VIBRATION DESIGN

[sedesigner06]

i was doing some research and found a artile stating that vibration design criteria only needed to be checked for idealized framing layout. This is a far different approach then what i have taken in previous projects where vibration designs were heavily dependent on deflection. I wanted to get some opinions on this approach.

Thanks, see link below for article.

https://communities.bentley.com/pro...on-analysis-in-the-ram-structural-system.aspx

 

[StoyanAndreev]

What are you designing? What vibration criteria are you wanting to implement?

 

[sedesigner06]

I am designing for sensitive equipment with a design criteria of the vibrational velocity >4,000 micro-in/sec

 

[JoshPlumSE]

I would take what's said in that article with a healthy dose of skepticism. That is an article largely meant to justify the methods that RAM uses. There is some truth in what they say. But, they take it a bit too far.

I may be able to offer some "historical" insight about why the article was written and what they were trying to accomplish with it. Granted, this is total speculation on my part. But, I feel confident in my speculation.

My guess is that this article was written in response to criticism that a competing program (my company's RISAFloor) was throwing their way about the limitations of their DG-11 implementation. Specifically their inability to perform any calculations for bays "irregular" framing. They want to paint what we (and their users) were seeing as one of RAM's weaknesses into a strength. That way they could reduce the number of their users who were ditching RAM in favor of RISAFloor. As such, my belief is that this is article was written with a dual goal.... marketing and technical.

Now, there is some truth to what they say in that article. So, don't completely discount what is said. They make some valid technical points. It's just that those points should be that this weakness of RAM's is not as severe as it first appears. Not that it should be viewed as a "strength" of their program. Really, that's a pretty transparent indicator of the article veers away from the technical goal in order to achieve its marketing objectives.

My company had been hammering them pretty hard on their weaknesses related to "irregular" framing. So, I understand their desire to combat the narrative we were putting out. And, after having discussed the subject with Dr. Murray I understand where they were coming from with their implementation.

Note:
There was a previous Eng-Tips discussion between me and Dr. Murray on the subject of irregular bays and floor vibration. Eventually, we came to the conclusion that RAM's exclusion of irregular bays was NOT necessarily appropriate. And, that the procedures of Design Guide 11 can and should be used to determine vibration or frequency problems in irregular bays.

Specifically, we discussed the example shown in Design Guide 11 figure 7.2. Where the "irregular" bays in a floor system exhibited the most severe vibrational response on the floor level. Enough to warrant adding in additional columns to change the frequency of the irregular framing. This is a case where RAM's limitation would have ignored a bay because of "irregular" framing but where RISAfloor would have identified a vibration problem.

 

[sedesigner06]

Thank you for the thorough response Josh, and that confirmed my way of thinking. I wanted to believe it since it would cut out about 1/3 of my calculation packages on most vibration controlled floor design. But some of the statements I could just not agree with and find the approach very flawed. Especially since the design guide they say they follow does not take the approach. I guess it makes sense if they wrote the article to match their limitations of the software. I will look for the previous discussion with Dr Murray and hopefully gain some more insight.

I looking for a quicker approach then DG 11 approach for irregular framing and talk to some engineers that just up the deflection criteria to L/600 which seems to be very conservative with some of the previous example I compared it too.

 

[JoshPlumSE]

That article (and my previous discussion with Dr. Murray) was geared mostly towards floor vibration due to walking excitation.

Sensitive equipment is a different animal. A lot of the underlying concepts are the same. Especially as far as calculating frequency of the floor system or beam or such. But, the acceptability criteria is quite different. Chapter 6 of the design guide rather than chapter 4 or 5.

The only experience I have with sensitive equipment equipment is from being in the cube next door to an engineer working on some support framing for an MRI. I absorbed some knowledge through osmosis. But, not enough to give you any real quality advice.

 

[TDIengineer]

I would urge any designer that is designing a floor, where that floor system has vibrational constraints, that certain parts of the flooring be checked by hand or by a spreadsheet that specifically applies the principles of design guide #11.

Use the software to identify the critical areas only, but then perform your own numbers to justify your design. Why wouldn't you when the success of your floor system is wholly dependent on the performance of its vibration and stiffness.

Let's not be lazy and trust the computer.. you will usually get best results when you do the thinking, as it appears you have done here.

 

[sedesigner06]

agreed TDIengineer and thats what i have been doing but it takes up about half of my design time to do the floors and was looking to reduce it. When a software says they do vibration design i definently think it is worth looking into but from what i have seen it does not do it fully to the extent that i do it by hand. Other engineers have seem to found much quicker and less sophisticated ways by putting more stringent deflection limitations but dont think that is correct either. In the time of design in quicker with less fee i try and explore anything i can.