JAMES D. PFLUGER PEDESTRIAN AND BICYCLE 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 population
soaring nearly 40% over the past ten years. Austin is also unique among large cities in that many of its
citizens are avid walkers, runners, and bicyclists, not only for recreation, but also as a practical alternative for
the daily commute.

The Town Lake area—formed along the southern edge of downtown Austin by the damming of the Colorado
River—is one of the most popular locations for recreational activity and one of the most heavily-traveled
commuter corridors. The existing Lamar Boulevard Bridge, an historic six span reinforced concrete deck arch
bridge 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 functionally
obsolete.

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

Final design work on the new bridge proceeded on an accelerated schedule to meet a deadline for obtaining
partially matching federal funds for construction. The design engineer and architect worked in parallel to
refine aesthetic concepts, while simultaneously developing construction plans. Several alternatives were
considered for implementing the “Double Curve” concept; in the end, the most attractive and practicable
scheme involved a weathering steel plate girder superstructure with a unique framing plan utilizing opposing
curvature exterior girders to achieve a constant deck overhang. Given the complex geometry of the bridge and
the constraints associated with construction over water, steel proved to be the most practical material for
achieving both the structural and architectural goals of this project. The design engineer used many standard
steel bridge design elements in innovative ways to simplify design and construction of the bridge while still
conforming to, and indeed celebrating, the architectural concepts associated with the “Double Curve” scheme
and 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 an
attractive signature structure well integrated with its surroundings. This paper will highlight how this bridge
used conventional materials and standard highway bridge elements in unique and innovative ways to achieve
the dual goals of a consistent aesthetic theme and an economical, readily-constructed design.


PROJECT HISTORY
Six major bridges—carrying a variety of traffic, including automobiles, trucks, trains and, of course,
numerous pedestrians and bicyclists—cross Town Lake. One of the major crossings is the existing Lamar
Boulevard 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 of
Historic Places in 1994. However, the Lamar Boulevard Bridge suffers from a degree of functional
obsolescence, with 10-foot traffic lanes and narrow sidewalks. Significant population growth in Austin over
the past decade has resulted in substantial daily vehicular traffic on the bridge. This growth, combined with
the bridge’s key location near the center of the north and south shore hike and bike trails, has also resulted in
heavy pedestrian and bicycle traffic. The net effect of combining heavy traffic with narrow lanes and
sidewalks was a facility that was less than ideal for many of its users. The City of Austin desperately needed
an improved facility for crossing Town Lake.

Recognizing this and other needs, the citizens of Austin approved one of the largest bond packages in the
city’s history, including $8 million specifically earmarked to widen the Lamar Boulevard Bridge. In the early
1990s, the City of Austin secured approximately $950,000 of matching federal funding for the project, as part
of the Intermodal Surface Transportation Efficiency Act (ISTEA).

In 1995, the City signed a contract retaining a consultant team—led by HDR Engineering, Inc.—to study
project alternatives. Under the original contractual agreement, the consultant team evaluated six options, all
variations on the theme of widening the existing Lamar Boulevard Bridge. The study phase included
inspection of the existing Lamar Boulevard Bridge, evaluation of existing site conditions, traffic modeling
and extensive public involvement activities. The consultant team held regular meetings with the City of
Austin, the Texas Department of Transportation (TxDOT), the Texas Historical Commission and other key
stakeholders.

