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RE: steel - vibration analysis
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- Subject: RE: steel - vibration analysis
- From: "Josh Plummer" <josh.plummer(--nospam--at)cox.net>
- Date: Tue, 12 Jun 2007 20:24:31 -0700
Jordan - 1) Allen's message essentially said that it was impossible to have vibration problems in non-orthognal areas of a building. The main point of my response was that this stance of his was patently absurd. I take it that you at least partially agree with me on that. 2) Please take a closer look at that example in 7.2 of Design Guide-11. Your characterization of that problem is COMPLETELY inaccurate. I'm not trying to flame you here... I just think you didn't read the example very closely. The text of that example actually says the following: "Unacceptable walking vibrations occurred throughout most of the floor, more so adjacent to the atrium". When you look at the figure, the area adjacent to the atrium is the one with the non-orthogonal framing. Now, look closer at the skewed girders. They added in new COLUMNS at the mid-points of the girders. That's a waaay more significant stiffening effect than what they did to the other girders. Personally, I'm confident that our RISAFloor program would produce results which show the skewed area being more sensitive to vibration than the rest of the floor. 3)The fact that you characterized non-orthogonal plate vibration as akin to a "non-linear non-prismatic curved member" is a fairly gargantuan exageration. Structural vibrations just aren't that complicated. If it were, then you'd need a PhD to understand modal analysis, even more so if you were doing a Response Specta Analysis. I understand that some folks (perhaps even you) can find vibrations to be a truly intimidating subject. But, we're talking about common framing configurations with girder to joist angles of less than 90 degrees. This just isn't that big of a difference. We're not talking about extending the rectangular panel mode concepts to one of those freaky Frank Gehry buildings that look like they're melting. Like you pointed out, a true modal analysis (which RISAFloor can do through it's interaction with RISA-3D) can be appropriate for many of the funkier non-orthogonal cases (like the Gehry buildings). But, doing that for a nearly orthogonal bay would be a shocking amount of extra work for something that is almost certainly going to produce the same kind of results as the DG-11 calculations. Finally, I said before that I thought that I had read every article that AISC has published on Floor vibrations. And, I still haven't seen anything in any of their articles that implies that non-orthogonal areas are immune from floor vibration problems. Nor have I seen any warnings or cautions about extending these concepts into systems that are not perfectly orthogonal. But, if I'm missing something important here, then I'd like to know about it. Sincerely, Josh Plummer, SE RISA Technologies -----Original Message----- From: Jordan Truesdell, PE [mailto:seaint1(--nospam--at)truesdellengineering.com] Sent: Tuesday, June 12, 2007 4:42 AM To: seaint(--nospam--at)seaint.org Subject: Re: steel - vibration analysis Josh - when I read Allen's post, I generally agreed with it, though I may have misread what I wanted to read. Where framing changes length within a bay, it is correct that the variation in frequency tends to reduce or eliminate footfall vibrations because of the large number of different vibration modes at play - there is no large area in which the vibrations may be sustained. The example in 7.2 is also a very regular structure, with the diagonal areas both on the perimeter and around the atrium apparently not affected by the vibration problems - the primary areas of stiffening were in back-to-back regular orthogonal bays. (I only skimmed the article to see what you were referring to, let me know if I missed some of it) You can still have issues in non-orthogonal bays if the primary frequency is related to the girder, but the joists/beams should tend not to promote resonance unless they are all of similar stiffness, or all fall in the lousy range. Vibrations of plates and shells is greatly complicated by non-regular geometry, and can effectively only be performed numerically. If you have an issue with an unusual structure, the first thing to do (barring obvious deduction) is to determine your mode shapes, frequencies, and modal mass participation - then you can start to isolate where the issues are. Your comparison of extending the vibration theory from wide flange to tube is not analogous- extending from a wide flange to a non-linear non-prismatic curved member would be more appropriate. And, of course, the theory doesn't extend to such members. (now where did I put that set of nomex khakis...) Jordan ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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