Forton Lake lifting bridge spans the mouth
of Forton Lake, connecting the new museum at Priddy's Hard with the Town of
Gosport. Part of Portsmouth Harbour Millennium Scheme, the bridge is designed as
a pedestrian and cycle bridge but can also carry a road train and emergency
vehicles.
The bridge was the subject of a design competition, which was won
by the team comprising Maunsell Ltd, Bennett Associates, Percy Thomas
Partnership and ERM. The objective was to produce a landmark structure that was
in keeping with the historical setting and flat landscape of Portsmouth Harbour.
The client Gosport Borough Council commissioned Bennett Associates
and Maunsell Ltd to design and produce detailed manufacturing drawings of the
support structure and mechanical components.
The movable span of this bridge is a single leaf bascule bridge
that utilises a counterweight with wheels which roll along a track, the shape of
which is designed to ensure that the bridge mechanism remains closely balanced
throughout the travel of the bridge. The bridge is lifted by means of chains
each side of the deck which pass over the motorised chain sprockets on the top
of the tower and which, in turn, support the counterweight.
The walkway of the bridge is 5.5m wide between parapets. The
bridge deck weighs approximately 28 tonnes and the counterweight weighs
approximately 23 tonnes. Operated by two 3 kW motors the bridge is designed to
physically rise in 5 minutes and lower in 5 minutes. This small power
requirement is achieved by the use of a well-balanced counterweight and the slow
operation time.
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