About the Chota Motala Bridge
The Chota Motala interchange, of which the bridge forms a part, straddles the N3 freeway as a major access route to the Pietermaritzburg CBD. It had been operating at above capacity for quite some time, evidenced by long delays in traffic on the Chota Motala Road (R33). Of even more concern was the dangerous situation created by the backing up of traffic on the off-ramps from the freeway, resulting in through traffic on the N3 suddenly being confronted by stationary traffic in the slow lane.
After evaluating several interchange upgrade options, authorities settled on a concrete bridge over the N3 which would create a much-needed third layer of traffic at this busy interchange. The South African National Road Agency Limited (SANRAL) and the Mzunduzi Municipality appointed Aurecon, in joint venture with Illiso, for the detailed design of the upgraded interchange, with Aurecon responsible for all bridge work.
The main determining factor in the conceptual design for the bridge was the client’s requirement that any new bridge over the N3 had to be constructed without disrupting or endangering the heavy traffic on the N3 freeway. It was for this reason that Aurecon proposed the construction of an incrementally launched bridge.
The incrementally launched construction method involves construction of the bridge superstructure section by section on one side of an obstacle and then launching them sequentially into their final position. “The incremental launch technique was considered ideal for this situation,” comments Aurecon bridge engineer, Pierre Van Der Spuy. “With very little risk of disrupting traffic at any stage during construction, it met the client’s requirements in every respect.”
Exceptional use of concrete
Reinforced concrete was deemed to be the logical choice of construction material for the Chota Motala bridge as it offers the lowest initial construction cost and lowest life-cycle maintenance costs. The only alternative was a launched composite deck involving the launch of open steel box-girder over the whole length of the bridge and casting a concrete slab on it at a later stage. However, this option was not considered further because it is generally more expensive than a concrete bridge deck, has much higher maintenance costs and the construction of the concrete top slabs would have posed a safety risk to traffic on the N3, negating the main advantage of the launch bridge.
The bridge was designed as a 220 m long deck consisting of a single cell prestressed concrete box girder with a total of seven spans varying in length between 25.15 m and 36.05 m, arranged symmetrically around the centre. The deck is supported on 6 piers, of which four are founded on piled foundations and two are founded on spread foundations.
The judges found the bridge fully deserving of a ‘Commendation’ for its clever use of concrete to achieve low costs, meet the need of ultimate safety during construction and ensure low maintenance costs during the lifespan of the bridge. The judges also commended Aurecon for superior input in terms of design, commenting: “Of special note was that the engineers, in the absence of an architect, paid particular attention to the aesthetics of the bridge and considered many aspects such as slenderness, span lengths, pier diameter, shapes of members, shadow and lighting. The result is an elegant and aesthetically pleasing civil engineering structure that showcases concrete in an excellent manner”.
“We believe our objective of providing the client with exceptional value for their investment has been achieved”, comments Aurecon project manager Hennie Niehaus. “This achievement is confirmation of Aurecon’s in-depth experience of designing fit-for-purpose bridge-related structures. Congratulations to both the bridge and broader project team on their dedication and a job well done.”
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