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

                                                                  The City considered many options for how
                                                                  to develop concepts for the new bridge,
                                                                  ranging from hosting a full-blown design
                                                                  competition among competing architecture
                                                                  and engineering firms to sponsoring a design
                                                                  idea competition open to the general public.
                                                                  Eventually, the City decided to hold a public
                                                                  workshop for key stakeholders.          This
                                                                  workshop took place in May 1998, and
                                                                  generated fifteen proposed concepts,
                                                                  including:

                                                                  •    Cable-stayed bridge options
                                                                  •    Arch bridge options
                                                                  •    Relocation of an existing, historical
                                                                       truss bridge
                                                                  •    Several variations on beam bridge
Figure 1: Original sketch from the May 1998 public                    options
workshop, showing what would eventually become the
concept for the final design of the James D. Pfluger             One of the most innovative and intriguing
Pedestrian and Bicycle Bridge.                                   concepts proposed was the “Double Curve”
                                                                 shown in Figure 1. This unique concept was
developed by a group of five workshop participants (Chas Tonetti, Tere O’Connell, Jamie Wise, Rush
McNair and Chris Hutson) and focused on the function of the facility suggesting the form of the structure.
                                                               The concept grew around the “paths of
                                                               travel”: the function of connecting the trail
                                                               system along the south shore of Town
                                                               Lake to activity centers at 5th and 6th
                                                               Streets and a future public activity center
                                                               on the north shore. The concept featured
                                                               two curved alignments, crossing over each
                                                               other at Town Lake and creating a wide
                                                               area that would serve as a “gathering
                                                               [place] at the river” for people to stop and
                                                               enjoy the view or watch lake activities.
                                                               This “curved” theme would then be
                                                               echoed in the design of the structure itself.

                                                              The City selected four of the 15 proposed
                                                              concepts for further development by the
                                                              consultant team’s architect (Kinney and
                                                              Associates + Carter Design Associates),
                                                              including the Double Curve concept. The
  Figure 2: The developed concept prepared by the consultant  architect set a criterion of “no straight
  team architect for the “Double Curve Alignment” option.     lines” and conceived a structure with an
                                                              hourglass-shaped deck plan resulting from
                                                              the    crossing’s    curved     horizontal
alignments and featuring helical ramps and curved connector spans at each end. The architect’s original
Double Curve concept is shown in Figure 2. The Double Curve concept remained true to the basic “curved”
concept from the public workshop – it was simply developed from a freehand sketch to a higher level of
refinement.

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 new
bridge. The Double Curve concept emerged as the selected alternate. With only six months remaining before
the matching ISTEA funding would expire, development of aesthetic concepts had to occur in parallel with
the preparation of the final Plans, Specifications and Estimates (PS&E).



DESIGN
Basic Themes and Concepts
As the first step in the final design
process, the consultant team’s engineers
further refined the architect’s developed
concept for the Double Curve structure.
The helical ramp at the south end of the
bridge was eliminated following a
geometric evaluation of the vertical
profile and existing topography, hike and    Figure 3: The “Double Curve Alignment” option, as refined
bike trails, and several surface streets.    by the engineering team.
Structural design, constructability and cost considerations resulted in the decision to change the north end
“triangle connector” span from curved to tangent. The engineering team established a span arrangement
sensitive to both the adjacent Lamar Boulevard Bridge and users of Town Lake for crossing the lake, trails
and streets at both ends of the bridge. As the plan developed, it stayed remarkably true to the original freehand
concept sketch from the public workshop.

Following these refinements, the consultant
team constructed a scaled physical model
of the proposed bridge. This model vividly
illustrated how the bridge itself would look
and, more importantly, how it would fit
into its surrounding environment. Figures
4 and 5 show the physical model.

