Response spectrum
Response spectrum
(OP)
Hi,
I have a question on response spectra. I have attached a figure showing an arbitrary response spectrum. Assume that my component's first mode is at the vertical red line. If I assume that all the mass is participating in that mode, and apply the acceleration (in my Nastran model) shown by the horizontal red line, is that conservative?
My thinking is that regardless of what other modes the item has, it should never see an acceleration higher than the horizontal red line. But I'm not completely sure and would really like someone to either confirm this or tell me I'm wrong.
Thanks!
I have a question on response spectra. I have attached a figure showing an arbitrary response spectrum. Assume that my component's first mode is at the vertical red line. If I assume that all the mass is participating in that mode, and apply the acceleration (in my Nastran model) shown by the horizontal red line, is that conservative?
My thinking is that regardless of what other modes the item has, it should never see an acceleration higher than the horizontal red line. But I'm not completely sure and would really like someone to either confirm this or tell me I'm wrong.
Thanks!





RE: Response spectrum
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Response spectrum
I'm not sure I understand your question. I'm coming from the perspective of using a static analysis rather than a response spectrum analysis. That's why I wanted to make sure that the approach I mentioned was conservative; if it's not then I need to find another way, or add some fudge factor to ensure I get a conservative acceleration.
I know that if my system has only one mode (a single degree of freedom system), then it will see the acceleration on the response spectrum curve that corresponds to its modal frequency. I'm less clear on multi-mode (real) systems, because if you have multiple modes you could end up with an acceleration higher than the curve. I guess I could rephrase my question to be something like: If the lowest mode of my system is at the vertical red line, can I obtain an acceleration value that would enable me to analyze it statically and conservatively?
RE: Response spectrum
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Response spectrum
NRC Reg Guide 1.92 has some discussion and a good list of references on response spectrum analysis if you need it.
I'm not sure I understand Greg's concern on fatigue. The performing a response spectrum just gives you the stresses; an appropriate number of cycles for your loading would need to be considered.
RE: Response spectrum
Thank you, that definitely helps. A lot of the math in the NRC document is over my head (I have only a humble bachelor's degree), but I'll see what I can learn from it. Maybe you can clarify something that has been bothering me for a while and is related to this subject. One of the methods I know of for seismic analysis is to take the peak of the response spectrum and multiply by 1.5, then apply this acceleration statically to the model. What is the basis for the 1.5? If all the mass of the structure participated in a mode at the peak, then the peak would be the actual acceleration. Is this related to the possibility of closely spaced modes? Can you actually have an acceleration greater than the peak?
Thanks again
RE: Response spectrum
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Response spectrum
That 1.5 number sounds familiar to me as well but I'm not sure of the basis, perhaps I'm missing something as well. I'll have look around a see if I have something on it.
The closely-spaced modes problem relates to not knowing when the peak responses of modes occur in time. Modes that are at nearly the same frequency tend to peak near the same point in time and the combination of the response of each mode should reflect this.
Greg,
It doesn't really have anything to do with fatigue and hence my confusion. Perhaps we are talking about two different things with similar names. In the language I'm used to a response spectrum is a plot of the peak response of SDOF oscillators to a given input motion. A response spectrum analysis uses that info to approximate the peak response of an MDOF system to that input.
RE: Response spectrum
My guess is that that is an excitation spectrum (ie what is applied at the foundation). Then the Op's post makes a glimmering of sense.
Well I still think that ignoring the enormous low frequency peak in that response spectrum is non conservative. So my answer is still no, it is non conservative, for general systems.
If your system has sufficiently low damping then the stress spectrum may be dominated by the response at resonance, but it may not be. How can that be a conservative conclusion?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Response spectrum
In my experience response spectrum is used for short duration random loading (e.g., seismic excitation), where I would think resonance is less of a concern because you aren't driving it at a constant frequency like you may in a more typical mechanical vibration problem.
RE: Response spectrum
The response is dominated by the non resonant low frequency stuff, NOT the response at resonance.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Response spectrum
My issue has to do with a system that is not quite at the "flat part," and will therefore experience some amplification. It would make sense to me that it would be conservative to just apply the value on the curve. However, I wonder whether several other modes nearby could make the actual response higher than the curve.
