Modal Frequency/ Harmonic Loading
Modal Frequency/ Harmonic Loading
(OP)
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.
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.





RE: Modal Frequency/ Harmonic Loading
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,
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RE: Modal Frequency/ Harmonic Loading
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.
RE: Modal Frequency/ Harmonic Loading
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.
RE: Modal Frequency/ Harmonic Loading
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.