pfluger bridge

7/31/2019 Pfluger Bridge

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JAMES D. PFLUGER PEDESTRIAN

AND BICYCLE BRIDGE

INTRODUCTION

Now the 18th largest city in the nation, Austin, Texas, is growing at a phenomenal rate, with its populationsoaring nearly 40% over the past ten years. Austin is also unique among large cities in that many of itscitizens are avid walkers, runners, and bicyclists, not only for recreation, but also as a practical alternative forthe daily commute.

The Town Lake areaformed along the southern edge of downtown Austin by the damming of the ColoradoRiveris one of the most popular locations for recreational activity and one of the most heavily-traveledcommuter corridors. The existing Lamar Boulevard Bridge, an historic six span reinforced concrete deck archbridge spanning Town Lake, is widely used by both commuters and recreational pedestrians and bicyclists,although the bridge was not originally designed to accommodate such use and is now considered functionallyobsolete.

The City of Austin spent several years with much public involvement studying viable options in search of aremedy. The City opted to design and build a new, stand-alone crossing of Town Lake adjacent to the existingLamar Boulevard Bridge instead of widening the historic bridge. An unusual Double Curve Alignmentconcept was proposed, using two curved alignments that meet and overlap over Town Lake, resulting in anhourglass-shaped deck.

Final design work on the new bridge proceeded on an accelerated schedule to meet a deadline for obtainingpartially matching federal funds for construction. The design engineer and architect worked in parallel torefine aesthetic concepts, while simultaneously developing construction plans. Several alternatives wereconsidered for implementing the Double Curve concept; in the end, the most attractive and practicablescheme involved a weathering steel plate girder superstructure with a unique framing plan utilizing opposingcurvature exterior girders to achieve a constant deck overhang. Given the complex geometry of the bridge andthe constraints associated with construction over water, steel proved to be the most practical material forachieving both the structural and architectural goals of this project. The design engineer used many standardsteel bridge design elements in innovative ways to simplify design and construction of the bridge while stillconforming to, and indeed celebrating, the architectural concepts associated with the Double Curve schemeand the adjacent historic Lamar Boulevard Bridge.

The result is a pedestrian and bicycle bridge that will satisfy the special needs of its users and serve as anattractive signature structure well integrated with its surroundings. This paper will highlight how this bridgeused conventional materials and standard highway bridge elements in unique and innovative ways to achievethe dual goals of a consistent aesthetic theme and an economical, readily-constructed design.

PROJECT HISTORY

Six major bridgescarrying a variety of traffic, including automobiles, trucks, trains and, of course,numerous pedestrians and bicyclistscross Town Lake. One of the major crossings is the existing LamarBoulevard Bridge, a six-span concrete deck arch bridge built in 1940. The bridge features restrained-but-elegant Art Deco detailing and is a historically significant landmark, added to the National Register ofHistoric Places in 1994. However, the Lamar Boulevard Bridge suffers from a degree of functionalobsolescence, with 10-foot traffic lanes and narrow sidewalks. Significant population growth in Austin over


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the past decade has resulted in substantial daily vehicular traffic on the bridge. This growth, combined withthe bridges key location near the center of the north and south shore hike and bike trails, has also resulted inheavy pedestrian and bicycle traffic. The net effect of combining heavy traffic with narrow lanes andsidewalks was a facility that was less than ideal for many of its users. The City of Austin desperately neededan improved facility for crossing Town Lake.

Recognizing this and other needs, the citizens of Austin approved one of the largest bond packages in thecitys history, including $8 million specifically earmarked to widen the Lamar Boulevard Bridge. In the early1990s, the City of Austin secured approximately $950,000 of matching federal funding for the project, as partof the Intermodal Surface Transportation Efficiency Act (ISTEA).

In 1995, the City signed a contract retaining a consultant teamled by HDR Engineering, Inc.to studyproject alternatives. Under the original contractual agreement, the consultant team evaluated six options, allvariations on the theme of widening the existing Lamar Boulevard Bridge. The study phase includedinspection of the existing Lamar Boulevard Bridge, evaluation of existing site conditions, traffic modelingand extensive public involvement activities. The consultant team held regular meetings with the City ofAustin, the Texas Department of Transportation (TxDOT), the Texas Historical Commission and other keystakeholders.

