The Vietnam Veterans Memorial Bridge is a 1453-m (4,766-foot)-long crossing of the James River in Virginia, directly downstream from the Port of Richmond. It is part of the 14-km (9-mile) -long Route 895 design-build toll-way project that will link Chippenham Parkway (Route 150) in Chesterfield County with I-295 in Henrico County near the Richmond International Airport.

The bridge spans the 61-m (200-foot) -wide navigational channel leading to the port, can accommodate a possible future widening of the channel to 91 m (300 feet), and provides the required vertical clearance of 44 m (145 feet). The total width of the river is approximately 206 m (675 feet).

The bridge comprises two 3-lane roadways over the river and an existing interchange of I-95 and the Chippenham Parkway. Additional ramps connecting I-95 and Route 895, also part of this project, are located on the west side of the river, directly adjacent to existing industrial plants and a municipal waste treatment plant. Right-of-way constraints and the height of the mainline above grade resulted in complex geometric arrangements for the ramps and relatively high piers. Many buried utilities were in the area of the west approach span because of the industrial character of the riverbank area, and the profile required relocation of high voltage power lines that crossed the alignment.

Figure 1: West Bound Bridge Elevation (Larger Image)

Figure 2: West Approach Span Plan and Section

PB prepared a bridge type in 1997 as a subconsultant to the ramp designer. We were selected subsequently by the design/build team to prepare the final design for the approach spans and river crossing bridges, which are shown in profile in Figure 1. The major elements of the project are the:

• Crossing Bridge. Cast-in-place, variable depth segmental concrete box girders with a maximum span of 206 m (675 feet)
• Approach Spans. Constant depth, precise segmental concrete box girders with a maximum span of 64 m (210 feet)
• Tangent Ramps. Constant depth, precast segmental concrete box girders
• Curved Ramps. Steel plate girders.

Approach Spans

The west approach spans were divided into two units:

• Unit 1 extends 415 m (1,360 feet) between the west abutment and the ramp gore area for Ramps F and G (Figure 2) and is comprised of seven spans.
• Unit 2 is more complex, extending 162 m (530 feet) from the ramp gore area to the river crossing and comprised of three spans of differing lengths to accommodate the existing I-95 mainline and Chippenham Parkway ramps directly below (also shown in Figure 2).

Substructure. The area is relatively low-lying with a high water table. Rock was encountered at a relatively shallow depth of approximately 6 m (20 feet) below grade at the west abutment and 15 m (50 feet) at the west riverbank, and hazardous materials were reported present in the soils. Vertical and battered steel H piles driven to rock were used for the foundation system with the bottom of the footing elevation located near existing grade to minimize or eliminate excavating existing soils and minimize hazardous materials issues.

Benefits of Hammerhead Pier Caps. Single column piers with hammerhead pier caps were selected for the approach piers, giving a relatively conventional substructure configuration. Because the deck cross slope varied and the deck segments were a constant depth, the cross slope of the caps was varied and the geometry standardized to eliminate adjustments in the cap forms and allow the use of a typical prefab reinforcing cage. We used these caps on the steel ramp structures also to have a common substructure type for the majority of piers on the project and improve the overall aesthetics by reducing the number of potentially visually conflicting elements.

Seismic Issues. The site is in an area with a design bedrock acceleration of 0.13 g. The AASHTO Seismic Performance Category (SPC) B classification required detailing the column bases for concrete confinement to provide for ductile hinge development, so we used circular columns to facilitate predictable hinge development and simplify construction. A709 (not the more conventional A615) reinforcing bar hoops with mechanical couplers were used in the ductile hinge areas. These couplers were adopted based upon their successful testing and application on many projects constructed by the California Department of Transportation (CalTRANS), which has some of the strictest seismic design standards in the U.S.

Superstructure. The bridge decks consist of two precast segmental concrete box girders connected by a longitudinal cast-in-place construction joint. Typical segments are 3 m (9.84 feet) long and 3 m (9.84 feet) in depth. To accommodate the varying width for Unit 2 in the vicinity of the gore area for Ramps F and G, the width of the cantilevered slab overhangs that extend from the trapezoidal box core of the girder was varied. This arrangement standardized the core forms for the casting machines and simplified formwork adjustments to accommodate variations in the transverse segment geometry. The standardized core form also made it possible to use the same casting machines to fabricate the approach spans and the ramp superstructures.

Complex Geometry. Horizontal curves were introduced in the girders adjacent to Pier 7 to limit the maximum overhang dimensions. There were also variations in the segment cross slope over the length of the gore areas. This complex 3-dimensional geometry was accommodated by the short-line, match cast, precasting equipment that was custom-fabricated for this project. The segments were transversely post-tensioned in the casting yardand longitudinal post-tensioned in the field during erection.

