R=1.0 with an Importance factor of 1.5 5

[WARose]

I have a situation where I have to use a Response modification factor of 1.0. (Long story as to how I got there.)….but along with that, the client wants to use an Importance factor of 1.5. You do the math on that and that’s getting into the elastic range in a seismic event.

I can't find anything in the code that will (explicitly) get me out of it……can anyone think of a code based argument that could get me out of it? (Or perhaps just a common sense type argument.)

 

[TehMightyEngineer]

There are efforts being made to generalize some calculation based on typical framing systems, but it's still early days.

Good to know. As I said; I only am familiar with it in passing so I'm glad you're here to correct my understanding.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[TLHS]

But basically, once you're outside of minimum code requirements (reasonably low probability of catastrophic failure during the agreed upon code level probabilistic seismic event (i.e. life safety during the seismic event) ), everything becomes a conversation. This is a discussion of risk and consequences in which people need to be careful not to over-promise.

Remember that even if you go elastic, you're still working with a load factor of one with really fuzzy input criteria when you start looking at the probabilistic models that are stacked on top of each other to get the design forces. That's not to say that the solution is to be more and more conservative, just that the solution involves lots of discussion and documentation!

 

[Agent666]

Real earthquakes don't give a damn about your code limit. Keep that in mind. So increasing the strength of the structure does have a direct correlation with minimising damage/repairs and increasing reoccupancy downtime under larger than code earthquakes.

In NZ given we have had two recent significant earthquakes with considerably higher spectrum than our design spectrum (over 2 times for some period ranges) there is a lesson to be taken away I'm sure for ensuring all structures are robust. Certainly there is a trend towards low damage structures because the economic reality is its the best insurance policy you can have when the big one hits.

 

[MotorCity]

Building on Agent666's comment above...…..I have often wondered what the net effect is in terms of the predicted seismic load compared to the actual seismic load. Some factors increase the seismic force, some factors decrease the seismic force, so after all the dust settles, how close are we? Granted, we do not design for the actual seismic force but rather some percentage of it. So perhaps the better question is how close are we to that percentage? It seems to me that given the complexity of our seismic codes, we are really just trying to bridge that gap of uncertainty by applying various factors. Are all of these factors just our attempt to explain away the uncertainty in seismic design? Are we kidding ourselves into believing we can predict earthquake damage? The only measurable way to confirm our accuracy is to quantify the damage and attempt to correlate the damage back to our original theory.

 

[BridgeSmith]

As others have said, once you're beyond the minimum code requirements it's a conversation of possible damage vs cost, but nothing is a guarantee. It could experience something beyond the MCE a week after it is finished or it might not see the design earthquake in its lifespan. The tricky part is coming up with cost figures of what it would take to survive earthquakes of various intensities, without expending extensive effort and time.

 

[KootK]

It seems to me that given the complexity of our seismic codes, we are really just trying to bridge that gap of uncertainty by applying various factors. Are all of these factors just our attempt to explain away the uncertainty in seismic design? Are we kidding ourselves into believing we can predict earthquake damage?

I almost included some thoughts about this is my post above but didn't want to muddy the waters. That said, if we're going to dive into this anyhow, I'm definitely interested in the conversation. Here's how I see it:

- I feel that the most important modern innovation in the realm of seismic design is the capacity design concept pioneered by the NZ rockstars. And if you study their work from the source documents, you will find that one of the major benefits of capacity design that is touted is that it isn't particularly sensitive to the magnitude of the earthquake response induced. You might design for a 7.5 earthquake but you'll probably be in pretty good shape for a 9.0 as well so long as you have that precious ductility. Park, Paulay, and Priestly felt that attempting to "know" your seismic demand with any accuracy is a fools errand. And I agree.

1) One of my beefs with the modern trend of performance based design is that it seems to me that method takes us back to needing confidence in our "knowing" the earthquake magnitude in order to justify a less ductile design. As I understand it, PBD is usually based on statistical analysis of site relevant ground motions. That's still a version of the "knowing" in my book. Seismic knowing = hubris.

2) A feather in the cap of capacity design is that it's been somewhat empirically validated via the success of Japanese building designed using capacity design that have performed well in some recent, strong earthquakes. What a shame it would be to move from the Northridge problems, to good performance in Tokyo due to capacity design, back to possible problems in the next earthquake because of the move to PBD.

3) Despite capacity design originating in NZ, it is my understanding that they are questioning whether or not high ductility design is really a good thing after all. That, given that capacity designed buildings in NZ have not collapsed in recent earthquakes but the resulting damage to the nation's building stock has proven quite unacceptable. I very nearly moved to NZ to study this issue with Dr. Kenneth Elwood who relocated there from British Columbia to study it himself (among other things). It will be interesting to see where this lands. Pardon the name dropping on this. My hope is to demonstrate a modicum of credibility in a jurisdiction that is not my own which is always risky business. We'll see how I do when the kiwis respond.

