Design-Construction Feature

Segmental Design ofThe Harbour Island

People Mover

Roy L. Eriksson*Project EngineerLEAP Associates International, Inc.Tampa, Florida

Stephen ZendeguiProject Manager

LEAP Associates International, Inc.Tampa, Florida

The Harbour Island People Mover isan elevated system (see Fig. 1)

which links the mainland of downtownTampa with Harbour Island, a billiondollar commercial development. ThePeople Mover is actually an automatedvehicle which transports pedestrians toand from the development. Visitors tothe island can park in the multilevel ga-rage at the mainland terminal (see Fig.2) and conveniently ride this "horizontalelevator" to Harbour Island.

The $2.8 million contract for the proj-ect was awarded on a design/build fast-

'Currently, Vice President, Library of Engineering Ap-plications Programs, Inc.

track basis to Misener Marine Con-struction, Inc. of Tampa. After estab-lishing the shape and design param-eters for the guideway structure, Mise-ner subcontracted with LEAP Asso-ciates International, Inc. to design andproduce the shop drawings for the pre-cast prestressed portion of the project.The fabrication of the prestressed ele-ments was subcontracted to Florida Pre-stressed Concrete and the vehicle oper-ational system was designed andsupplied by Otis Elevator Company,Transportation Technology Division.

The scope of work performed byLEAP was divided into two parts: a seg-mental portion and the balance of the

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!A1

a

Describes the design and construction features ofthe precast prestressed segmental span of theHarbour Island People Mover, part of a billion dollarcommercial development project in Tampa, Florida.

PCI JOURNALJJuly-August 1985 39

Fig. 1. Segmental span of the Harbour Island People Mover which links the mainland ofdowntown Tampa with Harbour Island, a billion dollar commercial development.

project. The two-piece segmental por-tion is the center span of a three-spancontinuous guideway which spans theCrosstown Expressway (see Fig. 3). Thebalance of the project, which actuallyconstitutes the bulk of the project, con-sists of a guideway supported by singleprecast prestressed concrete box beamsgrouped into units made continuous forlive loads.

The focus of this article is on theanalysis and design of the precast seg-mental portion of the project. It was thispart that proved to he the most interest-ing and challenging part with regard toLEAPS involvement in the project.

First, the basic layout of the entiresystem will be discussed includingsome of the reasons for selecting a seg-mental design. Then, the highlights ofthe analysis and design techniques willbe discussed which include the loadsconsidered, the analysis tools used, andthe actual design methodology. Someunique aspects involved in the produc-tion of the precast prestressed compo-nents also will he described. Finally, afew of the special problems encounteredduring construction, to which interest-ing solutions were found, will be pre-sented.

Structural SystemThe main structural system of the

people mover consists of a concreteguideway composed of precast pre-stressed voided box beams supported bycast-in-place concrete piers founded ondrilled shafts. A total of 34 spans wereused which were grouped into 12 unitsof one, two, three, and four spans whichare continuous for live load. The totallength of the project is roughly 2500 ft(762 m).

The spans of the system are basicallylinear with the stationing running adja-cent to and along the centerline ofFranklin Street (see Fig. 3). Two groupsof two cable-driven passenger carstraverse the system on cushions of air,each group traveling in the oppositedirection with respect to the other.

Most of the guideway of the system isa single track. However, at the bypassregion (see Fig. 3), where the cars passeach other enroute to their respectiveterminus points, there is a double lane.This region is characterized by a doublebox beam arrangement whereas theother areas have only one beam perspan.

A section taken near midspan of thesegmental span revealing the compo-

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Fig. 2. The Fort Brooke parking garage is at the mainland terminus of the People Mover.

PCI JOURNALJJuly-August 1985 41

nent parts of the system is shown in Fig.4. The four basic parts are the box beam,deck, flying surface, and rails. For pur-poses of comparison, a typical sectionthrough a nonsegmental part of the proj-ect is shown in Fig, 5. Note that thesidewalls of the segmental box werethickened to accommodate the post-tensioning ducts.

