PCI DESIGN AWARD WINNER

Precast Prestressed ConcreteStructure Provides Solution forGetty Center Tram Guideway

Mark G. Josten, RE.Senior Project ManagerA. T. Curd Constructors

Glendale, California

Wilfred L. Painter, Jr.P. E.Senior Vice PresidentPrecast OperationsA. T. Curd ConstructorsGlendale, CaliforniaThe Niobrara River CompanyRialto, California

James S. Guarre, RE.Vice President

BERGER/ABAM Engineers Inc.Federal Way, Washington

To solve the problem of transporting visitorsfrom an underground parking structure up asteep hillside to the new J. Paul GettyMuseum (near Los Angeles, California), apeople-mover system was devised. Thedesign and construction of the guide waystructure supporting this people-moversystem posed several unique challenges tothe designers and builders of the project.The answers to these challenges came inthe innovative use of precast/prestressedconcrete components combined withpost-tensioning. This article presents thedesign features, precast concrete fabrication,and construction highlights of the project.

precast and prestressed concrete played a prominentrole in the design-construction of a 1080 m (3535 ft)long, curved, U-shaped tram guideway near the city

of Los Angeles, California. This guideway structure supports a people-mover system, which rapidly transports visitors from an underground parking structure next to the SanDiego Freeway to the main plaza outside the new I. PaulGetty Museum. Fig. 1 shows an aerial view of the tramwayproject alongside the San Diego Freeway (Interstate 405).

The Getty Center is a cultural complex dedicated to thevisual arts and humanities. The facility will feature the newJ. Paul Getty Museum, a scholarly research library, scientific laboratories for conservation of cultural property, andheadquarters for the programs and administration of The J.Paul Getty Trust. The Center lies alongside Interstate 405in the Sepulveda Pass between the San Fernando Valley

24 PCI JOURNAL

,

r’

fr.%

-- 4

Fig. 1. The Getty Center Tram guideway winds up the Southern California hillside along the San Diego Freeway, and blends in

beautifully with the site.

and the Los Angeles basin in Los Angeles, California (see Fig. 2).

The new museum will complementthe existing museum at the Villa located on the Pacific Coast Highway inMalibu. The location of the GettyCenter presented the logistical challenge of moving a large number ofpeople from an entry point at an underground parking structure next tothe San Diego Freeway to the arrivalplaza of the Getty Center and to thenew museum atop an adjacent hillsideoverlooking the Los Angeles basin.

The site has limited access — onlyone service road and a fire road withextremely steep grades. The site isalso located in a secluded, quiet residential area of West Los Angeles. Thisresulted in restrictions on construction

activities and on people-mover systemconfigurations. With these problemsconfronting them, the Getty Trust decided that a people-mover systemwould be needed to shuttle Center visitors from the parking garage to themuseum on a dedicated guideway.

SYSTEM PROCUREMENT

After a competition among vehiclesystem suppliers that included pricing,guideway aesthetics, vehicle and trainappearance, pollution-free operatingconditions, and low noise, Otis TransitSystems Inc. of Farmington, Connecticut, was chosen as the vehicle systemsupplier. The Otis system uses cable-driven, air-cushion supported vehicles.

The Otis contract was a design-build

contract with the project contractor,Dinwiddie Construction Company(DCCo). Otis was to provide the vehicles, subsystems, and guideway designto DCCo; DCCo would then subcontract the guideway construction to aselect shortlist of qualified bidders.With the added responsibility ofguideway design, Otis contracted withBERGER/ABAM Engineers Inc. ofFederal Way, Washington, to designthe guideway structure.

THE PEOPLE-MOVERSYSTEM

The tram system guideway is singlelane with a double-lane off-center bypass section. Two separate trains willoperate in a shuttle configuration on

May-June 1995 25

the guideway. Each train will be attached to a drive cable supportedalong the length of the guideway by asupport sheave network. In turn, eachcable will be driven by an elevator-type geared drive system located in amachine room outside and under theMuseum Station. As illustrated in Fig.3, a cable tension device (tensionweight) is located at the entry station

just outside of the station area.The shuttle operation will be con

trolled by computerized equipment located in a control room next to the machine room. A schematic drawing ofthe cable drive system is shown in Fig.3. Each train will be 27.4 m (90 ft)long and will consist of three, two-module vehicles, each with a length of8.8 m (29 ft). Vehicles will be 2.24 m

(7 ft 4 in.) wide and 2.90 m (9 ft 6 in.)high. Vehicle capacity will be 14seated and 17 standing per vehicle or93 per three-vehicle train.