The reason for my doubt is the "static coefficient method," which is discussed in IEEE-344 (seismic qualification) and is quoted in many places on the internet. This method takes the peak of the curve and multiplies it by 1.5 to account for "the combined effects of multifrequency excitation and multimode response." I'm not sure what that means, and it is generally assumed that the system "can be represented by a simple model" and is "physically similar to beams and columns." The whole point of the static coefficient method is that you don't have to know the modes of the system.
This just occurred to me: could this factor (1.5) be due to the fact that the system could have multiple modes that act in different directions? Let's say I have 2 columns with a horizontal beam in between: the two columns could be excited out-of-phase with each other, which would increase the stress in the horizontal beam. You would not get this same effect if you applied a static acceleration. Perhaps that is where the 1.5 comes from?
RE: Response spectrum
http:/
It says that taking the peak value of the spectrum (i.e., factor of 1.0) can "usually be shown to give conservative results" and references an older paper by the author (which I have not found). This particular paper doesn't happen to point out the unusual case where it would not be conservative.
RE: Response spectrum
http:/
RE: Response spectrum
what is unclear is that you are tossing out an "arbitrary reponse spectra" that does not show the modes of concern.
what you've shown appears to be the excitation spectra, although the shape of the curve appears to suggest that non-linear structural effects may be present
all vibration analysis and testing of the system response always exhibits the presence of the structual modes, they are not invisible to such tests unless you happen to place the sensor at one of the nodes.
if indeed your "arbitrary spectra" is that of the excitation, and your red lines indicate the lowest system mode, then you are actually exciting those modes albeit with reduced intensity, however, the system response to that is governed by any damping present.
fatigue issues are another matter. to have a discussion in that regard you need far more specifics that you've provided
RE: Response spectrum
I'm not familiar with "excitation spectra"--is that a graph of the input acceleration (what would be the floor motion in my case)?
What I have is a response spectrum, which is the response of a spring-mass-damper mounted to the floor (the x-axis represents the frequency of the spring-mass-damper). The spectrum I posted is one I drew by hand in Microsoft Paint, but it illustrates the basic pattern of all the ones I've seen: low accelerations at low frequencies, a peak in the middle, and then sloping down to a flat line (the ZPA) on the right.
I just want to make sure we're talking about the same thing.
RE: Response spectrum
that is what was meant regarding excitation spectra,
for the response spectra, the peak you've drawn is the system resonance assuming a swept frequency excitation.
The vertical red-line is where you predicted the resonance to occur. you'll have to sort out why it does not, and that is where actual details of the measurement are critical, amplitude, frequency, etc.
the "shape" of your resonance curve, suggests frequency dependent effects beyond simple spring-mass resonance, and may depend on the amplitude of the excitation.
RE: Response spectrum
I think you are talking about something slightly different. A response spectrum is not the response of one system to excitation at many frequencies. It is the peak response of many systems to a given base input motion (e.g., a time-history record of an earthquake). A response spectrum for a given base input motion would be generated as follows:
1. Choose a certain damping level, say 5% of critical.
2. Solve the time history response of an SDOF system with natural frequency, f, and damping 5%.
3. Take the maximum acceleration (absolute value) of the solution from step 2 and plot the maximum acceleration vs. the natural frequency, f, as one point on the response spectrum curve.
4. Repeat steps 2 and 3 for many frequencies, plotting the maximum acceleration vs natural frequency points to build the response spectrum curve.
RE: Response spectrum
the spectra that i'm familiar with consists of shaker table, impulse testing, broad-band noise where the source consists of its own statistical description), and analysis of time-series data.
my appologies but I am not seeing a match between my own experience, such as it is, and the frequency spectra put forward initially.
how do you explain what appears to be a frequency dependent damping term in the indicated spectra?
RE: Response spectrum
RE: Response spectrum
RE: Response spectrum
RE: Response spectrum
RE: Response spectrum
RE: Response spectrum
http://en.wikipedia.org/wiki/Response_spectrum
I agree the term seems a little misleading if you haven't heard it used that way. It's not just spectrum of response of a given system to a given input. It's a spectrum of responses of multiple sdof systems to a given input.
=====================================
(2B)+(2B)' ?
RE: Response spectrum
It's not just spectrum of response of a given system to a given input. It's a spectrum of responses of multiple sdof systems to a given input.
should've been:
It's not just spectrum of response of a given system to a given input. It's is based on response of a spectrum of sdof systems to a given input.
=====================================
(2B)+(2B)' ?
RE: Response spectrum
=====================================
(2B)+(2B)' ?
RE: Response spectrum