From early in this process, the Texas Historical Commission clearly indicated its preference to avoid anyalterations to the existing Lamar Boulevard Bridge (currently owned by the State of Texas), citing its missionto preserve the historical integrity of this landmark structure. In addition, attendees at a 1996 public meetingindicated they did not want more traffic lanes added to the existing bridge. In its transportation work sessionin March 1998, the city council directed the project team to seek public input in the design of a separatepedestrian/bicycle bridge. Although constructing a separate bridge would not correct all the deficiencies of theexisting Lamar Boulevard Bridge, it would create a safer facility for pedestrian and bicycle traffic. The idea ofa separate bridge was passionately debated by the city council, but the council ultimately decided to proceedwith a design process.

The City considered many options for how

to develop concepts for the new bridge,ranging from hosting a full-blown designcompetition among competing architectureand engineering firms to sponsoring a designidea competition open to the general public.Eventually, the City decided to hold a publicworkshop for key stakeholders. Thisworkshop took place in May 1998, andgenerated fifteen proposed concepts,including:

Cable-stayed bridge options Arch bridge options Relocation of an existing, historical

truss bridge Several variations on beam bridge

options

One of the most innovative and intriguingconcepts proposed was the Double Curveshown in Figure 1. This unique concept was

Figure 1: Original sketch from the May 1998 public

workshop, showing what would eventually become theconcept for the final design of the James D. Pfluger

Pedestrian and Bicycle Bridge.


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developed by a group of five workshop participants (Chas Tonetti, Tere OConnell, Jamie Wise, RushMcNair and Chris Hutson) and focused on the function of the facility suggesting the form of the structure.

The concept grew around the paths oftravel: the function of connecting the trailsystem along the south shore of TownLake to activity centers at 5 th and 6th

Streets and a future public activity centeron the north shore. The concept featuredtwo curved alignments, crossing over eachother at Town Lake and creating a widearea that would serve as a gathering[place] at the river for people to stop andenjoy the view or watch lake activities.This curved theme would then beechoed in the design of the structure itself.

The City selected four of the 15 proposedconcepts for further development by the

consultant teams architect (Kinney andAssociates + Carter Design Associates),including the Double Curve concept. Thearchitect set a criterion of no straightlines and conceived a structure with anhourglass-shaped deck plan resulting fromthe crossings curved horizontal

alignments and featuring helical ramps and curved connector spans at each end. The architects originalDouble Curve concept is shown in Figure 2. The Double Curve concept remained true to the basic curvedconcept from the public workshop it was simply developed from a freehand sketch to a higher level ofrefinement.

The four developed concepts were presented to the Austin City Council in September 1998 for feedback,leading to the choice of one selected alternate that served as the base concept for the final design of the newbridge. The Double Curve concept emerged as the selected alternate. With only six months remaining beforethe matching ISTEA funding would expire, development of aesthetic concepts had to occur in parallel withthe preparation of the final Plans, Specifications and Estimates (PS&E).

DESIGN

Basic Themes and Concepts

As the first step in the final designprocess, the consultant teams engineersfurther refined the architects developedconcept for the Double Curve structure.The helical ramp at the south end of thebridge was eliminated following ageometric evaluation of the verticalprofile and existing topography, hike andbike trails, and several surface streets.

Figure 2: The developed concept prepared by the consultant

team architect for the Double Curve Alignment option.

Figure 3: The Double Curve Alignment option, as refined

by the engineering team.


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Structural design, constructability and cost considerations resulted in the decision to change the north endtriangle connector span from curved to tangent. The engineering team established a span arrangementsensitive to both the adjacent Lamar Boulevard Bridge and users of Town Lake for crossing the lake, trailsand streets at both ends of the bridge. As the plan developed, it stayed remarkably true to the original freehandconcept sketch from the public workshop.

Following these refinements, the consultantteam constructed a scaled physical modelof the proposed bridge. This model vividlyillustrated how the bridge itself would lookand, more importantly, how it would fitinto its surrounding environment. Figures4 and 5 show the physical model.

Several structural systemsincluding cast-in-place, post-tensioned concrete boxgirders; precast segmental concrete boxgirders; and solid or voided cast-in-place,

post-tensioned slab structureswereinitially considered for the superstructure.However, the need for quick, simpledesigncombined with the overridingcriteria of ease of constructability over thelake, ability to easily conform to geometriccomplexities and low construction costquickly led the engineering team to selectsteel plate girders as the best possiblechoice for the superstructure system. The architect requested use of weathering steel to provide an organicor natural appearance to fit with the wooded shorelines of Town Lake. Additionally, this meshed well withcost implications associated with initial and long-term maintenance of coatings systems.

