16.01: Bjørnafjorden suspension bridge TLP concept

The Bjørnafjorden TLP (Tension Leg Platform), a multi span suspension bridge solution, is one of the possible fjord crossings options considered as part of the ambitious “Ferry‐free Coastal Highway E39” under development in Norway. The original design concept for Bjørnefjorden was very much similar to that of a conventional suspension bridge using central locks between main cable and bridge deck to distribute movements equally to the bridge ends. However since the bridge is of the multi‐span type, the inherent flexibility of the pylons caused a severe of in‐plane stability of the original system leading to large vertical displacement of the deck. To mitigate this effect top cables have been introduced between pylon tops preventing these from excessive longitudinal displacements and thereby also limiting the vertical deck displacement from traffic load. In this paper the different concept challenges for the Bjørnafjorden TLP solution is outlined.

The suspension bridge has a total of four pylons – a northern and southern pylon of concrete located near shore, while two central steel pylons are supported by TLP's at 550m and 450m depth. The TLP's, as well known from off‐shore industry, consist of pre‐tensioned steel pipes anchored in the seabed and at the surface connected to a large steel “floaters” onto which the towers are placed. The main spans are 1300–1400m and the bridge has an overall length from anchorage to anchorage of about 5200m.

Due to the long tethers of the TLP the central pylons are relatively unrestrained to move up to 20m horizontally and thus in order to achieve sufficient stability of the bridge system a special feature in form of top cables are introduced. The top cables connect the tower top of all four pylons and are anchored to the bridge anchor blocks together with the main cable.

The bridge structural system is the outcome of innovative thinking and a fine example on how a few unconventional features can make a very conventional bridge system work in challenging conditions. As the bridge is floating it is essential that the structure is light and therefore the primary structures are in steel.