The 48 x 54 in. (1219 x 1372 mm)voided box beam has end regions whichare solid for approximately 5 ft 7 in. (1.71m) at each end. Two intermediate dia-phragms, located at approximately thirdpoints, were used in each of the 70-ft(21.3 m) segments which make up thesegmental span. The thicknesses of theend regions and the intermediate dia-

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Fig. 3. Sketch showing major areas of the project and the locations of the mainlandmarks in the vicinity.

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Fig. 4. Section through midspan of the segmental portion reveals the post-tensioningducts and prestressed strands.

phragrns were controlled by the shapeand placement of styrofoam voids (seeFig. 6).

A 28-day concrete strength of 5000 psi(34.5 MPa) was required in the design ofthe boxes. However, cylinder testsshowed strengths consistently wellabove this value throughout the project.

The deck is 6 in. (152 mm) thickdirectly above the box (plus a build-upof varying thickness) and tapers to about5 in. (127 mm) at the outer edges. Thenominal width of the deck is 12 ft (3.66m) and is centered on the vertical axisthrough the centroid of the box. Smallkeyways were formed near the edges onthe upper surface of the deck where therails are joined. The deck as well as theother cast-in-place components were

designed with 5000 psi (34.5 MPa) con-crete.

The rails, located at each extremeedge of the deck, have rectangular crosssections measuring 5 in. x 2 ft 10 in.(127 mm x 0.86 m). The flying surfacemeasures 8 ft x 4 in. (2.44 m x 102 mm)and, as with the deck, is centered on thebox beam.

Why Segmental Design?Length was one of the main design

constraints which affected the three op-tions considered for the span over theCrosstown Expressway. The Express-way Authority prohibited the placementof a permanent bent between the road-ways of the expressway. This meant that

PC JOURNAL/July-August 1985 43

Fig. 5. Section through nonsegmental portion shows thinner side walls in these spans.

the span length had to be a minimum ofabout 140 ft (42.7 m),

The first of the possibilities consid-ered was a steel box section. Designinga steel section to span 140 ft (42.7 m)would have been possible. However,since a high premium was placed on ar-chitectural impressions, this option wasquickly ruled out since it would nothave blended very well with the con-crete sections used in the balance of theproject.

Also considered was the same one-piece design used on the rest of theproject. Parallel strand patterns wereused in these cases, but this was notpossible for this long of a span because asuitable strand pattern could not befound to satisfy both service conditionsand ultimate strength requirements

without changing to a larger box crosssection. Moreover, it would have beenvery difficult to transport and erect asingle 140-ft (42.7 m) girder.

The solution adopted, which wassuggested by LEAP, was to use a post-tensioned segmental design. Two 70-ft(21.3 m) segments could be easily han-dled and once erected on a temporarybent, could be post-tensioned together.By using bundled strands in ducts, asufficient eccentricity at midspan couldbe achieved to satisfy design require-ments. A box section with the same ex-terior dimensions as the other boxescould be used and so the architecturalrequirements were easily satisfied withthis option. Additionally, the mainte-nance requirements would be lowerthan for steel.

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Fig. 6. Reinforcing scheme of a typical beam prior to the side forms being lifted into place.

Design AspectsThe loads acting on various parts of

the overall system were grouped intofive general categories: vehicle loads,dead loads, wind, thermal forces, andshrinkage and creep forces. These loadswere combined into three critical loadcases.

Case I consisted of the car, dead,shrinkage and creep, and thermal loads.Case II consisted of the same loads asCase I except that full wind load wassubstituted for the car load plus all loadswere reduced by 25 percent. Case IIIconsisted of car, dead, partial wind,shrinkage and creep, thermal, and brak-ing loads which were also reduced by 25percent.

As a consequence of the method and

sequence of construction of the seg-mental span, there was actually oneother component load which acted onthe segmental unit. This was the loadwhich was placed on the structure tosimulate the removal of the shoring. Asummary of the construction procedurewhich gave rise to this other load corn-ponent follows.

