| Better Bridges
Vietnam Veterans Memorial Bridge
Completed Quickly
The Vietnam Veterans Memorial
Bridge, which crosses the James River directly downstream of the Port of
Richmond, is the gem among the 15 bridges along the new 8.8-mile Route
895 Connector, known as the Pocahontas Parkway.
by Fred Parkinson
The new toll road was planned to alleviate congestion and traffic
delays by linking I-95 in Chesterfield County to I-295 in Henrico County.
 The
project cost $324 million, of which approximately $111 million was for the
Vietnam Veterans Memorial Bridge. Completion of the bridge was the key to
meeting the ambitious 45-month schedule for the entire parkway project.
Delivered under budget, and completed on schedule, the project was
implemented 15 years sooner than it would have been under the Virginia
Department of Transportation’s normal construction budget processes.
The 1,475-foot-long, high-level fixed bridge, which extends from the
west abutment in Richmond, over the Route 150 interchange and the I-95
mainline, to the east abutment in Henrico County, features a 672-foot main
span with 145 feet of vertical clearance for marine traffic using the
deepwater port. The bridge also includes nearly 3,500 feet of high-level
approach spans and three new, high-level ramp structures that connect to
I-95.
 A
series of complex criteria, from accommodating ocean-going ships and
complying with seismic requirements to maintaining river and vehicular
traffic during construction, were among the challenges that called for
innovative approaches to design and construction. Among these was the use
of three distinct concrete structure types — cast-in-place on the main
and approach spans; precast cantilever on the approaches; and precast span
by span on straight portions of the ramps (curved steel girders were used
on the tighter radii). To leave the river bottom undisturbed and to avoid
cofferdams as well as ship collisions, the bridge was designed without the
use of piers in the river.
Due to the magnitude of the project, alternative designs for steel and
concrete were evaluated for the main span and approaches. During the
conceptual design phase, Parsons Brinckerhoff, the designer for the river
crossing, considered several options for both alternatives. The steel
design alternatives evaluated for the main span included steel truss,
steel tied arch, steel plate and box girders, and steel cable-stayed
bridges. The concrete main-span alternatives evaluated included
cast-in-place balanced cantilever segmental concrete box girder and
concrete cable-stayed bridges, while the alternatives studied for the
approach spans included precast segmental box girders, prestressed
concrete AASHTO beams, and steel plate and box girder spans.
After further study of selected alternatives following conceptual
design, a hybrid segmental concrete option was chosen as the most
economical solution for the final design. For both the cast-in-place and
precast segmental spans, 6,500-psi compressive strength concrete was used.
Cast-in-place span
The bridge is believed to be the second-longest cast-in-place span in
the United States. To achieve the great length required for the main span,
the design team selected cast-in-place concrete-haunched twin cell boxes
cast in balanced cantilever. The girder depth ranges from 41 feet at the
piers to 16 feet at midspan.
The smaller span lengths of the approach spans permitted the use of
precast constant-depth boxes. The deck width varies between three and four
lanes, with full shoulders. This width was accommodated by placing two
boxes side by side and casting a wet joint between the wings. Over 1,300
segments were required, weighing between 32 metric tons and 50 metric tons
each.
Both the east and west approaches were erected as balanced cantilevers.
The west side primarily used an overhead truss to place the segments,
which allowed construction to proceed over the heavily used six lanes of
I-95 without interrupting traffic. The east approach was erected using
cranes.
The single-lane ramp structures use both precast concrete segments and
structural steel for the sharp radius portions of the superstructures.
These precast segments were erected using a span-by-span method and an
underslung truss.
Complex footings
A contaminated groundwater plume under portions of the western end of
the bridge required the construction of most of the footings above the
existing grade, supported on piles driven to rock. Single-column
cylindrical piers resting on steel H-pile foundations support the approach
span and ramp superstructures. The construction of the footings on top of
the ground and the use of the H-piles driven through the contaminated
plume avoided the need to excavate contaminated material or groundwater
and minimized disturbance of the existing soils. The single-column piers,
up to 130-feet tall, provided additional flexibility to cope with seismic
loads.
The main span foundations consist of blade-type pier shafts supported
by up to 20 6- or 8-foot drilled shafts. Fortunately, these shafts were
located in an area outside the contaminated plume. The twin-blade design
was used at the two main river piers to more effectively resist
out-of-balance construction loads. The river piers incorporate multiple
cylindrical rebar cages to provide for adequate confinement during a
seismic event. The drilled shafts penetrate to bedrock and are designed to
support the bridge in a 500-year scour event.
Mass concrete pours
 The
size of the foundations for this monumental structure required the
development of state-of-the-art specifications for the use of mass
concrete. All substructure elements for this project were considered to be
mass concrete. Pier footings up to 16-feet thick required single pours of
up to 3,900 cubic yards of concrete that each took 24 hours to complete.
The design team prepared specifications to allow the contractor to
monitor the curing rate and temperature history of these elements in order
to ensure that specified temperature rise and gradients were not exceeded
in the pours, and to determine a form stripping time. Studies were
undertaken to determine acceptable temperature gradients in various
elements, including the effects of various reinforcing ratios and form
insulation under a wide variety of ambient temperature conditions.
To avoid any potential weakening of the concrete due to overheating
during the curing process, the concrete supplier recommended the use of
blast-furnace slag in place of some of the cement to lower the heat of
hydration. The heat transfer analysis, use of blast-furnace slag, and
instrumentation monitoring avoided the use of cooling pipes in the pours.
The bridge’s columns and piles are designed to resist seismic loads,
including uplift. California Department of Transportation detailing
practices were adopted for the larger columns to more effectively resist
the seismic loads with simpler details. The project is located in a
Category B seismic zone, with a lateral acceleration coefficient of 0.13.
Multi-modal (response spectrum) analysis was used for the seismic
investigations in order to properly distribute the seismic forces to the
different height foundations.
Column detailing incorporated the first use in Virginia of
seismic-rated bar couplers for the seismic hoops. Cap and column shapes
and rebar details were developed with input from the contractor in order
to produce a design that was efficient and economical for the contractor
to build, and allowed for multiple reuses of formwork.
Innovative funding
 In
addition to the technical elements, the monumental structure was a
pioneering effort in other ways. It was the first project to be completed
under the Commonwealth of Virginia’s 1995 Public Private Transportation
Act, which allows for the Virginia Department of Transportation to accept
proposals from private entities to build needed transportation facilities
when public funding is not forthcoming. Just $26 million of the $324
million for the Route 895 project derives from public funds; the rest was
raised through the sale of private bonds and will be repaid through toll
revenues. The private partner, developer FD/MK LLC, a joint-venture firm
created by Fluor Daniel and Morrison Knudsen, was responsible for the sale
of the bonds.
Following the conceptual design phase, VDOT turned the project over to
the developers, which used a design-build method of delivery to complete
the entire project in the shortest possible time. Final design was
completed on a fast-track basis, and the design team also prepared an
eight-stage construction sequence scheme that allowed the contractor to
speed up construction activities while traffic moved unimpeded through the
site throughout the building process. The designers also assisted the
developer during construction by providing segmental experts on call, as
well as the more usual shop drawing reviews and responses to contractor
inquiries.
Fred Parkinson is a project manger for Parsons
Brinckerhoff in Norfolk, Virginia.
Reprinted from Better Roads Magazine
November 2003 |
