c strucperformance taylor mar12

8/13/2019 C StrucPerformance Taylor Mar12

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performance issues relativeto extreme events

STRUCTURALPERFORMANCE

Andrew Taylor, Ph.D., S.E.is an Associate with KPFFConsulting Engineers (Seattle,WA). Andy may be contacted [email protected] .

Ian Aiken, Ph.D., P.E. is aPrincipal at Seismic IsolationEngineering, Inc. (Emeryville,CA). Ian may be contacted at

[email protected].

By Andrew Taylor, Ph.D., S.E.and Ian Aiken, Ph.D., P.E.

Whats Happened toSeismic Isolation ofBuildings in the U.S.?

Seismic isolation is the strategy of placinga structure on a flexible foundation toeffectively decouple earthquake groundmotions from the motion of the build-

ing. It is a technically

elegant solution to thechallenging problemof minimizing or eveneliminating earthquakedamage in buildings.

Yet, despite the substan-tial benefits offered byseismic isolation and its

availability since the mid 1980s, while other coun-tries have readily embraced the technology, theUnited States has been slow to adopt seismic isola-tion. In the United States there are only about 125seismically isolated buildings, whereas in Japan

there are more than 6500 and a similar number ofbridges. In China there are estimated to be severalhundred buildings. After a promising start in themid-1980s, today seismic isolation of buildingsin the U.S. has nearly ground to a halt: pres-ently, only about four or five seismically-isolatedbuildings are constructed each year. Why havebuilding owners in the U.S. apparently ignoredthe most effective means available for protectingtheir investments from earthquake damage? Tereis no single reason, but rather a host of factors.In this article we explore these factors, and makesuggestions for removing some of the barriers to

the implementation of seismic isolation in theUnited States.Te concept of seismic isolation is not new.

More than one hundred years ago, in 1885,the Englishman John Milne designed andconstructed a seismic isolation system for abuilding in okyo that incorporated ball bear-ings and dished cast iron plates (Naeim andKelly, 1999). Te first use of rubber for a seismicisolation system was in 1969, when a school inSkopje, Macedonia was constructed on unre-inforced rubber blocks. Te first modern-era

seismically-isolated building was a governmentbuilding in Wellington, New Zealand, con-structed in 1981, which used laminated steeland rubber bearings. Tese bearings contained alead core that dissipates energy through plastic

deformation during an earthquake, a systemthat since has become one of the most widelyused in the world. Seismic isolation was firstintroduced to the United States in 1985 withthe construction of the Foothill CommunitiesLaw and Justice Center, in Rancho Cucamonga,California. Tis 170,000 square foot facilityalso uses laminated rubber bearings, calledhigh-damping rubber bearings, with a speciallyformulated rubber compound to provide theenergy dissipation properties to the system. Alsodeveloped in the United States in the mid-1980s

was a sliding seismic isolation system known

as the Friction Pendulum System, which hasa sliding surface in the shape of a sphericaldish and a low-friction articulated slider toprovide an elongated natural period for thesupported structure. Te first application of thissystem was the seismic retrofit of a wood frameapartment building in San Francisco that wasdamaged by the 1989 Loma Prieta earthquake.Te first design provisions for seismic isolation,

the Tentative Seismic Isolation Design Requirementsby the Structural Engineers Association ofNorthern California, were published in 1986.Tese evolved into the first formal building

code provisions for seismic isolation in the 1991Uniform Building Code (UBC) and are nowembodied in the International Building Code(IBC), through reference to ASCE/SEI 7, andexist in similar form for seismic isolation retrofitin ASCE/SEI 41.Tus, by the early 1990s, it appeared that

seismic isolation was poised to take off in theUnited States as the earthquake protectionsystem of choice, particularly for critical facili-ties such as hospitals, police and fire stations,and emergency operations centers, high-value

e seismically-isolated Ishinomaki Red Cross Hospital, about 75 miles (120 km) from the epicenter of the M9.0Great Tohoku Earthquake of March 11, 2011, was undamaged, and fully-operational throughout and after theearthquake and subsequent tsunami. Courtesy of SIE, Inc.


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buildings such as museums and data cen-ters, and socially important buildings suchas historic city halls. By the late 1990s,though, there were only about 50 seismi-cally-isolated buildings in the U.S., of bothnew and retrofit construction. Tis hardlyrepresented the revolution in seismicprotection technology envisioned by earlyadvocates of seismic isolation; on average,

between 1985 and 1997 only about threeseismically-isolated building projects werecompleted each year. In Japan, at the timeof the Great Hanshin (Kobe) Earthquakeon January 17, 1995, about 85 seismically-isolated buildings had been approved forconstruction. By the year 2000, there wereabout 600 (Aiken, et al, 2000). Trough thefollowing decade, thousands more seismi-cally-isolated structures were constructedin Japan.What has accounted for the slow pace ofadoption of seismic isolation in the United

States? While there certainly are technicalchallenges to the implementation of seis-mic isolation, these challenges have mainlybeen overcome. oday the barriers to imple-mentation in the U.S. are not primarilytechnical, but rather economic, culturaland regulatory.

