Jigsaw bridge puzzle

Innovative fabrication makes onsite assembly a breeze

Bridges Article March 21, 2002
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When the Virginia Department of Transportation needed to replace the mainline structure of the I-95 James River Bridge, they knew it had to happen without affecting rush-hour traffic through the middle of downtown Richmond.

The initial contract called for a traditional replacement of
each span with precast units consisting of individual girders and a deck, with
transverse and longitudinal closure joints for each piece. But this system was
not a good fit for the project’s tight 7 p.m. to 6 a.m. nightly schedule.

Under the original plan, the project team would come in at
night, cut out one piece, replace it, construct a temporary transition and open
the bridge in the morning. It would take two or three nights to complete each
section.

Joining forces with the Virginia DOT and Archer-Western Contractors Ltd., Fort Lauderdale, Fla., Parsons Bridge and Tunnel Division developed constructibility modifications for the project. For Parsons, the key to making the process work better was match-casting.

In match-casting, pieces are cast together and then
separated. The idea is that the pieces will fit back together at the jobsite
better than pieces that were cast separately. Parsons likes to compare
match-casting to creating a jigsaw puzzle: you make the picture first, then cut
it up, instead of making each piece separately.

Using a match-cast process, the James River Bridge team was
able to increase the size of the replacement pieces. This reduced the total
number of pieces in the project by approximately one-third. The precast
composite units (PCUs) were typically three girders across with a concrete
deck. They varied in length from 45 to 95 ft with a deck width of approximately
22 ft. Reducing the total number of replacement pieces allowed the team to cut
their work in half.

The team built each span completely and used a transverse
match-cast joint to pour one against the other. The bridge structure was cast
in three 22-ft pieces for a total width of 66 ft. This was the first time a
long, thin slab had been successfully match-cast. Implementing a match-cast
system, in which adjacent PCUs are match-cast longitudinally and erected side
by side to make up the full width of a given span, increased efficiency and
saved time. With the modifications and the match-cast approach, the I-95 crew
could replace three units—and complete an entire span—in just one
night.

 

Benefits of match-casting

As with Parsons’ work on the I-95 replacement, the
match-cast method allows a project team to accelerate the erection schedule.
With more traditional cast-in-place segmental construction using form
travelers, two segments—or 20-35 ft of superstructure—can be cast
in place every week. But with precast segmental construction, an entire
cantilever or span—100-250 ft of superstructure—can be erected in
the same time.

On the Santa Rosa Bay Bridge in Milton, Fla., Odebrecht
Contractors of Florida Inc., Coral Gables, and Metric Constructors Inc.,
Boston, erected seven 140-ft spans in one seven-day week using match-cast segments.
Traditionally precast members are cast straight, prestressed and erected with
cast-in-place joints or diaphragms between each member. AASHTO girders,
box-beams and T-beams are all examples of traditional precast members used for
bridge construction. With match-casting, the intermediate cast-in-place
closures and joints are minimized or completely eliminated from the bridge
structure, saving valuable time during erection.

After match-cast pieces are placed, they are fitted together
with post-tensioning. Each match-cast joint between segments can be dry, but it
is most often coated with an epoxy. The epoxy acts as a lubricant, assists with
alignment and serves as a sealant.

For span-by-span erection, in which the bridge structure is
erected one span at a time from pier to pier, one cast-in-place closure joint
at each end of the span is typically required. For balanced-cantilever
erection, in which the bridge structure is erected in both directions from the
pier segment, one cast-in-place closure joint is required at midspan between
two adjacent cantilevers. It is critical to cure and obtain the required
strength in the closure before post-tensioning the span so the allowable stress
in the cast-in-place closures is not exceeded.

The cast-in-place closure joints can be eliminated for
span-by-span erection if a means for sliding the pier segment is provided.
Regardless of the method or how many closure joints are provided, the segments
must be cast correctly to ensure a successful erection and achieve the desired
final geometry.

