HARRY H. EDWARDS AWARD WINNER

Design-Construction ofThe Paramount — A 39-StoryPrecast Prestressed ConcreteApartment Building

Robert E. EngIekrk,Ph.D., RE., S.E.Consulting Structural EngineerEnglekirk Systems Development, Inc.Los Angeles, CaliforniaAlso, Adjunct ProfessorUniversity of California at San Diego

At 39 stories and 420 ft (128 m) high, The Paramount (located in SanFrancisco, California) is the tallest concrete structure in addition tobeing the tallest precast, prestressed concrete framed building inSeismic Zone 4 (a double record). It is the first major high risebuilding to be braced by an architecturally finished exposed precastconcrete ductile frame. The reinforcement used to create this seismicductile frame includes post-tensioning and high strength reinforcingsteel. All this represents a major milestone in the development ofprecast/prestressed concrete. The building is basically an apartmentcomplex, although the lower floors accommodate retail space, vehicleparking and recreational amenities. This article presents the designconsiderations, construction highlights, research and development,and code approval process that led to the realization of this structure.

Long-time PCI Professional Member Robert E. Englekirk has been astrong advocate of precast concreteconstruction for many years. A PCIFellow, he is the author of numeroustechnical articles, three of which havewon the Martin P. Korn, Robert J.Lyman and Charles C. Zoliman PCIJOURNAL Awards. He has just completed a book titled “Seismic Designof Concrete and Precast ConcreteStructures to a Performance Criterion,” which will be published byJohn Wiley & Sons in early 2003.

Soaring majestically amidst theother high-rise buildings in SanFrancisco is The Paramount —a 39-story residential apartment towerthat reaches 420 ft (128 m) skyward(see Fig. 1). Costing nearly $93 million, the newly constructed building isprestigiously located at Third andMission across from the famousMoscone Center, further enriching thecity’s world renowned skyline.

What distinguishes this buildingfrom the other highrises surrounding itis that the structure incorporates a

novel precast hybrid moment resistingframe that is particularly effective inthe severest seismic regions of theUnited States and indeed the world.As such, The Paramount is not onlythe tallest concrete structure built inSeismic Zone 4, but it is also by farthe tallest precast, prestressed concreteframed structure built in a region ofhigh seismicity.

From a precast concrete perspective,it is the first major high rise buildingto be braced by an architecturally finished exposed concrete ductile frame.

56 PCI JOURNAL

Fig. 1. The Paramountat Third and Mission,San Francisco,California.Photo courtesy:Kwan Henmi,Architecture andPlanning.

The reinforcement used to create thisductile frame includes both post-tensioning strand and high strength mildsteel reinforcing with a yield strengthof 120 ksi (8280 MPa). This combination of materials also represents a significant technological breakthrough.

The accomplishment of these milestones is a credit to the courage andperserverance of the design-construction team:

• Owner: Third and Mission Associates, Inc. — For courageously accepting a concrete high rise buildingwith a brand new structural bracingsystem in a very severe seismicarea.

• Architects: Kwan Henmi, Architecture and Planning, and Elkus/Manfredi Architects, Ltd. — For veryimaginatively integrating the structural framing system into the archi

tectural design, and thereby creatinga very functional and beautifulbuilding.

• Structural Engineers: Robert Englekirk Consulting Structural Engineers, Inc. — For pioneering thestructural engineering concept anddeveloping the details of the bracingsystem for this building.

• Contractor: Pankow ResidentialBuilders II, Ltd. — For enthusiasti

July-August 2002 57

Fig. 3. Typical planof building.

cally embracing the seismic bracingsystem and sponsoring the necessary research to test the system atthe University of Washington.How the design-construction team

realized their dream is the subject ofthis paper.

SYSTEM SELECTIONThe obvious first question to ask is,

why, in such a pedantic industry,would any one of the team memberselect to follow such a difficult road?New systems are only developed whenit is clear that they will be less expensive and better than the standard theypropose to replace. The author is notsure whether the “better” is essential,but in this project, both conditionswere in fact met.

Economies in construction are produced by an efficient use of materialsand/or reducing the time required todeliver an occupiable building. TheParamount team fulfilled both of theseparameters. Fig. 2 shows an artist’srendering of the project, while Figs. 3and 4 show a plan and elevation, respectively, of the building.

Two expensive building components, namely, the architecturalcladding and the seismic bracing sys

-

Fig. 2. Artist’s rendering of The Paramount. Photo courtesy: Kwan Henmi,Architecture and Planning. Artist: W. Yeliseyev.

