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

 

[jochav5280]

Hi zekeman,

I think you simply are not familiar with ASCE 7-05; see Section 11.4 and 12.8 to better understand our discussion. Yes, we have the accelerations and site/soil classification, (SS, S1, Site Classification), but they correspond to a damping ratio of 5% and therefore they need to be adjusted for damping ratios other than 5%. This is particularly important when damping is less than 5% per the attachment that accompanied my post on 10 Apr 13 17:26; as you can see, a lower damping ratio increases the seismic forces across the range of natural periods.

I'm not sure what studies referenced, so I can't confirm what you've claimed. Nevertheless, if amplification of the ground accelerations is required to account for a difference in damping by code, then it simply must be done, unless one is willing to go off the beaten path which can be done at their own risk/burden.

Best regards,

jochav5280

 

[structSU10]

See attached for damping values. I would believe that unless your R = 1 and you design is super conservative stresswise, a damping of 5% is conservative. It is true that the lower the damping the greater the response, so you could amplfy if you feel better, but if you can use an R = 3 system, the inelastic response portion should allow for 5% damping to be sufficient.

Also see table 6 from this link

http://www.cs.wright.edu/~jslater/SDTCOutreachWebsite/damping properties of materials.pdf

 

[PicoStruc]

About the OP comment

« I think 2% is still more accurate for our industry.»

I agree with that .

Additionnal information about cliff234 comment

« 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»

There is a lot of literature and debate about this right now. New research seem to indicate that damping ratio, even for existing concrete building, seem more to between 1% and 2%.

--

Right now, (in building design - i dont do industrial), I still follow standards with 5%. But i am aware that a lot of research indicate otherwise.

For industrial structure, it's not clear. Do a standard other than building standard or design guide exist and could provide more information ?

 

[jochav5280]

Hi structSU10,

Per my practicing Dyanmics of Structures professor,the R-value has absolutely nothing to do with the damping of that particular structural system; this is a misconception. All the R-value accounts for is the ductility of the system, nothing more. This makes sense when you consider that the code could not possibly give a broad damping coefficient that works for an unique application of that structural system; there are likely many variables for each specific building design that would impact damping, which would be tough for the code to account for.

Damping is something that can really only be determined after the structure has been built, so generally, I believe the engineering community uses what has been observed in the past, (i.e. concrete damping = 5%, steel damping = 2%). With respect to what you mentioned regarding the R-value and it's impact on damping, I believe at present time the impact is not on damping, but on the natural period of the structure; someday we may be able provide more precise damping information for a given system but we've yet to arrive there. Since a structure's natural period is a function of stiffness and mass, a given structural system of the same height/mass will have a different stiffness than another structural system, and in the end, all this leads to is a different natural period and hence a different seismic force. Since it seems that there is really no good research regarding the damping ratio associated with a particular structural system, I believe the engineering community just uses the 5% and 2% ratios I mentioned previously.

Hi PicoStruc,

I'd appreciate if you could share some of your references. You've got me; not sure if any such standard exists. I got the impression from Larry Muir at AISC that there is still a lot of work to be done, especially in the industrial field.

Best regards,

jochav5280

 

[PicoStruc]

Personnaly, I didn't have those articles...

But a fast googe search got me these. In my opinion, damping is a parameter that has a big influence on the response and we know about nothing about !

http://peer.berkeley.edu/tbi/wp-content/uploads/2010/09/Building-Analysis.pdf

Page 12 of that document :
ftp://ftp.ecn.purdue.edu/ayhan/Fabian/Damping/Damping%20in%20buildings_TAMURA%20PPT.pdf

Conclusion of this one :

http://www.iitk.ac.in/nicee/wcee/article/13_48.pdf

 

[Robbiee]

A lot of confusion here and I hope I wont add to it. I don't think the researchers who wrote the code would have let this very fundamental issue slips under their radar. We should remember that the assumed damping ration is for earthquake design, where we expect the structure to behave inelastically. R and damping ratio don't mix. Yes, R leads to higher damping but we don't account for the R contribution in damping because R is accounted for in the non-linear behaviour we assume.
The limited available data for recorded damping ratio for different building materials indicate that buildings have higher damping at higher stress level. The damping ration that matters for (earthquake) design is that at just under yielding not at working stress level for example. I don't know how it was determined that industrial buildings have 1-2% damping, but records indicate that 5% at high stress levels is conservative. That said, your questions remains unanswered if you have experimentally determined that your structure is 2% damped at just under yield stress level.

