40 ASCENT, SUMMER 2000 SEISMIC DESIGN concrete seismicdesignsproven inthePRESSSresearch program buildinginSan Francisco apartment takesadvantage oftheprecast39-story Precast’s SeismicCapabilities Apartment SkyscraperShows seismic designapproaches andwillhave major improvement overprevious frame’s ability to controldamageisa Ltd. ofAltadena,Calif.“The hybrid manager forCharlesPankow Builders project,” saysJoeSanders, project was developedandisbeingused onthis seismic performance,andthat’s whyit beam connectionregion. quake isdissipatedinthebeam-to- the deformationenergyofanearth- the structurebackintoalignmentafter nal andtransversedirectionsbrings seismic resistanceinboththelongitudi- resistant frame.Theframeprovides system thatfeaturesahybridmoment- the newlydevelopedprecastconcrete general contractortookadvantageof seismic designinSanFrancisco,the restrictive coderequirementsfor ments. Toachievethisheightwiththe office spaceand36storiesofapart- floors ofretail,onefloorcommercial standing 425feettall,willcontaintwo a highseismiczone.Thebuilding, structure erectedontheWestCoastin Street willbethetallestconcrete 680 MissionApartmentson concrete seismicload-resistingsystems. techniques fordevelopingprecast general contractorusingnewlyproven taken recentlyonbothcoastsbythe The projectisoneofseveralunder- apartment buildinginSanFrancisco. tion ofan$87-million,39-story lighted thissummerbytheconstruc- T “This formatdefinitelyofferssuperior When finishedinOctober2001, the WestCoastarebeinghigh- concrete structuralsystemson he capabilitiesforusingprecast Rendering: Elkus/Manfredi resistance. Thestructural componentsalsoserveasarchitectural cladding. under construction usingaprecastconcretehybrid moment-resistantframeforstructural The 680Mission Apartments building,a39-story skyscraper inSanFrancisco,currently is Stanton ofthe University ofWashing- research. Pankow alsoworkedwithJohn California atSan Diegocoordinatedthe Nigel PriestleyoftheUniversity of California (PCMAC).Professor M.J. Concrete ManufacturersAssociation of (PCI) andthePrecast/Prestressed Precast/Prestressed Concrete Institute National ScienceFoundation(NSF),the Three full-sizemock-uppanelswereproducedfortheMissionApartmentsprojecttoreplicatedifferentfinishesandtexture the UnitedStatesandJapan scientific andbusinesscommunitiesin involving membersoftheacademic, It representsacoordinatedeffort research institutionsaroundthecountry. began in1990atvariousuniversitiesand Systems) ResearchProgram.Theresearch PRESSS (PrecastSeismicStructural Technology) inconjunctionwiththe (the NationalInstituteofStandards& Pankow’s developmentworkatNIST culmination ofresearchundertakenwith industry worldwide.” a dramaticimpactontheconstruction PRESSS ResearchWas Catalyst ‘ seismic performance.’ seismic performance.’ This format definitely This format The programissponsoredbythe The structuralsystemresultsfromthe offers superior . sive inspectionandrepairprocesses. producing time-consumingandexpen- randomly andatanunknownpoint, tures, thatdeformationcanoccur seismic event.Inothertypesofstruc- inspected andrepairedlocationaftera and flexingwithit,providinganeasily controllable focusforabsorbingforce comes fromusingthejointsasa shearwall). Thekeytotheirsuccess ductile framesandonejointed seismic load-resistingsystems(four 60-percent scalethatincludedfivekey and testingofafive-storybuildingat September 1999withtheconstruction of thePRESSSprogramculminatedin in thePRESSSprogram. ton, whowasoneofthekeyresearchers structural members. Thepost-elastic independent of theintegrityof absorbs seismic energyinamanner concrete moment-resistantframe that Mission Apartmentsconsistsof aprecast sophisticated constructionmarkets.” build inallbuttheworld’s most technologically, makingthemdifficultto have beenexpensiveandverycomplex explains Sanders.“Butthesesolutions seismic eventhavebeenfocusedon,” of thestructureitselfduringamajor that minimizethedamageordestruction “In recentyears,structuralsolutions The researchanddevelopmentphases The systemPankowadapted for the lent storydriftof6percent. even whentestedtoarotationequiva- typically consistingofminorspalling moment frameshowsminimaldamage, inelastic andelasticresponses,the structural member.Bycombiningboth connection regionratherthana performance isconcentratedinthe headroom oneach floorwithashorter cross-section. So weachievethesame to-floor heights becauseoftheshallower the buildingtobebuiltwithlower floor- from concrete,”Sandersnotes. “It allows general tobuildingaresidential building “Therearegreatbenefits in economical. its lifesafetyfeatures,itwas more hybrid frame.Foremost,inadditionto this system.” the structureswe’vepreviouslybuiltwith the applicationofhybridframefrom “Thisprojectisa quantumleapof zone. this substantialsizeinahighseismic new designtechniquesbutthefirstof the company’ssixthprojectusingthese efficient.” TheMissionApartmentsare create somethingnewandmore marrying thetwosystemstogetherto history,” Sanderssays.“Allwe’redoingis prestressing strands;theyhavealong the technologyofpost-tensioningand Reusing ExistingTechnology Numerous benefitsaccruefromthis “Many structuresinthepasthaveused s tobeincorporated. 