| Construction |
| Green Bay's Main Street Bascule Bridge (The
Ray Nitschke Memorial Bridge) |
| By John F. Elwell, Minneapolis, Minnesota 1-612-677-1266,
elwell@pbworld.com |
| Many factors had to be considered when designing
the Main Street Bridge, including determining the best alignment and
whether to use an open or closed deck, rolling lift or trunnion bascule
span and mechanical or hydraulic drives. |
|
The original Main Street Bridge in Green Bay, Wisconsin, built
in 1924, was one of three major east-west highway bridges crossing
the Fox River in the central business district. The four-lane, double-
leaf, Strauss articulated counterweight bascule spanned 38.1 m (125
feet) from center to center of the main trunnions. It had two steel
approach spans on the east and four steel approach spans to the west.
Ray Nitschkle Bridge: Partially Opened Position |
In 1992, the City of Green Bay decided to replace the structure with
a new bascule span on a slightly different alignment to the north
because:
- Prior to 1992, the bridge had been closed
a number of times for maintenance and rehabilitation.
- As a result of foundation deterioration,
the continual movement of the bascule leaves caused misalignment
of the machinery and the trunnions which, in turn, caused rapid
bearing and gear wear.
- The mechanical and electrical components
of the bascule unit were in poor condition.
- The alignment of the approach roadways was
poor.
- Increasing traffic demands were forecasted.
The Wisconsin Department of Transportation (WisDOT) retained PB along
with Graef Anhalt Schloemer, Inc. (GAS) to perform a study and preliminary
design of the available options for the new crossing. Upon completion
of the study, our team provided final design services.
Study and Preliminary Design
The study phase centered on three major tasks:
- Designing an alignment for the crossing
- Preparing more in-depth studies for the bascule
span alternatives once the preferred alignment was determined
- Minimizing impacts of the project on historical
resources in the area.
Bridge Alignment. We investigated a number of different alignments,
including pier placement and crossing locations, to determine the
optimal bridge and channel alignment. The selected alternative moved
the bridge to the north, eliminated the dangerous S-curve that led
onto the bridge, incorporated other improved approach roadway alignments,
and eliminated a complex and troublesome series of ramps at the southeast
end of the bridge.
Bascule Span. Span alternatives included:
- Open versus closed deck
- Rolling lift versus trunnion bascule
- Mechanical versus hydraulic drives.
Roadway Deck. Three criteria-initial cost, maintenance
cost and ride quality-were used to evaluate the following deck types:
- Conventional open grating
- Grating half-filled with concrete
- Exodermic.
The Exodermic deck was chosen because of reasonable initial cost,
low maintenance cost and excellent riding characteristics. A proprietary
system developed by Exodermic Bridge Deck, Inc., of Lakeville, Connecticut,
the Exodermic deck incorporates a reinforced concrete deck that is
cast compositely with the steel grating and the floor beams. The deck
system spans between floor beams spaced at 4.1 m (13.5 feet), thus
eliminating the need for longitudinal stringers.
The Main Street Bridge was the first project to use an Exodermic deck
on a bascule bridge. We worked closely with Exodermic Bridge Deck,
Inc. to develop and adapt details for use on this project.
Type of Bascule. Two types of conventional bascules
considered are:
- Scherzer bascules that roll back as they
rotate open
- Trunnion bascules that pivot about a fixed shaft or trunnion.
Scherzer Bascules. Scherzer bascules are supported on heavy tread
plates. Gravity and pintles or gear teeth in the tread plate prevent
the bridge from moving out of line as it rolls open. The drive machinery
is mounted on the movable leaf between the girders.
The Scherzer bascule requires a slightly smaller opening angle than
the trunnion bascule (described below) because it moves away from
the channel as it opens.
Trunnion Bascules. Trunnion bascules have the advantage of
simpler girder construction because they do not require a curved flange
segmental girder. Trunnion bearing support may be complicated, however,
so it requires additional steel framing that complicates the bridge
counterweight framing.
With the trunnion bascule, the machinery is mounted on the pit and
is stationary during the operation of the bridge. The trunnions provide
a point of fixity for the bridge alignment, which is not subject to
the walking action of Scherzer bascules.
The estimated construction cost for each type of bridges was determined
to be similar. The decision was made to use a Scherzer bascule because
most of the movable bridges in WisDOT's District 3 are of that type,
so maintenance would be somewhat easier with a familiar bridge type.
Ray Nitschkle Bridge: Operator's Tower |
Mechanical Drive Systems. The following three types
of mechanical drive systems were examined:
- A traditional electric motor and gear drive
system
- A hydraulic cylinder system
- A low-speed/high-torque hydraulic motor drive
system.
