Olifants River Bridge
Improvement of the National Route 7 (N7) Olifants River Bridge and associated works, Western Cape Province, 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,” said Hennie Niehaus, Zutari Technical Director.
In 2013, the South African National Roads Agency Limited (SANRAL) appointed Zutari to improve the Olifants River Bridge crossing and associated works on the National Route 7, Section 4 (N7/4), from Km 47.1 to Km 48.9. The N7 is an important route, as it provides an economic link between South Africa and Namibia through tourism and freight.
The existing road had limited pavement structure, substandard horizontal and vertical curves, steep grades, and a narrow cross section to deal with increasing traffic volumes along the N7. A major constraint was the existing bridge over the Olifants River. This bridge had insufficient horizontal clearance and during an extensive investigation it was found that the structural capacity was insufficient to deal with increased traffic loads if the road is widened. The only alternative was to re-align the road over this section, which required a new structure next to the existing bridge.
The existing bridge spans the main waterway by means of an arch, spanning nearly 100 m. Due to the proximity of the existing bridge, the new bridge follows a similar form. High maintenance items such as expansion joints and bearings were reduced by providing a continuous deck and integral piers. These, however, attract additional forces due to horizontal loads on the deck and bending in the arch, meaning two seemingly contradictory requirements had to be met when sizing the piers.
All the required unskilled labourers were employed from the local community
The piers had to be large enough to resist the vertical reactions and movements from the deck and be slender enough to avoid attracting large forces. For the tall piers that are supported on the ground, this could be achieved due to their natural slenderness, but for the shorter piers that are supported on the arch, this was a nearly impossible task.
The solution was to introduce a detail, namely the concrete hinge, that was developed 120 years ago, and not used very often by modern designers. By introducing a concrete hinge in the top of the short piers, the stiffness was reduced by a factor of four and the design moments were significantly reduced.
All the required unskilled labourers were employed from the local community and several skills training programmes were conducted over the duration of the project, resulting in as many employable skilled people as possible for future projects.
“The project was completed to high standards and to the client’s requirements and handed over in November 2018. This was only possible through the cooperation between the contractor, consultant and client,” says Hennie Niehaus.
*The Aurecon Africa business has been officially renamed Zutari as at 21 July 2020. Zutari acquired Aurecon Middle East on 20 November 2020.