THE GUIDEWAY DESIGN

Site Conditions

Site conditions imposed uniquechallenges on the guideway configuration and design. The Getty Trust dictated that guideway construction notscar the natural vegetation on the hillside. It was also important to positionthe guideway parallel to the accessroad that wound up the hillside to facilitate construction and minimize thevisual impact of the guideway onviews from the hillside on the oppositeside of the San Diego Freeway. Theseconditions produced a guideway alignment consisting of nine curves, withhorizontal radii of curvature varyingfrom 38 to 305 m (125 to 1000 ft), thatfollows a profile with an average vertical grade of 7.2 percent (see Fig. 4).

The total length of the tramway is1080 m (3535 ft). The rise in elevationfrom the parking garage station to themuseum at the station is 64 m (210 ft).The aerial structure is 800 m (2580 ft)long flanked by either at-grade or pile-supported grade beam sections approaching the two passenger stations.

This 800 m (2580 ft) long structureis divided into 9 four- to six-span continuous units. The average span lengthis 20 m (63 ft); maximum span lengthis 26 m (86 ft). At some locations, theguideway is supported on columnsover 13 m (42 ft) tall. At other locations, the guideway superstructure issupported directly on drilled pier capswithout columns.

Vehicle System Interfaces

The guideway supports the vehicleon a smooth “flying” surface and supports the guide rail, sheave supportsfor the drive cable, power rails, andcontrol subsystems. A U-shapedguideway with a horizontal bottomslab and vertical sidewalls was required to interface with the Otis vehicle system. Supports for a guidebeamfor maintaining the lateral position oftrains on the guideway, brackets tosupport the numerous rope sheaves re

Miles

0 4

North

Tension Weight

Fig. 3. Schematic of the cable drive system.

26 PCI JOURNAL

Fig. 5. The guideway/vehicle system interfaces.

quired to guide the propulsion cablearound numerous turns, and the powerrail for supplying electric power to theoperating vehicles are secured to theguideway sidewalls and supportingslab (see Fig. 5).

Preliminary Design

The guideway designer’s initial effort was to develop alternative guide-way cross sections that would meetproject design criteria. Alternate guide-

way cross sections included those sections shown in Fig. 6.

Alternative A — Precast concretedouble tees spanning column to column. The stems of these double teeswould be straight; the flanges of thedouble tees would be curved to makecurved guideway sections. The side-walls and flying surface would be cast-in-place concrete. Multiple double teeswould be used in the bypass area.

Alternative B — Cast-in-place U-section. This section would use the

necessary elements to interface withthe Otis vehicle system as structuralelements. Added structure beneath thesupporting slab would generally beeliminated. In the bypass area, a structure to stiffen the cross section wouldbe added beneath the supporting slab.

Alternative C — Precast concreteL-beams spanning from column tocolumn with a cast-in-place connecting slab to form the above U-section.Precast concrete stem beams extending below the supporting slab wouldbe used in the bypass area. Precastconcrete beam components would bedesigned to support the weight of theconnecting slab and associated form-work and construction loads withoutauxiliary supports. This alternativeminimized the amount of cast-in-placeshoring and minimized the pickweights of precast concrete elements.

Alternative C was chosen by theGetty Trust, the project architect, andthe project contractor because of cost,aesthetics, and constructability factors.

Design Criteria

The site for the guideway waswithin the jurisdiction of the City ofLos Angeles, so the Los Angeles Department of Building and Safety wasthe approving authority. Early discussions with the Department of Buildingand Safety indicated that the Department would be comparing the designfor the guideway with the Los AngelesUniform Building Code (LAUBC).The concerns of the Department centered on the seismic design of theguideway structure.

This review process presented adilemma to the design team becausethe guideway structure was more likea bridge than a building. The LAUBCgrouped this type of structure into thecategory of “Other” structures. Guide-way design engineers thought it wasessential to introduce the latest research regarding bridge seismic behavior and safety into the design ofthis structure. Therefore, prior to beginning the final design process, aGetty Center People-Mover GuidewayStructural Design Specification wassubmitted to the Department for approval as an acceptable analysis!design method. This specification

(North

Road

Fig. 4. The Getty Center site plan.