Traditional System. This system would include a combination
of open and closed gearing driven by electric motors. Preliminary
analysis indicated that four independent drives would be required
to provide redundancy-one at each main bearing. Routine maintenance
would include regular lubrication of the exposed gear sets, inspection
of gear mesh for misalignment, bearing inspection and periodic replacement
of the oil in the gear reducers.
Hydraulic Cylinder. Preliminary analysis indicated that four
large bore hydraulic cylinders would be required on each bascule leaf.
A large hydraulic fluid reservoir would be required to supply the
fluid volume demand. The cylinders would be mounted vertically under
the girders with spherical bearings at each end to prevent eccentric
loading. Routine maintenance would include checking fluid levels,
filters and leaks. Rod seals would have to be replaced every 8 to
10 years.
Low-Speed/High-Torque Hydraulic. The hydraulic motor drive
would consist of one hydraulic power unit, two hydraulic power motors
and two rack-and-pinion sets per bascule leaf. The motors would be
coupled to the pinion shaft by a shrink disc, thus allowing removal
of the motor from the shaft for maintenance. Routine maintenance would
include checking fluid levels, filters and leaks.
The low-speed/high-torque hydraulic motor system was determined to
have a slightly higher initial cost than the hydraulic cylinder system,
but one lower than that of the gear drive system. It was recommended,
however, because of its maintenance and operational advantages. Hydraulic
motors are capable of smooth rotation at very slow speed, allowing
very reliable and smooth span control during acceleration, full speed
and span seating.
Electrical power service is provided to both leafs, with no power
tie across the river. Hookups for portable, emergency power generators
are provided on both sides of the bridge. The hydraulic system is
powered by a pair of 75-hp electric motors on each leaf. The bridge
control systems are controlled by programmable logic controllers (PLCs).
Each bascule pier houses a PLC control system for the leaf, and the
far leaf is controlled by signals from the control tower transmitted
by radio modem. [Ed. note: For more information on PLCs, see "Leading
the Way in Applied Technology for Control Systems" by Rick Newcomb.]
Historic Issues. A major aspect to the design of this project
was its affect on historical resources in the area, particularly the
following three, which were identified within the area of potential
effect:
- The existing Main Street Bridge
- Broadway Historic District
- Fort Howard Military Reservation.
Several steps were taken to mitigate the most significant historical
impact-removal of the existing bridge. The new operator tower at the
east bascule pier was designed with an octagonal shape to reflect
the shape of the towers on the original bridge. The terra cotta cornice
details from the original bridge were salvaged and reused on the new
operator tower and a tile roof was used on the new tower to match
the original tower. The operator tower and piers on the new bridge
were constructed using a form liner and were stained red to match
the block pattern and color used on the original bridge piers and
tower. To further enhance the historical character of the new bridge,
the bridge railing was designed to closely match that of the original
bridge.
Significant effort was made in the geometric design of the approach
roadways to minimize the impact on the Broadway Historic District
and the Fort Howard Military Reservation. Through close coordination
with a number of local, state and federal entities, each of the design
issues was addressed satisfactorily.
Final Design
Final design was done on a compressed schedule, beginning in July
1995 and finishing in April 1996. Structural design was done in the
Chicago and Minneapolis offices, mechanical design was done in the
New York office, and electrical and control systems design was done
in the Tampa office. The project was let for construction in the fall
of 1996 and opened to traffic in October 1998.
As the design of the superstructure and mechanical and electrical
systems progressed, a foundation alternatives analysis was performed
to determine the optimal foundation type for the bascule piers. The
new bridge was determined to have potential local scour to depths
of nearly 14 m (46 feet) in the vicinity of the bascule piers. In
addition, soil conditions from the river bottom to the top of bedrock
were found to be extremely poor. [Ed. note: For more information on
scour, see "In-Depth
Scour Evaluations of Three Movable Bridges over New Jersey's Intracoastal
Waterway" by Tom Anella and others.]
We investigated three foundation types in depth-vertical steel piles,
battered steel piles and caissons. The steel piles lacked lateral
support because of the very poor soil conditions and would have been
prone to excessive lateral deflections and buckling. The selected
foundation type was 2.4-m (7.8-foot) caissons, socketed into sound
rock.
Conclusion
The Main Street Bridge Project involved a number of important technical
challenges and required effective communications between four PB offices,
several sub-consultants, local, state and federal agencies, local
business groups and special interest groups. We all worked together
to achieve a roadway that improved vehicular safety and at the same
time did not adversely affect navigation on the Fox River. The new
bascule bridge incorporates state-of-the-art technology while remaining
faithful to the unique architectural aspects of the original bridge.
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| John Elwell has 24 years experience in the design
of highway and railroad bridges, 10 of which have been with PB. In
addition to his role in the design of the Main Street Bridge, he has
had key roles in the Blatnik Bridge Project in Duluth, Minnesota and
the Dartmouth Bridge Project in Minneapolis, Minnesota. |
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