Designed as a cable-stayed bridge, the unique feature of this design is that it does not have a counterweight rear span. Typical cable stayed bridges – like, for example, the 2003 Steel Awards overall winner, the Nelson Mandela bridge – have a long steel clear span with a short counterweight span in concrete.
In the case of the 7th Avenue Bridge, the traditional short span concrete counterweight span is replaced with an outwards leaning concrete tower column which transfers the forces in the cable stays both axially and by bending moment into the footing. According to the submission, the absence of a counterweight span gave the team an opportunity to do something ‘very elegant’.
“When a functional concrete arch bridge spanning the N1 highway in the North-west corner of Johannesburg needs to be replaced and the replacement bridge is a combination of steel and concrete where the strengths of both materials are used exactly as they should be, we regard it as a coup for the industry,” the submission added.
The Steel Awards, held on the 15th September 2010 concurrently in Johannesburg, Durban and Cape Town was hosted by the Southern African Institute of Steel Construction (SAISC) with The Aveng Group the main sponsor.
In its brief, the client, South African National Roads Authority Limited (SANRAL) said, inter alia, that the 7th Avenue Bridge contract was an opportunity that should be used to create a special structure that would be seen as a ‘gateway’ to Johannesburg.
The judges said that the result certainly did justice to this request. “This is one of the most utilised pedestrian bridges in the greater Johannesburg area used, in the main, by scholars crossing the busy highway. We have no doubt that both SANRAL and Johannesburg will be proud of it from both aesthetic and engineering points of view.
Structurally, the walkway and cable stay ends are tied to a Toblerone circular hollow section truss creating a clear arched span over the freeway and while the absence of a counterweight span did give the team an opportunity to do something very elegant, a load path for all the forces at the top ends of the cable still had to be created.
In this regard, the concrete tower leans back from the road, creating an axial path down the concrete that redirects all those sloping cable forces through the concrete structure down to the ground. Another advantage of the concrete is its bulky mass with the ability to withstand the rogue forces a vehicle smashing into the concrete work would impart to the structure.
The use of a concrete (composite) walkway surface (deck) makes sure that concrete loads are all compressive in their nature to suit the strengths of the concrete. The steel carries the tensile forces where it is strong as well as some of the compressive forces.
The judges noted that dealing with the ends of the cables is one of the intricate issues in designing, detailing and building a bridge of this nature. Each slope is different and each angle of intersection between the cable and the concrete tower is different. “This requires immaculate attention to detail, which is an omnipresent feature of this job,” they said.
They added that the 7th Street Bridge was a great example of ‘the right material in the right place’ and this, combined with the quality of the steelwork and the excellent presentation overall made it a high quality and elegant solution.
“This is a wonderful addition to Johannesburg’s North Western gateway and the lighting at night makes it truly exceptional. It is clearly an example of excellent use of steelwork truly deserving this award in the bridge category,” they concluded.
Developer/ Owner: SANRAL
Architect: Professor Glen Mills / SANRAL
Structural engineer: SSI Engineers and Environmental Consultants
Main contractor: WBHO
Steelwork contractor: Omni Struct Nkosi (Pty) Ltd
Detailer: Omni Struct Nkosi (Pty) Ltd