Guiderail Support

Sheave Support

Rope Sheaves

Concrete Beam or Pavement

Power Rail Post

AirCushion

May-June 1995 27

combined provisions of the LAUBCand California Department of Transportation (CALTRANS) Bridge Design Specification.

Seismic design provisions of theStructural Design Specification statedthe following:• Internal seismic forces would be

computed using response spectrumanalysis methods.

• A site-specific response spectrawould be used with a return periodof 1000 years. A two-thirds verticalresponse would also be imposed onthe structure.

• Design methods of the CALTRANSBridge Design Specification wouldbe used for reinforced and precastconcrete components. The CAL-TRANS methods for seismic designof reinforced concrete componentsuse ultimate loads and ultimatecomponent design capacities.

• A minimum design base shear forcewould also be imposed on guidewaycolumns. This minimum force wouldbe computed per the LAUBC, basedon rules for “Other” structures inthe code.

• Effects of vehicle live load would bescaled from results of response spectrum analysis with only dead loadmass to account for the added mass.This was done because the vehiclewill be “secured” to the guideway.

Superstructure Design

Fig. 6. Alternate cross sections considered in designing the guideway structure.

The single-lane tram guideway superstructure was made from two Lshaped precast, prestressed concreteweb beams connected with a cast-in-place slab. Web beams were designedto support the weight of the slab andits formwork. Web beams were prestressed at the plant to support theirown weight and simple-span appliedloads.

The guideway design specified thatW30 beams would be used to tie theprecast concrete web beams togetherduring casting of the connecting slab.Spans were later post-tensioned intofour- to six-span continuous structureunits.

Flanged rectangular precast concretestem beams, extending below the slab,were used to stiffen the superstructurein the bypass area. One or two stem

beams were used, depending on theoverall structure width. Typically, thesingle-lane guideway is 4.62 m (15 ft2 in.) wide and 1.17 m (3 ft 10 in.) high.The guideway widens to 6.20 m (20 ft4 in.) in the bypass area (see Fig. 7).

Spliced girder construction wasused in the design to extend the spanlength of the 1.17 m (3 ft 10 in.) deepchannel section to 26 m (86 ft) over aslide area along the tram alignment.Two adjacent beam elements wereerected on a shore tower, joined witha cast-in-place closure pour. The elements were then post-tensioned in thefield to make up the 26 m (86 ft) span.The shore tower was removed afterthe span was integrated into a continuous span unit.

Three other long bypass spans weredesigned to use a touch shoring sys

tern to transfer dead load from thesimple-span, precast concrete elements to continuous span compositebeams. Full-span-length precast concrete elements were erected; shoreswere installed to just contact the bottom of these elements. Decks and closure pours between elements werecast. Touch shores were removed afterthe spans were integrated into continuous span units.

Inserts to support the vehicle guideposts and supports for the ropesheaves were cast directly into precast concrete L-beams. Over 10,000threaded inserts were to be cast intothese precast concrete components to atolerance of 3 mm (‘/8 in.). The designrequired formwork adjustments to offset predicted deflections and cambersas small as 12 mm (1/2 in.).

Cast-In-PlaceFlying Surface Slab

/

Cast-In-PlaceSidewall

Alternative A

Cast-In-PlaceFlying Surface Slab Cast-In-Place

Beam

Alternative B

Cast-In-PlaceFlying Surface Slab

Alternative C

28 PCI JOURNAL

Substructure Design

The guideway substructure generally consists of 1.07 m (3 ft 6 in.)diameter round columns supported on1.22 m (4 ft) diameter shafts. Thecolumns support a cross-head elementto absorb shear forces from precastconcrete web beam superstructure elements (see Fig. 8). The shaft and column use a common reinforcing steelcage to achieve ductility requirementswith a minimum amount of reinforcingsteel. Foundation shafts are generallydesigned to resist horizontal shear andmoment forces associated with hinging(yielding) of the base column section.

In the bypass area, dual columnsand shafts are used. These columns areconnected with a cross-head elementto transfer shears from precast concrete L and stem beams to supportingcolumns. Careful attention was paidduring the design phase to the ductilityrequirements of the joints betweencross-heads and columns.

