Driving on blades

James River Bridge in Virginia relies on blade-type pier shafts for earthquake protection

Bridges Article May 16, 2003
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An innovative approach by the Virginia Department of
Transportation (VDOT) and FD/MK LLC enabled the completion of the Rt. 895
Connector, now known as the Pocahontas Parkway, in Richmond, Va., some 15 years
earlier than using the traditional process for acquiring project funding. This
was the first project to be initiated and completed under the Public Private
Transportation Act, passed by the Virginia Legislature in 1995. FD/MK LLC, a
firm created by Fluor Daniel and Morrison Knudsen (now Washington Group), which
is the private sector owner/partner with VDOT, completed the $324 million
east-west tollway project connecting I-295 and I-95. A major part of the
project was opened to traffic in May 2002 and the remainder in September 2002.

It's not going anywhere

Initially, the four-lane Pocahontas Parkway is expected to
carry 24,000 vehicles daily with an eventual ramp-up to 50,000. The drivers
will pass through a toll plaza equipped with Smart Tag technology.

The 8.8-mile-long highway has several new bridges, the most
significant of which is the 4,765-ft-long James River Bridge. The bridge over
the James River was built by the cantilever system using cast-in-place concrete
segments. The 675-ft clear span over the river represents the third longest
cast-in-place cantilever construction in the U.S. The main span also provides a
vertical clearance of 145 ft over a 300-ft-wide channel for shipping. The
superstructure for the river crossing consists of twin cell boxes with depths
of 41 ft at the piers and 16.5 ft at midspan.

The main span foundations consist of blade-type pier shafts
supported by 8-ft-diam. drilled shafts. The piers adjacent to the river are of
a twin-blade design to more effectively resist out-of-balance construction
loads. The blade-type columns incorporate multiple cylindrical rebar cages to
provide for adequate confinement during a seismic event. The drilled shafts
penetrate over 15 ft into bedrock, and are designed to support the bridge in a
500-year scour event.

The approach spans of the James River Bridge consist of
precast concrete box segments of constant depth, joined together by
post-tensioning. Single-column cylindrical piers resting on steel H-pile
foundations support the approach span superstructure. The footings are
generally above the finished grade to minimize the removal of contaminated
soils from the site. The columns and piles are designed to resist seismic
loads, including uplift.

The project is located in a Category B seismic zone, with a
lateral acceleration coefficient of 0.13. Several features were incorporated
into the design of the James River Bridge to mitigate the impact of seismic
forces, including the following:

* Except for the end spans, the continuous spans of the
river crossing are fixed to the piers, which allow a more uniform distribution
of longitudinal forces;

* The continuous spans of the approach structures are pinned
to the piers, which allow a much better distribution of seismic forces to the
varying height foundations;

*   The
race-track-shaped piers for the approach structures, as developed in the
preliminary design, were too stiff requiring unreasonably heavy foundations to
resist the seismic forces. Alternative designs with hollow cylindrical columns
and solid cylindrical columns were investigated to make the piers more
flexible. This effort resulted in a substantial reduction in seismic forces
leading to more economical foundations and piers. Single-column cylindrical
piers were selected from a constructability point of view;

*   The
design of the steel pile foundations for the approach structures were subject
to large uplift forces. Tension load tests were performed in the field to
determine more accurately the frictional resistance and tension capacity of the
piles. These tests resulted in optimizing the design with significantly fewer
piles and smaller  pier footings;

* A three-dimensional seismic model, which considered the
various features of the structural system, including the configurations, sizes
and connections of the superstructure, piers and foundations and the behavior
of the soil/structure interaction, was used in the analysis of seismic forces.
The response spectrum analysis provided an accurate determination of the effect
of seismic forces, which helped in selecting the appropriate sizes for
structural members; and

* The use of large-diameter welded hoop-reinforcing steel
for the river piers, as required in the specifications, posed significant
problems for fabrication and transportation. An alternative design using
seismic-rated bar couplers, which had been tested in California, was
recommended and accepted by VDOT. Seismic-rated bar couplers were used for the
first time in Virginia.

Parsons Brinckerhoff of New York City, as a subconsultant to
Site Blauvelt Engineers of Richmond, Va., under contract with FD/MK LLC, was
responsible for the design of the James River Bridge. The joint venture of Recchi
America Inc. (now Condotte America), Miami, and McClean Contracting Co.,
Glenburnie, Md., under contract with FD/MK LLC, was responsible for the
construction of the James River Bridge.

The project has won several awards including:

*  2003
Engineering Excellence Award from the Consulting Engineers Council of Virginia;

* 2003 National Finalist from the American Consulting
Engineers Council;

* 2002 Outstanding Engineering Achievement Award from the
Engineers Club of Hampton Roads; and

*  2002
Excellence in Concrete Award - Innovation, from the Virginia Chapter of the
American Concrete Institute.

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