Several structural systems—including cast-
in-place, post-tensioned concrete box
girders; precast segmental concrete box
girders; and solid or voided cast-in-place,
post-tensioned      slab    structures—were
initially considered for the superstructure.
However, the need for quick, simple
design—combined with the overriding
criteria of ease of constructability over the
lake, ability to easily conform to geometric
complexities and low construction cost— Figure 4: Physical model of the James D. Pfluger Pedestrian
quickly led the engineering team to select and Bicycle Bridge.
steel plate girders as the best possible
choice for the superstructure system. The architect requested use of weathering steel to provide an “organic”
or “natural” appearance to fit with the wooded shorelines of Town Lake. Additionally, this meshed well with
cost 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 used
                                                              in unconventional manners. Basically, the
                                                              engineering team set out to produce a set of
                                                              plans closely resembling those for a major
                                                              steel plate girder highway bridge, with the
                                                              exception of the basic geometry and
                                                              aesthetic treatments. The scale of this
                                                              structure and the selection of steel plate
                                                              girders as the main structural system made it
                                                              clear that only heavy highway bridge
                                                              contractors would be bidding on this project.
                                                              In order to obtain low, competitive bids, the
                                                              engineering team had to produce plans that
                                                              would look familiar to these contractors to
                                                              give them confidence in their understanding
                                                              of the project and allow them to bid the
                                                              project without fear of surprises during
Figure 5: Physical model of the James D. Pfluger Pedestrian   construction.
and Bicycle Bridge.
Design Criteria
The scale of the James D. Pfluger Bridge suggested that although it is a “pedestrian bridge” covered by the
provisions of AASHTO’s Guide Specifications for Design of Pedestrian Bridges (1), it is more the same scale
and construction as a typical highway bridge, covered by the provisions of AASHTO’s Standard
Specifications for Highway Bridges (2) and AASHTO’s Guide Specification for Horizontally Curved
Highway Bridges (3). The design criteria selected for the bridge reflect this assessment of the bridge’s
character.

The basic bridge design criteria were AASHTO’s Standard Specifications for Highway Bridges (2) and
AASHTO’s Guide Specification for Horizontally Curved Highway Bridges (3). However, the live load
definitions were modified to reflect the nature of the structure as a pedestrian/bicycle facility, and a 100 psf
live load was used as the primary live load. This 100 psf load was applied both over the entire structure and
in various “checkerboard” patterns to determine maximum loading effects. 100 psf is a higher criteria than
that provided in AASHTO’s Guide Specifications for Design of Pedestrian Bridges (1) but reflects the criteria
used in several other similar structures where large groups of people are expected to congregate during major
recreational and civic events (such as regattas on Town Lake or Independence Day fireworks shows). In
addition, 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.

AASHTO’s Guide Specifications for Design of Pedestrian Bridges (1) also provides criteria for vibration
analysis of pedestrian bridges. The consultant team performed vibration analyses of the various units of the
James D. Pfluger Pedestrian Bridge and, as expected, found no cause for concern. These provisions of the
guide 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 has
commensurate dynamic characteristics. A simple analysis approach quickly demonstrated the bridge’s
fundamental frequencies were well above the threshold of concern.

                                                                    Framing Plan
                                                                    The Double Curve alignment resulted in
                                                                    an unusual plan for the bridge (see Figures
                                                                    6 and 7). At the south end, two ramp
                                                                    structures curve toward each other. The
                                                                    southwest ramp is a two-span continuous
                                                                    unit (Unit A; 86’-120’), while the
                                                                    southeast ramp consists of two single-span
                                                                    units (Units B and C; 48’ and 111’
                                                                    respectively). Units A and C each utilize
                                                                    three concentric, horizontally-curved
                                                                    composite plate girders. Unit B utilizes
                                                                    three concentric, horizontally-curved
                                                                    composite rolled beams. Unit B has a
                                                                    relatively short span, allowing the use of
                                                                    rolled beams to achieve a shallow
                                                                    superstructure depth, required to maintain
Figure 6: Drawing and photograph showing the plan for the           adequate vertical clearance over the hike
main river crossing of the James D. Pfluger Pedestrian and          and bike trail below. Units A, B and C are
Bicycle Bridge (photo courtesy of White Photographic                relatively narrow (for this structure), with
Services, Austin, Texas).                                           a total out-to-out width of 23’-0”.
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

Units A and C meet at Interior Bent 3 and the bridge continues out over Town Lake on Unit D, a three-span
continuous 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 in
this 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 from
Interior Bent 6 to Interior Bent 7W to the northwest; Span 6E (109’ span; 26’-0” width), a single span unit
curving from Interior Bent 6 to Interior Bent 7E to the northeast; and Span 6X (49’ span; 18’-0” width), a
single span unit spanning between support brackets on the exterior girders of Spans 6W and 6E. Spans 6W
and 6E each utilize three concentric, horizontally-curved composite plate girders, while Span 6X utilizes three
tangent composite rolled beams. The use of the relatively shallow rolled beams in Span 6X was possible due
to the comparatively short span length and was desirable since it simplified the detailing of the beam ends at
their supports.

