Elastic Seismic Design? 3

[Logan82]

Hi,

From what I have seen, codes typically ask that the building must resist earthquakes of X shake intensity, using inelastic deformations to absorb seismic energy. Inelastic deformations can imply important building damage, but it is cheaper to design using inelastic deformations.

How can we perform seismic design while remaining in the elastic realm and without damaging the building? Does anyone have good references or codes for that kind of design? I am thinking about the kind of design for important buildings such as power plants or hospitals. They are the type of buildings that need to be operational even after the "design earthquake shake" happens.

 

[phamENG]

I can't speak for Canadian codes, but in the US that would typically be done by setting R, the response modification factor, to 1. R is a way of approximating the inelastic response of the building. If you set it to one, you're saying there will be no inelastic response. But then we also have the seismic importance factor. For the buildings you're describing, we design them for 150% of the calculated base shear. The structure can still experience some inelastic deformation, but it generally won't be enough to cause collapse or render the building unusable - at least not for the duration of the immediate disaster.

 

[WARose]

From what I understand, if you go with R=1 and a importance factor of 1.5, you are pretty much at the elastic level of seismic design. (Although I am not aware of any code that specifically says that.)

I got into this a few years ago on this thread:

https://www.eng-tips.com/viewthread.cfm?qid=447981

 

[strucbells]

The way current design practice handles this is through the use of an importance factor associated with the type of structure (Ie). Ie = 1.5 essentially increases your seismic design loads by 50% for hospitals, for example, and the intention is generally to maintain operability in that way.

Look into "Performance Based Design" for a more specific/tailored approach. You could always use R = 1.0 to truly remain elastic in a design earthquake, but it is likely to drive up costs considerably.

 

[Logan82]

Thank you everyone for your answers! It is spot on what I was looking for.

 

[skeletron]

Elastic and w/o damage to the building = high cost (almost unreasonable cost). You are likely to use modern technologies to control the response (ie. base isolation, dampers, etc). I would think that you would still want to have *some damage* as a means to dissipate energy. It would be interesting to hear back to what you find.

 

[Deker]

Depending on your needs, you may find it useful to review some of the more recent literature on resilience-based design.

Link #1
Link #2
Link #3
Link #4

 

[JoshPlumSE]

How can we perform seismic design while remaining in the elastic realm and without damaging the building? Does anyone have good references or codes for that kind of design? I am thinking about the kind of design for important buildings such as power plants or hospitals. They are the type of buildings that need to be operational even after the "design earthquake shake" happens.

Some thoughts on this subject:
1) R = 1.0, with an I = 1.5 does not mean that the building will be elastic and damage free. It means that this is what you're TRYING to do. But, that doesn't mean that this is what will happen.
2) Refer to the 1995 Kobe earthquake in Japan where a lot of there infrastructure collapsed because they tried to design it this way. I think that pretty much ended any debate as to the relative merits of brittle (but elastic) design versus ductile (but plastic) design.
3) You would not be able to get away with this type of design for critical projects like hospitals. Probably not for power plants either, though there are portions of power plants that will likely be designed that way to some extent.
4) It sounds like you're interested in more of a "performance based" design criteria rather than strictly relying on the code requirements. This generally involves a much more involved non-linear analysis with various criteria for different limit states to determine the level of damage and whether the facility can remain operational during an event, or can be quickly up and running after an event. You may design it for a certain type of very rare event to prevent collapse (by ductile deformation), but to be operational or quickly up and running after lesser events. To me, this would be a more rational approach than merely setting R = 1.0 and I = 1.5.
5) Another option is to look at some of the criteria used for nuclear power plants and such.

 

[JoshPlumSE]

I should have mentioned that we the intent (whent R = 1, I = 1.5) is that the design be essentially elastic. We just know that this won't actually be the case and that if important portions of the structure are not designed properly (connections, confinement, and such) then the you will have non-ductile failures. That (as I understand it) was sort of the lesson of the Kobe earthquake.

 

[Logan82]

I like the dampener idea, however I have no clue where to start to include these kind of equipment in structure designs. Do you have a lead where I could start looking into it? I haven't seen that in North American codes.

Deker, that resilience design is interesting.