The relatively tight construction budget led to the basic theme for the engineering design of the bridge:conventional materials and techniques usedin unconventional manners. Basically, theengineering team set out to produce a set ofplans closely resembling those for a majorsteel plate girder highway bridge, with theexception of the basic geometry andaesthetic treatments. The scale of thisstructure and the selection of steel plategirders as the main structural system made itclear that only heavy highway bridge

contractors would be bidding on this project.In order to obtain low, competitive bids, theengineering team had to produce plans thatwould look familiar to these contractors togive them confidence in their understandingof the project and allow them to bid theproject without fear of surprises duringconstruction.

Figure 4: Physical model of the James D. Pfluger Pedestrian

and Bicycle Bridge.

Figure 5: Physical model of the James D. Pfluger Pedestrian

and Bicycle Bridge.


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Design Criteria

The scale of the James D. Pfluger Bridge suggested that although it is a pedestrian bridge covered by theprovisions of AASHTOs Guide Specifications for Design of Pedestrian Bridges (1), it is more the same scaleand construction as a typical highway bridge, covered by the provisions of AASHTOs StandardSpecifications for Highway Bridges (2) and AASHTOs Guide Specification for Horizontally Curved

Highway Bridges (3). The design criteria selected for the bridge reflect this assessment of the bridgescharacter.

The basic bridge design criteria were AASHTOs Standard Specifications for Highway Bridges (2) andAASHTOs Guide Specification for Horizontally Curved Highway Bridges (3). However, the live loaddefinitions were modified to reflect the nature of the structure as a pedestrian/bicycle facility, and a 100 psflive load was used as the primary live load. This 100 psf load was applied both over the entire structure andin various checkerboard patterns to determine maximum loading effects. 100 psf is a higher criteria thanthat provided in AASHTOs Guide Specifications for Design of Pedestrian Bridges (1) but reflects the criteriaused in several other similar structures where large groups of people are expected to congregate during majorrecreational and civic events (such as regattas on Town Lake or Independence Day fireworks shows). Inaddition, the bridge was checked for H10 truck loads (to reflect the occasional maintenance truck on the

bridge), but it proved to control only in a few local loading checks.

AASHTOs Guide Specifications for Design of Pedestrian Bridges (1) also provides criteria for vibrationanalysis of pedestrian bridges. The consultant team performed vibration analyses of the various units of theJames D. Pfluger Pedestrian Bridge and, as expected, found no cause for concern. These provisions of theguide specification are more intended for lightweight pedestrian bridge structures (most often constructed).The James D. Pfluger Pedestrian Bridge is of a scale, size and weight more akin to a highway bridge and hascommensurate dynamic characteristics. A simple analysis approach quickly demonstrated the bridgesfundamental frequencies were well above the threshold of concern.

Framing Plan

The Double Curve alignment resulted inan unusual plan for the bridge (see Figures6 and 7). At the south end, two rampstructures curve toward each other. Thesouthwest ramp is a two-span continuousunit (Unit A; 86-120), while thesoutheast ramp consists of two single-spanunits (Units B and C; 48 and 111respectively). Units A and C each utilizethree concentric, horizontally-curvedcomposite plate girders. Unit B utilizesthree concentric, horizontally-curvedcomposite rolled beams. Unit B has arelatively short span, allowing the use ofrolled beams to achieve a shallowsuperstructure depth, required to maintainadequate vertical clearance over the hikeand bike trail below. Units A, B and C arerelatively narrow (for this structure), witha total out-to-out width of 23-0.

Figure 6: Drawing and photograph showing the plan for the

main river crossing of the James D. Pfluger Pedestrian and

Bicycle Bridge (photo courtesy of White Photographic

Services, Austin, Texas).


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Units A and C meet at Interior Bent 3 and the bridge continues out over Town Lake on Unit D, a three-spancontinuous steel plate girder unit (114-114-114). Unit D has variable width (minimum width of 31-3;maximum width of 42-0) and utilizes a very unusual hourglass framing plan (discussed in detail later inthis paper). Unit D ends at Interior Bent 6, where two ramps split off to the northeast and the northwest.

