Processing structure bar grating and deck plate vibration

[ldeem]

I have a project where the vibrating equipment (screen) was upgraded to a larger unit. The frequency is about the same but the dynamic forces are 5 times higher. During design I focused on keeping the main structure frequencies away from the vibrating equipment. This has worked and the structure itself is performing well. The issue is bar grating and deck plate between supports in some areas are vibrating so bad it feels like your teeth are going to shake out. This is only in areas between supports and when you move over to stand on a support (C6 channel) the vibration feels acceptable.

I am wondering if anyone else had experienced something like this and what the solution was. I am thinking of trying to isolate the floor from the supports somehow but that would mean possibly having different floor elevations since I only have objectionable vibration in certain areas. Welding down the bar grating is a possibility but if that didn't work it would be a lot to take up the bar grating and put down some kind of isolation system.

As a second question when dealing with high dynamic forces does anyone have any design references for flooring design? I have seen a lot of screening structures built with concrete floors but I always assumed that to be for ease of cleaning.

 

[r13]

Have you tried different type of clips/fasteners?

 

[human909]

It sounds like the you did you job on the structure but forgot about the grating. So increase the fundamental frequency of the grating by installing intermediate supports.

In my experience you generally need beams about twice as stiff as you would otherwise design them purely for strength requirements.

 

[bootlegend]

Have you checked the natural frequency of the walkways between the supports? Human909's suggestion of adding supports is probably where I'd look first also.

I don't claim to have a solution, but I've designed many screen supporting structures and generally the approach of keeping the natural frequencies away from the screen operating frequency works for me. I usually check with a minimum mass (dead load only) and then also with any live load that is likely to be there during operation. Beyond that I check walkways and individual bracing members for natural frequency. I have had some instances of a particular walkway, a single brace, or a random handrail vibrating over the years and it is certainly frustrating. Most issues I've had are during shutdown when the screen will pass through several of those frequencies. The more accurate you can make your structural model the more accurate your calculated natural frequencies. Also, I would verify your screen operating frequency using motor speed and sheave ratio. I've found this can be as much as 60rpm different from the nominal value on the data sheets.

I've seen concrete floors a few times but never used them on my designs.

 

[ldeem]

Thanks for the input everyone. I generally check the main structure and then primary supporting beams. I like the idea of doing the main bracing members as well. I have never had an issue with bracing but I can see the value in checking.

As for looking at the fundamental frequency of grating or deck plate between supports - do you use a range of mass to simulate a person standing on the grating? I thought about doing this but then decided there was too big a range of potential masses to make it meaningful. I was thinking I would have self weight only, person standing in middle (could range from say 175 lbs to maybe 250 or more) then have support conditions varying from pin-pin to 3 or 4 span depending on the location. Secondly, in this situation the grating was fine on the previous screen and the new one at similar frequency but higher dynamic load is a problem. So to me that indicates the fundamental frequency is ok (not resonating).

I am going to make a spreadsheet to look at the various SDOF conditions on bar grating and deck plate to see what I come up with.

bootlegend - I have always taken the screen speed as accurate but I will look into motor speed and sheaves. In this case I used some free vibration measuring apps on my phone and found the frequency to be off but I don't know if that is the phone or screen. My phone measurement was 8% off which I thought was pretty good when you consider the design range I use is 25% minimum (usually more like 30 to 40% is my final number).

 

[WARose]

It's always the little things that get you in this kind of work. (I.e. What you didn't model: piping, deck, grating, light fixtures.) So no, this isn't unusual.

The first thing to find out is: what sort of frequencies is your grating shaking at? You might could modify the grating (maybe welding a angle diagonally on the underside)....or perhaps change it's boundary conditions (i.e. how it's connected).