Is ductile design preferable over elastic because in case of a bigger earthquake shake than the design earthquake shake, the ductile structure may deform even more than what was designed while not necessarily collapsing?
Other than that, why would a properly designed structure using only elastic deformation for seismic loads be less adequate than a properly designed structure using inelastic deformation?

 

[Harbringer]

Is ductile design preferable over elastic because in case of a bigger earthquake shake than the design earthquake shake, the ductile structure may deform even more than what was designed while not necessarily collapsing?

Exactly.

Other than that, why would a properly designed structure using only elastic deformation for seismic loads be less adequate than a properly designed structure using inelastic deformation?

First you need to specify what you mean by a properly designed structure. In this case the issue is that designing for an elastic load can erroneously lead to designs that are unable to safely experience inelastic deformation. This can lead to catastrophic collapses because the code seismic forces are just statistical modeling and the design event may be exceeded in real life. For instance if you convince yourself that everything will for sure stay elastic, you may design mistakenly design strong beam/weak column.

In other words IMO you can design for the whatever seismic force you want as long as it exceeds the code minimum AND you use all of the seismic provisions in your detailing to provide ductility...uh and you have a jurisdiction that will allow it and a client that will pay the huge sum for it.

 

[WARose]

2) Refer to the 1995 Kobe earthquake in Japan where a lot of there infrastructure collapsed because they tried to design it this way. I think that pretty much ended any debate as to the relative merits of brittle (but elastic) design versus ductile (but plastic) design.

That's a good point.

Something else: we don't know if the design event (in the code) is the strongest it will see. (Probabilities are it is....but probabilities can be wrong.) Ergo even if you are doing a super conservative elastic design.....you don't know if that actually is the event it will see. So there is something to be said for ductile design.

 

[HTURKAK]

I could not look to the previous posts so not sure that i'm repeating some similar concepts .. I just want to remind that , it is not reasonable to keep the structure elastic at high seismic zones..The practice is , define seismic use group (SUG ) and then Seismic Design Category (SDC ) which is a function of Risk Category and design level spectral accelerations Sds and Sd1. Moreover, additional strength is provided for risk-critical facilities in this case for hospital Ie=1.5. I would suggest you to look performance based seismic engineering documents.

Another point, keeping the structure operational or immediate occupation level does not mean only the structure performance but MEP , architectural elements shall also be operational..


I will suggest also you to look seismic isolation concept.

http://richardson.eng.ua.edu/Former...otes/FEMA/Topic15-7-SeismicIsolationNotes.pdf

 

[JoshPlumSE]

Is ductile design preferable over elastic because in case of a bigger earthquake shake than the design earthquake shake, the ductile structure may deform even more than what was designed while not necessarily collapsing?

I'd phrase it a little bit differently. One of two ways where ductile design is preferable:
1) It's about whether the structure is capable of dissipating the energy that's put into it. That's why the hysteresis loops are so important for the testing of structural systems, because they demonstrate the energy absorbed by the structure directly.

2) It's also about a subtle difference between the seismic event imparting DEFLECTION on a structure rather than a force. In Northridge, for example, we found that our short bridge bent columns were failing. The reason being that they were not capable of moving when the ground forced them to move. The taller, leaner columns moved very well, but the short ones failed when a similar deflection was imposed on them.

 

[Logan82]

Ductile seismic design appears to be the preferable approach for some very good reasons you guys provided.

AND you use all of the seismic provisions in your detailing to provide ductility.

Is ductile design mandatory in North American Codes?

HTURKAK, I will read that presentation document for sure.

 

[moments]

I work in a relatively high seismic area, and when my office expanded, the company wanted the new building to practice what we preach, put the structure on display, and provide an immediate occupancy building performance to serve as a command center after a big earthquake. With those goals in mind, the office is base isolated using friction-pendulum isolators. (They are also visible from the sidewalk.)

We’ve used a number of other techniques on projects to address the issue you are asking about, apart from just designing for enormous loads. Self-centering PT shear walls is one type of lateral system that comes to mind. Another is the use of strongback frames on pinned bases to provide “mode shaping” for the ductile resisting elements, which are essentially elastic vertical trusses intended to promote even story drift over the full building height.

One resource that comes to mind for more examples / techniques is the US Resiliency Council. They have been doing a series of presentations to highlight projects where building performance was a goal, rather than basic life safety, in the seismic design.

