Avoiding Resonance in Structures

[eit09]

I use RISA3D to calculate the natural frequency of the structure in the vertical & horizontal directions. I try to keep the natural frequency of the structure at least 30% below the frequency of the screen. I would like to know how others design to prevent resonance. I deal with structures that support screens that run between 800-900 rpm. These screens are usually located three to four stories high. I run several separate RISA3D models as listed below for frequency checks to assure that resonance will not be a problem. I am very interested in how others approach these types of cases and if there are other suggested checks I should be performing.

1. Full structure model
2. Level supporting screens only model
3. Beams supporting screen only model
4. One single screen supporting beam model

 

[cessna98j]

Hi eit, is this an aggregate screen for a concrete plant?

We've designed support structures for these in the past, and we typically design our structure so that there is at least a 25% difference between the operating frequency of the screen and the nearest structural modal participation based on a RISA dynamic analysis. I typically perform analysis for the whole model only.

Another thing to consider is the ratio of the forcing function (operating frequency of screen) to the natural frequency of the supporting structure. This ratio in conjunction with the damping ratio (typically 5%) is used to determine the dynamic load factor. This factor can then be multiplied by the operating weight of the screen to determine how much additional load to account for dynamic effects. There are graphs that you can use to determine the dynamic load factor given the forcing function/natural frequency ratio and damping ratio. I use the one in "Dynamics of Structures" by Anil K. Chopra. I know there are other graphs out there, I just wasn't able to find anything online to link to.

 

[cessna98j]

Something else to consider:

You should probably account for a plugged condition as well and assume that the rotating mass will be significant if this is a rotary screen. Either way, a plugged condition will have a higher operating weight, thus your fundamental frequency for the supporting structure will be lower. The manufacturer should be able to provide you with a plugged weight.

This should be analyzed as a separate condition.

 

[eit09]

cessna98j,

yup, the screens are 8x24 with 4 Decks.

 

[eit09]

These are inclined vibrating screens mounted on springs so I do not consider the weight of the screen or a plugged weight when checking the frequency. The few articles that I have come across state that when checking frequency only account for weight that is rigidly attached. Are the screens that you work with mounted on springs? Are the structures that you design to support screens multi story or are they mainly for the screen supports?