Unit E is a triangular unit consisting of Span 6W, a single span unit (104 span; 21-0 width) curving fromInterior Bent 6 to Interior Bent 7W to the northwest; Span 6E (109 span; 26-0 width), a single span unitcurving from Interior Bent 6 to Interior Bent 7E to the northeast; and Span 6X (49 span; 18-0 width), asingle span unit spanning between support brackets on the exterior girders of Spans 6W and 6E. Spans 6Wand 6E each utilize three concentric, horizontally-curved composite plate girders, while Span 6X utilizes threetangent composite rolled beams. The use of the relatively shallow rolled beams in Span 6X was possible dueto the comparatively short span length and was desirable since it simplified the detailing of the beam ends attheir supports.

Two further units, Units F and G, were designed running to the northwest over the hike and bike trail, CesarChavez Boulevard and Sandra Muriada Way. Unit F is a four-span continuous steel plate girder unit utilizingtwo concentric, horizontally-curved composite steel girders. Unit F runs from Interior Bent 7W to InteriorBent 11W, where Unit G begins. Unit G is a nine span, conventionally-reinforced concrete slab and T-beamunit that continues to Abutment 20, where a retained fill section with a switchback ramp leads to the sidewalkof the existing Lamar Boulevard. Units F and G were fully designed and detailed for this project but wereidentified as deductive alternates during bidding and were not included in the final construction contract.

Provisions were made at Interior Bent 7E for a future ramp running to the northeast toward the existingSeaholm power plant, which is planned to have a future public facility. Interior Bent 7E also serves tosupport a short-span, conventionally-reinforced concrete slab and beam span that links Span 6E to the HelixRamp, a conventionally-reinforced helical ramp structure that rotates through 540 degrees to link up with thehike and bike trail below. A second link to the hike and bike trail is provided via a stairway from Span 6X tothe trail below.

Figure 7:

Unit A = Spans No. 1W and 2W (Abut. 1W to Bent 3); Unit B = Span No. 1E (Abut. 1E to Bent 2E);

Unit C = Span No. 2E (Bent 2E to Bent 3); Unit D = Spans No. 3-5 (Bent 3 to Bent 6);

Unit E = Spans No. 6W 6E and Cross Bridge (Bent 6 to Bents 7W and 7E);Unit F = Spans No. 7-10 (Bent 7W to Abut. 11); and Unit G = Elevated approach section north of Abut. 11


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Units A, B, C and E utilize framing plans and design and construction techniques that are indistinguishablefrom those used in typical highway bridge construction. Beyond their unique pedestrian live load criteria,these bridges are identical to their highway bridge brethren.

Due to its highly unusual framing plannot at all typical of highway bridge constructionUnit D meritsfurther discussion. The unique framing plan of Unit Dwith a tangent center girder and two exterior girders

with opposing horizontal curvaturederives from its unusual hourglass plan. The hourglass shape of theUnit D deck plan is a direct result of the refinement of the original Double Curve alignment concept into aworkable design plan. The final deck plan was essentially unchanged from its first incarnation as adimensioned engineering drawing, except for the replacement of the re-entrant corners at the alignmentoverlap locations with radiused edges. The final framing plan for the girders, however, was the second of twoconcepts examined.

Originally, the engineering team proposed using two tangent girders for Unit D. The girder spacing was set atapproximately 20 to provide a nominal 3-3 overhang at the narrowest points of the deck plan. With thisarrangement, adding a third, center girder would have resulted in an inefficient girder spacing. Thisarrangement appeared to have merit because it resulted in simpler, cheaper-to-fabricate tangent girders anduniform diaphragm dimensions. However, the engineering team was struggling with the deck design, since at

the widest points of the deck plan, the deck overhangs were up to 12. The team studied various options,including conventionally-reinforced, cast-in-place decks; precast, prestressed deck panels; and cast-in-placepost-tensioned decks. The cast-in-place options suffered from constructability concerns related to relativelylong span shoring and construction over the lake, while the precast option suffered from complicated detailingfor the interfaces between panels and the interfaces with the shear studs provided to obtain composite actionin the girders.

The tangent girder option also met with disapproval from the project architect, who disliked the unevenshadow line created from the variable width deck overhang and requested a constant width deck overhanginstead. The engineering team was initially reluctant to consider such a plan, since it resulted in girders withcomplex reverse curvature and variable girder spacing. However, after a few minutes of discussion, theengineering team realized this was actually a win-win suggestion that not only resolved many of the

architects concerns, but many of the engineering teams concerns as well. The resulting framing planprovided a short, 3-3 deck overhang and more reasonable interior deck spans the girder spacing nowvaried from 12- 4 minimum to 17-9 maximum. This allowed the use of a conventionallyreinforced, 12thick concrete deck that could easily be constructed using typical highway bridge deck construction materials,equipment and techniques.