 

[Boof19]

I work in a relatively high seismic area, and when my office expanded, the company wanted the new building to practice what we preach, put the structure on display, and provide an immediate occupancy building performance to serve as a command center after a big earthquake. With those goals in mind, the office is base isolated using friction-pendulum isolators. (They are also visible from the sidewalk.)

TSE?? StructureMag had an article on their new office a few years ago. Love what they did there, I've always been a sucker for projects that highlight structural elements.

 

[KootK]

Ductile seismic design appears to be the preferable approach for some very good reasons you guys provided.

Indeed it is. While I agree with what other have said on the topic, I like to hear it said this way:

1) A ductile design can absorb a lot of inelastic drift and is, therefore, quite forgiving. A ductile design usually will not give out right away at design drift/capacity.

2) An elastic design can absorb little inelastic drift and is, therefore, relatively unforgiving. Go past the design capacity and connections may well start tearing apart.

3) Seismic loads and deformation requirements are very difficult to estimate (2/3 factor anyone?). As such, forgiveness is good. With an elastic design, you pretty much have to feel confident that you've placed a reliable upper bound on what the seismic demand might be. Most of us find that confidence hard to come by.

 

[haynewp]

You asked about referenced documents. The U.S. Government has certain buildings that it tries to keep the system elastic but there is some more to it than just the R value and importance factor. This document, that says it is canceled, is actually still being referenced for the Category V structure designs by the one that superseded it. I have never designed such a building but I have used R=1 and I=1.5 as part of the seismic rehabilitation for an existing structure where it had a very unusual existing system.

https://www.wbdg.org/FFC/DOD/UFC/ARCHIVES/ufc_3_310_04_2013_c1.pdf

 

[bones206]

Anyone interested in the approach used for US nuclear plant seismic design can look here:

https://www.nrc.gov/reading-rm/doc-collections/cfr/part100/part100-appa.html

It is essentially a Performance Based Design approach. The design earthquake is based on a rigorous site-specific geologic study and response spectrum that envelopes the time histories of several earthquake motions. But after all that, it's still just an educated guess... Here's an excerpt (bold emphasis added by me):

VI. Application to Engineering Design

(a) Vibratory ground motion—(1) Safe Shutdown Earthquake. The vibratory ground motion produced by the Safe Shutdown Earthquake shall be defined by response spectra corresponding to the maximum vibratory accelerations at the elevations of the foundations of the nuclear power plant structures determine pursuant to paragraph (a)(1) of section V. The response spectra shall relate the response of the foundations of the nuclear power plant structures to the vibratory ground motion, considering such foundations to be single-degree-of-freedom damped oscillators and neglecting soil-structure interaction effects. In view of the limited data available on vibratory ground motions of strong earthquakes, it usually will be appropriate that the response spectra be smoothed design spectra developed from a series of response spectra related to the vibratory motions caused by more than one earthquake.

The nuclear power plant shall be designed so that, if the Safe Shutdown Earthquake occurs, certain structures, systems, and components will remain functional. These structures, systems, and components are those necessary to assure (i) the integrity of the reactor coolant pressure boundary, (ii) the capability to shut down the reactor and maintain it in a safe condition, or (iii) the capability to prevent or mitigate the consequences of accidents which could result in potential offsite exposures comparable to the guideline exposures of this part. In addition to seismic loads, including aftershocks, applicable concurrent functional and accident-induced loads shall be taken into account in the design of these safety-related structures, systems, and components. The design of the nuclear power plant shall also take into account the possible effects of the Safe Shutdown Earthquake on the facility foundations by ground disruption, such as fissuring, differential consolidation, cratering, liquefaction, and landsliding, as required in paragraph (d) of section V.

The engineering method used to insure that the required safety functions are maintained during and after the vibratory ground motion associated with the Safe Shutdown Earthquake shall involve the use of either a suitable dynamic analysis or a suitable qualification test to demonstrate that structures, systems and components can withstand the seismic and other concurrent loads, except where it can be demonstrated that the use of an equivalent static load method provides adequate conservatism.

The analysis or test shall take into account soil-structure interaction effects and the expected duration of vibratory motion. It is permissible to design for strain limits in excess of yield strain in some of these safety-related structures, systems, and components during the Safe Shutdown Earthquake and under the postulated concurrent conditions, provided that the necessary safety functions are maintained.