Design of the Isando Cable Stayed Pedestrian Bridge – the Walking Wonder

This paper describes the conceptual and technical design process followed by the employer, engineer and architectural advisor during the design of a cable stayed footbridge in Johannesburg, South Africa. Dubbed the “Walking Wonder”, the concept design was the selected solution out of 6 alternatives submitted. The client, the South African National Roads Agency SOC Ltd. (SANRAL), identified the importance of creating an interesting aesthetic that responded to the site’s prominence. For this reason the bridge consultants were asked to come up with several different solutions and to approach three architects for their concepts. One of the two designs submitted by the consulting engineers was selected. Seeing value in a collaborative design approach, the consulting engineers then selected one of the architects to act as an architectural advisor.

The urban planning inputs of the site played a large part in the structure’s final form. The crossing is located at an intermodal transport junction where train commuters, taxi users and pedestrians meet to access work opportunities on the other side of a busy highway. The new footbridge replaces two structurally deficient structures and serves some 9 000 pedestrians daily. With long inclined suspended access ramps, it has a total length of 446 m whilst the main structure crossing the highway is a 126 m long cable stayed bridge with a walkway width of 4.5 m. The bridge has a unique appearance in that the two unsupported towers are inclined at 11 degrees in opposite directions.

The design team identified the importance of translating the concept design through the detailed design stage and onto the construction drawings. The final structure has remained true to the original design intent of providing an aesthetically pleasing and unique structure that fits well into the airport environment. The towers leaning in opposite directions support a thin, elegant deck. The stay cables are connected to outriggers with blade like ties which support the composite deck.

The structure now acts as a visual marker for pedestrians heading to and from work. The intent of the leaning towers is to give the bridge a dynamic feel that mimics the urgency of commuters who hurry across it. The layout gives the illusion that the tower ‘legs’ move when motorists pass beneath the bridge, giving the structure its name and a unique visual appearance. The bridge was completed and opened to the public in December 2013.