The two segments were first erectedon the permanent supports and the tem-porary bent and then post-tensioned to-gether (see Figs. 7 and 8). This causedboth pieces — now acting as one unit -to lift up off the temporary bent ap-proximately V in. (6.3 mm). The bentwas then raised enough to come firmlyinto contact with the 140-ft (42.7 m) gird-er — the so-called touch shored condi-tion.

PCI JOURNAL1July-Au9USt 1985 45

Fig. 7. Traffic was detoured on the Crosstown Expressway while one of the two 70-ft(21.3 m) segments is being erected. Note the temporary bent at the left.

Fig. S. Closeup view at the temporary support side of the other 70-ft (21.3 m) segmentbeing set into place.

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At that point, the deck and rails weresequentially cast. Since the temporarybent was snugly in place, all additionalweight (the deck and rail weight) actingon the box beam had a two-fold effect:negative moments developed over thesupport and the reaction at the tempo-rary bent increased with each incrementof load added.

The resulting reaction at midspan wasthen viewed as simply an upward pointload required to maintain zero net de-flection. Consequently, if a load oppositein direction and equal in magnitudewere superimposed at the points of reac-tion, the result would be zero. That is,the effect of the support would be-come zero. Therefore, the removal of thetemporary support was modeled as adownward point load equal in mag-nitude to the reaction at the temporarysupport.

Role of Computersin the Design

Perhaps the single most useful too] inengineering analysis aside from the cal-culator is the computer. This proved tobe especially true in the analysis anddesign of this project. The leveragingeffect of the available manpowerprovided by the computer provided foran economical design that otherwisemay not have been feasible, given thefast-track time constraints of the job.

Although numerous programs wereused throughout the design, there werethree major ones in particular that arenoteworthy. The first is PM, a programwhich was written to simulate themovement and effects of the passengercars as they traversed the system. Theheart of PM is CON-BEAM, a continu-ous beam analysis program which LEAPpreviously developed.

The moment and shear envelopesgenerated by this program were inputinto the second program, BRIDGE,which is for the design of prestressedbridge girders. Finally, frame analyses

which were performed by MisenerMarine using McAuto's STRUDL wereused to gain supplementary informationfor the design.

PM is essentially a routine which in-crementally steps a uniform load ofgiven length across a continuous beamwhile observing such things as theboundary conditions of the structure.The shuttle groups (each composed oftwo cars linked together) were designedto ride on a cushion of air and weretherefore simply modeled as uniformloads 80 ft (24.4 m) in length.

As these loads moved from the endspan to the segmental center span to theother end span of the three-span contin-uous unit, the maximum positive andnegative moment envelopes andmaximum shear envelopes were cap-tured for each span. The maximumreactions at the piers, both positive andnegative, were also saved.

The results from PM were then in-put directly into BRIDGE (which wasthe precursor to the SPAN program).For all spans except the segmentalspan, the main reinforcement wasdesigned using BRIDGE. For the seg-mental span, however, BRIDGE wasonly used to design the prestressed re-inforcement which supported only thehandling and construction loads. Thepost-tensioned reinforcement, the mainreinforcement for the segmental span,was manually designed for the momentand shear envelopes provided by PM.

Misener Marine performed com-prehensive lateral load analyses for eachof the continuous units which make upthe People Mover using STRUDL.Separate analyses for braking and cor-nering forces induced by the cars andwind loads of 45 and 110 miles per hr(72 and 176 kmlhr) were performed. Addi-tionally, analyses for plus 30°F (17°C) andminus 40°F (-22°C) changes in tempera-ture were performed.

The results of the thermal analysisobtained from the STRUDL outputwere used to calculate the forces and

PCI JOURNAL1July-August 1985 47

moments caused by the shrinkage andcreep of the concrete. The strains due toshrinkage and creep were calculatedand, knowing the thermal properties ofconcrete (the concrete strains due tochanges in temperature were easily cal-culated from these properties), a constantof proportionality was determined. Thisconstant of proportionality was then usedto estimate the forces and moments due toshrinkage and creep from the forces andmoments induced by thermal strains.