4) For any structure built in a region of high seismic risk, an outcome that I would find unacceptable would be for that structure to be designed at R = 1.0 but non-ductile / limited ductility. That, because even though it may feel conservative to design this way because the design would be to very high forces, the method would be reliant on "knowing" the seismic forces. In such a situation, I would want the design to be to R=1.0 while retaining at least detailing consistent with the moderately ductile systems. I think that this approach might also be where NZ lands. Keep the ductile detailing for the most part but also design for higher forces that would reduce damage.

5) Thankfully, I think that ASCE7 mostly takes care of #4 by requiring the use of detailing that forces a minimum amount of ductility regardless of your R value choice. If WArose is using R=1.0 with non-ductile detailing, I'm sure that he's doing it at a site where seismic risk is not severe.

 

[MotorCity]

KootK - In the US, I would say that PBD is in its infancy (at least in terms of actually being implemented on a regular basis) owing to the fact that our NZ and Japanese counterparts have done more research in this area. Quite frankly, I believe a lot of my peers in the low and moderate seismic areas of the Midwest would have the "deer in headlights" look if asked about PBD, as most get away with ELF procedures. After re-reading my post, it occurred to me that it could be construed as oversimplifying all of the research that went into deriving our seismic theories, but my intent was actually quite the opposite. All of this research is conducted behind the curtain by the best minds in our field and the end product.....multiply the seismic forces some factor, then divide them by another factor, etc.

In reality, all of our codes are somewhat performance based, right? Big earthquake hits = 20 more pages in ASCE 7. New wind tunnel testing = 30 more pages in ASCE 7.

 

[KootK]

Quite frankly, I believe a lot of my peers in the low and moderate seismic areas of the Midwest would have the "deer in headlights" look if asked about PBD, as most get away with ELF procedures.

As it should be. PBD is much to heavy of a hammer to be swinging at modest building in low seismic zones.

In reality, all of our codes are somewhat performance based, right?

Not in the sense in which I've been thinking of it. In my mind, PBD = attempting to accurately (statistically) estimate the forces induced in the members during the design seismic event and design the members for that. And ELF = design for ductility and toss in enough strength that you'd have a fighting chance of mobilizing the yield mechanism that you've assumed.

Let's call a spade a spade here. The prime motivations for PBD are so that you can save money and please architects by:

- Providing less ductility.

- Providing less redundancy.

- Cheating the irregularity limitations.

PBD will facilitate designing to owner's wishes for limited damage which is an admirable goal. But for the most part, I see PBD being used as I've just described. The tool of the devil I say!! Case(s) in point, PBD is being used up and down the west coast to justify tall, concrete condos that skirt around the need for hybrid lateral systems. Ergo happy architects and contractors at the expense of ductility and redundancy. And sadly, market pressure will force all of us to follow suit. We shall see how that turns out and how society feels about it post hoc.

The thing that I valued most about the NZ rockstars is that they struck me as engineers first and academics second. PBD has too much of a science project feel for my liking. Heck, the consensus seems to be that, with conventional methods, not enough checking is happening and designers are too software reliant. Just imagine how PBD will affect that. The only guy in your office qualified to design your shear walls will have a PhD but won't know which way to turn a rebar hook. I'm not saying that everybody with a PhD is a dud as an engineer. I think it undeniable, however, that a lot of PhD's wind up "silo'd" in terms of their experience in ways that are unhealthy. They train to be highly specialized and, voila, they are.

 

[Agent666]

KootK, certainly in the February 2011 Christchurch earthquake a number of buildings did collapse, one in particular the CTV building caused over 100 deaths. Whether these buildings were sufficiently designed in the first place using robust capacity design principles is debatable based on the findings of the royal commission, there were a lot of technical and human failings in the design and construction. When I look at the drawings for this structure its almost criminal detailing, and so avoidable in hindsight.

There was a huge amount of damage to all types of buildings throughout the region. Many buildings while they survived, they ultimately were knocked down due to the economics of repair vs rebuild. The center of Christchurch nearly 8 years on is still a bit of a wasteland with limited rebuilding going on, certianly a shadow of its former self. There are buildings for example that are still partially standing and simply fenced off awaiting resolution of insurance claims.

Yet in some ways we seemed doomed to repeat the failings of the past, in Christchurch for example in the past there was never a peer review process at all. As I understand it you submitted some plans and you can go off and build it. Now they have 'performance consenting', one of your buildings gets checked, then if no issues you are free to make as many mistakes as you want on future ones as none of them are checked. In Auckland every single design is checked by contrast, yet we have the lowest seismic design loading of the entire country. Other territorial authorities have varying practices. If the peer reviews I have done have taught me anything, everyone makes mistakes, but you only find out if someone is looking for mistakes.

