Russky Island cable-stayed bridge

Assembling HDPE ducts of more than half a kilometer length and hauling them more than 300 m in the air

Welding of the ducts took place on the bridge deck, which in itself was a challenge given that ambient temperatures, regularly below freezing and potentially as low as -30 °C, are several degrees lower on the exposed deck which sits 70m above the Eastern Bosphorus Strait. Special measures included setting up a heated and insulated shipping container to create shelter and a controlled environment where the duct welding could be carried out.

The extreme height of the bridge towers and length of the cable ducts also meant that the standard installation procedure of using a single strand to haul and maintain the duct at the correct alignment could not be applied. Instead these longest ducts had to be pulled up on two strands to fix them in position for the remaining strand installation.

A very compact cable-stayed system using compact strands in an HDPE duct

To reduce the wind fairing area of the cable layers and thus limit the transverse wind force on the towers, Freyssinet installed a very compact cable system using compact strands in an HDPE duct of optimized diameter surrounding the strand bundle. For the installation, new threading tools had to be developed to fit into the limited space available.

Freyssinet’s Iso-tensioning equipment was upsized and upgraded with a stroke of 300 mm – increased by 50% – to cope with the excessive length and large sag that was characteristic of these cables.

Freyssinet designed internal and external dampers which are fitted at the base of the shorter and longer cables respectively. The external dampers ensure an additional in-plane logarithmic decrement of up to 6% for the longest cables, within a temperature range of -40 °C to +65 °C and even at very low vibration amplitudes.

Reliability and preventive measures

At the tower top, where those tasked with threading strands into the anchor blocks were fully exposed, special measures were necessary to safeguard them. No solo working was permitted, and staff were trained to spot any signs of frostbite or hypothermia in their colleagues. The low temperatures also had the potential to affect hydraulic fluids in installation equipment, making it prone to breakdowns. With no give in the programme, reliability was key and preventative measures involved applying insulation to the equipment and heating the oil.