 

[cliff234]

Robbiee,

You make a good point. I read somewhere that the damping ratio of a structure at low levels of loading is smaller than when a structure is exposed to loading in the inelastic range as would occur during the design earthquake. So, while a steel framed structure might have a 2% damping ratio under small wind loads, that damping ratio might be larger when the building is racking back and forth in the inelastic range during the design earthquake. You also make a good point that those who decided on using the 5% value must have had a good reason for doing so. Codes are generally somewhat conservative, so I would be surprised if there was not good justification for using the 5%. Perhaps industrial structures such as those that jochav5280 is designing might warrant the adjustment, however I am comfortable with the 5% for building structures.

 

[structSU10]

The R value is related to damping, here is how -

The R value allows for the structure to behave inelastically. From that, the R values allow you to use a smaller design load, which through detailing allows the structure to yield in a controled manner, forming areas where energy dissapation is done safetly. That is why a structure to have 1/4 its yield stress during a design earthquake (something with a reccurance of hundreds of years) is basically impossible.

The damping values we use are viscous damping, which is just mathematically convient. Real damping is found not be related to velocity as our general equations of motion are formulated - really the damping we use is made up based on experimental results and curve fitting. Real damping can be attributed to resistance with the air, yielding of cross section, crushing of concrete, etc. .There have been attempts to quantify damping in a more realistic sense - the term is escaping me at this moment - but it is extrodinarly complicated, and viscous damping is 'good enough'. That is why values are all over the board - it is highly structure dependant.

 

[jochav5280]

Hi Robbiee,

That is a good point. For clarification, per my Dynamics of Structures professor, 2% damping applies generally to steel buildings, not just industrial structures. Similarly, 5% damping applies to concrete structures in general. Since there is no good direction, like most engineers I suspect, we have chosen to go the more conservative route of 2%; in seismically active zones I think it's better to go that route until a good basis for going a less conservative route is justified. While in-elasticity does increase damping, there doesn't appear to be any definitive answer as to how this should be quantified.

While quantifying damping is still a mystery, my question as to how to adjust the design response spectrum for a given damping ratio appears to have been answered; see Table 18.6-1 of ASCE 7-05. I plan to read through Chapter 18 to make sure this the correct interpretation.

Best regards,

jochav5280

 

[Gumpmaster]

My co-workers and I have designed many, many buildings, concrete, wood, steel, whatever with 5% damping. It is not common practice to design all steel buildings with 2% damping, even in incredibly active seismic zones. This happens not just at my company, but most.

Scaling your response spectra is not common at all. It seems pretty silly to scale an acceleration on a series of guesses. Most of the seismic stuff is a guess (backed up by some research) and after the next big one we'll throw it all away and come up with new guesses. Don't overthink it.

I think there's no reason to scale your response spectra unless you're intentionally altering the damping of your facility (viscous dampers...), or you have some unusual design requirements for performance.

 

[PicoStruc]

I agree 100% with Gumpmaster previous comment.

I, myself, still follow the 5% prescribe by the code.

Using lower damping values might increase a lot the cost of the structure and foundation !

Using 2% damping ratio might be a good idea only if it is a demand from the client !!!

Don't forget that you will have a certain overstrength anyways !

 

[smshaw]

If you get a copy (free online) of FEMA 273: NEHRP Guidlines for The Seismic Rehabilitation of Buildings there is a section that specifically addresses this. It is Section 2.6 Sesimic Hazard. The section shows you how to create a Design Spectrum for (a) Any hazard level (i.e. 2in50, 5in50,10in50) (b) Any Site Class and (c) Any Damping Ratio. For steel buildings that probably have a damping ratio of about 2% the guidlines tell you to essentially increase your spectral accelerations by 25 percent (code says divide by 0.8).

 

[structuralnuke]

I have attached a portion of NUREG/CR-6728 which provides guidance on scaling of the 5% horizontal damped spectrum to either higher or lower damping levels.
The full document can be downloaded for free from NRC website.