In areas where the guideway is supported close to the ground withoutcolumns, multiple shafts are used. Shaftmoments and shears were designed toresist the maximum elastic seismicshears and moments from responsespectrum analysis. The guideway superstructure is connected to the substructure through bearings and large steel barlugs that were cast directly into thecross-head elements (see Fig. 9).

The completed tram. guidewaystructure survived the January 17,1994, 6.8 magnitude Northridge earthquake without damage (see Fig. 10).During the earthquake, measured horizontal accelerations within 20 miles(32 km) of the epicenter ranged from0.24g to 1.82g. The Getty site is 10miles (16 km) from the epicenter ofthe earthquake.

THE GUIDEWAYCONSTRUCTION

Initial guideway construction documents were issued to a select shortlistof qualified bidders in the summer of1992. Final guideway constructiondocuments were bid in September andOctober 1992. A. T. Curd Constructors of Glendale, California, was selected to be the guideway constructionsubcontractor.

Fig. 7. Cross section of the superstructure.

Site Logistics and Planning

The site conditions were the majorfactor driving construction sequencesand methods. The guideway alignmentroughly follows the contours of asteep hillside, passing over several environmentally sensitive areas thatcould not be disrupted by constructionactivities. While the vertical gradientof the guideway along its final path oftravel is approximately a constant 7percent, the slopes and cross slopes ofthe hillside at the foundations are assteep as 1:1 in some locations. Creating access for drilling 1.22 m (48 in.)diameter-deep caissons and erectinglarge precast concrete elements was ofprimary importance.

A series of narrow access roads wasplanned to provide access for caissondrilling. These roads were limited toareas where partial canyon fills werealready planned; hence, regrading andrevegetation after construction was afeasible alternative. It was not possible, however, to construct a road wideenough to allow a suitable erectioncrane to travel around cross-heads thattopped support columns. Several sections of the alignment were over sensitive areas that could not be disturbedwith access roads, so all constructionhad to be reached over the top withlarge cranes.

Erection of precast concrete ele

ments from the nearby parallel permanent roadway had several drawbacks.This road was the only access to thetop of the hill where the main buildingconstruction was underway. Constant,heavy construction traffic needed to bemaintained almost the entire day. Onlybrief time windows were available toblock this road with precast componenterection activities. Also, the size andnumber of erection cranes requiredwould be significantly larger if erec

Cast-In-PlaceFlying Surface Slab”\

Precast Webs

Cast-In-Place Slab

Single-Lane Section

Cast-in-PlaceFlying Swlace Slab

Precast Webs

Cast-In-Place Slab

Bypass-Area Section

Fig. 8. Typical guideway substructure.

May-June 1995 29

Fig. 10. Nine horizontal curves make up the guidewayalignment. The structure survived the nearby Northridgeearthquake (January 17, 1994) without damage.

tion took place from the main road.The key to solving the access prob

lem was the guideway contractor’s decision to convert the original cast-in-place cross-heads to precast concreteelements. The guideway contractor requested that the designer modify thecross-head design to provide for precast concrete cross-head elements instead of cast-in-place cross-head elements. This allowed the contractor touse a narrow access road alongside thesupport columns for the erection crane(see Fig. 11). After the crane hadpassed by the support column, thecross-heads were erected just beforethe beams (see Fig. 12).

The guideway was divided into fivegeneral areas for overall constructionsequencing in the following order:

1. The first 16 spans of elevatedguideway at the uphill end

2. The last 11 spans of elevatedguideway at the downhill end

3. The 14 spans of elevated guide-way in the bypass area

4. The on-grade guideway adjacentto the entry station

5. The on-grade guideway between

the south abutment and the museumstation

Foundation andColumn Construction

Work at the site began with construction of the temporary accessroads. These roads were generally aseries of short spurs off of the mainroad, in most areas only wide enoughfor crane travel without outriggers.Special outrigger pads were created onthe slopes at the points where thecrane needed to set up. A few of theroad areas required shoring to create alarge enough flat area. Several sections of the access roads had 20 percent gradients.

Deep caisson drilling followed theroad construction. Drilling was accomplished with conventional earth augersand rock cutting buckets. Drilling wasmuch more difficult than originally anticipated. There was significant timespent coring through fractured rock layers. The bottoms of all of the caissonswere cleaned out by hand. This required long continuous casings to be

installed temporarily in each hole during cleanout operations. These casingshad to be transported up and down theaccess roads during drilling.

