INTRODUCTION :

The navigational span arch bridge of Mumbai Coastal Road Project (South), Package 2 consists of two steel arches, Left Hand Side (LHS) and Right Hand Side (RHS) arches, with largest length of 136 m. The arch bridges were designed with the assumption of using elastomeric bearings as the supports. Each arch is designed to be supported at 4 locations over elastomeric bearings. In most of the other spans of this project (both package 1 and package 2) considerably large elastomeric bearings (up to 1050 x 1050 x 298 mm) have been used, which might have prompted the designer to envisage elastomeric bearings for this span as well. The supports are required to be flexible supports so that the horizontal force transferred to the substructures are limited to certain values in different conditions. The challenge to provide the support system was not limited to addressing the gravity load of about 33,000 kN in ultimate limit state and accommodating static displacements and rotations of the bridge supports, but extends to the dynamic behavior of the bridges during seismic actions.

STRUCTURAL AND FUNCTIONAL REQUIREMENT OF THE SUPPORT SYSTEM

STRUCTURAL REQUIREMENT :

The project specification specified 8 numbers of bearings, 4 under each arch. Each bearing was to carry a ultimate vertical load of about 33,000 kN. A rotation of 0.01 radian was to be allowed on each bearing. A longitudinal movement of ±209 mm (in extreme event) and transverse movement of ±155 mm (in extreme event) were to be allowed.

FUNCTIONAL REQUIREMENT :

The two arches were designed with flexible bearings at the supports to reduce the seismic horizontal forces to be transferred to the substructures. They were designed considering elastomeric bearings at the supports. According to the project requirement, the bearings system should provide the following functional solutions:

 1.            Providing adequate lateral flexibility under gravity load to achieve the reduced force response

2.             The bearings should have the horizontal stiffness values as close as to the design values

3.             To restore the structure back nearly to its original position after events like an earthquake

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