Feature: Third Bridge for the Forth

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The Queensferry Crossing, opening for business in Spring, will be the longest three-tower, cable-stayed bridge in the world, echoing back to the proud tradition of Scottish engineering. Its innovative design provides extra strength and stiffness, allowing the towers and the deck to not only break records but also to be eye-catchingly slender and elegant. It will also have to stand against high wind, freezing temperatures and salt spray far into the next century.

With a length of 1.7 miles (2.7km) the structure will also be the largest – by far – to feature cables that cross mid-span. It will include 150,000 tonnes of concrete and 30,000 tonnes of steel, with cables protected by high density polyethylene (HDPE) pipes. Amongst many innovative features, these cables can be easily replaced as the need arises. Further protection includes a dehumidification system inside the box girders to control and reduce moisture.

Signature cables

Contractor Forth Crossing Bridge Constructors (FCBC), working for client Transport Scotland, is a consortium including Hochtief Solutions, American Bridge International, Dragados, and Morrison Construction.

Their spokesman says: “In the main, we are using tried and tested techniques and high quality materials. The main factors are constant movement caused by high winds (the Forth is one of the most exposed marine locations in the UK), expansion and contraction of the steel structure caused by temperature variations and traffic load. Vibration is also a consideration.”

The cables are a signature feature of the crossing design. After being cut to the correct length they are lifted off the deck and into position at the tower anchor box using an enormous tower crane. FCBC continue: “The stay-cable outer pipes are white, made of HDPE. HDPE was chosen for its longevity, resistance to UV rays, strength and ability to withstand constant movement due to varying load factors on the bridge, principally wind, traffic and weather related expansion.

“Each stay-cable consists of multiple strands (up to a max of 109). A strand is made up of 7 high tensile, galvanised steel wires, 4.8mm in diameter and individually wax coated, twisted in a helix pattern and contained within a black HDPE casing. Strands are delivered to site pre-prepared.”

Box girders

The main crossing consists of a new road bridge across the Forth estuary spanning 2637.5 metres between abutments.

Protection against ice formation is a major consideration. FCBC say: ”The entire road deck (two lanes in each direction plus hard shoulders) is waterproofed using a spray-applied Stirling Lloyd eliminator product which is laid directly on top of the reinforced concrete deck surface.

”Most coating is done off-site, except for the welding of stay-cable outer pipes. Principal deck segments arrived on-site pre-painted.   Once lifted into place 60m above the sea, segments are welded together leaving a seam which has to be painted.”

Maintenance

Design life for the bridge is 120 years, while the stay-cable out-pipes are expected to last for 60 years. The road deck waterproofing membrane is expected to be good for 100 years.

FCBC say the future maintenance of the Queensferry Crossing will be handled by Forth Bridges Operating Contractor, AMEY, who also have responsibility for operating the existing Forth Road Bridge. Six under-slung travelling gantries will be used for inspections and maintenance of the underside of the road deck structure. Cradles will meantime ascend each tower to allow for above-road inspections and maintenance.

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