Two further units, Units F and G, were designed running to the northwest over the hike and bike trail, Cesar
Chavez Boulevard and Sandra Muriada Way. Unit F is a four-span continuous steel plate girder unit utilizing
two concentric, horizontally-curved composite steel girders. Unit F runs from Interior Bent 7W to Interior
Bent 11W, where Unit G begins. Unit G is a nine span, conventionally-reinforced concrete slab and T-beam
unit that continues to Abutment 20, where a retained fill section with a switchback ramp leads to the sidewalk
of the existing Lamar Boulevard. Units F and G were fully designed and detailed for this project but were
identified 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 existing
Seaholm power plant, which is planned to have a future public facility. Interior Bent 7E also serves to
support a short-span, conventionally-reinforced concrete slab and beam span that links Span 6E to the Helix
Ramp, a conventionally-reinforced helical ramp structure that rotates through 540 degrees to link up with the
hike and bike trail below. A second link to the hike and bike trail is provided via a stairway from Span 6X to
the trail below.
Units A, B, C and E utilize framing plans and design and construction techniques that are indistinguishable
from 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 plan—not at all typical of highway bridge construction—Unit D merits
further discussion. The unique framing plan of Unit D—with a tangent center girder and two exterior girders
with opposing horizontal curvature—derives from its unusual hourglass plan. The hourglass shape of the
Unit D deck plan is a direct result of the refinement of the original Double Curve alignment concept into a
workable design plan. The final deck plan was essentially unchanged from its first incarnation as a
dimensioned engineering drawing, except for the replacement of the re-entrant corners at the alignment
overlap locations with radiused edges. The final framing plan for the girders, however, was the second of two
concepts examined.

Originally, the engineering team proposed using two tangent girders for Unit D. The girder spacing was set at
approximately 20’ to provide a nominal 3’-3” overhang at the narrowest points of the deck plan. With this
arrangement, adding a third, center girder would have resulted in an inefficient girder spacing. This
arrangement appeared to have merit because it resulted in simpler, cheaper-to-fabricate tangent girders and
uniform 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-place
post-tensioned decks. The cast-in-place options suffered from constructability concerns related to relatively
long span shoring and construction over the lake, while the precast option suffered from complicated detailing
for the interfaces between panels and the interfaces with the shear studs provided to obtain composite action
in the girders.

The tangent girder option also met with disapproval from the project architect, who disliked the uneven
shadow line created from the variable width deck overhang and requested a constant width deck overhang
instead. The engineering team was initially reluctant to consider such a plan, since it resulted in girders with
complex reverse curvature and variable girder spacing. However, after a few minutes of discussion, the
engineering team realized this was actually a win-win suggestion that not only resolved many of the
architect’s concerns, but many of the engineering team’s concerns as well. The resulting framing plan
provided a short, 3’-3” deck overhang and more reasonable interior deck spans – the girder spacing now
varied from 12’- 4” minimum to 17’-9” maximum. This allowed the use of a conventionally–reinforced, 12”
thick 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 framing
plan with a tangent center girder and two exterior girders with both opposing and reverse curvature would
appear 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 be
accounted for: global overturning and local lateral flange bending. The global overturning effect results from
the center of gravity of any horizontally-curved bridge span being offset from a chorded centerline drawn
between the two supports. This global overturning moment manifests itself as an increase in the vertical
loading of the girder on the outside of the curve and a decrease in the vertical loading of the girder on the
inside of the curve. The “shifting” loads are carried in the diaphragms, which in a typical horizontally-curved
highway bridge become primary load-carrying members.
The lateral flange bending effect results from
the axial forces in the flanges being applied to
structural elements that are curved. Thus
flanges in compression experience lateral
flange bending moments that tend to bow the
flanges more, while flanges in tension
experience lateral flange bending moments that
tend to pull the flanges straight.