Five caisson locations could be accessed only by foot (see Fig. 13).Drilling with conventional equipmentwas impossible. Instead of hand excavating these shafts, the contractor useda special long reach drilling apparatus.A small remote-operated diesel augerwas suspended over the side of the hillfrom a large truck crane located on theaccess road directly uphill from thedrilling location.

Continuous spiral-tied reinforcingcages were installed in the cleanedholes (see Fig. 14). The cages werecontinuous from the bottom of thehole to top of the cross-head. Caissondepths ranged up to approximately24 m (80 ft) and column heights varied up to 13 m (42 ft). Some of the resulting reinforcing cages were nearly30 m (100 ft) long. All of the caissoncages were fabricated off-site due tospace restrictions on site.

A critical aspect of the cage installation was orientation of the vertical re

Fig. 9. Multiple steel lugs and bearings were required totransfer earthquake shear forces from the guidewaysuperstructure to this pier.

30 PCI JOURNAL

inforcing bar pattern. Because thecage was continuous to the top of thecross-head, the reinforcing bar protruding above the top of the caissonshad to be in the correct pattern. The

pattern had to be in the correct horizontal location and at the correct azimuth orientation; the top of the reinforcing bar, as much as 13 m (42 ft)above ground level, had to be at the

correct elevation. The tolerances weremuch tighter than normal for drilledcaisson construction, so the contractorused special double templates for fabrication and installation of the caisson/column cages.

The columns were cast in placeusing 1.07 m (42 in.) diameter steelforms. A special double reinforcingbar template was used on the top ofeach column form to ensure accuratealignment of each vertical dowel passing into the precast concrete cross-heads (see Fig. 15).

The typical reinforcing bar patternincluded 16 vertical #9, #10, or #11bars. These dowels matched up with aset of vertical corrugated sleeves castinto the cross-heads. After erectionand alignment of the cross-head, thesesleeves were filled with a highstrength grout. At the bypass areas,larger cross-heads were supported bytwo columns. These had to be alignedas a matched set when the columnswere poured because they shared acommon cross-head piece.

At some locations, the terrain elevation matches the elevation of the bottom of the guideway (see Fig. 16). Inthese locations, precast guidewaybeams were supported directly oncast-in-place pile caps at grade level.These pile caps were constructed concurrently with the cast-in-placecolumns in each area.

Cross-head andSuperstructure Construction

A significant contribution was a full-scale mock-up that was constructed inthe precast manufacturing yard prior toproduction. The mock-up verified architectural details and constructionmeans and methods (see Fig. 17).

Precast concrete erection followedthe columns. Most of the erection wasaccomplished with a single 160-t(180 ton) conventional truck crane.Several picks, however, required a225-t (250 ton) crane or both of thecranes working together. Cross-headswere erected on the station ahead, reinforcing bar sleeves were grouted,and web and stem beams followed onthe cross-heads.

Special lifting frames were requiredfor the beam elements to prevent them

.,“-, 7.

-

Fig. 11. The erection crane was able to back out a narrower access road withoutcross-heads atop the columns. A precast concrete cross-head is being rigged forerection.

May-June 1995 31

from rolling over. The lengths andcurvatures of many of the elements resulted in the center of gravity of thecomponent being located outside thecenter of gravity of the available lifting points. The lifting frames allowedthe crane connection point to be offsetfrom the cross section of the beam.The frames were configured to automatically compensate for any of thecurvature conditions encounteredwithout readjustment (see Fig. 18).

Special heavy diagonal braces andlateral clamps were also fabricated to

transmit the torsional force from theprecast concrete beam end to the cross-head. After two parallel webs wereerected, they were tied together with0.76 m (30 in.) deep, wide flange beamsin the transverse direction. These largebeams served to hold the web pairs incorrect horizontal and vertical alignment with each other (see Fig. 19).After installation of the web beams, diagonal braces could then be removedand used again on the next span.

Other special erection and bracinghardware was devised by the contrac

tor to keep the beams from movingdownhill in the longitudinal directionprior to connection to the permanentbearings and steel lugs through cast-in-place closures. Temporary verticalbearing pads and support shelves werealso required because the precast concrete webs and stems had minimalphysical overlap above the top of thecross-heads. The ends of the precastconcrete sections were joined over thecross-heads by cast-in-place closures;closures included the permanent slideand fixed bearings and lateral restrainer lugs (see Figs. 20 and 21).

