Mtentu Bridge, Eastern Cape province, South Africa
South African National Roads Agency Limited’s N2 Wild Coast Toll Road (N2WCTR) serves as a shining example of a catalytic infrastructure project that combines socio-economic benefits and engineering ingenuity.

Challenge

The bridge’s structure must be able to support its own weight and the anticipated traffic loads while resisting wind and seismic forces. Engineers must employ sophisticated structural analysis techniques to ensure the bridge’s stability and integrity. The bridge’s foundations must withstand the challenging geological conditions of the site, including unstable rock formations and potential seismic activity. 

 

During the concept and preliminary design stage, the team faced the challenge of creating a bridge that minimised environmental impact while adhering to aesthetic and cost-effective principles. Given the bridge’s location in a valley with steep sides, different vertical grades for the approach roads, ranging from 4% to 6%, were evaluated to optimise the alignment and reduce the bridge’s length.  

 

Solution

More than seven alternatives were assessed, all aimed at decreasing wind and seismic forces, reducing the size of the foundations, and employing well-established and uncomplicated design and construction methods. The optimisation process factored in cost, schedule, visual appearance, environmental impact and design and construction risks. 

 

The final bridge design consists of three balanced-cantilever spans with a 260 m main span. The approach spans are incrementally launched with 66 m spans, resulting in a total deck length of 1,133 m. The deck accommodates four traffic lanes, shoulders and two separate 1.4 m wide sidewalks. The bridge’s horizontal alignment is straight, while the approach spans follow vertical sag curves, necessitating the incrementally launched spans to follow a vertical arc. The main spans maintain a consistent gradient and all foundations are spread footings on rock, with main piers reaching heights of 148 m and approach span piers up to 80 m. 

 

The deck is continuous, with expansion joints only at the abutments. This required the main piers to support significant loads during construction and seismic events while accommodating temperature-induced deflection. To achieve this, the main piers were constructed as twin-bladed hollow box sections. The deck’s cross-section at the front face of the main piers for the balanced-cantilever spans is supported on bearings, and these piers are also constructed as hollow box sections. The deck’s cross-section for the main spans varies from 15 m deep at the piers to 5 m deep at mid-span, while the approach spans are 5 m deep.  

 

Impact

Mtentu Bridge will form part of the N2 Wild Coast Toll Road which also includes Msikaba bridge and 112 km of new highway. Standing at 223 m and stretching 1.13 km, including a 260 m long main span, the Mtentu Bridge will rank among the world’s longest main-span balanced cantilever bridges. As the appointed bridge designers, SMEC South Africa and HVA JV partners CH2M and Axis, have played a pivotal role in the Mtentu Bridge project since its inception. 

 

Mtentu bridge will enhance accessibility to the Wild Coast region, a popular tourist destination. Improved transportation links are likely to attract more tourists, generating additional revenue for local businesses. The bridge will also improve the efficiency of the regional supply chain by reducing travel times and costs for transporting goods. This will benefit local businesses and consumers alike. 

3
hours
Estimated time savings for road users
1133
metres
Length of Mtentu Bridge
223
m
Height of Mtentu Bridge

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