Adjustment of the Design Response Spectrum for damping ratios other than 5% 3

[jochav5280]

Good Morning,

In regards to Section 11.4.5 of ASCE 7-05, would someone please advise of how one would adjust the Design Response Spectrum curve for a damping ratio other than 5%? I’ve had multiple contradicting statements such as, “the system R-value accounts for the damping associated with that structural system”, or “Table 18.6-1 should be used.” This is not clearly addressed in ASCE 7-05, as there is no reference as to how this curve should be adjusted. Table 18.6-1 obviously comes out of Chapter 18, which is for structures with a formal damping system, but would it apply to just regular structures as well? Does ASCE 7-05 address how to approximate the damping ratio or are these values generally just based upon a building being a steel (damping=2%) or concrete structure (damping=5%)?

A reference to ASCE 7-05 supporting your answer would be much appreciated. I've had (2) PHD's give me conflicting answers and depending on which way you go, the seismic forces could be substantially higher in the case of 2% damping per Table 18.6-1, which calls for increasing the spectral response accelerations by 25%, (1/0.8).

Thank you in advance!

Best Regards,

jochav5280

 

[PicoStruc]

Why not use time History analysis instead ? Use more that seven TH and use average response !

Scaling design spectrum can be complex and generally required time history anyways !

 

[PicoStruc]

Note : I don't know about USA general practice on the subject.

But Where I live, it's quite easy to find design spectrum for 1%, 2% & 5% in PDH and article litterature for major cities.

 

[jochav5280]

Our office is not familiar with applying a Time History analysis, so your response just went over my head. Instead, we always use the Equivalent Lateral Force method described in Chapter 12 of ASCE 7-05. This is for a project in the remote Peruvian Andies and so I'm not sure there are even any seismic records available.

Can you elaborate on your third sentence; sounds like you know where to go to scale the design spectrum.

Thank you,

jochav5280

 

[PicoStruc]

In research, To develop a spectrum for different damping ratio :

1) You select compatible ground acceleration record (Soil acceleration time history) with your location
2) You do pseudo spectrum time history analysis for 1 degree-of-freedom for a specified range of period and for a specific for each time history.

For canadians, this is THE reference for time history selection :

http://www.seismotoolbox.ca/index.html

 

[PicoStruc]

Don't know about ASCE, there might be a simplier method. I never searched about that subject in US Litterature and standards.

 

[structSU10]

You are talking about the response spectrum which is not needed unless you do a modal analysis. If you are using ELF procedure, the response spectrum is not used.

 

[jochav5280]

Hi structSU10,

I don't believe your statement is correct. If you look at the equivalent lateral force method, the base shear value from Eq. 12.8-2 of Section 12.8 is a function of SDS, R, and I. The SDS conservatively corresponds to the plateau in the design response spectrum curve, or the maximum seismic force a structure will see. This makes sense since the fundamental period of a structure is unknown initially when design commences.

The dillema that I am facing is that we used SDS as a basis for our seismic loads, yet recently discovered that the design response spectrum SDS may need to be amplified by 25% to account for the fact that our steel structure is only 2% damped as opposed to the 5% that the curve is based upon, (reference Table 18.6-1 of ASCE 7-05). In view of this, we have done a modal analysis to determine the fundamental period of our structure in order to see if we are really in the plateau region of the curve. Our hope is that we won't be and after amplifying the corresponding accerlation, we will still be below the plateau value that we originally designed for.


Thank you,
jochav5280

 

[PicoStruc]

Of course, ELF use uniform design spectra, See equation 12.8-2 to 12.8-4 !!!!

 

[jochav5280]

Hi PicoStruc,

Agreed!

I appreciate the responses, but the fundamental question I posed originally still has not been answered. Any further comment would be appreciated.

Thank you,

jochav5280

 

[zekeman]

"In regards to Section 11.4.5 of ASCE 7-05, would someone please advise of how one would adjust the Design Response Spectrum curve for a damping ratio other than 5%? I’ve had multiple contradicting statements such as, “the system R-value accounts for the damping associated with that structural system”,

This may be naive on my part but why can't you simply take the 2 single degree systems( 5^% and 2 % damping) and get the peak response of the expected shock? This this called the shock spectra.Or am I missing something?

