| Construction |
| Merseyside's Poulton Swing Bridge |
| By Richard Sansbury, Taunton, U.K., +44-1823-253291, sansburyr@pbworld.com
|
| Replacing an old moving bridge over the busy Merseyside
docks in the U.K. gave our bridge design team the opportunity to present
a full range of services-from conceptual planning through detailed
design to contract administration. |
|
The docks at Merseyside contain several moving bridges that date
back to the early 1900s when the Port of Liverpool was a major gateway
for international trade. The docks are quieter now, but are still
active and remain a focus for heavy industry in the centre of a large
population. A busy network of roads with a high proportion of heavy
trucks crosses the docks, and maintenance of the original bridges
requires a considerable effort from the local highways authority.
The original bridge is demolished |
The Need to Replace Poulton Bridge
Poulton Swing Bridge crossed the docks carrying a main road from Birkenhead
to Wallasey. Built originally in 1926, its primary use was to carry
a single quayside railway track. The quayside railways were torn up
long ago but the original steel truss bridge remained, although with
dimensions more suited to railway traffic than road traffic. With
only 3.9 m (12.8 feet) of headroom and 5.4 m (17.7 feet) width between
kerbs, the steelwork was damaged frequently by passing vehicles.
In 1993, we were commissioned by the local authority, the Metropolitan
Borough of Wirral, to carry out a feasibility study for easing the
width and headroom restrictions on the bridge. It soon became apparent
that any attempt to keep the existing bridge trusses would not be
cost effective. A wider structure would have to be set back further
from the dock edge in order to give full clearance to passing ships
and, thus, the main span would be increased. The trusses would have
needed strengthening and the cross girders would have had to be either
lengthened or replaced. Moving the pivot point of the swing span would
require extensive modifications to the mechanism and foundations.
Therefore, a new bridge structure was recommended.
Although other forms of moving bridges were considered, a new swing
bridge was chosen to replace the original. A swing bridge suited the
site because it would have a shallow foundation depth and the empty
land required for the bridge to swing over was already available.
A 200 Tone section of the new Poulton Bridge is craned into
position on the new foundation |
Detailed Design
Following the successful feasibility study, the Borough extended our
commission to include the detailed design of the new structure. It
was agreed that the bridge deck and foundations would be fully designed
at this stage, but the mechanical and electrical (M&E) works would
be included as a design/build element of a construction contract due
to their specialised nature.
Deck and Foundations. Detailed design of the new structure
was carried out in our Taunton office. The bridge superstructure design
comprises a 860 tonne (846 ton) steel box girder deck with a 42-m
(138-foot) -long main span and 24-m (79-foot) -long back span. The
decking is an "orthotropic" steel deck with steel cross
beams and main box girder beams to either side of the road.
Because of the shallow construction depth required, the box beams
are partly above and partly below road level and are protected from
traffic damage by a crash barrier. The walkways are supported on cantilevers
to the outsides of the main beams, safely away from the traffic. The
counter balance is provided by a 380-tonne (374-ton) concrete slab
that also acts as the deck of the back span, and the foundation design
is a reinforced concrete slab on bored piles.
M&E Works. We prepared an outline M&E design and performance
specification in which the bridge was to be rotated by hydraulic rams
acting on a chain around the central pivot. Before rotating, the bridge
first tilts back slightly so that the nose end lifts clear of the
end bearings. This tilt is achieved by a system of jacks and wedges
at the tail end that allows the tail bearings to slide clear which,
in turn, allows the tail end to drop down onto rollers.
The new bridge as it nears completion |
Construction
The £3 million construction contract commenced in 1995. A traditional
British form of contract was used with our firm acting as "the
Engineer" with responsibility for contract administration, approval
of the M&E design and full time site supervision. The contractor
was Birse Construction Limited and their designer for the M&E
works was Sir William Arrol, part of the Rolls Royce group. Their
design for the main hydraulics differed slightly from that specified
in that the chain around the pivot was replaced by a system in which
a pair of main hydraulic rams aligned at 45° to the bridge to
act directly on the central cross girder.
Construction took 15 months, slightly longer than intended. There
were obstacles encountered during piling that required redesign of
the foundations, and some issues with the design/build element of
the works that delayed the installation of fundamental M&E components.
Despite these problems, however, the bridge opened in the first quarter
of 1996 and is successful in providing a much improved highway across
the docks. |
|
| Richard Sansbury BA (Cantab) CEng MICE is a
bridge design engineer based in PB's Taunton, U.K. office with eight
years experience in design and assessment of bridges of all types.
He was part of the design team for Poulton Swing Bridge and one of
the site supervisors during construction. |
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