Jigsaw bridge puzzle

March 21, 2002

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.

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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