roducing precast concretestructures that meet strict coderequirements for seismic condi-

tions has long challenged designers andprecasters. But a new design approvedfor use in western regional codes hasproven it can meet the needs whilesaving time, material and money. Inthe process, says the team of engineersand contractors who devised it, thedesign has the potential to alter thefundamental way in which Type Istructures are designed and built inregions of high seismic activity.

“Virtually any type of building cantake advantage of this connection sys-tem,” says H.S. Lew, chief of the struc-tures division of the National Instituteof Standards & Technology (NIST),United States Department of Commerce.NIST, in a cooperative effort withCharles Pankow Builders Ltd., Altadena,Calif., and the University of Washington,developed and tested the system todetermine its performance. The systemwas designed to Uniform Building Codeseismic Zone 4 requirements.

Based on the test results, NIST andEnglekirk & Nakaki, Irvine, Calif.,developed a set of design guidelines forthe use of this system. Using these tests,design procedures, guidelines and othersupporting information, this systemwas accepted after thorough evaluationby the International Conference ofBuilding Officials Evaluation Service(ICBO-ES) early last year.

While Pankow’s intent is for unre-stricted use of the frame, currentlyICBO requires Pankow’s supervisionfor all applications of the frame. “Cur-

Hybrid Precast FrameMeets Seismic ChallengesPrecast concrete framing system combines high-strength and mild steel to both resist forcesand absorb energy, proving superior to monolithic frames in speed, cost and durability

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The new hybrid precast moment-resisting frame has been put into practice in a parkingstructure built at Roosevelt Field Mall in Garden City, N.Y. The system was created in acooperative effort of Charles Pankow Builders Ltd., the University of Washington, and theNational Institute of Standards & Technology (NIST).

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rently, there are highrise structuresbeing designed with this system, as wellas parking structures that have beenconstructed,” says Lew. “All types ofbuildings will benefit, and we expectmore will be using it soon, because itshould cost less than other conventionalsystems and speed construction.”

Connection Absorbs Energy In essence, the system consists of a

precast concrete moment frame thatabsorbs seismic energy in a manner thatis independent of the integrity of thestructural members, explains DavidSeagren, manager of research and devel-opment at Charles Pankow Builders Ltd.,who began collaboration with NIST in1991. The system’s post-elastic perfor-mance is concentrated in the connec-tion rather than in a structural member.

The hybrid frame relies on theconnection of its precast columns andbeams which use both standardreinforcing steel and high-strength,post-tensioning steel cables to performtwo functions. This combinationprovides both the inelastic actionnecessary to absorb energy throughmovement of the joint and the elasticaction required to provide the shearand moment resistance that hold thejoint together. The high-strength, post-tensioned steel gives the joint its strengthto resist applied dead, live, and seismicloads, while the mild steel used acrossthe joint serves as an energy dissipaterby yielding under seismic loading,Seagren explains. “It isolates and separ-ates the strength and energy-absorption

components within the joint.”By keeping the initial post-

tensioning force low relative to thestrand’s ultimate strength, the high-strength steel does not yield, stayingwithin its elastic range with a largereserve capacity for deformation. Thisprovides a restoring force that allowsthe post-tensioning steel to make thejoint self-centering, resulting in noresidual drift or building lean after aseismic test. “In simpler terms,”Seagren says, “the elasticity of the jointallows it to open and close, much like aspring-loaded door, in order to accom-

modate the seismic ground motion.”(For details on how the system installs,see the sidebar.)

The result is a hybrid precast framethat can be constructed economicallywhile exhibiting higher performancethan conventionally reinforced cast-in-place concrete frames, he says. Its driftcapacity exceeds six percent, demon-strating its reserve capacity, he adds.Only negligible cracks occurred at thishigh level, and these closed up againupon the removal of the load, confirm-ing the elastic response of the columnand beam elements.

This elevation shows how the combination of mild steel and partially bonded post-tensionedstrands run through the connection. Diagram courtesy of NIST

The new framing design helps the Roosevelt Field Mall parking structure resist wind loads, as there are no special seismic needs required in thisregion. It also met the owner’s desire for open spans unobstructed by conventional shear walls. The project was part of a $150-million capital-improvement program at the mall.