The formwork for the cast-in-placedeck was supported by hangers attached to the precast concrete beam elements. After the decks were placed,groups of individual spans were post-tensioned together to form a series ofmultispan bridge structures (see Fig.22). The on-grade guideway has across section matching the elevatedguideway; however, the sidewalls arecast-in-place concrete rather than precast concrete.

Flying Surface Construction

The entire guideway, including boththe precast and cast-in-place concreteelements, is topped by a superfiat concrete flying surface upon which theair-supported vehicle glides. The tolerances for smoothness and flatness forthis surface are critical because theyaffect the vehicle ride quality andwear life of the air-cushion pads.

There was considerable concernabout the ability to achieve high-tolerance flatwork at such a demandingsite, where concrete was placed on a 7percent grade, and where the geometrically correct shape of the flying surface is a helix, not a flat plane, in all ofthe curves. There was also some difficulty in defining the proper tolerancesand measurement methods to achievethe desired surface. The contractor, together with the vehicle system supplierand a superflat floor consultant, tookas-built measurements from an existinginstallation, analyzed the data, and devised a measuring scheme and tolerances to be used to control the qualityof the flying surface.

Concrete placement on the flyingsurface was successful due to a number

Fig. 13. Some column locations were only accessible by foot.

Fig. 14. Reinforcing steel cages for drilled shaft foundations and columns were

over 30 m (lOOft) long.

32 PCI JOURNAL

of special quality control measures.These measures included tight controlof the concrete mix proportions andtiming of deliveries, high precisionformwork measuring and levelingtools, special wet finishing techniques,prompt and thorough wet curing, andimmediate measurement and reportingof results so daily corrections could bemade (see Figs. 23 and 24).

PRECAST CONCRETEFABRICATION

The structural solution to the designchallenges was largely due to the efficient use of precast concrete components. In total, 134 components wereused as follows:• L-shaped web beams• Flanged rectangular stem beams• Single-lane cross-heads• Bypass single column cross-heads• Bypass dual column cross-heads

Table 1 shows the shape, numberand principal dimensions of the precast concrete components.

Precast Concrete Cross-heads

As previously noted, a key conceptin solving this project’s constructiondifficulties was casting the precastconcrete cross-heads. Typical precastcross-heads were 4.67 m (15 ft 4 in.)wide and weighed 140 kN (31,000ibs); bypass cross-heads measured upto 7.82 m (25 ft 8 in.) and weighed350 kN (80,000 lbs). Forming cross-heads was straightforward, using standard steel forms similar to those usedfor cast-in-place cross-heads on top ofthe columns. Tying the reinforcingbars was more complicated becausethe ductility requirements called for aseries of closed, interlocking, progressively tapered welded loops requiringa systematic placement sequence thattook a few tries to figure out.

The real challenge, however, andthe key to the successful use of theprecast concrete cross-heads was thetemplating system devised to ensurethat they would meet with a cast-in-place column in the field. The connection detail was designed to fit thecross-head over the extended columnvertical steel, 16 bars in a 0.9 m (3 ft)diameter circle.

Fig. 15. Precise jigs were used to ensure that reinforcing steel protruding from thetops of cast-in-place concrete columns aligned properly with mating sleeves inprecast concrete cross-heads.

Fig. 16. At some locations, the guideway profile was nearly at ground level.At these locations, the guideway superstructure was supported directly oncast-in-place pile caps.

May-June 1995 33

Table 1. Breakdown of precast and prestressed concrete components used in tram guideway.

L-shaped precast prestressedweb beams: 84 componentsMinimum length — 13.9 m (45 ft 6 in.)Maximum length —22.1 m (72 ft 6 in.)

Flanged rectangular stem beams:19 componentsMinimum length — 13.9 iii (45 ft 6 in.)

Maximum length— 21.5 rn (70 ft 6 in.)

Single-lane cross-heads:19 componentsWidth of cross-head — 1.5 m (5 ft)

Bypass single-columncross-heads: 7 componentsWidth of cross-head — 1.5 m (5 ft)

Bypass dual-column cross-heads: 5 componentsWidth of cross-head — 1.5 m (5 ft)

Varies 22-2 to 25-8

Poet or Stem Beam

2-4”

15-4”

c9

9C.,

Varies 15-4” to 19-8-1/2”

Ii

j

Pocket for Stem Beam

34 PCI JOURNAL

In order to clear cross-head reinforcing, sleeves were limited to 40 mm(1.5 in.) diameter corrugated tubes for#11 bars, leaving very little clearanceper sleeve. The fabricator of the miscellaneous metal components deviseda precise jig that remained in his shop.All templates for the project, bothfield and shop, were made against thatmaster template to guard against progressive creep in dimensions.