In Unit D, the global overturning moment
effect did not exist. Since the girders were
essentially symmetrical in plan, the center of
gravity of each span essentially fell on the
centerline of the supports. Thus, the only effect
of curvature requiring consideration was the
lateral flange bending effect, which could
easily be included in the design calculations
using the simple lateral flange bending moment
formula presented in the V-Load curved girder          Figure 8: Drawing showing the rolled beam diaphragms.
design methodology (4):

                                               Md 2
                                     M Lat =
                                               12 Rh

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 D
                                                                     could be designed as tangent girders with
                                                                     the lateral flange bending moment effects
                                                                     added in manually. This is exactly what
                                                                     the engineering team did for the
                                                                     preliminary design of the Unit D exterior
                                                                     girders. First, a tangent girder design was
                                                                     performed       using    the    commercial
                                                                     STLBRIDGE tangent plate girder design
                                                                     program (5). Then, the stresses resulting
                                                                     from this design were imported into a
                                                                     spreadsheet. Other key parameters, such
                                                                     as the radius of horizontal curvature,
                                                                     diaphragm spacing and girder depth, were
                                                                     input and the lateral flange bending
                                                                     stresses calculated and compared to stress
                                                                     limits calculated in accordance with
                                                                     AASHTO’s Guide Specification for
Figure 9: Photo showing the rolled beam diaphragms.                  Horizontally Curved Highway Bridges (3).
The diaphragm spacing and flange sizes were then adjusted to result in an optimized design. This approach
proved quite successful, and these preliminary designs exhibited excellent correlation with the results of a
                                                                 later, detailed 3D finite element analysis
                                                                 performed using the proprietary BSDI
                                                                 3D System computer modeling service
                                                                 (6).

                                                                    Diaphragm Detailing and Design
                                                                    Given the variable girder spacing in Unit
                                                                    D, the detailing of diaphragms warranted
                                                                    special attention during the design
                                                                    process. The engineering team initially
                                                                    examined several options, including plate
                                                                    diaphragms, rolled beam diaphragms, X-
                                                                    frames, W-frames and K-frames.

                                                                      Frame diaphragms are perhaps the most
                                                                      typically used in highway bridge design,
 Figure 10: Sketch of two column bent.                                but would have been cumbersome for this
                                                                      bridge due to the variable girder spacing—
                                                                      a 12’-4” minimum to a 17’-9”
maximum—in Unit D. With such wide girder spacings, the diagonals of X-frames or K-frames would have
inefficiently shallow angles from horizontal. Similarly, W-frames would either have shallow diagonals or
require multiple W’s to span between the girders. Combined with these inefficiencies, there would have been
virtually no identical diaphragms anywhere in Unit D (because of the 15 degree skew, half of the
commonality due to symmetry was also lost). Finally, all the different bracing details would have been
aesthetically distracting for recreational boaters under the bridge on Town Lake.

As a result, the engineering team selected
rolled beam diaphragms (see Figures 8 and
9). In addition to eliminating problems
with inefficient diagonal angles in the
frame options, rolled beam diaphragms
also offered substantial advantages in
fabrication. Instead of developing jigs for
many different frame diaphragms, the
fabricator simply cut rolled sections to the
lengths needed for the variable girder
spacing in Unit D. In order to reduce
weight and keep the diaphragm stiffnesses
reasonable, shallow rolled beams were
selected for the diaphragms.            The
difference between the diaphragm depth
and the depth of the girders was made up
using a simple curved gusset detail at the
ends of the diaphragms.
                                                Figure 11: Photo of two column bent.
Beyond the greater efficiency and simpler
fabrication of the rolled beam diaphragms, they also provided a much simpler and cleaner appearance. The
curved gusset detail also worked extremely
well with the overall curved theme of the
bridge. Thus, the choice of rolled beam
diaphragms proved to be another example
of a win-win solution that best satisfied
aesthetic, engineering and construction
criteria.