Better Than Monolithic“The new precast frame proved to

be at least equal to the monolithicframes in almost all respects, andsuperior in most,” says Geraldine S.Cheok, research engineer in NIST’sbuilding and fire research laboratory.She and two engineers—John Stanton,professor of civil engineering at theUniversity of Washington, and WilliamC. Stone, senior research engineer atNIST—described the details of thestudies they performed and their designimplications in a paper released this

spring. (For details, see “A HybridReinforced Precast Frame for SeismicRegions” in the March/April 1997 issueof PCI Journal.)

Others who have participated inreviewing and testing the system alsohave been impressed with its potential.“The precast hybrid frame usestraditional materials in a unique waythat vastly improves both the reliabilityand performance of a buildingconstructed with this system,” saysSuzanne Dow Nakaki, president ofEnglekirk & Nakaki Inc. “This benefitsnot only building owners but theoccupants and users of the buildingalso.” Adds Stanton, “The technicalinnovation is a remarkable break-through.”

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‘The result is a hybridprecast frame that can

be constructedeconomically whileexhibiting higher

performance.’

This detail shows the angles and locations ofgrouting for the connection. Column bars havebeen omitted to provide clarity of the system. Diagram courtesy of NIST

This diagram of the partial bonding of the post-tensioned strand shows how the system workswithin the building’s frame. Diagram courtesy of NIST

How It InstallsThe hybrid connection system is described in detail in a paper presented in

the March/April issue of PCI Journal. It is authored by John Stanton, professorof civil engineering at the University of Washington in Seattle; William C.Stone, a senior research engineer in the Building & Fire Research Laboratoryat the National Institute of Standards & Technology (NIST) in Gaithersburg,Md., and Geraldine S. Cheok, a research engineer at NIST.

A key appeal of the system comes from its use of precast beams and columnswithout the need for permanent corbels, the three explain. The column isassumed to be a single, multi-story unit to save the cost of splicing. The beamhas a solid, rectangular cross-section at its ends and a trough at the top andbottom in its central region. The beam-to-column connection is made withgrouted reinforcing bars and post-tensioning that is either partially or totallyunbonded. The bar steel is placed in ducts at the top and bottom of the beam,while the straight post-tensioning tendon is located at the beam’s mid height.

In the construction sequence, the columns are erected first and are equippedwith temporary steel corbels on which the beams will be set. The beams thenare erected, and the reinforcing bars are placed in the trough. The bars then arepassed through the ducts in the solid ends of the beam, which line up withmatching ducts in the column. The gap between the beam and column then isgrouted with a fiber-reinforced grout. The ducts containing the reinforcingbars may be grouted at the same time.

When a line of beams is in place and the grout at the beam-to-columninterface has gained strength, the post-tensioning steel is installed and stressed.The temporary corbels are removed and the floor system is then installed.

The intent of the system is to have the beams and columns act as rigidbodies, with the deformation of the system concentrating at the beam-to-column joints. The end of the beam rocks against the column face and a singlecrack opens there. The post-tensioning remains elastic through story drifts of3.5 percent due to debonding and to the location of the post-tensioning steel.

Key AdvantagesThe system offers owners and

designers a host of advantages overother systems, including providinganother option when deciding amongfundamental building materials withwhich to build. Others include:

• Lifecycle costs: “Long-term costsof the system offer a quantumimprovement over other systems,” saysSeagren. Following earthquakes, astructure using a precast frameconstructed with this process willrequire only normal inspection andminimal, if any, repair of structuralmembers, he says. That’s a dramaticchange from what typically isexperienced following a seismicdisaster. For instance, after theNorthridge, Calif., earthquake, arelatively minor seismic event, it wasrevealed that conventional steelmoment-resisting frames experiencedwidespread brittle failure, encom-passing more than 100 buildings.Repair costs averaged more than$10,000 per joint. In addition, someseverely damaged concrete-frame

buildings were demolished due toprohibitive repair costs. “The precastmoment-resistant frame results in abuilding whose value and usefulness isnot diminished or decimated by aseismic event,” Seagren says.

• Simplicity: “The beauty of thisnew technology is that it is quitetransparent to the designer, simplifyingthorough understanding,” says Nakaki.“The science behind the systemrequires that the designer truly

understands the expected post-elasticperformance of the system. This isknowledge that is deeply hidden in theintent of the prescriptive sections of ourexisting building codes for other typesof system.”