 

[cliff234]

Your question is a reminder that the older I get, the more I realize how little I know.... (I have been designing building structures for over 30 years.) I'm on the east coast and most of the projects I work on are ones where "wind controls". I have never given any thought to that "5% of critical damping" notation on the ASCE 7 acceleration maps - and I suspect that I am not alone. I think I found an article that provides an easy way of adjusting the design acceleration. Here is the link,

http://peer2.berkeley.edu/ngawest2_wg/wp-content/uploads/2010/09/Naeim-Kircher-2001.pdf

The building code requires us to list the important seismic design parameters on our drawings. I suppose the damping value should be added to that information. It appears to be an important variable.

Thanks for asking a great question.

 

[jochav5280]

Thank you cliff234,

This article looks promising and Table-2 on the third page looks just like Table 18.6-1 from ASCE 7-05, which I referenced in my inital post. If Table 18.6-1 is the table we're supposed to use to adjust the spectral accelerations, ASCE 7-05 has some improving to due as this is not referenced in Chapter 11 at all. Additionally, this table is in Chapter 18, which is for structures with damping systems, which has thrown us for a loop since we do not have an actual damping system beyond the materails natrual damping.

You definitely are not alone as none of our senior engineers have seen this either. Yes, I think the assumed damping ratio would be important since it impacts the seismic response of any structure. I am currently taking "Dynamics of Strutures" in graduate school, which is how this topic reared its head. I've attached a scanned page that shows how the Design Response Spectrum varies depending on the damping ratio. As you can see, damping ratios above 5% can dramatically increase the seismic forces.

Thank you very much for your helpful post!

Best regards,

jochav5280

 

[jochav5280]

Hi cliff234,

My apologies; in my previous post, I indicated in the last sentence of the second paragraph that damping ratios above 5% increases leads to higher seismic forces. This was an error; I meant to have wrote that damping ratios below 5% lead to higher seismic forces.

Best regards,

jochav5280

 

[cliff234]

I've talked to several other engineers in our office about this. They noted that although the damping ratio of a steel frame might be 2%, the damping ratio of the entire system (including façade, partitions, building contents) is probably much higher. That said, the 5% value is probably accurate enough for most building structures. If you have a very unique structure where the actual damping ratio (considering everything) is less than 5% then the adjustment that you looking to make is probably warranted. Otherwise I suspect that the mapped values based on 5% is accurate enough for design of building structures.

 

[jochav5280]

Hi cliff234,

I've had similar conversations and direction from my colleagues, however, since we engineer heavy industrial steel structures, there is typically no facades or internal components to increase the damping ratio, so I think 2% is still more accurate for our industry. The area that we are working in has monthly earthquakes that control our designs, so it's important that we don't underestimate seismic. Thank you, I appreciate your comment though.

Best regards,

jochav5280

 

[cliff234]

Jochav5280, Yes. Based on the type of structures you are designing (and their proximity to frequent earthquakes), I agree with what you are doing. It is interesting how what you were looking for was buried deep in chapter 18. I agree that the there should be a bit more of an explanation on this issue (an explanation on why the 5% damping ratio was used and how to make adjustments if the actual damping ratio is less than 5%) in ASCE 7. Thanks again for posting an interesting question.

 

[zekeman]

So this big mystery amounts to getting the published spectra depending on where the structure is located, period.And what happens when you don't have that seismic data? If you don't, then I doubt that you can make a meaningful extrrapolation in going from .02 to .05 damping.

 

[jochav5280]

Thank you cliff234.

Hi zekeman,

The mistery for me was where do you find the ASCE 7-05 amplification values for adjusting a system with a damping ratio other than 5%, which I believe cliff234 turned me onto. The site accerlations are usually provided by our client and if not, we hire a geotechnical group to produce these values. So getting the actual site parameters is not typically an issue. Once these values are known along with the amplification values, the site specific, damping specific design response spectrum curve can be generated, allowing us to determine the seismic loads.

Best regards,

jochav5280

 

[zekeman]

".. The site accerlations are usually provided by our client and if not, we hire a geotechnical group to produce these values. So getting the actual site parameters is not typically an issue. Once these values are known along with the amplification values, the site specific, damping specific design response spectrum curve can be generated, allowing us to determine the seismic loads."

If you have this data, why are you so concerned about socalled amplification factors "derived" from the studies which correlate so poorly ( look at the linear fit; its a joke) that in my opinion the scatter is so great that they are useless as a design tool.

Again, why go further than your actual supplied site data? I am truly puzzled.Or am I missing something.