For the cross-heads, a template inthe form’s baseplate matched the inside diameter of the corrugatedsleeves. A similar template at the top,with “handles” that would support itfrom the form sides, aligned thesleeves vertically and in plan. PVCpipes inside the corrugated sleevesacted as stiffeners. In the field, allcross-heads fit and aligned correctly.

Precast Concrete Weband Stem Beams

The L-shaped precast concrete beamsections, or webs, ranged from 11.6 to22.0 m (38 to 72 ft 4 in.) long, withcorresponding weights of 132 to 250kN (29,500 to 55,800 lbs). Geometrywas complex, as even an 11.6 m (38ft) beam might include a tangent, spiral, and constant radius alignment. Radiuses were 64, 69, 75, and 305 m(210, 225, 245, and 1000 ft) each withspiral transitions into and out of thetangent or straight sections.

The necessary alignment came froma profile grade line (PGL) at the centerof the flying surface. In two locations,the PGL also required a vertical curve(VC). The design allowed the webs to“chord” between cross-heads and compensate for the VC with the cast-in-place flying surface. The embedments,however, needed to follow the PGL,which meant that at these two locations, embedments were not a constantheight relative to the web section.

Web form layout was clearly amajor challenge. The entire structurewas drawn in three dimensions usingAutoCAD 12.0. The individual beamsections were cut out on the computer,and rotated into a horizontal, castingposition, which resulted in a twistedshape relative to the beam centerpoint.Horizontal and vertical offsets werethen calculated from control lines that

could be used in the plant for formlayout and control.

The forms had to be able to twist tomatch the required shape. The formpallet was built with 0.6 m (2 ft) oncenter back-to-back channels 2.4 m (8ft) long resting on screw jacks set intoa longitudinal track (see Fig. 9). Tomake a flexible base over the doublechannels, flat 0.6 x 1.8 m (2 x 6 ft)timber sleepers with a 19 mm (3/4 in.)

plywood sub-base and 13 mm (‘/2 in.)high density overlay (HDO) plywoodwere laid down for the casting surface.

Setting the table was a two-step process. First, each double channel was“aimed” at the center of curvature bysliding the screw jacks perpendicularto the channels. Thus, for example atthe outside of a curve, the center-to-center dimension might be 620 mm(2.034 ft) and the inside 595 mm

I

Fig. 17. Prior to production, a full-scale mock-up was constructed at the precastmanufacturing yard.

Fig. 18. Special lifting frames were fabricated to allow picking outside the center ofthe web beam cross section.

May-June 1995 35

Beam sides were controlled bybuilding a series of 90-degree framesof glued and screwed plywood over0.6 x 1.8 m (2 x 6 ft) timber (see Fig.26). Side forms, like the pallet, wereflat 0.6 x 1.8 m (2 x 6 ft) plankssheathed with 19 and 13 mm (3/4 and‘/2 in.) HDO plywood. Again, this created a flexible form that would not deflect between supports but was limberenough to follow curves and spirals accurately.

The 90-degree frames were bolteddown to the double channels so that thebeam sides were always perpendicularto the twisting pallet below. A castingsequence was developed in a databaseof all components, sorted by curves andlengths, to minimize minor and majorform reconfigurations. Templates forembedments consisted of plates withthe exact, tight eight-hole pattern setthrough oversize holes in the formsides, so complete embedment assemblies could be shifted to cope with thelayout differences caused by the horizontal and vertical curves.

Two forms were built. The typicalproduction cycle started at 3 a.m. witha small stripping crew, pulling formsand yarding a beam into the storagearea. There was sufficient mild reinforcing steel in the design to allowstripping and handling without prestressing. Meanwhile, the form wascleaned and oiled, and ready for theform setting and reinforcing bar crewsat 6 a.m. At the same time, the stripping crew went over to the other form,and became the concrete placing crew.They would be “poured out” and readyto go home by lunchtime.

