Analysis of tether anchored floating suspension bridge subjected to extreme environmental loads

Abstract

The Norwegian public road administration wants to replace eight ferry crossings on the main road of western Norway with fixed straight crossings. Many of these crossings are too long, and too deep for conventional bridges, requiring novel technologies and new methods. One of the challenges these bridges face is environmental loading.This paper studies the response of a tether anchored floating three span suspension bridge over Bjørnafjorden subjected to environmental loads with a return period of 100 years. The crossing has a length of over 4000 m and depths in excess of 500 m. With eigenperiods spanning from 100 s and downwards, the bridge is susceptible to severe dynamic amplification from many types of environmental loads. The two largest load contributions are due to wind and waves. Wind loads are included with a stochastic wind field over the middle span of the bridge. This includes both temporally - and spatially varying wind speeds in all three directions. Second order slowly varying wave drift forces on the floaters are also taken into account.The analysis is performed by time domain dynamic simulation using the computer program USFOS. The bridge girder, pylons cables and hangers are modelled with beam elements. The key response quantities studied and reported in the paper are horizontal displacement of both floaters and the midspan, accelerations of the floaters, both at the bridge girder and at the top of the superstructures and at the midspan, and the tension force both in the main cables and in the tethers. The simulations are based on the contour line method, which is commonly adopted by the offshore industry. Thirty one-hour time domain simulations were conducted to estimate the 100-year response of the bridge to environmental loads.