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One of the greatest challenges presented by the balanced cantilever bridge construction method is the design of the “hinge detail” at expansion joint locations, where the tips of the cantilevers from each side meet at mid-span. This hinge detail must be able to withstand not only the continuous movements of the adjacent spans but also the continuous fatigue loading from every vehicle that crosses the bridge – transferring its weight from one cantilever to the other, with resulting reversal of vertical loading. A modern high-tech solution to this challenge has recently been implemented in the construction of a new multi-span bridge over the River Brahmaputra near Tezpur in eastern India, as illustrated in the attached pictures.
With its overall length of 3,040 m, including 24 spans of 120 m each plus several shorter spans at the approaches, the bridge was designed with a prestressed concrete box girder superstructure, to be constructed by the balanced cantilever method. The superstructure consists of nine continuous parts, separated by expansion joints (also supplied by mageba) at eight locations. Since these expansion joints (mostly TENSA®MODULAR joints with four gaps each) are at mid-span locations, 360 m apart, a suitable hinge detail to transfer vertical loads and prevent relative vertical displacements was required at each of these locations along the bridge’s length. A separate solution, in the form of vertically-installed RESTON®DISC bearings (specially detailed to prevent the sliding material from falling out) was supplied to transfer transverse forces across the mid-span discontinuity and prevent relative transverse displacements.
The hinge detail supplied by mageba for this project consists of a pair of central hinge bearings at each expansion joint location – one at each web of the precast concrete superstructure – as shown in the pictures. The design challenges included:
Each central hinge bearing required to be designed for various project-specific loads, movements and rotations, including vertical load reversals of +/-390 kN. Since these load reversals result from live loading, as trucks cross the bridge, the entire bearing solution required to be designed to withstand fatigue throughout a long service life.
Each central hinge bearing is subjected to high accumulated movements over time – not only due to superstructure expansion/contraction etc., but also (and predominantly) due to the passage of every vehicle crossing the bridge. This required the bearing’s sliding interfaces to be designed accordingly, with high resistance to wear and facilitating easy replacement.
To overcome these and other challenges, the central hinge bearings were designed with prestressed anchorages to the superstructure and with a high-performance special sliding material (rather than PTFE) used at the sliding interfaces. Specially designed RESTON®POT LIFT-CONTROL bearings were placed at the top and bottom interfaces of the central hinge bearings to allow pre-compression during the installation stage and further compression during the service life to counteract any loss of compression due to wear and abrasion at the sliding interfaces. Each interface is also equipped with a sensor which can be connected to an automated monitoring system to continually measure the pre-load at any time – enabling any loss of pre-load to be recognised and then counteracted by injection to increase the height of the interface detail.
This sophisticated central hinge bearing solution can be used not only to facilitate the construction of new balanced cantilever bridges but also to rehabilitate such bridges that were constructed in the past with hinge details that have not performed well. It can thus enable this relatively economical method of bridge construction to be used in the future with greater confidence in the structure’s durability – a significant development for the bridge construction industry.
Contractor: M/s Gammon India Ltd. – SP Singla Constructions Pvt. Ltd. (JV)