The Angus L. Macdonald Bridge is one of Halifax’s most important structures, connecting the Halifax Peninsula to Darmouth across an inlet from the Atlantic Ocean. It opened to traffic in 1955, and was determined in recent years to require major renovation works – with emphasis on the replacement of the existing deck and stiffening trusses with an entirely new orthotropic deck/truss arrangement. The method developed to undertake the deck replacement was very noteworthy. It involved cutting and removing operating deck segments, followed by lifting new replacement segments off barges into place. The new superstructure was ready for traffic immediately after installation, having been prefabricated, complete with road surfacing, right down to the road markings needed to guide traffic. Thus, full bridge closures occurred during week nights and on weekends but the bridge remained open during weekdays during peak traffic flows. It was only the second time such a replacement procedure had been implemented, the other time being the Lion's Gate Bridge in Vancouver. Substantial completion of the refurbishment of the superstructure is planned for late August / early September this year. The completion of the project, including dehumidification of the main cables, is scheduled for January 2018.
mageba was honored to have the opportunity to support this important and challenging project from an early stage – firstly, in the provision and use of a structural health monitoring (SHM) system to evaluate the bridge’s movements, and secondly, in planning the new structure’s optimal expansion joint solution. A ROBO®CONTROL automated SHM system was installed and used to record deck movement and rotation data over a period of one complete year, thus including data from all seasons. This data was then used, not only by the bridge’s designers to verify their computer modelling of the deck and optimize the superstructure’s design, but also by mageba in addressing the bridge’s expansion joint needs. Having established, for example, that the joints at the towers would have to accommodate total longitudinal movements of 797 mm, transverse movements of +/-31 mm and vertical movements of +/- 25 mm, and knowing the precise rotations that would also arise, mageba was able to recommend the use of modular joints rather than sliding finger joints, which had also been previously considered as a potential solution.
TENSA®MODULAR expansion joints were selected for use, having previously successfully passed the specified strenuous prequalification testing in accordance with AASHTO Bridge Construction Specifications, Appendix A19. Most significantly, this type of joint was the only type available on the market to have been successfully subjected to the required fatigue testing in the “infinite life regime”, doing that with six million load cycles per specimen.
The joints were designed, in accordance with demanding Canadian standards, to meet all project requirements. The demand for low noise emissions under traffic was addressed by the addition of noise-reducing surfacing, known as “sinus plates” – which limit the transverse and vertical movements of a modular joint to some degree, but which can still be designed to accommodate larger transverse movements than finger-type joints. The joints at the towers were designed with seven movement gaps per joint – one less than had been believed to be required prior to the use of the SHM system, resulting in lower supply and future maintenance costs. The joints also had to be designed for connection by bolting to the bridge’s steel deck, rather than by concreting. A very important design consideration was the very large accumulated movements that the expansion joints will experience over a service life of several decades. Having established that the bridge deck will move much more than a typical suspension bridge, with movements not only due to daily thermal cycles but also due to traffic movements throughout the day, it was essential that the expansion joints’ sliding materials should be able to withstand hundreds of kilometers of sliding movements without excessive wear and abrasion. The TENSA®MODULAR expansion joint, using ROBO®SLIDE instead of PTFE at these interfaces, offered the ideal solution, thanks to ROBO®SLIDE’s far greater strength and much higher durability (2.5 times the sliding capacity of PTFE). It is also noteworthy that the joints were designed to be installed, like the deck segments lifted from a barge underneath, as a single, easily installed “block”. To facilitate this, the joints were delivered full length without any need for splicing on site, and with the spaces between support bar boxes, which normally project from the joint into the structure’s deck, infilled to create a block without projections.
Having been fabricated accordingly, with hot dip galvanizing for corrosion protection as specified, the expansion joints have already been delivered to site and are currently waiting to be installed in July and August 2017 – new joints in the new superstructure of the old Macdonald bridge.
mageba acknowledges the invaluable cooperation of all those who have been working on this project, whose professionalism and expertise has contributed greatly to the project’s success to date – especially the bridge owner/operator, Halifax Harbour Bridges, the design team led by COWI, and the constructors joint venture led by American Bridge.