1 79’-O”

9

* DDC Reinforced Beams

‘—Hybrid BeamStressingProgram

0

58 PCI JOURNAL

tern, were efficiently combined on thisproject. This not only resulted in lowerconstruction costs, but also produced amore water resistant exterior, an especially important benefit in San Francisco. When the exterior water barrieris applied to the frame of a buildinglocated in a seismically active area, itmust be designed to accept the movement expected during an earthquake.

This means that slip joints must beintroduced that can accommodate differential displacements of as much as21/2 in. (64 mm). This is usually accomplished with the introduction oflarge caulked joints which, in additionto being expensive, require significantmaintenance.

The construction of the Paramountexterior avoids this problem, becausethe precast components are rigidly attached to each other and designed sothat the pieces move together. Further,the beams are post-tensioned to 760

EL. +175—3‘1

,—, EL. .155—11‘..1

ELEVATION

EL. .195—3 1/2’FLOOR a

Fig. 4. Elevation of building showing heights of various floors. Drawing courtesy:Kwan Henmi, Architecture and Planning.

I I

11111 II I I I

EL. .520—6 5/SIP’

_________________

‘ENTHOUSE ROOF

hEL. .487—1’

FLOOR 40 MECH.

EL. .4J9’—5

FLOOR 35

EL. *392—0 1/2”

FLOOR 30

EL. +347-1W

FLOOR 25

EL. ,302’-4iFLOOR 20

EL. .257’—4

FLOOR 15

EL. +212—lI /2”FLOOR 10

EL. +212—1’

FH ROOF IT.O.S.)

BUILDING FACILITIESThe building contains 486 apart

ment units comprising a total of660,000 sq ft (61380 m2) area ofrentable space. The lower eightfloors and one basement level accommodate a variety of functionswithin a floor area of 31,000 sq ft(2880 mT). Retail space occupiesmost of the first and second floors.Residential amenities include aleasing office and business centeron the third floor with a fitnesscenter and outdoor swimming pooion the fourth floor.

Floors 3 to 7 of the north side ofthe building accommodate parkingfor 350 vehicles, including an all-valet parking station served by elevators instead of ramps. Residential units are located on the southside of the building at the fourththrough seventh floors.

The eighth floor serves as apodium for the typical 13,700 sq ft(1274 m2) residential floors aboveon Floors 9 through 33. The building steps back at the 34th floor to a9900 sq ft (920 mT) floor for Floors34 through 39. Level 40 is an outdoor recreation deck area.

O EL. +136—7”El

EL. .165—7”

FLOORS

I

July-August 2002 59

psi (5.2 MPa) and the columns are, ofcourse, always subjected to high compressive stresses.

The result is a watertight enclosure.An added level of protection is provided by caulking the beam-to-columnjoints. Because the joints are small[about 3/4 in. (19 mm)], and are alwaysprestressed, maintenance should beminimal.

The façade of the building consistsof 732 sandblasted precast beams and478 two-story precast moment framecolumns. In addition to the framemembers, 641 architectural precastpanels, 68 precast gravity columns,and 312 precast, prestressed beamswere fabricated off-site. By using suchan extensive quantity of precast components, the construction schedulewas significantly expedited.

Fig. 6. Seismicinduced movement

(exaggerated) ofprecast framecomponents.

The fact that the structural systemprovided was superior from a seismicperspective was easily (and often)demonstrated to the developer, investors, lenders, and insurers by showing them the post-test conditions oftested models. Fig. 5 illustrates thecondition of a cast-in-place beam (seeFig. 5a) after it was subjected to earthquake-like deformations, while Fig.5b shows the post-test condition of aprecast frame subjected to similar deformations.

The improved structural behavior isa characteristic of the manner in whichprecast concrete members are assembled. Fig. 6 shows how the beam andcolumn interact during an earthquake.In essence, they must rotate so as toallow the building to sway and withstand the shock of the earthquake. In

the precast system, this is accomplished by opening a gap (0) betweenthe beam and column.

In the cast-in-place system, this gapdistributes itself over a finite regionand causes the outer shell of the concrete to spall (see Fig. 5a). Demonstrating the improved behavior of theprecast system and the basis for itmade the system an easy sell to the design-construction team, its financialbackers, and the building officials.

The final building system was theresult of many design iterations thatconsidered both functional, aestheticand construction needs. The floorplan for the typical floors (up to Floor34), as well as the setbacks requiredof Floors 35 through 39, is shown inFig. 3. The entire perimeter of everytower floor including the setbacksconsists of precast concrete spandrelbeams and two-story precast concretecolumns. The floor was cast-in-placepost-tensioned concrete stressedthrough the top of the slab along thesouth edge where the slab span isshort.

Two types of structural lateral forceresisting systems are utilized in thebuilding. Below the eighth floorpodium, the varying slab elevationsand the required occupancy separations between parking and livingspaces created a natural location forshear walls. Consequently, a shearwall system and a precast and cast-in-place moment frame bracing system

d -

(a) Cast-in-place beam

Fig. 5. Post-test condition of cast-in-place beam and precast ductile beam-column frame.