Detailed Design of the Unit D Girders

The design of the Unit D girders themselves was surprisingly simple. At first glance, the unusual framingplan with a tangent center girder and two exterior girders with both opposing and reverse curvature wouldappear to be quite complex to design. However, it was actually simpler to design than a typical horizontally-curved steel plate girder highway bridge.

In a typical horizontally-curved steel girder highway bridge, there are two main curvature effects that must beaccounted for: global overturning and local lateral flange bending. The global overturning effect results fromthe center of gravity of any horizontally-curved bridge span being offset from a chorded centerline drawnbetween the two supports. This global overturning moment manifests itself as an increase in the verticalloading of the girder on the outside of the curve and a decrease in the vertical loading of the girder on theinside of the curve. The shifting loads are carried in the diaphragms, which in a typical horizontally-curvedhighway bridge become primary load-carrying members.


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The lateral flange bending effect results fromthe axial forces in the flanges being applied tostructural elements that are curved. Thusflanges in compression experience lateralflange bending moments that tend to bow theflanges more, while flanges in tension

experience lateral flange bending moments thattend to pull the flanges straight.

In Unit D, the global overturning momenteffect did not exist. Since the girders wereessentially symmetrical in plan, the center ofgravity of each span essentially fell on thecenterline of the supports. Thus, the only effectof curvature requiring consideration was thelateral flange bending effect, which couldeasily be included in the design calculationsusing the simple lateral flange bending moment

formula presented in the V-Load curved girderdesign methodology (4):

Where:MLat = Lateral Flange Bending Moment at Section Under Investigation

M = Primary Vertical Bending Moment at Section Under Investigation

d = Diaphragm Spacing

R = Radius of Horizontal Curvature

h = Depth of Girder, from Center of Top Flange to Center of Bottom Flange

In fact, the exterior girders in Unit Dcould be designed as tangent girders withthe lateral flange bending moment effectsadded in manually. This is exactly whatthe engineering team did for thepreliminary design of the Unit D exteriorgirders. First, a tangent girder design wasperformed using the commercial

STLBRIDGE tangent plate girder designprogram (5). Then, the stresses resultingfrom this design were imported into a

spreadsheet. Other key parameters, suchas the radius of horizontal curvature,diaphragm spacing and girder depth, wereinput and the lateral flange bendingstresses calculated and compared to stresslimits calculated in accordance withAASHTOs Guide Specification for

Horizontally Curved Highway Bridges (3).

Rh

MdMLat

12

2

=

Figure 8: Drawing showing the rolled beam diaphragms.

Figure 9: Photo showing the rolled beam diaphragms.


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The diaphragm spacing and flange sizes were then adjusted to result in an optimized design. This approachproved quite successful, and these preliminary designs exhibited excellent correlation with the results of a

later, detailed 3D finite element analysisperformed using the proprietary BSDI

3D System computer modeling service(6).

Diaphragm Detailing and Design

Given the variable girder spacing in UnitD, the detailing of diaphragms warrantedspecial attention during the designprocess. The engineering team initiallyexamined several options, including platediaphragms, rolled beam diaphragms, X-frames, W-frames and K-frames.

Frame diaphragms are perhaps the most

typically used in highway bridge design,but would have been cumbersome for thisbridge due to the variable girder spacinga 12-4 minimum to a 17-9

maximumin Unit D. With such wide girder spacings, the diagonals of X-frames or K-frames would haveinefficiently shallow angles from horizontal. Similarly, W-frames would either have shallow diagonals orrequire multiple Ws to span between the girders. Combined with these inefficiencies, there would have beenvirtually no identical diaphragms anywhere in Unit D (because of the 15 degree skew, half of thecommonality due to symmetry was also lost). Finally, all the different bracing details would have beenaesthetically distracting for recreational boaters under the bridge on Town Lake.