The system is so basic and easy toconstruct, Seagren adds, that costsavings result when it is used instead of

other existing systems. For example, inone recently constructed parkinggarage, savings were estimated atapproximately one percent of the totalconstructed cost, with a 60-percentsavings over the number of man-hoursthat would have been needed toconstruct a conventional lateral-bracingsystem. “These savings, achieved by thenew frame, will only increase as itsconstruction techniques are perfected.”

• Speed: Savings also are expected tomount from the reduced time neededto install the system over conventionalsystems, Seagren notes. A recentlycompleted 550,000-square-foot, 1,719-car parking lot used the system and waserected in six and one-half months,compared to an estimated 12 monthsto finish it with a conventional system.That resulted in a 45-percent savings inproject delivery time, bringing thestructure on line quicker andgenerating revenues faster, in additionto cutting time on construction loansand labor costs.

• Safety: Perhaps best of all, thesystem’s toughness and resiliency helps

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‘Long-term costs offer aquantum improvement

over other systems.’

Several advantages are offered by the hybrid system, which combines standard reinforcing steel with high-strength, post-tensioning steel cables toprovide both inelastic flex to absorb energy and elastic action to offer shear and moment resistance. The benefits include long-term costefficiencies, simplicity of design, speed of erection and safety enhancements.

protect occupants during a seismicevent. “It can significantly reducestructural and architectural damage tonew buildings that use the framingsystem in eathquake zones,” says NeilM. Hawkins, professor of civilengineering at the University of Illinoisat Urbana-Champaign, who watchedthe system develop.

In parking garages, it also aids day-to-day security and attractiveness of theproperty for security-conscious patrons,adds Charles H. Thornton, principal inThornton-Tomasetti/Engineers in NewYork City, which helped test thesystem. “Using this precast moment-frame joinery system in a parking

structure provides the additionalbenefit of offering lateral load resistanceunder wind or seismic loads whilesupplying an open plan, free of shearwalls,” he explains. “This significantlyenhances the security of the parkinggarage.”

Designers Tough To ConvinceConvincing designers and engineers

to adopt the system and achieving codeapproval proved tough challenges,notes Seagren. It’s a process that willcontinue for some time. “Current codesand design procedures for seismic zonesare predicated on structures protectingthemselves by dissipating seismicenergy by inflicting damage tothemselves during post-elasticresponse,” Seagren explains. “Whilethis goal has been largely successful, theimpact of severe damage inflicted uponstructures by major seismic occurrenceshas been devastating.”

That attitude is beginning tochange, as building owners, designers,insurers, and investors realize howdevastating such an approach can be.“Structural solutions that minimize thedamage or destruction of the structureitself are being focused on,” Seagrensays. “But solutions have beenexpensive and very complex tech-nologically.” In addition, he says,“engineers who have grown up with

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Expert ReactionsCharles Pankow Builders Ltd. has nominated its precast hybrid connection

system for the 1997 National Medal of Technology awarded by the Presidentthrough the Office of Technology Policy. In its supporting background,Pankow submitted the technical opinion of a number of engineers and otherexperts. Here are some of their comments on the system:

• “Pankow’s forward thinking on new ways of construction has led toinnovative systems that will revolutionize the precast concrete industry.”— H.S. Lew, chief of the structures division, National Institute of Standards

& Technology

• “Four aspects of this achievement make it unique and deserving of thisaward. They are the technical innovation, the regulatory difficulties that hadto be overcome, the economic success, and the teamwork required to pull offthe whole venture.”— John Stanton, professor of civil engineering and director of the Structural

& Geothermal Engineering & Mechanics program, University of Washington

• “This system has very special relevance to our industry in light of thewidespread failure of the ‘tried and true’ steel Special Moment ResistingFrame during the Northridge earthquake.” — Charles H. Thornton, principal, Thornton-Tomasetti/Engineers

• “While most of the construction industry continues to look for ways to makebuildings cheaper, Pankow has been able to incorporate a new technology ina cost-effective way that also significantly improves seismic performance.” — Suzanne Dow Nakaki, president, Englekirk & Nakaki Inc.

• “One of the big roadblocks to encouraging privately funded research anddevelopment in building structures and civil engineering in general is that itis extremely difficult for a sponsor to exclusively capitalize on a successfulresult. In this case, Pankow has chosen to allow its R&D results to inure tothe public domain.”— Norman L. Scott, chairman, The Consulting Engineers Group Inc.