(b) Precast beam-column frame

60 PCI JOURNAL

was implemented from the mat foundation to the eighth floor. The shearwalls terminate at the eighth floor.

Above the eighth floor, a perimeterprecast moment frame was developedusing both the Precast Hybrid MomentResistance Frame (PHMRF) Systemand the Dywidag Ductile Connector(DDC) System®. The PHMRF systemis the predominant frame system utilized for the multi-bay frames. TheDDC System was used at single-bayframes that occur at re-entrant cornersof the building where the effectivepost-tensioning force required by thePHMRF could not be developed.

Fig. 7 is a schematic of the post-tensioning anchorage details for an exterior column, while Fig. 8 shows the reinforcing cage of a precast column.The newly developed DDC reinforcedframe beam is shown in Fig. 9. A typical corner column detail is shown inFig. 10. Fig. 11 shows the post-tensioning jacking operation.

Typical frame columns are 36 in.(914 mm) square up to the 20th leveland reduce to as small as 24 x 36 in.(610 x 914 mm) above. Frame beamsare typically 24 x 36 in. (610 x 914mm) and were set flush to the outsideface of the columns.

THE DESIGN PROCESSNeither the building nor any portion

of its seismic system fit within theframework of the existing building

Fig. 7. Reinforcing details of exterior column.

code. The functionally logical bracingprogram for the building included ashear wall base, which extended fromthe foundation to the eighth level, forthis was the top of the parking structure that occupied the north side ofthe building. The precast concreteductile frame braced the buildingabove Level 8.

Since buildings over 160 ft (48.8 m)[now 240 ft (73.1 m)j require the existence of a ductile frame or a dual sys

tern (shear wall plus ductile frame), anexception was necessary to allow astacked bracing system-frame on topof the shear wall. The Hybrid System,which provided the bulk of the lateralsupport, had yet to be approved, whilethe DDC® System represented a modification of the precast system approved by the International Conference of Building Officials (1CBO).1Accordingly, the project lacked awell-defined seismic criterion.

Wrap Mild 6Steel Rebar

I —Fig. 8. Reinforcing cage of precast column. Fig. 9. DDC reinforced frame beam.

July-August 2002 61

In some ways, this lack of a specificcriterion was an advantage. This is because building codes should not beused as the exclusive criterion for thedesign of a complex structure. Currentbuilding codes have as their focus theelastic behavior range. In essence, thestructure is designed to a strength criterion (F0) (see Fig. 12).

The design engineer understandsthat the structure will respond whensubjected to a major earthquake in adisplacement range many times thatassociated with the elastic design load(F0, 4,) (see Fig. 12). The probable response range is understood to be in thevicinity of 4 (see Fig. 12) and this, ofcourse, should be the region of primary concern to the design engineer.

Current building codes bridge thisdisplacement gap (A0

—A0) by pre

scriptions intended to cover all eventualities. Obviously, it is not possible togenerically cover all possible systemsby prescription. Consequently, conflicts tend to arise, and this often resuits in the acceptance of less than optimal behavior.

The design approach that has as itsfocus the region of behavior interest(see Fig. 12) is now referred to as Performance Based Design, and only attempts to codify this approach arenew, for many designers including theauthor have used it for 30 years. Theessence of Performance Based Designis to predict member strain states atthe anticipated level of building drift.

Fig. 10. Corner column detail.

Region ofBehaviorInterest

Regionof DesignInterest

Force F0 -

I0

Fig. 12. System behavior idealization.

Displacement

Granted

As shown, PI’ steel ispartially bonded

Fig. 13. Hybrid beam system as developed by N 1ST.5

62 PCI JOURNAL

Strain objectives or limit states areestablished experimentally, and thelinkage between analysis and component behavior is established throughthe testing of large-scale models ofmembers and subassemblies. On thisproject, exclusive reliance on a Performance Based Design approach wasadopted by the design team, althoughthey, of course, were required todemonstrate compliance with the intentof the code to obtain a building permit.

PRECAST SEISMICBRACING SYSTEMS

The combining of precast components by post-tensioning is not new.Professor Robert Park (University ofCanterbury, Christchurch, NewZealand) and his associates tested several post-tensioned subassemblies inthe 1960s. Clearly, the concept preceded the ability of builders to exploitit, so the idea was not pursued.

In 1978, the author proposed the development of a post-tensioned assembly and subsequently presented theconcept at several workshops.2-4H. S.Lew at the National Institute of Standards and Technology (NIST)5 obtained a grant to develop what becameknown as the Hybrid System. Ultimately, the system described in Fig.13 was produced after a number of iterations at the NIST test facility.

