The overall width of the deck is 39.8m and is configured as a three-corridor arrangement with the towers and stay-cables located in the central zone between the two carriageways. The bridge deck carries two general lanes of traffic and a hard shoulder in each direction. The hard shoulders also provide the flexibility to carry buses displaced from the Forth Road Bridge during periods of high wind, and other forms of public transport should it be required in the future.
The deck is also impressive in scale. It is 2.7km in length and is continuous from abutment to abutment to eliminate movement joints at the marine supports. This delivers a smoother driving experience and means there are only four expansion joints at the ends of each carriageway to be inspected and maintained.
The deck was designed both in the final condition and during construction using software that calculates loading in all elements of the bridge. Computer models were created to analyse the whole bridge, with additional three dimensional models for load cases applied to the model to simulate the self-weight of the bridge, various traffic loadings and climatic conditions such as temperature effects and wind.
A total of 1,300 construction phases were identified and analysed for the erection of the main spans, which saw the segments being aligned, bolted together with plates at 24 locations and welded into position.
The cable stayed deck is formed of 134 composite steel and concrete segments, made of 110 standard segments, 12 starter segments and 12 approach viaduct north segments. Virtually all of the average 750 tonne deck segments were fabricated in China and topped with surface concrete, either in-situ or in casting bays at the Rosyth Docks. The deck sections were fully prefabricated in the Chinese factory before being shipped to Rosyth for final fit out and installation.
Both the south and north approach viaducts were launched, overcoming the challenges associated with access for superstructure erection over steep terrain and shallow water. The launch of the Northern Approach Viaduct in particular was one of the most complex engineering challenges on the entire project. Towards the end of the launch the nose needed to be raised some 2m to clear the final support pier. This was achieved by pivoting the whole deck around the middle launch support and controlling this pivot by sliding the rear supports down on purpose built ramps in the abutment structure.
It’s estimated that this approach saved the removal and replacement of 100,000 tons of bedrock and considerable local disruption. Vertically, the deck is supported at the central tower, the approach viaduct piers and at both abutments. Away from the towers the approach deck spans are supported on a series of V-shaped piers. The height of the piers varies significantly from 10.1m to 49.4m on the southern approach spans and 24.2m to 46.5m on the northern approach spans.
The abutment structures are the final supports of the bridge at each end and form the vital transition interface between the bridge spans and the land. These two-storey reinforced concrete structures also:
- Support the approach viaduct deck spans and integration of the structural movement joints permitting up to ±1000mm longitudinal movement
- House electrical, mechanical and structural health monitoring equipment, including emergency backups essential for the day-to-day management and operation of the bridge
- Provide maintenance facilities, equipment, and material storage space
- Provide a safe access route into the deck for maintenance personnel
- Provide an emergency escape route for the public from the motorway central reservation down to ground level
The high winds that are a feature on the Forth Estuary caused regular closures and speed restrictions on the Forth Road Bridge, resulting in careful consideration in the design of wind shielding on the new crossing.
The wind shielding design on Queensferry Crossing resulted in 3.3m high sides to shield vehicles from wind speeds of up to 184km/h as they cross the wake of the towers, where a rapid change in wind force can occur. These wind shield measures have built in great resilience to the bridge, making the crossing much less susceptible to closures during high winds, without compromising on its appearance or the views of the users.