From a structural design perspective, this arrangement also called for complex analysis. The longitudinal girders are torsionally stiff and restrain the connecting deck slab. A 3-dimensional finite element analysis was prepared to determine the internal forces and stresses resulting from dead and live loads. Time dependent material effects such as creep and shrinkage were investigated by use of specialized segmental design software. Service load combinations specified by AASHTO were investigated, as were ultimate strength requirements.

The box girders are supported on pot bearings to allow for longitudinal movement. Seismic pintels are placed at the bottoms of the pier diaphragms to restrain seismic movements of the superstructure relative to the substructure.

Unusual Feature. One unusual feature of this project is the combination of the precast segmental concrete box girder mainline superstructure with the structural steel plate girder ramp structures. Steel ramp structures are used primarily in areas adjacent to the mainline due to tight horizontal curve geometry or tall columns on curved areas. As a result, the gore areas had to be detailed to accommodate both bridge types.

Durability. Durability of the segments was addressed by the use of epoxy coated deck reinforcing 45 MPa (6.5 ksi) concrete with slag cement, galvanized bars for the parapets, and coated transverse prestressing hardware in the upper areas of the deck slab. In addition, the precast segments were steam cured. Further protection was provided by a 50 mm (2-inch) latex or micro-silica deck overlay.

Figure 3: West Approach Span Construction — Note the overhead truss used to place segments.

Figure 4: East Approach Span Plan and Section

Figure 5: East Approach Span Construction — Note the cranes used to place segments.

Overhead Erection Method. Overhead erection methods were used because access from below was limited due to the existing roadways and temporary detours directly beneath the bridge. Erection of the approach span segments in cantilever was done with an overhead erection truss (Figure 3) used previously on a precast segmental concrete cantilever bridge in Florida. This self-launching truss places segments on opposing longitudinal faces of the pier segment, minimizing longitudinal unbalanced moments on the substructure. The truss also supports stressing platforms necessary for installing longitudinal tendons in the upper portions of the segments.

For the taller columns, there were limitations on transverse unbalanced moments and deflections that resulted from erecting one entire cantilever in advance of erecting any of the segments on the adjacent girder. The truss was equipped with a chassis system that allowed for transverse launching or “side shifting” at piers. As a result, a portion of one girder was erected, the truss side shifted, a portion of the second girder erected, and the truss side shifted again to finish erection of the first girder.

East Approach Spans. The 6-span east approaches extend 351 m (1,150 feet) and are located on curved horizontal alignments (Figure 4). Span lengths range from approximately 41 m (135 feet) to 64 m (210 feet). In contrast to the west approach spans, the east approaches were located in good soils at a height of approximately 15 m (50 feet) above the James River floodplain, and the water table was further below existing grade, as is the bedrock. The steel H pile foundation system was used here also, however, to simplify installation and provide better driving resistance through cobbles and boulders.

The east approach spans have many of the same features as the west approach spans, although the geometry was not as complex because there were no gore areas. The primary difference was in the erection, which was done in cantilever from the piers using ground based cranes (Figure 5) because the relatively open terrain on the east side of the river posed few limitations on contractor access. Longitudinal stability of the partially completed cantilevers required use of cable stays anchored into the footings to resist out-of-balance erection moments.

River Crossing Bridges

The eastbound crossing is three-span cast-in-place segmental box girder with span arrangement of 115 m, 205 m, 24 m (377 feet, 672 feet, 406 feet). The 205 m (672 foot) main span enabled us to locate the piers directly adjacent to and behind the river banks, thereby limiting the hydraulic impacts of the foundations and eliminating the need to design the foundations for ship impact.

The 4-span westbound bridge is considerably longer and has a significantly different span arrangement of 75 m, 129 m, 205 m, 118 m (246 feet, 423 feet, 672 feet, 388 feet) because the existing Chippenham Parkway interchange ramps limited pier placement locations and resulted in the necessity to add the fourth span. The main span piers are located on a common foundation with the eastbound main span piers (Figure 6).

Figure 6: River Crossing Bridge Plan and Section

Figure 7: Pier 13 of the East Bound Bridge Under Construction

Both the eastbound and westbound bridges have ramps that merge with the mainline. Both ramp terminals required lane tapers and drops, resulting in variable-width roadways on both bridges. In addition, a horizontal curve and super-elevation transition is located on the east side of the river within the limits of the river crossing structures. These roadway geometric requirements resulted in unusual, complex geometry for the two long-span structures.

Substructure. As with the west approaches, rock was encountered at a relatively shallow depth of approximately 15 m (50 feet); however, hazardous materials were not reported in the soils in the vicinity of the foundations. All foundations were 2.5-m (8-foot) -diameter concrete-filled drilled shafts. Some of the advantages of using drilled shafts as opposed to H piles were a reduced foundation footprint, improved resistance to seismic forces for these heavily loaded piers, and the stability of the foundation system during potential scour events.