This particular model, however,needed additional work to satisfy theneeds of the Paramount project becausethe angles that anned the corners of thebeams (see Fig. 10) were not acceptable. Also, the concrete strain limitstates required to effect a performancebased limit state were not established asa consequence of arming the corners.Further, the strength of the beam-to-column joint was not established because the strength of the beam-to-column joint tested significantly exceededthe demand imposed on it.

Subsequently, the Hybrid Systemwas tested as one of the four precastframe systems used in the five-storyPRESSS building conducted at theUniversity of California, San Diego, inSeptember 1999. The PRESSS research program was funded primarilyby NSF with strong industry supportfrom PCI and PCMAC.

Fig. 14. Interior Hybrid beam test assembly.

Additional testing was required todevelop a performance based designcriterion for the Hybrid System, andthis was undertaken at the Universityof Washington. Interior, exterior, andcorner subassemblies were tested. Theattempt was to follow the guidelinescontained in ACI’s proposed acceptance criterion.6 The interior test subassembly is described in Fig. 14.

This subassembly was modeled soas to represent a two-thirds scalemodel of the frame proposed for theParamount building. The ACT acceptance criterion required that the subassembly be designed prior to testingso as to predict its strength and deflection, as well as the point at which thesubassembly would start losing itsstrength. The subassembly (see Fig.

12-4”

• 18x20Column

16’x21Beam

Fig. 15. Behavior of Hybrid beam test assembly (see Fig. 14).

July-August 2002 63

14) was constructed and assembledstrictly following the procedures proposed for the project. The experimental model was then subjected to displacements of increasing magnitude,and each displacement was repeatedthree times before proceeding to thenext level of deformation.

Fig. 15 describes the behavior of thetest specimen. The strength that wasattained exceeded that predicted byanalysis. The predicted nominal flexural strength was 90 percent of that experimentally attained, while thestrength predicted for the joint was 83percent of that established by the test.Accordingly, the validity of the designprocess was demonstrated, It is interesting to note that a recently proposedconsensus design criterion will allow

the use of only 60 percent of the developable strength.

From a displacement perspective,the performance of the subassemblyalso exceeded the desired objectives.Normally, in Seismic Zone 4, the subassembly is designed to drifts of 2 to2.5 percent. The acceptance criterionlooked into the deformability of thesystem and required that the test program be able to demonstrate thatdrifts of 3.5 to 4 percent could be attained without significant loss ofstrength. At a drift of 4 percent, thethird cycle loss of strength was lessthan 30 percent, while no loss ofstrength was experienced in the expected drift range (2.5 percent), andthis was considered acceptable.

For years, the author had studied the

behavior of cast-in-place beams subjected to post-yield seismic deformations in an attempt to predict the behavior of limit states (see Fig. 5a).About 15 years ago, it occurred to himthat the best approach was to avoid theprincipal causative action responsiblefor the deterioration of the concretedescribed in Fig. 5a.

The high concrete strains imposedon the unconfined shell of the beamdescribed in Fig. 5a are exacerbatedby the tendency of the once over-strained reinforcing bars to buckle outward when subjected to compressionloads. It seemed virtually impossibleto prevent this outward displacementof the bar clearly visible in Fig. 5awith confining ties.

The solution seemed so obviousonce it occurred to the author whynot simply move the yielding elementout of the frame beam and into the column where the yielding bar could berecompressed without damaging thesurrounding concrete? This relocationwas made possible through the development of a forged ductile rod whichcould be placed in a column (see Fig.l6a). A high strength bar [120 ksi(8280 MPa)1 would then be screwedinto the end of the ductile rod (see Fig.9). Ultimately, this was the system thatwas used in the Paramount project.

The ductile rod concept seemed appropriate for use in all precast buildings, so the author worked with Dywidag Systems International to developthe precast subassembly described inFig. 16b, which was the originally proposed short and single span solution(locations marked with an asterisk inFig. 3). The builder was concerned,however, that bolt alignment might become a problem, so he opted to use thesystem described in Fig. 9.

Alignment problems should not beanticipated in the precast system (seeFig. 16b) because ‘2 in. (12.7 mm)tolerances are provided for in the connection. During this same period(2000-2001) the DDC System (seeFig. 16b) was used to brace the Hollywood Highland project, and 6700bolts were placed absent any misalignments despite the fact that memberswere assembled in self-stabilizingtowers more than 100 ft (30.5 m) high.

Tests had been performed on a cast-

Fig. 1 6a. Precast concrete column Fig. 1 6b. Precast frame application of forgedshowing forged ductile rods. ductile rods.