 

As good as it’s cast

While more bridge designers are approaching match-casting
for different types of projects, especially larger bridges in environmentally
sensitive areas or with short timetables, the process is not a cure-all. The
structure is only as good as the pieces that are match-cast. Using either a
long-line or  short-line process,
the contractor must ensure that the correct horizontal and vertical roadway
geometry—as well as casting camber—is achieved. In theory, to
achieve the desired geometry, the contractor needs to provide small horizontal
and vertical angle breaks between segments, and each segment should be
trapezoidal or pie-shaped in plan and elevation.

For long-line match-casting, segments are cast along a bed
that exactly reproduces the geometry of the span or cantilever. As each segment
is cast in the long bed, the bulkhead is moved ahead to the next position for
the next segment. The previously cast segment remains in the casting bed and
the bulkhead face of the previous segment is used as the bulkhead form for
creating the next segment.

While only one segment per bed can be cast per day because
the concrete cures overnight, long-line provides an easier means for setting
and monitoring the geometry control. Bigger segments can be cast, but precast
piece sizes are still dictated by transportation and erection limitations. One
disadvantage to long-line casting is the substantial space that is required for
fabricating a long-line bed: it needs to be at least as long as half the span
length.

Short-line match-casting uses a stationary casting form to
create each successive segment (“wet-cast segment”) against the
previously cast piece (“match-cast segment”). After a contractor
casts and cures one segment, he moves it into position to cast the next
segment, and the forward face of the match-cast segment becomes the bulkhead
for the rear face of the wet-cast segment. Each wet-cast segment is cast flat
while the match-cast segment is positioned to provide small horizontal and
vertical angle breaks between segments. The match-cast segment can be shifted
horizontally or vertically to ensure the correct geometry.

If there is a superelevation transition between two
successive segments, the match-cast segment also can be rotated about its
longitudinal axis. The contractor surveys the position of the match-cast
segment to determine the as-cast geometry of the wet-cast segment and
subsequent setup for casting the next segment.

Short-line casting is ideal for projects with time and
space constraints. Project teams can cast one segment per day per casting bed (allowing the wet-cast segment to cure overnight), and the casting forms are smaller and can accommodate different geometry requirements. For congested urban interchange projects with multiple ramps that curve and thread through the
interchange, short-line match-casting is a valuable tool. On bridge projects in
Las Vegas, Albuquerque and Dallas, the short-line method proved invaluable.
(See “Match-casting in action” below.)

 

Geometry is key

In a process where achieving the desired geometry is key, it
is necessary to establish reference points on each segment. Typically six
control points per segment (four for elevation control and two for horizontal
control) are used to physically position each segment and to determine as-cast
positions after segments are cast.

For vertical control, a minimum of three bolts or rivets
(but typically four) are cast into the deck near the segment joints.
Contractors use a level to record the theoretically level position of the
wet-cast segment and the after-cast position of the match-cast segment. The
actual length of the wet-cast segment also is measured and recorded during the
after-cast survey. Any casting errors then must be included with the desired
geometry to determine the setup for casting the next segment.

For horizontal control a hairpin wire is cast into each end
of the segment on the centerline, near the segment joint. During the after-cast
survey, the longitudinal segment centerline is transferred from the survey
control by placing a punch or sawcut marks in the hairpin wire of the wet-cast
segment using a surveying instrument called a theodolite. The after-cast
position of the match-cast segment also is recorded using a theodolite and a
scale.

 

Not just for superstructures

While match-casting is widely used for superstructure
projects, it can be beneficial for substructure production and erection as
well, especially on jobs with site limitations. For example, pier columns can
be match-cast offsite and erected quickly by stacking the precast pieces.

Bayshore Concrete Products Corp., Cape Charles, Va.,
employed a match-cast process for pier columns during the C&D Canal Bridge
project in Delaware. GLF Construction Co., Miami, similarly match-cast the
substructure for the Seabreeze Bridge in Daytona Beach, Fla. Martin K. Eby
Construction Co. Inc., Wichita, Kan., also is currently using match-casting for
the pier columns in the Dallas-Fort Worth Airport Automated People Mover.

