Industrial equipment on a steel structure 3

[marwin23]

Good morning,

I am looking for a piece of advise how to solve the issue when placing industrial equipment on an existing steel structure.

In details: 4sty existing steel frame (columns, beams + metal deck w/concrete) carries currently some flour mill equipment (old, mostly to be replaced). Some units of the new equipment to be placed on the top floor (can not on ground) have an operating frequency between 7-14HZ, whereas it seems that structure natural frequency is close to 8.5HZ. As per manufacturer, I can not provide dampers in between unit and my floor - however here if someone has any experience to argue this statement, then it would be good.

My first thought is:
- Unit operating weight is roughly 5kips. If I pour a block of additional 15kips, then (since mass is in square root (Sqrt(k/m)->(Sqrt(k/(m+3m))) I may be able to reduce the system output frequency twice to 3.5-7HZ. Or maybe it does not work that way, and concrete block would gladly transfer 14HZ from equipment to my deck without damping it (that would be against newton laws probably, but just want to assure myself).

I probably can someone support this weight on the current structure or reinforce as necessary. But at the same time such block would reduce my natural frequency of whole structure (but that would be just a fraction of whole weight). Does this approach make sense?

If someone has any experience - I would appreciate your input!

 

[marwin23]

We failed. Or in fact I failed. We can not deliver solution which owner is willing to accept. He refuses adding concrete on top of the slab, equipment sales representative does not allow on using dampers, and I am unable to provide any other meaningful solution. Over an over again expectation is that I shall add more beams (but not columns) what would only increase stiffness and natural frequency. Contractor seconds the opinion that he participated in project requiring adding mass, but here it means that we need to stop operation for the other (existing to remain) part of mill, which is not acceptable.
Owner asked for providing all drawings and calculations for probably other engineer to take it over. The closest what I was able to achieve was approx. 7.9HZ for the whole floor, but equipment manufacturer was not willing to reduce his factor of safety (disallowed range)to 8HZ. We decided that we are unwilling (as a company) to take the risk, and believe that there will be someone who will sign existing structure without major modification to be suitable for this load. Probably if I were 30 years older and retired, I also would certify such thing (maybe experience, maybe knowing that before it fails I may be somewhere else), but not today. The worst - I feel bad since I was unable to provide acceptable solution.

Have designed over 200 multistory buildings in my life, and many times had to explain others different issues. Some of them were taller, some smaller (but rarely below 6sty), and often on a lot line with other structure (NYC) and 1 to 3 floors underground. Almost always cast-in-place, steel or sometimes CMU. Some were existing, some were new. But here on Midwest I failed with a simple steel framing for a mill. No seismic (such area), no wind (inside other building), quite simple loads, no underpinning, no MTA (subway), no architect to argue for column location. And I could not find a solution.

Thank you all for very helpful pieces of advise I received!

 

[r13]

Sorry to see you've decided to walk away from this project. You should list all feasible alternatives in your report, and as a last shot - agree on a least intrusive modification that is acceptable for all, and retain the right to assist in subsequent troubleshooting and calibration, if necessary. It seems to me the owner was willing to go that direction. But, yes, it involves risks.

 

[human909]

It sounds like you did you best at considering most options. Sometimes it is better to walk away rather than accept responsibility for something you don't have complete control of. Or to others who have more experience in niche areas.

Sorry to see you've decided to walk away from this project. You should list all feasible alternatives in your report, and as a last shot - agree on a least intrusive modification that is acceptable for all, and retain the right to assist in subsequent troubleshooting and calibration, if necessary. It seems to me the owner was willing to go that direction. But, yes, it involves risks.

Agreed.

Post installation rectification might be an option that could be considered. When the building is existing I don't see the issue of installing the item and observing the results. Any induced vibration should quickly become apparent. There is enough unknowns and uncertainty that it could be much ado about nothing. (It is a different story if you are building from scratch.)

Also:

-Was adding lump masses considered? This wouldn't result in down time.
-What about the measurement of the existing natural frequency of the floor?

 

[ldeem]

I would not feel bad at all. You proposed sound solutions to the problem that the client rejected. You are far better off walking away then moving forward with a compromised design and getting a phone call in a few months because the equipment vendor is blaming you for some problem.

There is lots on uncertainty in dynamics which makes it very difficult from a design perspective. Maybe it will work fine with a few braces or maybe not. One way you look overly cautious and the other a genius.

I am not surprised by the equipment sales rep's position, there is no way they will deviation from the manufactures specification. The manufacturer is not going to make any exception to what has worked for them in the past on a single order.