Since the diaphragms perform as primary
load-carrying members, particularly in the
curved ramp structures of Units A, B, C
and E, they were carefully designed using
diaphragm forces obtained from the results
of BSDI 3D finite element analyses.

Substructure Design
The substructure system for this bridge is Figure 12: Photo of single column bent.
quite conventional in its materials but quite
unconventional in its form. All substructures are conventionally-reinforced, cast-in-place concrete bents, with
pile caps straddling conventionally–reinforced, concrete drilled shaft foundations. Depending on the
superstructure width at any given bent location, either two column frame bents or single column hammerhead
bents were used.

In following the “curved” theme for the structure, the two column bents ended up with a rather unusual
appearance. Following an exhaustive aesthetic design process, the final selected bent form included the use
of curved columns and a haunched bent cap (see Figure 10 and 11) to allow views of scenic Town Lake
through the bents. In and of itself, this two column bent, although not typical in appearance, was not overly
unconventional. However, the narrower superstructure sections utilized a single column hammerhead bent.
                                                                   To stay consistent to the established
                                                                   theme, these bents also used curved
                                                                   columns; hence the asymmetrical curved
                                                                   column hammerhead bents (see Figure
                                                                   12).

                                                                   Although unconventional in appearance,
                                                                   these bents posed no unusual challenges
                                                                   from a design standpoint. The engineering
                                                                   team strove to find ways to simplify the
                                                                   construction whenever possible to help
                                                                   keep construction costs low. For example,
                                                                   the architect had originally drawn
                                                                   essentially freehand curves for the bent
                                                                   cap soffits and column alignments. The
                                                                   engineering team approximated these
                                                                   curves with circular curves set with given
                                                                   PC and PT locations that allowed the
                                                                   contractor to build and reuse one set of
Figure 13: Erection of girders at the north end of the main        forms for multiple bents on the bridge.
river crossing.
BIDDING AND CONSTRUCTION – LESSONS LEARNED
                                                                     The construction contract was bid twice.
                                                                     The first round of bidding involved four
                                                                     bidders, all heavy bridge construction
                                                                     contractors. The bids were relatively
                                                                     widely separated and all bids exceeded the
                                                                     City of Austin’s allowable budget for
                                                                     construction of the project. However,
                                                                     detailed evaluation of the bid tabulations
                                                                     revealed some lessons.

                                                                   While the high and low bids were
                                                                   separated by 24 percent, comparison of
                                                                   only the 20 bridge-related bid items
                                                                   (which essentially included all items
                                                                   except the helix ramp, retaining walls at
                                                                   the trails, architectural bridge railing,
                                                                   lighting details, colored concrete wearing
                                                                   course, and landscaping and associated
 Figure 14: Formwork for the deck at the south end of the main     site work) revealed the three low bids
 river crossing.                                                   were 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 that
allowed the City to restructure the project after bidding in an effort to fit the project within the construction
budget. This second round of bidding
included two bidders and resulted in a
winning bid that, with Deductive Alternate
1 (which eliminated Units F and G [see
Figure 7]), fit the City’s construction
budget. The construction contract was
awarded to Jay-Reese Contractors, Inc. of
Austin, Texas, in April 2000, after an up-
to-the-last minute debate on project options
and costs in the Austin City Council.