Design of theSegmental Span

Once a design procedure was estab-lished and the necessary analyses wereperformed, the actual design of the seg-mental span was very straightforward.The design was divided into three mainparts. First, the necessary reinforcementand connections to handle the verticalloads were designed. Next, lateral loadsresulting from both wind and corneringof the cars were investigated. And fi-naIly, the torsion reinforcement wasdesigned.

Each 70-ft (21.3 m) segment has anominal number of ½-in. (12.7 mm) di-ameter low-relaxation strands in it to re-sist handling and construction loads.The strands were arranged in a parallelpattern with some of the strandsdebonded near the ends of the segmentsto control top tensile stresses near theends resulting from the large eccentric-ity of the centroid of the strand group.By using this arrangement in place of adepressed strand pattern, a significantsavings in time and expense in castingthe girders was realized.

Whereas prestressed strand was onlyconsidered minor temporary reinforce-ment in the segmental span, it was themain flexural reinforcement in the otherspans of the project. The same parallel/debonded strand patterns were used onthose spans as well.

The design of the post-tensioned

reinforcement for the segmental span,the main reinforcement for bendingabout the horizontal axis, was essen-tially divided into five main parts. Themidspan stresses from all load compo-nents were first calculated and thensummed. With this information, therequired total final post-tensioning forceand tendon profile were calculated suchthat there was zero tension at the bottomof the joint between the segments atfinal conditions.

The stresses immediately after post-tensioning were then checked againstthe allowable limits. Next, a completestress history at final conditions wastabulated and again checked against theallowable limits. Lastly, the ultimatecapacity was checked for structuraladequacy.

Once the total required final post-tensioning force and parabolic tendonprofile were established, the actualnumber of tendon ducts and numbers ofstrands in each duct had to be deter-mined. This was done by DywidagSystems International, the suppliers ofthe post-tensioning system for the proj-ect.

Three parabolic ducts in each sidewall (see Fig. 9) and one straight duct inthe bottom flange of the box beam wereused. Within these seven ducts wereplaced a total of fifty-seven 0.600-in. (152mm) diameter, low-relaxation strands.The post-tensioning strands were fullygrouted within the corrugated gal-vanized steel ducts.

Lateral bending was actually of littleconsequence in the design of the boxbeams, The main reinforcement for lat-eral bending was longitudinal mildreinforcement located in the deck.Nevertheless, the possible adverse ef-fects of lateral bending on the boxbeams themselves were investigated.

Torsional forces, on the other hand,required much more detailed consid-eration. These forces resulted from cor-nering loads of the cars and wind actingon both the structure and the cars. The

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Fig. 9. End view of one of the segments. Note the three ducts and base plates in each

side wall.

primary mechanism for the torsion wasthe differential bending of the pierswhich were of varying height.

Closed stirrups anchored in the deckwere used as the reinforcement for shearand torsion. In calculating the contribu-tion of stirrup area due to torsion, thevoided box was taken as a solid section,as allowed by the provisions of the ACI318-83 Code.

Production Aspects ofPrecast Segments

The precast concrete componentswere manufactured by Florida Pre-stressed Concrete at their plant inTampa, about 10 miles (16 km) from theproject site.

A high degree of precision was main-tained in casting the members becausethe dimensional tolerances had to heexact for the segmental operation to pro-ceed successfully.

An interesting production problemarose in that the void location of the pre-cast sections had to he controlled withinya in. (6.3 mm). Production concepts in-vestigated included casting the lowerlayer, then fixing the voids and castingthe remainder, or fixing the voids verti-cally and horizontally to allow the con-crete to flow across the 4 ft (1.2 m)dimension under the void.