— —— : —

106 11) &Peak Values12

Strengthr.______.._____ I —

Coa

w0

0

I> -100w

-150w-200

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9AVERAGE VERTICAL BEAM DISPLACEMENT (IN)

Fig. 1 7. Precast frame application of forged ductile rods.

64 PCI JOURNAL

in-place DDC subassembly. These testresults are described in Reference 7. Inthis case, the nominal strength of thesubassembly was maintained througha drift of 6.7 percent [öb = 8 in. (203mm)1. The resulting damage was oniycosmetic (see Fig. 5b).

The DDC system required no additional testing in support of its use onthe Paramount project, and this is because a special product approval hadbeen granted Dywidag Systems International by the ICBO in the early1990s.’ Note that the ICBO approvalprocess is based on submitted experimental and analytical evidence whichsupports the design process, as well asproduction control procedures. The resuit is a quality assured product anddesign procedure. This approval process is essential to the responsible advancement of precast concrete, for thedays of closely guarded secret homemade connections are gone forever.

BUILDING DESIGNThe adopted Performance Based

Design required that the probablerange of building displacements bepredicted. This was done through theuse of several analytical procedureswhich have evolved over the last 30years. Response spectrum based procedures are the key conceptual designtool. Once the design has been developed, elastic three-dimensional timehistories are performed.

This process involves modeling the

anticipated ground motion both interms of intensity and characteristics.The result is a number of sets of earthquake ground motions, which are thenfed into the base of the analytical modelof the building. This process suggeststhe extent of building movement.

In the case of the Paramount apartments, the movement is expected to beabout 40 in. (1016 mm) at the roof ifthe design ground motion were tooccur. However, do not rush to the

roof in wild anticipation of an excitingride, for this design earthquake is expected to reoccur at intervals of about500 years!

The analytic testing of the buildingdoes not stop with elastic behaviorpredictions because the anticipatedlevel of ground motion will cause theframe beams to reach the inelastic behavior range (see Fig. 15). To predictthe extent of post-yield demand on theframe beams, inelastic time history

Fig. 18. Mockup of architecturai precast panel assembly atprecaster’s yard.

Fig.building façade.

1/ /

Fig. 20. Progress view of precast frame erection. Photo courtesy: Kwan Henmi,Architecture and Planning.

July-August 2002 65

Table 1. Breakdown of precastconcrete components.

Note: 1 ft = 0.3048 m; 1 sq ft = 0.093 m2.

east and west frames in both the elastic and post-yield behavior range. Thebalance attained is much more important than compliance with any prescribed strength objective. Thus, it isimportant to ensure that the balancingoperation not reduce the availablelevel of ductility in the weaker frameelement (west frame in this case).

The design of the precast concreteframe systems is fairly straightforward. The design of a frame beam isdescribed in Appendix B.

analyses were performed on the vaxious bracing frames. The design teamconcluded that post-yield rotationswould be in the 1.0 to 1.2 percentrange, and this is well below the experimentally confirmed limit state of 4percent (see Fig. 15).

Most buildings tend to be rectangularin plan in which one dimension is often2.5 to 3 times the other. This is a characteristic of the Paramount apartments.The design of this type of configurationmust consider the torsional response,especially in the post-yield range.

When a building responds to earthquake excitations that drive it into thepost-yield behavior range, the center ofrigidity will gravitate to the strongerbracing element. For example, theeastern-most frame of the Paramountbuilding would become the torsionalpivot, and thus would cause the westframe to respond in an undesirablemanner from a displacement perspective. Further, any imbalance in strengthwould limit the restoring force.

This concern was mitigated by balancing the strength and stiffness of the

CONSTRUC11ONHIGHliGHTS

The fabrication, transportation anderection of the precast concrete components, together with the overall construction operation and schedule, wascarried out by Pankow Builders. Theprecast components were fabricated inCorcoran, California, starting in January 2000. They were shipped to theproject site by truck-trailer, a distanceof about 200 miles (330 km). In all,2231 precast pieces were produced(see Table 1).

Fig. 18 shows a mockup of an architectural precast panel assembly at theprecasting yard. Fig. 19 shows a viewof the architectural precast façade installed on the building. Figs. 20 and 21show various erection phases of thebuilding. Fig. 22 is a completed viewof the building amidst the other high-rises in San Francisco.

Concrete strengths were specified asfollows:

• 478 Precast Moment FrameColumns[3x3x l8ft]

• 732 Precast Moment FrameBeams [2 x 3 (12 to 24 ftlong)]

• 68 Precast Gravity Columns[2 x 2 (40 ft long)1

• 641 Architectural PrecastPanels [50 to 100 sq ft each]

• 312 Prestressed Beams[40 ft long (average)]

2231 Total Components

Fig. 21. Erection of precast perimeter frames nearing top of building.Photo courtesy: Kwan Henmi, Architecture and Planning.