As a result, the engineering team selected

rolled beam diaphragms (see Figures 8 and9). In addition to eliminating problemswith inefficient diagonal angles in theframe options, rolled beam diaphragmsalso offered substantial advantages infabrication. Instead of developing jigs formany different frame diaphragms, thefabricator simply cut rolled sections to thelengths needed for the variable girderspacing in Unit D. In order to reduceweight and keep the diaphragm stiffnessesreasonable, shallow rolled beams were

selected for the diaphragms. Thedifference between the diaphragm depthand the depth of the girders was made upusing a simple curved gusset detail at theends of the diaphragms.

Beyond the greater efficiency and simplerfabrication of the rolled beam diaphragms, they also provided a much simpler and cleaner appearance. The

Figure 10: Sketch of two column bent.

Figure 11: Photo of two column bent.


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curved gusset detail also worked extremelywell with the overall curved theme of thebridge. Thus, the choice of rolled beamdiaphragms proved to be another exampleof a win-win solution that best satisfiedaesthetic, engineering and construction

criteria.

Since the diaphragms perform as primaryload-carrying members, particularly in thecurved ramp structures of Units A, B, Cand E, they were carefully designed usingdiaphragm forces obtained from the results

of BSDI 3D finite element analyses.

Substructure Design

The substructure system for this bridge is

quite conventional in its materials but quiteunconventional in its form. All substructures are conventionally-reinforced, cast-in-place concrete bents, withpile caps straddling conventionallyreinforced, concrete drilled shaft foundations. Depending on thesuperstructure width at any given bent location, either two column frame bents or single column hammerheadbents were used.

In following the curved theme for the structure, the two column bents ended up with a rather unusualappearance. Following an exhaustive aesthetic design process, the final selected bent form included the useof curved columns and a haunched bent cap (see Figure 10 and 11) to allow views of scenic Town Lakethrough the bents. In and of itself, this two column bent, although not typical in appearance, was not overlyunconventional. However, the narrower superstructure sections utilized a single column hammerhead bent.

To stay consistent to the established

theme, these bents also used curvedcolumns; hence the asymmetrical curvedcolumn hammerhead bents (see Figure12).

Although unconventional in appearance,these bents posed no unusual challengesfrom a design standpoint. The engineeringteam strove to find ways to simplify theconstruction whenever possible to helpkeep construction costs low. For example,the architect had originally drawn

essentially freehand curves for the bentcap soffits and column alignments. Theengineering team approximated thesecurves with circular curves set with givenPC and PT locations that allowed thecontractor to build and reuse one set offorms for multiple bents on the bridge.

Figure 12: Photo of single column bent.

Figure 13: Erection of girders at the north end of the main

river crossing.


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BIDDING AND CONSTRUCTION LESSONS LEARNEDThe construction contract was bid twice.The first round of bidding involved fourbidders, all heavy bridge constructioncontractors. The bids were relatively

widely separated and all bids exceeded theCity of Austins allowable budget forconstruction of the project. However,detailed evaluation of the bid tabulationsrevealed some lessons.

While the high and low bids wereseparated by 24 percent, comparison ofonly the 20 bridge-related bid items(which essentially included all itemsexcept the helix ramp, retaining walls atthe trails, architectural bridge railing,

lighting details, colored concrete wearingcourse, and landscaping and associatedsite work) revealed the three low bidswere separated by only about five percent.Since all bidders on this project were

heavy bridge construction firms, they all felt comfortable bidding the project very tightly on the 20 bridge-related bid items because they looked like the typical highway bridge plans they routinely bid.

For the second round of bidding, the construction contract was restructured using deductive alternates thatallowed the City to restructure the project after bidding in an effort to fit the project within the constructionbudget. This second round of biddingincluded two bidders and resulted in a

winning bid that, with Deductive Alternate1 (which eliminated Units F and G [seeFigure 7]), fit the Citys constructionbudget. The construction contract wasawarded to Jay-Reese Contractors, Inc. ofAustin, Texas, in April 2000, after an up-to-the-last minute debate on project optionsand costs in the Austin City Council.

Groundbreaking occurred on May 15,2000, and the contractor was allocated aone-year schedule with incentive bonuses

for early completion. After allowances forfoul weather days and change orders, thecontractor completed the project ahead ofschedule on June 16, 2001. Constructionusually progressed very smoothly, atestament to the philosophy followed inpreparing the bridge plans.

Figure 14: Formwork for the deck at the south end of the main

river crossing.

