Bridges

The Northern Marmara Motorway consists in a 115 km long ring road around Istanbul. It includes the iconic 3rd Bosphorus Bridge designed to be the symbol of modern Turkey, but also not less than 37 bridges spread over European and Asian sides.

Most of these bridges are made with precast Igirders, but Freyssinet managed to convince the main contractor ICTAS?ASTALDI to build three of these bridges using the incremental launching method (ILM). Several advantages are offered by the ILM technique, including quantity saving, overcome the difficulty of precast beam erection in deep valley, enhancing the safety during construction and introducing innovative seismic design approach.

This paper gives on overview of the design challenges associated with the ILM bridges of the Northern Marmara Motorway. It provides details about the prestressed concrete box girder designed for 55m spans on piers as high as 80m. But it also focuses on the pier design governed by the seismic conditions. Innovative pier shape was proposed to increase its flexibility in the transversal direction while providing enough energy dissipating capacity through the creation of plastic hinges.

Longitudinally, the deck is fixed on several piers and additionally fluid viscous dampers are placed at the abutments to reduce the seismic demand. Traditional multimodal response spectrum analysis was undertaken along with linear effective properties for the dampers. But upon request from the Turkish administration, the response spectrum analysis was completed by a nonlinear time history analysis in which the fluid viscous dampers were modelled with their intrinsic nonlinear force displacement constitutive model and the plastic hinges were represented using a multi?fibre beam element based on mechanical properties of concrete and steel reinforcement.

The results of these two analyses are compared in terms of forces and displacements, showing that response spectrum analysis is more conservatively representing the seismic action.