66 PCI JOURNAL

Fig. 22. Finishedview of TheParamount amidstother high risebuildings in SanFrancisco.Photo courtesy:Kwan Henmi,Architecture andPlanning.

• For the columns: 6000 to 8000 psi(41 to 55 MPa).

• For the beams: 5000 psi (34 MPa).Formwork for casting the architec

tural panels, beams and columns wasprovided by Hamilton Form Company, Inc.

The entire construction comprised a26-month schedule. The foundations ofthe building were started on November15, 2000, and subgrade work was completed in March of 2001.

The superstructure was completed

in 16 months. Erection of the precastcomponents started slowly but, aswork progressed, an average production rate of 2’12 floors per month wasattained.

A major time benefit was the rapidenclosure of each floor which madepossible the installation of electricaland mechanical accessories as well asother fixtures. This made it possiblefor tenants to occupy the premises byOctober 26, 2001 — less than 2 yearsafter the start of construction.

From the mat foundation to theeighth floor, a cast-in-place momentframe bracing and shear wall systemwas used. This was necessary becauseof the varying slab elevations and non-repetitive elements involved. Abovethe eighth floor, a perimeter precastmoment frame was used.

The floor construction cycle, whichultimately took 5 days, started with theplacement of half of the two-story precast column and ended with the raising of the flying forms by one story.

July-August 2002 67

These forms were designed to supportthe weight of the precast beams. Mostof the beams were hybrid and, as aconsequence, were connected to thecolumns by a concentric post-tensioning system together with grouted mildsteel bars.

The precast beams were typicallypost-tensioned by 19 — 0.6 in. (15.2mm) diameter strands. Baumesh wasused to facilitate the assembly of thecolumn cages. Short spans, whose locations are marked by an asterisk inFig. 3, were constructed using Dywidag Ductile Rods®, which were castin the precast columns, and Thread-bars®, which were turned out of the interior couplets and into the ductilerods. Note that precast panels servedas the outside form.

Two tower cranes were used toerect the precast members and relocate the flying forms. Despite the prototypical nature of the project, thecontractor reported that the erectionand assembly process went verysmoothly, and that noise and pollutionwere very minimal.

One of the few problems thatneeded to be resolved was the cornerconditions where, in the absence of amore viable solution, two exteriorstressing assemblies would need to beplaced. Offset assemblies were testedand proved to be effective. However,they were abandoned in favor of an“around the corner stressing.”

The author developed a piece ofhardware consisting of bent pipe sections, restrained by straps connected toan anchoring angle. The stressing program involved a significant amount oftesting to ensure that strands could beplaced and stressed without damage.

The adopted program involved sequentially tensioning the strandgroups in increments from both ends.Columns were connected using SpliceSleeves® (supplied by Splice SleeveNorth America, Inc.), which are capable of developing the breakingstrength of the bar.

The total cost of the project was$92.7 million. The precast cost was$8.9 million. Based on gross squarefootage of rentable area, this amountsto $140 per sq ft. This cost figure isfairly good considering the building’slocation and seismic environment.

CONCLUDING REMARKSThe construction of this innovative

project flowed smoothly and wascompleted on schedule. The credit forthis achievement lies with the Pankowproject management team. Assemblyprocedures were developed in the precasting yard and carefully documented, as were the procedures for ensuring high quality.

An even wider use of precast components is possible. The floor systems,for example, could have been constructed using pretopped hollow-coreslabs or double tees. All that is reallyrequired is imagination, teamwork,and careful planning.

From a design perspective, the performance and the assembly advantageassociated with the development ofyielding precast concrete frame systems is clear. The emphasis to date,insofar as code development is concerned, has been on emulative assemblies where precast concrete is assembled so as to perform as though it werecast-in-place concrete. The codification process must strive to encouragethe yielding approach and avoid therestrictive provisions that dominatecurrent codes.

This design philosophy is consistentwith the performance objectives of thenext generation of seismic codes. Sucha course is particularly important because the structural systems used onthe Paramount apartment building arejust examples of what can be accomplished. Hopefully, they will engendera whole new approach to construction.

Precast building assemblies mustcontinue to be explored, and theseshould logically include bearing wallsystems.’°’11 Contractors and especially precast contractors must be willing to spend the time to explore alternative systems, for the immediatereturn seems attractive and the futurelooks extremely bright.

EPILOGUEIn retrospect, the author finds it hard

to believe that during the span of hisprofessional engineering career, Precast Concrete, as a seismic bracingsystem, has evolved from a prohibitedstructural system to a system of preferred choice. The successful comple

tion of this building, as well as otherstructures currently under construction, attests to this metamorphosis.Clearly, the curved façade andsculpted finish do not fit the traditional stoic image of structural precastconcrete and attest to the fact thatstructural precast concrete has trulycome of age.