In the 2016 Kaikoura earthquake there was the rupture of multiple faults, and its described as the most complex rupture sequence that has ever been studied. In Wellington in particular the shaking exceeded the design basis earthquake (DBE) by something like 2-2.5 times within a certain period range due to local effects (local geology acted like a big bowl of jelly intensifying the shaking). This range corresponded to 1-2 second structures, main higher rise structures, all the earthquake prone unreinforced masonry (shorter period) structures which everyone thought would be first thing to go basically came though it unscathed for the most part. Several of these taller structures were damaged to the point where they had to be demolished. There were a few localised collapses of floors in at least one structure.

Coming from NZ and being taught from first principles about capacity design and why its required, it surprises me that more codes do not wholly embody the principles. Certainly before these earthquakes people were a bit blase about rigidly sticking to all the rules regarding ductile structures, its improved since but still a lot of room for improvement. It's made worse by the general decline in the standard of those building the buildings. I often joke that I don't go into the buildings I design, not because the design was bad, but because I know how it was built! I'm happy to live in denial about other peoples structures.....

Traditionally our codes have been based on achieving life safety using probabilistic approaches as many other international codes do, generally achieving a balance between life safety and the cost to construct. In NZ at least it came as a big surprise to the majority of the general public that while there building didn't collapse, the damage they sustained and sometimes simply to the non-structural fitout (etc) made it uneconomic to repair or unable to be repaired, and they were subsequently demolished. These structures were built to code, and achieved the intent of the code in terms of protecting life safety, yet we seem to have failed communicating this to the general public.

The public expected no damage, yet engineer strives for most economic initial build cost typically. Often selecting some level of ductility to achieve a more economic structure for a similar level of protection against collapse. The fact that if a design level event comes along and the building is damaged and needs to be rebuilt never even enters the mind of those developing the building in the first place. Its the economics and expectations of the public that is driving for lower damage designs, whether this is the use of base isolation at one end of the scale or simply adopting a elastic design approach at the other end in an effort to ensure the structure can be reoccupied in as short amount of time as possible following a seismic event.

I was involved in a lot of peer review work, and I'd say that even after the earthquakes that we aren't much further ahead on people correctly applying and understanding the capacity design approach in a consistent and correct manner in NZ. Certainly I feel a lot more comfortable about some companies designs when I see they are designing it as being elastic vs a ductility of 4, because many have no idea that they don't know what they are doing. Awareness is higher, but people always take the path of least resistance either intentionally or through ignorance, they don't know what they don't know.

The thing with earthquakes is the load your structure sees either carries on increasing until either a mechanism forms, or it responds elastically, or a mix of the two. The hierarchy of strength is almost more important than the strength you provide, then provided you have sufficient displacement ductility to deal with the estimated drifts, plastic hinge curvatures and a dependable mechanism forms without your members failing or suffering degraded strength then you are good to go. But you have to accept that this means damage in some form or another, and I think we make this decision about ductility based on technical considerations (like limiting force to make the design easier/more economic) and don't really communicate this well to those who pay for or will occupy the structure to ensure it aligns with their expectations.

 

[Deker]

The discussion above regarding PBD reminded me of an article and response I read a few years ago that I feel is germane to some of the concerns that were raised.

I believe it is a common misconception to assume that capacity design and PBD are mutually exclusive. Capacity design is necessary in any form of high seismic design and is integral to PBD when proportioning and verifying the performance of the structure, although strict adherence in the code-based sense is admittedly not required in the final product.

To me, the biggest selling point of PBD from an engineering perspective is that we are using rigorous and rational analysis methods to verify that our designs will likely behave as intended--to meet code performance objectives or enhanced performance objectives, if desired--rather than following prescriptive methods, hoping that we have detailed and proportioned the structure adequately, and waiting around until the next earthquake to see how we did. Capacity design alone doesn’t keep the designer from exercising poor judgement in laying out and proportioning the lateral system. PBD, on the other hand, at least attempts to mitigate that possibility by explicitly requiring verification that ductility demands aren't excessive along with a rigorous peer review process. It’s certainly not perfect, but in the hands of an experienced designer, I think it’s reasonable to expect a PBD building to behave more favorably in an earthquake than a prescriptively designed building.

Additionally, as KootK pointed out, PBD allows designers the opportunity to assess the performance of structures that are outside the scope of building codes. And so, for example, rather than providing a secondary frame as part of a dual system to resist lateral forces induced by residual drift in a high-rise condo, we can check the impact of residual drift directly, build in adequate margin of safety, and achieve the same level of performance intended by building codes without being bound by their limitations. I get the sense from this forum that many engineers yearn for the days when building codes were 10 pages long and they had room to exercise their judgement rather than follow cookbook provisions. I actually think PBD is a step in the right direction on that front.

 

[KootK]

I wish to respond to Decker's last comment but I've acquired some reading material that I wish to digest first. However, I'm worried that the thread will time out before I get around to that.

Can anyone tell me:

1) How long does it take for a thread to time out and get closed off to new contributions?

2) When somebody adds a new comment to a thread, does that reset the clock? Or is it based purely on thread inception?

 

[TehMightyEngineer]

I don't know definitively but based on other threads I'm pretty sure comments reset the clock.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[JAE]

KootK
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