Figure 1: Bolle Bridge, with Melbourne in the Background |
The $A 2.0 billion Melbourne City Link Project (MCLP) was Australia’s
largest infrastructure road ever undertaken under the build-own-operate-transfer
(BOOT) process. The project featured 22 km (13 miles) of eight-
and six-lane new, widened, or existing freeway. Its landmark feature
is Bolte Bridge, an architectural and engineering accomplishment
that is unique because of its twin 140-m (462-foot)-high non-structural
towers above the center span (Figure 1). The outer barrier of the
bridge has a metallic edge strip, and both the superstructure and
the towers are painted. The second major feature of the project
that involved segmental construction is a 4.5-km (2.7-mile) -long
viaduct constructed from precast match-cast elements.
PB was a member of the Office of the Independent Reviewer (OIR),
which had a statutory role in the design and construction of the
MCLP. The OIR had a unique contractual arrangement as it was under
contract to the state government of Victoria as well as the owner/operator
of the 34 year concession, Transurban. PB was brought into the role
because of its experience in big bridges and mega projects.
Design and Construction of Bolte Bridge
Bolte Bridge is one of the largest balanced cantilever cast-in-place
box girder bridges in Australia. The balanced cantilever method
of construction involves casting large concrete box units symmetrically
from the top of substructure, thus maintaining a balanced condition.
It utilises a complex temporary travelling formwork.
Bolte Bridge comprises twin structures, each a 490-m (1,618-foot)-long,
4-span bridge with two main spans of 173 m (571 feet) and side spans
of 72 m (238 feet). The bridge superstructure has a single-cell
box girder 8 m (26 feet) wide and up to 12.7 m (42 feet) deep. It
provides a minimum vertical clearance of 29 m (69 feet).
Foundation Design and Construction. The steel pile
foundation used for the bridge had 1200-mm (4-foot) -diameter piles
up to 50 m (165 feet) long driven through the compressible Coode
Island silts. The top 15 m (50 feet) were mucked out and filled
with concrete to provide the flexural capacity and connection to
the pile cap.
The center pier island was man-constructed to provide a work platform
and protect the bridge from ship collision. Its pile cap contained
more than 3000 m3 (3,900 cubic yards) of concrete and was placed
in one 24-hour continuous pour.
Superstructure Design and Construction. Interesting
superstructure features included large fillets at the top of each
web to house the cantilever prestressing anchorages. A total of
102 prestressing ducts were located in three levels within the top
flange and a bottom flange that varied from 300 mm to 1400 mm (1
foot to 4.75 feet) at the center pier.
Figure 2: Center Pier Construction with Traveler Forms |
Major temporary works involved in construction were the eight deck
form-work travellers fabricated for this project (Figure 2). Four
operated from the main center pier and four from the side piers.
The travellers consisted of three elements:
- A top support structure that carried the external forms and
was advanced by hydraulic equipment
- External forms made up of a bottom flange soffit, external
walls and the top flange wing soffit
- Internal forms that were self contained and consisted of support
beams and a rolling form for the top flange support and inner
walls of the web.
Each new segment was post tensioned onto the existing structure
by two tendons, one located at the top of each web. The other end
of the tendons was anchored in face anchors at the leading end of
construction on the other side of the pier. All prestress of the
cantilevers during construction was in the top flange. The cantilevers
were joined by a closure pour, which was post tensioned in the bottom
flange from “blisters” in the bottom flange of the cantilevers.
The superstructure was cast integrally with the hollow center piers
to form a fixed connection. The pier heads at piers 2 and 4 had
sliding bearings that used a temporary prop on the out of balance
side and temporary fixing connections during construction (Figure
3).
Figure 3: Side Pier-Balanced Cantilever with Prop |
Figure 4: Elevated Roads - Segmental Construction |
Figure 5: Mainline Substructure Construction |
Tensioning Procedures. Ninety-eight tendons are
in the top flange for the main spans and 32 in the bottom flange.
There are 15 to 19 strands in each tendon. Tendons were grouted
as soon as practical, but not more than two weeks after placing
in sheathing. Grout for filling prestressing sheathing comprised
of cement, water and an admixture methocel was added. Just prior
to grouting, the sheathing was blown clean with dry oil-free compressed
air.
Elevated Roadway: Match Cast Segmental Design and Construction
The twin elevated roadways weave their way from the Tullamarine
Freeway over the Upfield Railway line, along Moonie Ponds creek
to the Bolte Bridge, and contain five interchanges. The original
concept for the viaduct utilised a superstructure with precast super
‘T’ beams; however, the contractor opted to use a match
cast, precast, prestressed segments (Figure 4) This was the first
structure in Australia to fully utilise external longitudinal post-tensioning.
The twin viaducts included 203 45-m (149-foot) -long spans, each
consisting of 13 segments weighing up to 77 tons. There were two
pier segments, eight typical sections and three deviator segments.
The superstructure was simply supported on more than 200 mainline
piers supported by some 4,500 precast concrete piles (Figure 5).
The piers ranged from 5 m to 20 m (16 feet to 66 feet) above the
ground. The spans were supported on elastomeric bearings, and were
post tensioned with external tendons encased in a single sheath
system through deviator blocks and pier segment diaphragms.
The superstructure was designed to AASHTO Guide Specifications,
and included time dependent construction sequencing, permanent loads,
and the effects of long term forces due to creep, shrinkage, and
long term relaxation. These were combined to establish the controlling
loads.
A typical span consisted of 10 tendons with 31 strands. One tendon
was anchored to the first deviator block from the pier segment.
The vertical and horizontal deviations occurred at the outer deviation
blocks while in the center, the tendons passed with no deviation.
The pier segments and the deviator blocks deviated the tendons and
distributed the forces in the superstructure.
Including nine ramps, also constructed segmentally, more that 3600
segments were cast at a yard that was the largest in the southern
hemisphere during its operation. All segments were cast in a 20
month period. Average production was about 12 segments per day from
14 moulds. Segments weighed between 45 and 80 ton and varied in
width from 9.6 m to 17.3 m (32 feet to 57 feet). The moulds were
aligned in bays such that each mould would cast a trapezoidal unit
against the previous cast unit, that is, against the same unit to
which it would be connected.
The first spans took up to two weeks to erect to overcome the start
up difficulties, the learning curve as well as the site conditions
encountered, as the spans were erected over electrified tram and
commuter train lines, crossroads, and creeks. As the work became
systematic, an average erection rate of five spans per week was
achieved.
Figure 6: Mainline Underslung Erection Truss |
Spans were erected utilizing a pair of underslung trusses for each
carriageway. The precast segments were placed onto carriers on the
underslung truss, and supported on a three point system on the trusses
(Figure 6). As segments were placed on the truss, the segments were
aligned and joined “dry.” Multiple shear keys provided
shear transfer and facilitate alignment. The keys were not visible
from outside of the completed span for aesthetic purposes.
Fine tuning was performed with small hydraulic jacks and shim plates.
After all segments were aligned, the tendons were stressed. The
load was then transferred from the trusses onto the bearings at
the pier head. The trusses were advanced to the next span. All stressing
was usually done before the trusses were moved.
Concluding Remarks
The Western Link of the MCLP, which included Bolte Bridge and the
elevated viaduct, opened to traffic in April 1999. The delivery
of these two elements in an ambitious program was a reflection of
the confidence and reliability in the concrete segmental technology. |
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