Existing screening building with vibration issues 1

[gyung]

Hi everyone
Writing this question to seek any advices/comments about vibration issues on an existing steel building, perhaps sharing similar experience as well.

A steel building includes two vibration screens on higher levels and crushing machine at the lower level. Excessive vibrations were reported when first commissioning. The designer then came back and did a few modifications to the existing structure, such as additional steel bracing and columns. It did improve the vibration issue, but it was still not acceptable from people' comments on site.

Another consultant got involved to study the vibration issue, and a few reports were done. First report, based on purely natural frequency study and some vibration measurements on site, concluded that the vibrations likely damage the structure. Of course, the owner would not happy to hear that, so another set of vibration measurements were obtained on site. Then, second report, purely based on the second set of vibration measurements, stated that the equipments were safe to run. However, it did not mention about the structure health regarding to the vibration issue.

My first question: Is there any vibration guideline or limit for buildings subjected to vibrating machinery? I found most of them are talking about human acceptance rather than structural integrity. Vibrations were measured in peak to peak displacement, velocity, and acceleration.

Then, we engaged to re-look at the vibration problem and the client consider to upgrade the existing screens with even heavier in weight and higher operational frequency. First attempt was to use the existing model in SPACEGASS, which only does natural frequency analysis. The results showed that the building possibly get excited within the operational frequency of the screens. Unfortunately, the natural frequency analysis doesn't tell how bad is the vibration unless we run dynamic response analysis (such as harmonic analysis/ transient dynamic analysis). I learned it in a hard way, I should have modelled it in FEA package so that I could run the dynamic response analysis to target the vibration problem. Now, we had built a FEA model using Strand7 and had some vibration results.

My second question: FEA results show that the building will have excessive vibrations (like velocity 200mm/sec) during the operational frequency. The vibrations in FE model are much larger than the site measurements. I have put all the mass of equipments (including equipments, concrete slab, grating, chute works, etc) but, I have yet to put the stiffness of chutes and conveyors in the model. Actually, I don’t know how much stiffness I should put in for those components. Any thoughts about how to verify the FE model with the vibration measurements on site. I know it will be difficult to replicate the existing situation. Should I try to simulate the existing situation or I am wasting of my time here?

 

[steve1]

To answer your first question: according to our vibration consultant the maximum allowable accelerations to prevent damage to a steel building is 10 mm/sec.

 

[msquared48]

Interesting that in your description of the vibration problem, there is no mention of any dampers or spring isolators in the system.

Was this an original design oversight, or an omission in your description?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[gyung]

Thanks Steven1, I found similar limit (10mm/s) in dynamic textbook as well, but it was related to transient vibration. Vibration screen is more like steady-state vibration, will they have different limit. Correct me if I am wrong.

Additional to that, a vibration audit report was done by someone else, and they used peak to peak amplitude to justify the vibration issue, and they reported as dangerous to the structure. Should they look at velocity rather than amplitude?

 

[hokie66]

Generally, a structural frame designed for vibratory screens and crushers will be very robust, at least it should. But that doesn't mean that vibrations will not make it uncomfortable to work there. All vibratory screens use some type of isolation, be it steel springs, air springs, whatever. If the screens are big, it is also common practice to use a heavy frame under the screens which is itself isolated from the structure.

While I have designed frames to support this type equipment, the dynamics was left to the specialists. Sounds like you have disagreement between the "experts". We have a couple of those guys on this site, and perhaps they will chime in in due course.

 

[gyung]

msquared48:

The vibration screens are just supported with springs that come with the screen, but not on an isolated frame. I believe the dynamic forces specified by the screen manufacture are after their damping system. They are very small, like 2kN dynamic force in both vertical and horizontal, but I guess if it hits the resonance frequency, it could be 100 times.

The original design was done by other consultant few years ago. Many vibration measurements and studies had been done by others as well. Interesting that, I have tried many times to understand what actually went wrong, and how bad is the current vibration on site. But, no one wants to clearly say anything. We have also been asked to ignore the previous reports and measurements, just do our engineering to satisfy ourselves.

 

[bradleyelwood]

First, using velocity as a criteria is a good way to compare one industry to another, or to determine if a vibration level is acceptable. 200 mm per second is quite high. What were the actual, measured values? Even without measuring devices, you should be able to run the machines one at a time and determine if a few machines are causing the problems.
Vibratory screens operate at 600 to 1200 cpm and are normally supported on isolator springs and operate at speeds way above the natural frequency. They can be quite "violent" when started and even worse when stopped, but I assume that you are not concerned with this. Screens normally do not transmit large vibrations to the surrounding structure during the time they are running full speed.
Jaw crushers operate with horizontal shafts at lower speeds like 90 to 180 cpm and if they are supported with structures resonant with the "one per rev" the responce is very large and cannot be balanced out of the machine. The only solution is to fix the supporting structure.
Cone crushers wobble their liners about a vertical shaft and can impart a significant vibration to the surrounding structure.
Most structural-vibration problems like you are describing occur because the horizontal forces and horizontal structural stiffnesses are underestimated.
High speed vertical shaft impactor crushers are seldom a problem unless they are out of balance.
It might be helpful to get a list of machinery, type, model, size, etc and some idea of how each machine is supported.