Groundbreaking occurred on May 15,
2000, and the contractor was allocated a
one-year schedule with incentive bonuses
for early completion. After allowances for
foul weather days and change orders, the
contractor completed the project ahead of
schedule on June 16, 2001. Construction
usually progressed very smoothly, a             Figure 15: Concrete placement for deck at the sound end of
testament to the philosophy followed in         the main river crossing
preparing the bridge plans.
Generally, construction of the main lake
crossing (Units A through E) followed the
typical construction sequences and
appearances of routine, large highway
bridge construction projects (see Figure
13). The contractor was well-versed in this
type of construction and the plans were
relatively error-free. There were only two
minor added-charge change orders during
the main lake bridge construction. In fact,
there were relatively few problems with the
main lake bridge construction at all. One
of the minor problems that did occur is
highlighted below as a “lesson learned.”

The single biggest construction problem
was associated with the alignment of the
single column bent caps. At the insistence
of the architect, all bent caps for the main Figure 16: View of the new James D. Pfluger Pedestrian and
lake crossing were detailed with widths Bicycle Bridge from under Bent 7E.
identical to the thickness of their
supporting 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 column
forms would shift and twist slightly. The rotational twist resulted in the corners of the tops of the column
being out of position only ¼” 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 ¼”
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 bent
                                                                     caps, it was practically impossible to
                                                                     recover the correct alignment, as the
                                                                     forms could not be deformed to move the
                                                                     ends of the bent cap back into their
                                                                     proper positions. This misalignment
                                                                     complicated all other bent cap details,
                                                                     such as the positions of anchor bolts and
                                                                     bearings. In retrospect, the engineering
                                                                     team should have been more adamant
                                                                     about disallowing this questionable
                                                                     detailing.

                                                                     Many other unique or unconventional
                                                                     details worked well during construction,
                                                                     but for the most part, the smooth
                                                                     construction was due to detailing
                                                                     consistent with typical highway bridge
                                                                     detailing. This made it easy for an
  Figure 17: View of the new James D. Pfluger bridge looking         experienced, qualified highway bridge
  south at deck level. Note planters (not complete in this photo)    contractor to construct this bridge.
  that help to delineate “gathering” places from travel lanes.
CONCLUSION
This was a challenging, exciting and rewarding project
for the design team, even though the number of
stakeholders often made the project process arduous. The
short schedule for final design and the need to prosecute
detailed structural design simultaneously with conceptual
aesthetic design added to the challenges. However, the
design team had a truly inspired and well-conceived basic
concept from which to work and the unusual nature of the
bridge always kept the project interesting.

The engineering team successfully used conventional
materials and techniques in unconventional manners to
produce a set of plans that resulted in very competitive
bidding for construction. The unit cost for the main lake
crossing worked out to be approximately $150/SF. This is
high compared to typical highway bridge construction
costs in Texas (which can range as low as $34-40/SF),
but given the aesthetic and functional requirements of the
project and in comparison to equivalent projects this unit
cost relatively low. This approach also greatly facilitated
the construction phase, with very few Requests for
Information (RFIs), and no significant change orders
during construction of the main lake bridge. Assembling
the plans for this unique pedestrian/bicycle bridge to look
                                                               Figure 18: On June 16, 2001, hundreds of
like the plans for a typical highway bridge was a gambit
                                                               runners sprinted over the new James D.
that paid off well. Pedestrians and bicyclists will now
                                                               Pfluger Pedestrian and Bicycle Bridge,
have a new, safe, attractive route across Town Lake and
                                                               marking the bridge’s grand opening. The
the bridge’s owners and users are sure to be delighted
                                                               bridge was named in honor of the Austin
with their new facility.        All in all, this project
                                                               architect who conceived the trail system on
demonstrates        that      aesthetics,     functionality,
                                                               either side of Town Lake.
constructability and cost effectiveness need not be
mutually exclusive goals for a bridge project.
REFERENCES
1. American Association of State Highway Transporation Officials (AASHTO), Standard Specifications for
   Highway Bridges, 16th Edition, 1996, with Interim Updates through 1998.

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

3. American Association of State Highway Transporation Officials (AASHTO), Guide Specifications for
   Horizontally 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.

				
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