Economic Barriers

With few exceptions, building constructionin the U.S. is driven by first-cost con-siderations rather than life-cycle or riskmanagement cost-benefit considerations.

When the primary objective of a buildingproject is to keep the initial cost of construc-tion to a minimum, then seismic isolation

does not make economic sense. Te cost ofseismic isolation varies, and the actual costsare a function of the building configuration,total floor area, and seismic design perfor-mance objectives. For a typical medium-sizedata center or laboratory building, a roughestimate of the cost of seismic isolation is 5 to15 percent of the cost of the structural fram-ing system (note that this is not the total projectcost; depending on the type of facility, seismicisolation often amounts to only a few percentof the total project cost). When it comes toconsideration of seismic isolation, this added

cost almost always ends up being a dealbreaker; why would a building owner add5 to 15 percent to the structural cost of theirproject with no clear economic incentive?

Why would a developer add seismic isola-tion as a feature when the cost of isolationmight make their project un-competitive?

When a life-cycle cost evaluation is per-formed, considering the full expected lifespan of a building and assuming the occur-rence of a code-based design-level earthquakeevent during that life span, a far differentconclusion is reached. For such an event, aseismically-isolated building can be expectedto experience essentially no damage, a far dif-ferent outcome than for an ordinary building.

Furthermore, in the event of a design-level,or even beyond-design-level earthquake,most isolated facilities can be expected toremain fully functional, eliminating lossescaused by down-time, lost production,lost data, and lost building contents. Animpressive example of this is the fully-oper-ational performance of the Ishinomaki RedCross Hospital in the M9.0 Great ohokuEarthquake of March 11, 2011, dramaticallydemonstrated in this publicly available video:

www.youtube.com/watch?v=Pc1ZO7YwcWc.Viewed in such a light, seismic isolation

almost always shows itself to be economi-cally worthwhile.Seismic isolation would become more

economically attractive to building ownersif property insurers recognized the ben-efits of isolation in reducing earthquakedamage. o date, however, insurers have

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been unwilling to grant premium benefitsfor seismically-isolated buildings. Whereasinsurers routinely offer premium discountsfor protection measures such as fire-resistantconstruction, fire and theft alarms, hurricaneresistant windows, and other building fea-tures that reduces potential losses, to datethey have been unwilling to recognize thebenefits of seismic isolation. If insurers wereto provide premium incentives to buildingowners to use advanced protective measuressuch as seismic isolation, they would help to

encourage the use of the technology in thesame way that they have encouraged the useof fire-resistant construction.

Cultural Barriers

In the United States earthquakes tend to beviewed as regional hazards, affecting mainlyseismically active areas such as California,Nevada, Oregon, Washington State, and

Alaska. Anyone familiar with seismic haz-ards in the United States, however, knowsthat significant seismic hazards actually exist

throughout much of the country. Still, thegeneral perception is that earthquakes arenot a national problem, but a localizedone. In other countries, such as Japan,New Zealand, and Italy, earthquakes arerecognized as a threat to the safety andeconomic well-being of the entire nation. Inthese countries there is a heightened aware-ness of earthquake hazards and therefore a

willingness to spend resources to mitigateearthquake damage through implementa-tion of seismic isolation.

In the United States there is little interestin devoting time and money to prepare forearthquake events that are viewed (oftenincorrectly) as having only a small probabilityof occurrence during the life of a structure.

When it is proposed to building developersthat they may want to consider designing theirbuilding for a seismic performance level abovethe code-mandated minimum, the authorsare consistently met with a blank stare: Whywould I want to do that? Doesnt the buildingcode make my building earthquake-proof?

When it is explained that the building codeprovides only a minimum level of safety, andthat additional design and construction costsare required to provide improved seismic per-formance, the typical response is Lets just go

with the code. It is human nature to believethat bad things will happen to the other guy,and in a country the size of the United States,it is even easier to imagine that earthquakedamage will happen to someone else in someother place. In other countries, where earth-quake hazards exist throughout the nation,it is not as easy to rationalize away the threat

of earthquakes.Tis cultural difference in the perception of

seismic hazards, and the willingness to pay forimproved seismic performance, applies notonly to developers of commercial buildings,but also to individual home owners. Variousestimates have that anywhere from 150,000to 250,000 people live in seismically-isolatedbuildings in Japan, and these people haveall paid a premium to do so. For a typicallarge condominium building in Japan, theaccepted additional cost for seismic isolation

is on the order of the price of a small car,about $15,000 or about 5 percent more than acondominium without isolation. In contrast,the number of people in the U.S. who live inisolated buildings is about two dozen.Countries such as urkey, Chile and Italy are

now experiencing a dramatic upswing in theadoption of seismic isolation for both critical

(hospitals, emergency operations centers) andnon-essential commercial applications (suchas multi-unit residential structures). Why?In these countries, recent large earthquakeshave caused profound economic and loss-of-life disasters. Seismic isolation has beenrecognized as the best technology to protectcore components of societys infrastructure.It is also interesting to note that in urkeyseismic isolation devices must be imported,due to the lack of local manufacturers, whichmeans the costs to implement seismic isola-tion are even higher than in the U.S. Even

so, this cost premium is not proving to bean impediment to the use of the technology.