41 ASCENT, SUMMER 2000 Five Other Projects Use System Charles Pankow Builders Ltd. has constructed five other projects using the precast concrete hybrid frame being installed in the Mission Apartments building. They com- prise: A three-story New York City parking structure completed in 1995. “That was the first time the components were put together using this principle,” explains Joe Sanders, project manager for Pankow. “It gave us the opportunity to review Beams cast for the project are being stored in the precaster’s yard in Corcoran, Calif., until the details and economics of the needed to ensure erection moves smoothly and rapidly once the site is ready. system in a low-seismic zone.” A four-story, 264-car parking building, saving material costs.” After labor,” he says. “This approach not only structure in Eugene, Ore., com- budgeting the project with a structural uses new materials that aren’t stretched pleted in November 1996. steel frame and then with the hybrid to capacity but they can be assembled by A three-story, 1,504-car parking frame, Pankow estimated the precast more efficient crew sizes while achieving structure at Stanford University in concrete approach saved approximately a higher level of quality.” Palo Alto, Calif. “That was our first $4 million. It also helps control costs over the use in a high-seismic zone using the design process. “Steel prices are very hybrid moment-resisting frame.” ‘Precast concrete is a volatile, which means that a design that The West Side Media Center in begins assuming certain costs is at the Los Angeles, a four-story office very city-friendly mercy of later price rises. So there’s a building under construction. Struc- significant risk of moving forward tural work is finished but architec- material and a good without knowing the pricing that will tural interior work is still underway. be in place when you need to buy steel.” An 8-story office building in Daly approach overall.’ Pricing on precast concrete, in contrast, City, Calif., which currently is remains stable and uses local labor and under construction with foundation The cost savings becomes greater once materials. “Precast concrete is a very work completed. “The significance the post-earthquake costs are added in, city-friendly material and a good of this project is that Daly City sits Sanders says. “In steel systems, the approach overall.” directly on the San Andreas fault deformations can destroy any part of the line,” Sanders says. “This project is system, which requires an in-depth Several Connection Systems Used as close to ground zero in an earth- inspection following a significant seismic Because of its unique shape and quake as one can get. That will be a event. This precast concrete system also combined functions, the Mission real test for how well the system offers a self-righting force due to the Apartments project actually uses several performs—and it definitely will have post-tensioning strand. That allows the connection systems. Its shape features a ASCENT, SUMMER 2000 to perform someday.” building to undergo deformation or wide base that rises to the 34th floor 42 seismic drift and then restore itself. So before narrowing on each side for five the long-term benefits after a seismic more floors, creating what the architect event add more savings — and we know terms “shoulders” on the lower floors. there will be a seismic event in this area.” The façade also includes a curving face that added more architectural interest. Expanding The Market The project, which is owned by The Precast also offers a dense material Related Companies of California, that absorbs vibration between floors, Irvine, Calif., was designed by cutting disturbances from neighbors, he Elkus/Manfredi Inc. in Boston with notes. Fire-resistance requirements also Kwan Henmi Architecture/Planning are achieved without the need for extra Inc. in San Francisco serving as fireproofing due to concrete’s inherent executive architect. non-combustible composition. Another To express the unique architectural more subtle advantage comes about design on the exterior, the precast because the system creates a new design concrete structural load-resisting approach that allows a wider variety of members were designed to serve as the companies and craftspeople to bid on façade cladding as well. This meant that This moment-frame beam form shows the center PVC duct that holds the prestressing the structural systems. the structural components were cast tendons and the six corrugated tubes for the “San Francisco has so much construc- and finished as architectural precast mild steel bars used in the hybrid system. tion work that the market is stretched for concrete pieces, killing two birds with one stone but creating more challenges in the casting process. PRESSS Research Proves Systems’ Viability “We have always tried to achieve dual The system detailed in this story is one of five seismic load-resisting systems use of concrete structural components for tested during the PRESSS research for the past decade. The study involved five façade design as well,” he says. “This seismic load-resisting systems (four ductile frames and one shearwall). They project has a significant architectural included: element to it that adds interest and A Hybrid Frame Connection with unbonded post-tensioned strands distinction.” Two finish colors were used running through a