When match-casting pier column pieces, the contractor
positions match-cast segments on a leveling table in the same orientation it
was cast, and the forms for the next segment are positioned on top of the
match-cast segment. These forms are oriented so that the center line of the
segment on both the minor and major axes coincides with the true vertical
alignment of the pier column.

 

Match-casting in action

Following are some projects that have successfully used the
match-cast approach to erect structures in fast schedules, tight spaces and
environmentally sensitive areas:

 

GCRTA Conrail Bridge (Cleveland)

The use of match-casting to manage time and geometry demands
can be illustrated by the requirements of the Greater Cleveland Regional
Transit Authority (GCRTA) Bridge. The bridge provides access for the Waterfront
Transit Line over the Conrail tracks in Cleveland’s waterfront district.
With 72 precast match-cast segments, the five-span structure had to be cast and
erected within six months from the Notice-to-Proceed in time for
Cleveland’s bicentennial celebration in the summer of 1996. In addition
to the time constraint, the bridge curvature included a 142-ft radius. The
segments were match-cast in an enclosed warehouse 130 miles south of the site
during the winter. The Kokosing Con- struction Co., Frederickton, Ohio, cast
and erected the segments, which were erected in balanced cantilever with a
ground-based crane.

 

Baltimore-Washington International Airport (Baltimore)

Kiewit Construction Co., Omaha, Neb., is currently exploring
variations of the long-line casting process for a curbside extension of the
Baltimore-Washington International Airport. The relatively straight geometry
makes the long-line casting economical for this project. Each precast piece
measures 61 ft wide and 11 ft long and is part of an 8-in.-thick slab with
4-ft-deep edge girders and transverse steel girders. The wide flange steel
girders are cast compositely into the bottom of the deck slab with shear studs.
The segments over the bent and pier locations are match-cast with the span
segments. The result is one cast-in-place closure per span.

 

Each of the following multi-ramp interchange projects had
traffic and space constraints and used short-line match-casting with different
erection means:

 

“Spaghetti Bowl” (Las Vegas)

Walter Construction, Calgary, Alberta, added four match-cast
ramps with an assortment of curves and span lengths through the interchange of
I-15 and U.S. 95. The team erected three different cross-sections by both
span-by-span and balanced cantilever with an overhead gantry. Using an overhead
gantry to place match-cast segments allowed the team to quickly construct the
Spaghetti Bowl with minimal interruption to traffic.

 

“Big I” (Albuquerque, N.M.)

Just a year and a half after being awarded the contract for
the Big I interchange, Twin Mountain Construction, an Albuquerque subsidiary of
Kiewit, opened eight new precast ramps. The firm used match-casting to erect
the ramps for the extremely fast-paced project, including more than 650 precast
segments. An adjacent casting yard allowed the team to increase the size of
each segment to 80 tons and to use a balanced-cantilever approach with a
ground-based crane to erect the structure.

 

“Dallas High Five” (Dallas)

In a joint venture with Rizzani de Eccher, Udine, Italy, and
H. B. Zachry Co., San Antonio, Parsons Corp. is currently redesigning the
LBJ/Central Expressway Interchange to eliminate bottlenecking, loop ramps and
confusing left-hand exits. Referred to as the “Dallas High Five,”
the new interchange of I-635 and U.S. 75 will provide a five-level junction
with an assortment of flyover ramps, highway widening and high-occupancy
vehicle lanes to accommodate over a half million vehicles per day. By match-casting the segments offsite, the team can complete the interchange earlier with less disruption to traffic. The ramps will be erected in balanced cantilever using a specially designed rubber-tire segment erector on top of the cantilever.

With increasing demands on building bridges faster without
disrupting traffic or damaging the environment at the bridge site, the
match-cast process offers many advantages. Match-casting provides an innovative
alternative method for bridge construction that can be varied to meet the
requirements of the project as well as the contractor’s strengths.

About the author: 
Mehle is a regional bridge engineer with the Denver office of the Parsons Bridge and Tunnel Division of Parsons Transportation Group, Broomfield, Colo.
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