Regulatory Barriers

Regulatory impediments to the acceptanceand implementation of seismic isolationpersist today. Numerous experimental andanalytical research programs in the 1980sat many prominent research laboratoriesworldwide conclusively verified the effec-tiveness of seismic isolation, and establisheda strong technical basis for practical design.

Building code provisions for seismic isolation,originally established in the 1991 UBC, havesystematically evolved since then. It cannot,however, be said that code requirements havebeen improved in ways to facilitate morestraight-forward and widespread use of thetechnology. In Japan, the 2000 revision of theBuilding Standard Law, the national build-ing code, incorporated new provisions forseismically-isolated buildings that allowed forresponse spectrum, rather than time-historyanalyses, along with other simplified designrequirements for certain types of isolated

structures. Tese provisions are now usedfor the design of about one-third of all isolatedbuildings in Japan. Whilst a number of effortshave been made over time to codify simpli-fied procedures for seismic isolation designin U.S. codes, none have ever been adopted.Instead, seismic isolation design codes havebecome increasingly complex, and thereforeless intuitive and more difficult to use.In the U.S., the codified performance basis

for seismically-isolated structures is higherthan that for ordinary structures; that is, the

Overall view of the isolation basement of the Ishinomaki Red Cross Hospital. e isolation systemcomprises about 100 natural rubber bearings and U-shaped steel dampers. (Photo: SIE, Inc.)



playing field is not level. Clearly, the analysisand design of a seismically isolated buildingpresents greater technical challenges than foran ordinary building, but the building codesplace significantly higher requirements onboth the seismic performance expectationsand on the level of technical review requiredof the designer. Building owners often do not

understand that these additional requirementsexist, or why they lead to increased construc-tion costs and design fees. Seismic isolation isa mature technology with a 30-year trackrecord of successful implementation. Itstime that onerous code requirements forcomplex analysis, multi-party peer review,and full preliminary prototype testingof isolation bearings on every project bereduced, as they lead to superfluous costsand schedule delays that inhibit the adop-tion of seismic isolation.

SummaryIn short, the authors believe that thefollowing actions would increase theadoption of seismic isolation of build-ings in the U.S.:

Move away from soleconsiderations of first cost

when planning building projects,and give fuller consideration tobuilding life-cycle costs. Tis isone of the bases of the greenbuilding movement, where

potential increases in first costsare accepted to achieve long-termsustainability objectives.

Property insurers should beencouraged to recognize thebenefits of seismic isolation(and other enhanced seismicprotection technologies) inpreventing earthquake damage. Ifreduced insurance premiums werefactored into the life-cycle costs ofseismically-isolated buildings, thetechnology would become moreeconomically attractive.

Te design team (engineers,architects, and planners) shouldnot be afraid to promote seismicisolation as a means to reduceor eliminate seismic hazards,increase reliability, and lower totallife-cycle costs. While seismicisolation is still a somewhatunusual approach to earthquakeprotection in the U.S., it has been

widely accepted in other countries

to the point that elsewhere seismicisolation design and construction areconsidered routine.

Regulatory barriers to seismicisolation should be reduced. Inparticular, better simplified methodsfor seismic isolation design shouldbe implemented in building codes,

and requirements for peer reviewand project-specific prototype testingshould be streamlined.

Readers are invited to submit their ownthoughts on why the U.S. has been slow toadopt seismic isolation, while other countrieshave more readily adopted the technology([email protected]). Te authorsare particularly interested to hear from thoseof you who have promoted, but not managedto implement, seismic isolation for a project.

Tese ideas will be collected and will form thebasis of a follow up article in a future issue ofSRUCURE magazine.

References

F. Naeim and J.M. Kelly, Design of Seismic Isolated Structures, From Teory to Practice, JohnWiley and Sons, 1999.

Clark, P.W., Aiken, I.D., Nakashima, N., Miyazaki, M. and Midorikawa, M., Te 1995Kobe (Hyogo-ken Nanbu) Earthquake as a rigger for Implementing New Seismic Designechnologies in Japan, Lessons Learned Over ime, Learning From Earthquakes Series Vol.III, Earthquake Engineering Research Institute, Oakland, California, March, 2000.

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