Two Pier Types. The nine piers supporting the main span units are of two general types — double wall and single wall. The four interior piers that supported the main spans needed to resist large unbalanced loadings during cantilever erection while also accommodating longitudinal superstructure movements. A longitudinally flexible substructure would also reduce the superstructure period and limit longitudinal seismic forces in the foundation, so we used twin wall columns for the main piers (Figure 7). The four transition piers with expansion bearings at the ends of the river crossing bridges use single columns. Pier WB-11 also used a single column due to the relatively smaller out of balance erection moments and limitations in foundation placement resulting from the proximity to existing ramps.

Standardized Columns. The columns supporting the river crossing bridge were standardized as much as possible. Each twin wall column has four circular bar cages with additional face bars and cross ties, allowing for prefabrication of a large portion of the cage steel. Also, the use of circular cages accommodated seismic confinement details at the base of the columns required by AASHTO SPC B. Column cap dimensions and reinforcing details were also standardized.

Standardized Superstructure. The superstructure box girder cross sections for the two bridges were also standardized as much as possible to allow for reuse of equipment and formwork. The typical “core” areas of the cross sections consist of two cells, three vertical webs, and a constant soffit width of 14 m (46-feet). The deck slab is cantilevered past the exterior webs and the width of the overhang varies to accommodate the variation in roadway widths. The depth of the girder varied from 12.8 m (42 feet) at the piers to 4 m (13 feet) at midspan, and provided the required 44.2 m (145-foot) vertical navigation clearance at the edges of the future 91.46-m (300-foot) -wide channel.

Variations in the deck cross slope were accommodated by variations in the height of the three girder webs. Thinner webs are used in the midspan areas with relatively low shear forces to optimize the cross section and limit the dead load moments during the cantilevering erection. Segment lengths near the piers were shorter than the lengths further out on the cantilever arms as a result of the larger and heavier cross section required near the piers.

Analysis. As with the approach spans, both a three-dimensional finite element analysis and an analysis using specialized segmental design software were prepared to determine the internal forces and stresses resulting from service and erection loads. The analysis also showed that the variation in web heights resulted in differing degrees of end restraint of the deck slab by the webs, and resulted in the need to thicken the deck slab near the piers. The effect of shear lag on the transverse distribution of normal stresses in the soffit near the piers was also investigated.

Unusual Feature. A very unusual feature of this project is the connection of the structural steel plate girder ramp structures with the far larger segmental concrete box girder mainline superstructure. From a geometric perspective, the gore area required a large variation in ramp width over a relatively short distance, but large variations in the width of the main bridge box girder core were not possible because of limitations on the forms and other construction equipment. Steel plate girders were selected for the framing of a majority of the gore areas because the extent of the widening exceeded the maximum capacity of the segmental concrete deck slab cantilevers. The ramp gore framing is supported at one end on one of the ramp piers and at the other end on a cantilevered concrete bracket projecting from the outer face of the girder diaphragm. The deck slab on top of the gore area is detailed with a longitudinal closure joint to limit the effects of the box girder stiffness on the vertical deflection of the steel plate girders during deck slab placement. Design of this area required a finite element analysis to determine the transverse and longitudinal bending moments in the deck slab and cantilever overhangs from the box girder.

Figure 8: East Bound River Crossing Bridge Construction Sequence

Erection Method. Erection of the segments in “balanced” cantilever was done with traveling forms supported by the previously completed portions of the superstructure. These space frames supported the forms and wet concrete weight of the segments, the largest of which weighed on the order of 3,692 KN (830 kips) and was approximately 12 m (40 feet) tall. The travelers also support platforms required to finish the concrete surfaces and install transverse and longitudinal prestressing tendons. Because of the unequal length of the side span cantilever arms relative to the main span arms and the very large rotational stiffness of the twin wall piers, temporary support towers were required under the side spans at two locations to limit the erection impacts of on the substructure. The construction sequence is shown in Figure 8.

Unique Feature. A unique feature of this project was the use of progressive cantilevering for the westbound bridge span over I-95 between Piers WB-10 and WB-11. It was not possible to balance the segment weight at Pier WB-11 because of the span arrangement, so one arm of the cantilever was made continuous with the adjacent arm well before the opposite arm was completed. A vertical jacking operation was also introduced on this arm to control service stresses and assist in limiting moments in the single column support pier.

Conclusion

This project has been technically innovative in that it combines three structure types (steel, precast segmental, and cast-in-place segmental) into a single job of this magnitude. Adding to the complexity was the fact that the structure was required to span over a railroad, an existing freeway, and an active navigation channel without impacting any of those transportation links. The west approaches, east approaches, and river crossing bridges were erected simultaneously on this recently completed bridge.

With respect to project delivery systems, this project has also been innovative in that it will be the longest span bridge built in the U.S. using a design/build project delivery method. The successful completion of this project will show that design/build contracts are appropriate for projects of this magnitude.