The use of the system in a recent projectresulted in a 45-

percent savings indelivery time.

Wide-open spaces can be achieved in parking structures with the hybrid system because it offerslateral load resistance under wind or seismic loads while supplying an open plan free of shearwalls.

and lived with codes have these innatepreferences for continuing to do what iscodified and not to venture into newterritory. Overcoming the resistance ofa profession, both codified andpersonal, has not been trivial.”

Present building codes discriminateagainst discretely jointed precast framesfor high-seismic zones, explainsNorman L. Scott, chairman of TheConsulting Engineers Group Inc. ofMt. Prospect, Ill. “Most of the priortechnical-development work has beenfocused on monolithic cast-in-placeconcrete frames with the intention ofhaving rotation occur in energy-absorbing hinges in the beams somedistance away from the column face,”he says. “It is not very economical toemulate monolithic-frame behaviorwith a precast concrete design. Still, upto this time, the codes have tended toforce precast concrete in this direction,so many designs have employedcruciform elements that are expensiveto cast, ship and assemble.”

Adds Nakaki, “In an industry suchas ours, where building owners possessrelatively little knowledge about therisks of natural hazards such as earth-quakes, it is extremely difficult to evenattempt innovation that improvesperformance, much less make it suc-cessful as you move across the country.”

Theory In ActionTo prove the system would work in

actual field conditions, Pankow beganconstruction on a structure using thesystem in early 1995. In the first ofthree projects built as part of a $150-million capital-improvement con-struction project at Roosevelt Field

Mall in Long Island, N.Y., Thornton-Tomasetti/Engineers designed aparking garage that incorporated thesystem, which fulfilled the owner’sdesire for a clear and open plan thatwas unobstructed by conventional shearwalls.

Moment frame members consistedof 18" x 24" columns and 18" x 36"beams with a maximum contiguousmultiple frame length of 90 feet. “Thecompleted structure demonstrated theconstructability of the system,” Seagrensays. “It also confirmed the anticipatedfavorable cost and schedule impacts.”

Successfully taking the project from

theory to the field represents a majorstep that Nakaki hopes is the beginningof a continuing trend. “Designengineers, as a breed, are hesitant toembrace new technology, as it requiresthem to take risks many are unwillingto take,” she says. “It is a majoraccomplishment not only to succeed inobtaining regulatory approval throughICBO, but more importantly toachieve the approval of many differentdesign firms across the country nowwilling to professionally stand behindthe concept.”

Certainly, Seagren anticipates thesystem will grow in use as its implica-tions become better understood. “Thehybrid frame’s ability to control damageis a major improvement over previousseismic design approaches,” he says. “Itwill have a dramatic impact on theconstruction industry worldwide. Inaddition to the development’s remark-able performance, the new systemopens the way for the use of precast inseismic regions, with its attributes ofeconomy, high quality and speed ofconstruction.” ■

— Craig A. Shutt

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10 Years Of TestsThe development of a precast moment-resisting frame began in 1987 when

the National Institute of Standards & Technology (NIST) started anexperimental program on precast beam-column connections subjected to cyclicinelastic loading. The program was conducted with input from a steeringcommittee consisting of individuals from academia and the private sector.Several papers, reports and presentations resulted from this effort.

In the last phase of the test program, a cooperative effort between NIST,Charles Pankow Builders Ltd., John Stanton of the University of Washington,and John A. Martin & Associates, dealt with the concept of a hybrid systemthat combined the use of high-strength steel and mild steel. Funding for thisphase of the program was provided by NIST, Charles Pankow Builders Ltd.,and the Concrete Research & Education Foundation (ConREF) of theAmerican Concrete Institute (ACI).

Three potential designs were investigated in 1992-1993 at NIST’slaboratories. Two of the designs were developed by NIST and one by JohnStanton with Charles Pankow Builders supplying constructability review. Ofthe three designs, the one identified as having the most potential was furtherrefined and simplified in a follow-up cooperative effort.

“This paradigm shift of combining both elastic action for resistive strengthand inelastic action for energy dissipation proved to be a critical developmentin the success of the new hybrid frame,” says Pankow’s Dave Seagren. Four testspecimens were fabricated under field conditions and tested in 1993–1994 aspart of this effort.

The connection system opens new possibilities to designers because it will make precast concreteconstrution in high-seismic zones faster and more economical.