The second method was chosen inconjunction with a high-range water-reducing agent (superplasticizer). Avoid holding apparatus was developedto ensure lateral location. Two verticalrods were secured at 4 ft (1.2 m) throughthe void and the form to the permanenthold-down locations under the form toprevent void floating.

Shipment of the precast componentswas on a truck trailer where a specialbolster was mounted to the fifth wheeland a steerable dolly was used in therear.

PCI JOURNALJJuly-August 1985 49

Fig. 10. Bearing failure cracks formed in some of the girders cast early in the project. Thecause was quickly identified and a solution to the problem was found.

Special Problemsand Solutions

Several rather interesting problemswere encountered on this project. Al-though there were many small routineproblems to which straightforward so-lutions were found, two in particularare noteworthy. The first was the ef-fects of shrinkage and creep of the boxbeams and the second was a spallingproblem at the bottom ends of some ofthe beams cast early in the project.

It was estimated early in the projectthat the magnitudes of the shrinkage andcreep forces would be significant, Thestrains from shrinkage and creep wereevaluated for each span, and then thecorresponding forces and moments weredetermined by taking a proportion of thethermal forces and moments which wereprovided in the STRUDL analyses, asdiscussed earlier, For a continuousstructure with high levels of prestress,the amount of reinforcement required toresist these forces can be and often isrelatively large.

In this particular case, eight #6 bars

were required in the ends of the boxesto resist the axial forces and positivebending moments at the piers (negativemoments are resisted by mild steelreinforcement in the deck). Althoughnot large in absolute terms, these barswere large enough to require specialattention as to how to place them so thatthey could adequately develop withinthe very limited space available.

Fig. 10 shows several vertically in-clined cracks at the extreme ends of thebeams that appeared at release. Closeexamination of these cracks indicatesthat they are characteristically similar tobearing failure cracks. With this clue inmind, a probable mechanism for thecracking was determined_ Release of thestrands caused the member to camberup and simultaneously shorten elasti-cally. The member was then supportedonly at its extreme ends.

As the member began to shorten, ten-sile forces developed in the hearing re-gion which were caused by friction be-tween the support points of the mem-her and the casting bed. The shearcapacity of the concrete was exceeded,

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Fig. 11. Finished view of Harbour Island People Mover which is now fully operational.

and thus cracks formed.While the effects of these cracks were

very localized at the ends of the beamsand, therefore, structurally insignificantonce the beams were in place, it was,nevertheless, important that a solutionbe found because of the aesthetics in-volved. The remedy that was chosenwas to create a chamfer strip trans-versely across the ends of the beams andfor a short distance longitudinally alongthe span.

This solution effectively cured thebearing problem with minimal time andexpense. The beams that did have minorcracks before the problem was iden-tified were patched with a high strengthgrout that blended well with the originalconcrete.

Closing CommentsConstruction of the structural system

for the Harbour Island People Moverbegan in February 1984 and was com-pleted last August. The complete systemis today fully operational and open tothe public (see Fig. 11).

The total design/construction cost ofthe elevated structure (excluding themechanical system) was $2.8 million. Of

this amount the precast prestressed por-tion represented about 20 percent,

The Harbour Island People Moverprovides an outstanding example of oneof the many uses of prestressed con-crete. From its practicality as a con-struction medium to the strong ar-chitectural impressions it creates, thedesirability of prestressed concrete isexemplified in many ways in this proj-ect. As one of the focal points ofdowntown Tampa, these attributes arecertain to provide convincing and con-tinuing proof of its use as a viable con-struction material.

CreditsOwner: Harbour Island, Inc., Tampa,

Florida.Designer/Contractor: Misener Marine

Construction, Inc„ Tampa, Florida.Precast Engineer (Elevated Segmental

Structure): LEAP Associates Interna-tional, Inc., Tampa, Florida.

Precast Concrete Manufacturer: FloridaPrestressed Concrete, Tampa,Florida.

Post-Tensioning Supplier: DywidagSystems International, USA, Inc.,Lincoln Park, New Jersey.

PCl JOURNAL/July-August 1985 51