CREDITSOwner: Third and Mission Associates,

Inc., Irvine, CaliforniaArchitect of Record: Kwan Henmi,

Architecture and Planning, SanFrancisco, California

Design Architect: Elkus Manfredi,Boston, Massachusetts

Structural Engineers: Robert Englekirk Consulting Structural Engineers, Inc., Los Angeles, California

General Contractor: Pankow Residential Builders II, Ltd., Altadena, California

Precaster: Mid-State Precast, L.P.,Corcoran, California

POST-SCRI PTSince its completion last year,

The Paramount has received manyaccolades from the design community and visitors from around theworld, in June of this year, a juryof peers judging the 2002 PCI Design Awards Program, bestowedupon the structure the Harry H. Edwards industry AdvancementAward. The jury comments wereas follows:

“The successful completion ofthis 39-story precast, prestressedconcrete building brings to fruitionthe culmination of a ten-year research effort in which the bestminds developed an innovativeseismic lateral force resisting system. This structure is a classic example of combining the knowledgeand wisdom learned fromacademia, engineering, architecture and construction in a landmarkproject. The path is now open forothers to apply this technologywith precast/prestressed concretestructures in high seismic areas.”

68 PCI JOURNAL

REFERENCES

1. Englekirk, R. E., “An Innovative Design Solution for PrecastPrestressed Concrete Buildings in High Seismic Zones,” PCIJOURNAL, V. 41, No. 4, July-August 1996, pp. 44-53.

2. Englekirk, R. E., “An Evaluation of the State of the Art in theDesign and Construction of Prefabricated Buildings in Seismically Active Areas of the United States,” Earthquake ResistantReinforced Building Construction, June 1978.

3. Englekirk, R. E., and Selna, L.G., “Criteria for the Design ofPrefabricated Concrete Systems,” ATC Proceedings of aWorkshop on Design of Prefabricated Concrete Buildings forEarthquake Loads, April 27-29, 1981, pp. 639-656.

4. Englekirk, R. E., “Overview of the ATC Seminar on Design ofPrefabricated Concrete Buildings for Earthquake Loads,” PCIJOURNAL, V. 27, No. 1, January-February 1982, pp. 80-96.

5. Cheok, G. S., and Lew, H. S., “Model Precast Concrete Beam-to-Column Connections Subject to Cyclic Loading,” PCIJOURNAL, V. 38, No. 4, July-August 1993, pp. 80-92.

6. ACT Innovative Task Group I and Collaborators, “AcceptanceCriteria for Moment Frames Based on Structural Testing andCommentary,” ACT Proposed Provisional Standard (ACT Ti.1-XX), American Concrete Institute, Farmington Hills, MI, undated.

7. Warcholik, G., and Priestley, M. J. N., “Structural Systems Research Project: High Strength Concrete Joints Tests — Series 3,Report No. TR-98/12,” University of California, San Diego,July 1998.

8. Englekirk, R. E., Seismic Design of Concrete and PrecastConcrete Structures to a Performance Criterion, John Wiley& Sons, Inc., New York, NY, 2003.

9. Nawy, E. G., Prestressed Concrete: A Fundamental Approach,Third Edition, Prentice Hall, Upper Saddle River, NJ, 2000.

10. Rahman, A. M., and Restrepo, J. I., “Earthquake Resistant Precast Concrete Buildings: Seismic Performance of CantileverWalls Prestressed Using Unbonded Tendons,” Civil Engineering Research Report No. 2000-5, University of Canterbury,Christchurch, New Zealand, August 2000.

11. Holden, T. J., “A Comparison of the Seismic Performance ofPrecast Wall Construction: Emulation and Hybrid Approaches,” Civil Engineering Research Report No. 2001-4,University of Canterbury, Christchurch, New Zealand, April2001.

12. ACI Committee 318, “Building Code Requirements for Structural Concrete (ACT 318-99),” American Concrete Institute,Farmington Hills, MI, 1999.

APPENDIX A — NOTATION

a = depth of rectangular stress block= area of concrete= area of prestressed reinforcement in tension zone

