This diagram shows the differing forms of construction that could be used to span the proposed crossing for Route 6A.
The principle of Suspension Bridge design is that all of the dead weight of the bridge and the traffic is conducted via suspension cables anchored into rock galleries. These anchorages transfer the load into the surrounding rock with the main towers in compression, transmitting load down them into the rock below. This design can be used in bridges with spans in excess of 2km. These bridges were first used in the early 20th century.
Suspension bridges have traditionally been used for heavy trafficked multi-modal crossings, but because of their complex construction and recent problems with corrosion, they have proved costly to maintain.
Cable Stay Bridges, unlike suspension bridges, rely on all of the weight on the bridge structure being transferred down the mast of the bridge into the rock below. This method allows the bridge to be built incrementally from each mast using a balancing technique. The largest currently built has a span of 900m but the design is capable of spans in excess of 1.5km. These bridges are very much a 21st century design with the first major bridge of this type being built in the mid 1990’s. These bridges have been used mainly as road bridges, but the design could be adapted for multi-modal use. The bridges are made up of simple elements that are easy to maintain; the cables are designed to be replaced at regular intervals.
The Tsing Ma Suspension Bridge in Hong Kong is a multi-modal bridge
similar in span to that proposed by FETA. This bridge has a box girder deck with road transport running on the upper deck
and the Hong Kong Metro light rail running within the box girder. There are also two vehicular lanes in the box
which can be used during Typhoons. This bridge was completed in 1997 at a cost of 7.1Bn HK $ which equates to £0.5bn which,
if updated for inflation, would be in excess of £1.0bn at today’s prices. A bridge like this would take up to seven years
to build, and the masts would be about 680ft high. There are no suspension bridges in the world that can take heavy rail.
The loading characteristics are such that they do not suit this form of bridge.
The Tatara Cable Stay Bridge is in Honshu, Japan.
This road transport bridge is the largest of its kind built in the world, and at a span of 900m it is 100m short of
what is proposed in the Scottish Development Department (SDD) consultative document. The SDD Bridge would have the main span crossing from the south shore
to a mast on the south shore adjacent to the west break water at Port Edgar, onto a mast on Beamer Rock.
From there a side span would cross the channel into Rosyth onto the north shore, west of the Queensferry Hotel.
The mast of a bridge this size would be in excess of 900ft high. If the span proposed by FETA is adopted then this
bridge would certainly be the largest cable stay in the world with masts towering 1350ft in the air.
This would make this bridge a 21st century designs, contrasting the 20th century road bridge and the 19th century rail bridge.
However, the down side is that in would probably cost up to £1.0bn. With the exception of cable stay bridge in Boston USA
there are no bridges currently being built or being designed to take light rail.