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Building operational after seismic event 4
- Thread starter BAGW
- Start date
Depends on what you mean by "operational". Obviously the "still standing" goal of the codes (after a major seismic event) for a commercial structure isn't as appealing in some industrial structures. In some industrial settings, based on a client requests, I've had to go to a elastic level loading to insure everything is still going.
So you you need to define what "operational" really means (for yourself and the client).
So you you need to define what "operational" really means (for yourself and the client).
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Harbringer
Structural
Use importance category IV (Ie=1.5). If the client has a bigger budget than a hospital and wants a structure that doesn't get any "credit" for it's inherent ductility set R=1.0. This will get you to the elastic curve on the design spectra.
If you have a client that wants a guarantee of operability after a design level event you need to tell them that all seismic design is based on probability. A lot of the operability of the structure will depend on construction quality and quality/level of detailing.
Also it takes a lot of inspection and design work to ensure things like MEP are still operational after a large earthquake. The whole design team should have experience in the design of critical structures in order to achieve this goal. I'd recommend looking at the OSHPD amendments to the California Building Code as a good place to start.
If you have a client that wants a guarantee of operability after a design level event you need to tell them that all seismic design is based on probability. A lot of the operability of the structure will depend on construction quality and quality/level of detailing.
Also it takes a lot of inspection and design work to ensure things like MEP are still operational after a large earthquake. The whole design team should have experience in the design of critical structures in order to achieve this goal. I'd recommend looking at the OSHPD amendments to the California Building Code as a good place to start.
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HTURKAK
Structural
- Jul 22, 2017
- 3,407
If the applicable code at your zone is ASCE 7, As far as I understand ,the procedure for seismic design criteria is as follows ;
1- Choose Risk Category of Building or Structure from Table 1.5-1 or IBC Table 1604.5,
2- Assign importance factor I, from Table 1.5-2 ( I is between 1.0 to 1.5 )
3- Assign seismic design category SDC ( from A to F),
4- Make assesment for displacements and story drifts.
5- Choose analysis method from Table 12.6-1 Permitted Analytical Procedures .
The analytic procedures allowed are ; ( ELF, RSA, Linear Response History, Non linear Response History ) and Push Over analysis is not mentioned for new buildings .More over, apparently not allowed for bldgs without seismic isolation
I have copy pasted the SEAOC performance based seismic design levels below,
It is worth to look Guidelines for Performance Based Seismic Design of Tall Buildings at link ;
And a short summary for PBSD ;
- Thread starter
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- Thread starter
- #9
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- #12
HTURKAK
Structural
- Jul 22, 2017
- 3,407
It is not practical and will not be economical to design a structure for R=1.0, literally elastic forces having service life say 50 yrs for a risk of 2%.
However, my opinion is, the codes and standards are minimum, threshold values which shall be assured and the codes are also to provide legal shield to designer . ( e.g. ASCE 7; Minimum Design Loads and Associated Criteria for Buildings and Other Structures )
You may design for elastic forces for frequent earthquakes (50% in 50 yrs). If the structure hazardous and essential, the importance factor will be I=1.5 . That means the standard design event for period 500 yrs will be 2500 yrs.
IMO, to make sure for fully operational, make an assesement study for the structure , MEP, Architectural elements and fix the displacement requirements.
bones206 said:
The end of section 12.2.1 (ASCE 7-16) states,
"Nothing contained in this section shall prohibit the use of
alternative procedures for the design of individual structures that
demonstrate acceptable performance in accordance with the
requirements of Section 1.3.1.3 of this standard."
alternative procedures for the design of individual structures that
demonstrate acceptable performance in accordance with the
requirements of Section 1.3.1.3 of this standard."
I believe this is the closest you are going to get to a provision that "allows" this.
You might find this relevant:
Not Safe Enough: The Case for Resilient Seismic Design
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
Not Safe Enough: The Case for Resilient Seismic Design
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
dauwerda said:
This is just my opinion, but I think it's a bit reckless to set R = 1.0 and you would be going out on a limb if you think 12.2.1 would cover you. You're not really using an alternative procedure, you're just bastardizing an already established procedure beyond the parameters on which it's based. The Equivalent Lateral Force method is probabilistic and has been calibrated to expected seismic risks and structural behavior. If you set R = 1 and use that same ELF procedure, you are now outside that calibrated probabilistic arena even though you may think you're still in it. For example, if you set R=1.0, what's your overstrength factor and where would you apply it?
