A new award-winning bridge on the N7 in South Africa
“The Olifants River Bridge uses an ancient structural form in an innovative way, making use of modern materials and analysis techniques to provide an aesthetically pleasing structure,” – Hennie Niehaus, Aurecon Technical Director.
The N7 in the Western Cape is an important route providing an economic link between South Africa and Namibia for tourism and freight. The existing bridge had limited structural capacity which could not deal with increasing traffic loads.
In 2013, Aurecon was appointed by the South African National Roads Agency (SANRAL) to improve the Olifants River Bridge crossing and associated works on National Route 7 Section 4 (km 47.1 to 48.9). After extensive investigation on the structural capacity of the existing bridge, the team concluded that the only option to improve the service level of the road was to provide a new structure next to the existing bridge.
The existing Olifants Bridge, adjacent to the proposed new bridge, was a major influence on the conceptual design of the new structure. Due to aesthetics and hydraulic reasons, a similar arch type structure was chosen. The deck type that was found to be the most efficient was a twin spine beam. Instead of the wall type piers on the existing bridge, two columns, one beneath each spine beam, were proposed. On the arch portion of the bridge, the columns are supported on a twin arch structure, connected with cross beams at each pier.
High maintenance items such as expansion joints and bearings were reduced by providing a continuous deck and integral piers. Integral piers do, however, attract additional forces due to horizontal loads on the deck and bending in the arch. This meant that that two seemingly contradictory requirements had to be met when sizing the piers.
First, the piers had to be large enough to resist the vertical reactions and movements from the deck and secondly, be slender enough to avoid attracting large forces. For the tall piers that are supported on the ground this could be achieved because of their natural slenderness, but for the shorter piers that are supported on the arch, this was a near impossible task.
The solution to the problem was found by introducing a concrete hinge at the top of the short piers. This design change led to the stiffness being reduced by a factor of four and the design movements being significantly reduced.
Giving back to the community and environment
The entire unskilled labour force was employed from the local community. Several skills training programmes over the duration of the project were conducted, leaving as many employable skilled people as possible for future projects. The project team also ensured that all environmental factors were carefully considered.
In June 2019, the project won a Fulton Award for Infrastructure projects up to R100 million in value. It also won Technical Excellence, Divisional (Structural) and People’s Choice awards at the Western Cape Regional Awards of the South African Institution of Civil Engineering (SAICE).
“The project was completed to high standards and to the client’s requirements. In spite of various changes in construction details in response to unexpected geotechnical conditions, the bridge was completed and handed over to the client as required in November 2018. This was only possible through the cooperation between the contractor, consultant and client,” says Hennie Niehaus.
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