The existing Forth Road Bridge forms a key link in Scotland’s transport network. The crossing currently carries some 66,000 vehicles per day which includes over 70 percent of travellers across the three Forth bridges (Kincardine, Forth Road Bridge and Forth Rail Bridge).
In 2007, the Employer – the Scottish Ministers – announced that a Replacement Forth Crossing would be promoted by the Scottish Government. Previous work undertaken by Transport Scotland included consideration of alternative corridors and structures for the Replacement Forth Crossing and on 19 December 2007, the Scottish Ministers announced that the Replacement Forth Crossing would cross the Firth of Forth immediately upstream of the existing Forth Road Bridge and would be a cable-stayed bridge.
The Jacobs-Arup commission is for the management and delivery of the Replacement Forth Crossing Project inclusive of all roads and other infrastructure associated with such a structure.
This report outlines the development and assessment of Scheme Options for the Main Crossing. The preparation of this report has been carried out in association with:
- Dissing + Weitling
- Flint & Neill
- Professor Niels Gimsing
1.1 FRCS Reference Design
A reference design was developed during the Forth Replacement Crossing Study which is documented in Report 4: Appraisal Report, Appendix C - Bridge at Corridor D. Two options were considered during the FRCS, a 1300 m main span suspension bridge and a cable stayed bridge with two main spans, each of 650 m. The cable stayed bridge option was carried forward as the reference design for the Forth Replacement Crossing.
Drawings illustrating the Reference Design are included in Appendix A.
1.1.1 General Arrangement
The bridge has three towers with the central tower located on Beamer Rock. The 650 m southern main span places the south tower clear of the Forth Deep Water Navigation Channel in approximately 20m depth of water. A symmetrical arrangement results in a 650 m northern main span placing the northern tower well clear of the Rosyth Navigation Channel and in approximately 10m depth of water. 325 m side spans are adopted equal to half of the main span length. Each side span includes one additional anchor pier to provide additional stiffness.
A 635m long southern approach viaduct connects the cable stayed bridge to the south abutment via a nine span structure. The northern approach viaduct is significantly shorter at approximately 115 m long and is a two span structure.
The deck superstructure is a single cell orthotropic steel box girder with the stay cables provided in a fan arrangement and anchored along both edges of the bridge deck.
Each tower consists of a pyramid with four concrete legs extending above deck level, joined together in the zone where the stay cables are anchored. The spacing between the legs of the central tower in the longitudinal direction is greater than for the flanking towers. The shape has been developed in response to the requirement to provide additional stiffness for a double main span cable stay bridge.
The foundations shown for the main bridge towers are large caissons. For the central tower the caisson is founded on Beamer Rock which is to be levelled at the start of construction. For the flanking towers the caissons are founded at approximately 40m below water level on the sandstones and mudstones below the soft alluvial and glacial deposits in the Firth of Forth.
1.2 Development of Scheme Options
A Concept Design workshop was held from the 11th to 15th February 2008 in Transport Scotland’s offices to develop a short list of design concepts that would be taken forward for further development. The workshop included a site visit and a number of technical briefings (geotechnical / environmental / alignment / structural) to provide the background data necessary for concept development. Architectural visualisations were developed illustrating, in a series of photomontages, the basic concept of the design options to be carried forward.
Concept Design Options
Subsequent to the design workshops, further analysis and investigation has been carried out to develop the short listed design concepts into a number of Scheme Options. The options can broadly be considered in terms of:
- Functional Cross Section – What the bridge is required to carry and how that will be arranged on the deck in terms of location of traffic lanes etc.
- Deck Type – The construction material and structural arrangement of the deck.
- Tower Form – The appearance of the towers which will be the major aesthetic impact of the bridge.
- Approach Bridge Type – The construction material and structural arrangement of the approach bridge.
- Foundation Type – The construction form of the foundations.