distance of 20 m, to a height of 7.5m, braced at intervals of 2m with CHS sections. The whole structure is fixed to the side of a concrete bridge. We have been told that we need to design it for harmonic loading.
Currently, we are using Frame (Prokon) to model the structure, and now we are set to run the analysis for Dynamic:Modal and then for Dynamic:Harmonic.
We have a design wind speed of 275kmphr.
Has anyone ever run similar analysis and what are we looking for exactly? We are particular curious about the damping ratio and mode shapes as required by the sofware.
Can anybody run me through the procedures?
The model is herewith attached in dwg.

 

[Qshake]

Did you receive any information for the harmonic loading? certain period? Amplitude? Damping?

Articles attached to bridges are not typically designed with harmonic loading, they are designed using static loads based on a certain wind speed and height of the structure. Gusts are a different matter and factors used to account for speeds above the basic speed.

Seems like you should have more information and exactly what expectations are desired by your client.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Lutchmanen]

Basically, all the client is worried about is whether there will be a whooing sound caused by the attached structure everytime a car runs along the bridge. Or whether it will sound again whenever there is strong winds.
The structure shall be wholly fabricated and we are to design it such that no vibration (noise and detrimental to fixings) is caused by either the wind or cyclones we usually have, or just the movement of traffic alongside it.
We havent obtained any more info.

 

[YelEeCoyote]

For the wind issue I'll guess your event is going to be a sine dwell with amplitude equal to the wind force on your structure under max wind. Or maybe you can find something in the structural code you are designing to for specifications to help you with dynamic loading. Maybe they will give you a minimum allowable modal frequency as function of wind environment.
For the car wind load I'd smack your structure with a sine wave of amplitude equal to the wind force generated by a standard vehicle and frequency calculated based on a steady stream of cars at the posted speed limit. Your structure should be stiff enough that its first constrained modal frequency is well above that.

If you can't find code to guide you then I'd argue that the methods above, combined with engineering judgment and a significant factor of safety should settle your client's worries. Hopefully the structure is nice and stiff and vibrations aren't going to be a problem.

 

[Lutchmanen]

Indeed, I couldnt find any appropriate document or guidelines to see me through the specifics of this design. And finally, we found it satisfactory to simply design the structure to the maximum wind speed and go for sections seclections such that there is no deflection on the structure. Which we suppose is an indication that the attained stiffness shall be sufficient to resist any vibration.
This however led to connections with moment restraints, but we were able to deal with that.
You are right, sometimes, when designing, engineering judgement can provide better solutions than simply trying to follow prescriptive guidelines. Always at the expense of something else, yes, like money, but hey.