duct in the center of the beam and through the columns. on the precast components, with the Mild steel reinforcement runs through ducts at the top and bottom of the “shoulders” using a darker color than the beam in the connection region and is sleeved through the column and grouted. rest. All of it features a sandblasted finish, This system is used in the Mission Apartments. with components produced by Pankow’s A Pretensioned Frame with partially debonded strands set on one-story precasting division. columns. The columns’ reinforcing steel extends through the sleeves inside the The building was conceived using a beams. As the frame displaces laterally, the debonded strand remains elastic, tubular post-tensioning frame with a recentering the structure after a seismic event as with the Hybrid Frame. hybrid moment-resistant frame around A Tension-Compression Yielding Gap frame, which leaves a small gap the perimeter. But because the building between the end of the beam and the face of the column, with only the bottom is not rectangular, several connection portion grouted to provide contact between the beam and column. Post- systems were used. The hybrid frame tensioning bars are centered in the grouted region and clamp the frame gains efficiency when frames are long together. This keeps the gap open on one side of the column as it closes on the and join a number of bays, Sanders other during a seismic event, eliminating frame elongation. explains. In some areas that offered A Tension-Compression Yielding frame, which models a traditional only one bay, the Dywidag brand connection similar to one emulating cast-in-place methods. But the yielding is ductile-connection system was used. concentrated at the connection by debonding the beam reinforcement over a In these sections, making up less than short length at the beam-to-column interface. one-quarter of the entire frame, the A Jointed Shearwall System, using connection devices between the vertical components are bolted together rather joints of the wall panels to provide hysteretic damping. In the initial than post-tensioned. The first eight experiments, researchers used a previously tested U-shaped connection. Now floors, including one level of under- that the system has shown its capabilities, the connection itself will be ground parking, were supported with improved to lower costs and make it less complicated. cast-in-place shearwalls and moment frames. field, although Pankow has tested it That approach, combined with the past design needs at the University of significant benefits offered by the Sleeve System Used Washington, Sanders says. “Stressing system, gives Pankow great optimism A unique aspect of the post- two sides in the same space would have for the future use of these techniques. tensioning system comes in the way created an engineering complication,” “We see this system potentially Pankow handles corners. Rather than he explains. “One of the features that becoming the dominant approach in post-tensioning each side separately, allows the building to attain the height many areas,” he says. “We see great creating two sides that must stressed in it does is that the tube frame remains advantages from it. We’ve always the same space, the company devised uninterrupted around the perimeter of recognized the benefits that precast 90-degree angled tubes that let the the building. This tube system allows concrete provides to the economics of a strands turn the corner and continue us to deal with the corners.” project. You can control the cost of the ASCENT, SUMMER 2000 along the adjoining side. This allows Mission Apartments also will see the components much better than with 43 post-tensioning to occur in only two first application of the frames using grout- others that are more volatile; you can locations per floor, directly opposite to filled columns with splices, he adds. The achieve higher quality with factory-cast each other. other projects the company has created components and you can produce a This application will be the first time could use one-piece precast columns from system that is more economical in first the sleeve system has been used in the footing to roof. In a 39-story building, cost and in the long term. When you add that was impossible. Instead, the columns in the superior seismic performance, it are being cast in two-story lengths and are offers a strong option — especially since locked together with NBM Splice Sleeve it simply uses existing ideas and brand connections. components and connects them in an “This system requires a more technical innovative way.” ■ approach to concrete construction than a cast-in-place approach would need,” — Craig A. Shutt Sanders says. “But our efforts were designed to ensure the system was not Pankow Builders has incorporated a precast too technically daunting so ordinary concrete hybrid moment-resistant frame into craftsmen could accomplish it. Some For more information on PRESSS the Mission Apartments project, which technical application is needed in casting research program, see the Winter combines prestressing steel with mild steel reinforcement within fiber-reinforced grouted and putting together the elements, but it and Spring 2000 issues of ASCENT. tubes. is not a technologically complex design.”