Figure 15: Concrete placement for deck at the sound end of

the main river crossing


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Generally, construction of the main lakecrossing (Units A through E) followed thetypical construction sequences andappearances of routine, large highwaybridge construction projects (see Figure13). The contractor was well-versed in this

type of construction and the plans wererelatively error-free. There were only twominor added-charge change orders duringthe main lake bridge construction. In fact,there were relatively few problems with themain lake bridge construction at all. Oneof the minor problems that did occur ishighlighted below as a lesson learned.

The single biggest construction problemwas associated with the alignment of thesingle column bent caps. At the insistence

of the architect, all bent caps for the mainlake crossing were detailed with widthsidentical to the thickness of theirsupporting columns. For the two column bents, this did not cause problems. However, the asymmetrically-curved single columns had a recurrent problem of shifting forms; during concrete placement, the columnforms would shift and twist slightly. The rotational twist resulted in the corners of the tops of the columnbeing out of position only on the worst column, but since the bent cap was the same width as the column,there was no tolerance for this out-of-alignment twist and the cap forms had to conform to it. As a result, a misalignment at the top corners of the column cased a misalignment of up to 2 at the end of the bent caps.

Since steel forms were used for the bentcaps, it was practically impossible torecover the correct alignment, as the

forms could not be deformed to move theends of the bent cap back into theirproper positions. This misalignmentcomplicated all other bent cap details,such as the positions of anchor bolts andbearings. In retrospect, the engineeringteam should have been more adamantabout disallowing this questionabledetailing.

Many other unique or unconventionaldetails worked well during construction,

but for the most part, the smoothconstruction was due to detailingconsistent with typical highway bridgedetailing. This made it easy for anexperienced, qualified highway bridgecontractor to construct this bridge.

Figure 16: View of the new James D. Pfluger Pedestrian andBicycle Bridge from under Bent 7E.

Figure 17: View of the new James D. Pfluger bridge looking

south at deck level. Note planters (not complete in this photo)

that help to delineate gathering places from travel lanes.


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CONCLUSION

This was a challenging, exciting and rewarding projectfor the design team, even though the number ofstakeholders often made the project process arduous. The

short schedule for final design and the need to prosecutedetailed structural design simultaneously with conceptualaesthetic design added to the challenges. However, thedesign team had a truly inspired and well-conceived basicconcept from which to work and the unusual nature of thebridge always kept the project interesting.

The engineering team successfully used conventionalmaterials and techniques in unconventional manners toproduce a set of plans that resulted in very competitivebidding for construction. The unit cost for the main lakecrossing worked out to be approximately $150/SF. This ishigh compared to typical highway bridge constructioncosts in Texas (which can range as low as $34-40/SF),but given the aesthetic and functional requirements of theproject and in comparison to equivalent projects this unitcost relatively low. This approach also greatly facilitatedthe construction phase, with very few Requests forInformation (RFIs), and no significant change ordersduring construction of the main lake bridge. Assemblingthe plans for this unique pedestrian/bicycle bridge to looklike the plans for a typical highway bridge was a gambitthat paid off well. Pedestrians and bicyclists will nowhave a new, safe, attractive route across Town Lake andthe bridges owners and users are sure to be delightedwith their new facility. All in all, this projectdemonstrates that aesthetics, functionality,constructability and cost effectiveness need not bemutually exclusive goals for a bridge project.

Figure 18: On June 16, 2001, hundreds of

runners sprinted over the new James D.

Pfluger Pedestrian and Bicycle Bridge,

marking the bridges grand opening. The

bridge was named in honor of the Austinarchitect who conceived the trail system on

either side of Town Lake.


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REFERENCES

1. American Association of State Highway Transporation Officials (AASHTO), Standard Specifications forHighway Bridges, 16th Edition, 1996, with Interim Updates through 1998.

2. American Association of State Highway Transporation Officials (AASHTO), Guide Specifications forDesign of Pedestrian Bridges, August 1997.

3. American Association of State Highway Transporation Officials (AASHTO), Guide Specifications forHorizontally Curved Highway Bridges, 1993, with Interim Updates through 1995.

4. National Steel Bridge Alliance (NSBA), V-LOAD Analysis,Highway Structures Design Handbook,Volume 1, Chapter 12, pg. I/12/16, December 1996.

5. Bridgesoft, Inc., STLBRIDGE Design of Continuous Steel Bridge Girders, Omaha, Nebraska, 1997.6. Bridge Software Development International, Ltd. (BSDI), Bridge-System (SM), 3D System,

Coopersburg, Pennsylvania, 1987.

pfluger bridge

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