ASCE may allow this in the future (I think a low-seismic, limited scope R = 1.0 provision was on the ballot for ASCE 7-16 but was voted down: Link), but for now I would try to stay within the bounds of the permitted analysis methods.
One thing to look at closely in the beginning is what are the Designated Seismic Systems required to remain operational. Once those systems are identified, ASCE 7 Chapter 13 has the design/performance requirements. A lot of that is just holistic seismic design: making sure your equipment is inherently rugged, MEP lines have flex connection in the right places, adequate bracing, beefy anchorage, etc. Other aspects include relative displacement limits (avoiding things smashing into other things), which are just another form of performance-based design. You can use a lot of methods to achieve these targets, but a good first step is defining those targets.
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Harbringer
Structural
Do you understand what setting R=1 does? It will increase your Cs (seismic coefficient). Tell me, what is reckless for designing for a seismic load that is greater than code minimum?bones206 said:
The response modification factor is used to reduce the actual seismic force down to the design seismic force and is representative of the inherent ductility and overstrength of the system.
You can design for any seismic force you want as long as it EXCEEDS the minimum code requirements and you have an informed client that is willing to pay for it.
I didn't say it would lead to a reasonable design. As the OP didn't give any indication of the Sds or any information about the project it's up to him to decide what to use.
TLDR R=1 is super duper conservative for most SFRS
Well, you could end up in a situation where a beyond-code seismic event happens and your structure that was designed to behave elastically for what you thought were super duper huge loads could fail in a brittle manner. If you aren't intentionally designing for ductility as a structural feature, who knows what the failure mechanism would look like. I'm not saying you can't design something for elastic-level forces using some other method, but that's not the underlying premise of the Chapter 12 ELF procedure. What Chapter 12 considers conservative is inelastic energy dissipation.
bones206 said:
That's the salient point for me. The genius of capacity design is that it is not particularly sensitive to the scale of the design seismic event. And that's peachy because the scale of the design seismic event is very difficult to predict via simplified models. 2/3 MCE for starters?!? During a seismic event, I would personally rather find myself in an R=8 building designed with a defined failure hierarchy than in an R=1 building lacking such a hierarchy, praying that the structure doesn't "find" something to damage that the EOR didn't consider.
IN SHORT
[R=1 with CAPACITY DESIGN] is something that I can support technically, even if the costs would be prohibitive.
[R=1 without CAPACITY DESIGN] is to foolishly throw out the baby with the bathwater in my opinoion.
With regard to the other, non-structural stuff that might be important during and earthquake, it's a mixed bag:
Greater ductility usually means greater drift which can be bad.
Elastic response usually maximizes the acceleration amd duration of motion felt by such elements which is its own kind of bad.
BAGW said:
There are no guarantees, only probabilities. An R=1 design will generally allow a building to endure a larger range of seismicity without crippling structural damage. Performance based seismic design is also no guarantee but, in my opinion, is a more relational way to achieve a similar end. That said, undertaking a performance based design is costly in terms of design effort. So one must weigh the relative values of design costs, material costs, and reliability.
BAGW said:
Before taking this approach, I'd try to procure methodological approval from the authority having jurisdiction. As others have mentioned, it's hard to imagine that they'd object to you jacking up your seismic forces. I suspect that, if there's a sticking point, it will be in regard to the capacity deign stuff that I mentioned above.
Even nuclear plants find themselves having to reassess whether or not they are still conservative when understanding of seismic hazards evolve. They can go decades thinking they are conservatively designed, only to find themselves scrambling when someone finds a new faultline or seismic hazard (Tsuruga, Diablo Canyon, Akkuyu, Central/Eastern US plants), or the design response spectrum gets increased at a certain frequency (AP1000), or even experiences an earthquake that exceeded it's design basis (North Anna). As engineers, we sometimes have to put hubris aside and acknowledge that we can't always know what the "worst case scenario" is.
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