= area of non-prestressed tension reinforcement

c = distance from extreme compression fiber toneutral axis

d = distance from extreme compression fiber tocentroid of non-prestressed tension reinforcement

d’ distance from extreme compression fiber incentroid of compression reinforcement

fse = effective stress in prestressed reinforcement (afterallowance for all prestress losses)

f = increment of post-yield stress in prestressing tendonf, = specified yield strength of non-prestressed

reinforcement

= force in reinforcement at nominal strength= extent of reinforcement capable of sustaining

post-yield strainM = nominal moment strength

Tpse = effective prestressing force= tensile yield strength of ductile rod

= post-yield elongation of rod or strando = story drift or beam rotation

= overstrength factor

July-August 2002 69

APPENDIX B — FRAME DESIGN

The design and analysis of the Hybrid and DDC Systemsis fairly simple. Consider the Hybrid test assembly described in Fig. Bi. These Hybrid beams were reinforcedwith nine 1/2 in. (12.7 mm) diameter, 270 ksi (1862 MPa)strands (concentric unbonded post-tensioning) and three No.6 (Grade 60) mild steel reinforcing bars bonded in the topand bottom of the beam. The grouted interface is 16 in. wideand 20 in. deep (406 x 610 mm). The nominal flexuralstrength is developed from the effective force in the strandand the yield strength of the mild reinforcing steel.

The following properties of materials are assumed:

= 1.38 sq in. (890 mm2)

fse = 162 ksi (1117 MPa)

= Apsfse

= 223 kips (1032 kN)

= 1.32 sq in. (852 mm2)

=AL

= 79.2 kips (352 kN)

a=Tpse

0.85f’b

— 2(3172)()— 117.5= 62.7 kips (279 kN)

Two important design objectives are to:1. Provide a restoring force in the post-tensioning strand

that exceeds that which is developed by the mild reinforcingsteel. This will tend to restore the frame to its original preearthquake position (z 0).

> M

1865> 1307 kip-in. (148 kN-m)

2. Furnish a post-tensioning force on the order of 700 to1000 psi (4.83 to 6.90 MPa).

= 223

A 16(20)

= 0.7 ksi (4.83 MPa)

By achieving these two design objectives, it becomes easyto develop an initial frame beam size.

Post-yield strain states in the prestressing strands can alsobe easily checked. Assume that one wants to check thestrand strain state at a subassembly drift of 2 percent.

Start by assuming a neutral axis depth of 5 in. (127 mm).The neutral axis depth must consider the additional strengthprovided by the strand elongation and, depending on the extent of the rotation, the strain hardening in the mild steel.Consider the rotation described in Fig. B2.

Fig. Bi.Interior hybrid

beam testassembly.

2Mj-

o1

223— 0.85(5)(16)= 3.3 in. (84 mm)

= Tse(!— .) + F,,(d — d)

= 223 — + 79.2(16.5)2)

= 1855 + 1307

= 3172 kip-in. (358 kN-m)

The resultant column shear would be:

12’.4

Fig. B2. Seismic induced movement (exaggerated)of precast frame components.

70 PCI JOURNAL

Fig. B4. Post-test conditionof beam-to-columnframe.

O = 0.02 radian

o=o(- _c)

= 0.02(10 — 5)

= 0.1 in. (2.54 mm)

The total strand elongation developed in both beams is 0.2in. (5.1 mm). The increase in strain (r,) is:

2ô5

— PS

0.2

148= 0.00135 in. per in.

The associated increase in the post-tensioning force is:

Af5 =

= 0.00135(28,000)

= 37.8 ksi (261 MPa)

fps,2% = f5€ +=162 + 37.8

200 ksi (1379 MPa)

This stress is significantly less than f, [230 ksi (91586MPa)].

Obviously, space does not permit a full development ofthe design of the Hybrid System, but this will be available ina book which will be published by Wiley8 in early 2003. Inaddition, a design example is contained in Reference 9.

The design of the DDC System is even simpler. Each ductile rod can develop a yield force of 141 Idps (627 kN). Theframe beam used in the Paramount building (see Fig. B3)would be designed as follows:

T= 141N

where N is the number of ductile rods, which in this case, is4 (see Fig. B3).

For the 36 in. (914 mm) deep frame beam of Fig. B3, thenominal flexural strength is:

M = 564 (2.5)= 1410 kip-ft (1912 kN-m)

Since the plastic hinge region occurs within the column,the shear strength of the beam may include the contributionof the concrete because the beam is not damaged by post-yield rotations (see Fig. B4).

The strain state expected in the ductile rods of the beamsof Fig. B3 at a drift of 2 percent would be estimated in thefollowing manner:

0.85ff’ b

— (1.25)(564) — 564

— 0.85(5)(20)

= 1.68 in. (43 mm)

Therefore, the neutral axis must be at a depth of about6 in. (152 mm) in order to yield the compression rods:

örod = O(d — c)

= 0.02(33 — 6)

= 0.54 in. (14 mm)

6rod6p,rod —

0.54

90.06 in. per in.

This strain is well within the strain capabilities of the ductile rod.8

Note that the design of the beam-to-column joints for either the Hybrid or DDC Systems can follow the provisionsin the ACT Building Code.’2

Fig. B3. DDC reinforced beam frame.

Ma T(d_d’)

July-August 2002 71