The cleaned form pallet would bereconfigured if necessary, then the reinforcing bar crew would set the pretied cage, the post-tensioning ducts,and the stressing heads where applicable. They would then pre-tie the following day’s cages. The form crewthen buttoned-up the forms, locatedembedments, and checked details before the end of the shift. The formwould thus be ready for concreteplacement the next morning.

After any necessary dry finish tookplace, the beams were sent to the post-tensioning area. Every Monday, thepost-tensioning subcontractor did thefirst stage, simple-span post-tension-

Fig. 19. W30 steel sections were used to tie the web beams securely together aftererection.

(1.952 ft). Screw jacks were then adjusted vertically so that one end mightbe plus 4 mm (0.013 ft) and the otherminus 4 mm (0.013 ft). Each shopdrawing had these dimensions calcu

lated in tabular form, so that layoutcould be done with a simple tape andlevel approach. This combination ofradial and vertical adjustment of thebeams set the correct warp to the pallet.

36 PCI JOURNAL

concrete cross-heads.

ing and grouting, in the yard. Thisschedule allowed the weekend cureeven on Friday’s casting, met earlystrength requirements without overlyaggressive mix designs and heatingsystems, and gave the post-tensioningsubcontractor at least five beams for afull day’s work cycle. The balance ofthe prestressing was done in the field.

The small “T” beams that supportedthe deck at the wide, bypass locationswere dubbed “stems.” Although ofcomplex shape, as these poured into

the deck, the tolerances were less challenging than the webs. A similar bedsystem was used (see Fig. 25).

In all, over 500 shop drawings wereprepared to detail the 134 precast concrete components. Putting the effortinto details, especially into a simplified layout system, paid dividends inthe production cycle and achievedgood dimensional tolerances. No inserts cast directly into the precast concrete units had to be reworked becauseof out-of-tolerance positioning.

CONCLUDING REMARKSThe J. Paul Getty Trust is construct

ing a world-class art facility and is demanding the highest quality in designand construction. The tramway ridequality, passenger comfort, and durability were important requirementsthat necessitated the complex designand close tolerances.

The construction of the Getty Center guideway was a model of cooperation between owner, designers, andbuilders. Direct lines of communication were established early to facilitate a response to questions and toensure that design intent was beingfollowed and that guideway requirements for the vehicle system werebeing achieved.

Use of precast, prestressed concretemade the construction feasible andcost effective. The resulting finishedproduct is a graceful and dramaticstructure suspended along the hillsidethat will provide an exciting, welcoming experience for visitors to theJ. Paul Getty Museum.

The project won an award in the1994 PCI Design Awards Program.The citation of the jury read: “Thisproject really fits the site and environmental conditions, and through the innovative use of precast/prestressedconcrete elements provides an extremely efficient solution to a difficulttransportation problem.”

It is expected that the people-movertramway system (see Fig. 27) will beoperational by the end of 1995.

w

Fig. 21. Stem beams in the bypass area were set into notches cast into the precast

__._JFig. 22. After precast concrete beamswere erected, closure pours were castto make the precast units into four- tosix-span continuous beam units.

Reinforcing steel placed in the flying surface at the Entry

May-June 1995 37

Fig. 25. Two stem beams in the storage yard are ready fortransport to the site. Precast concrete beams were post-tensioned in the yard to carry their own weight andconstruction loads.

Fig. 26. - b beam forms were made from plywood securedto a series of 90-degree timber frames. Side forms were flatplanks sheathed with plywood, thus creating a flexible yetnot too rigid form.

38 PCI JOURNAL

CREDITS

Owner: The J. Paul Getty Trust, SantaMonica, California

Getty Center Project Contractor: Dinwiddie Construction Company, LosAngeles, California

People-Mover System Contractor:Otis Transit Systems, Inc., Farming-ton, Connecticut

Tram Guideway Contractor: A. T.Curd Constructors, Inc., Glendale,California

Guideway Designer: BERGERJABAMEngineers Inc., Federal Way,Washington

Aerial Photography: Warren AerialPhotography Inc.

Getty Center Project Architect:Richard Meier & Partners Architects, Los Angeles, California

Precast Concrete Manufacturer: TheNiobrara River Company, Rialto,California

Other Photography: Construction Documentation Services, Inc.

May-June 1995 39

Fig. 27. Panoramic view of Getty Center tram guideway project nearing completion.