Construction World December 2018

T his was a complex multi-disciplinary project involving Structural Engineers, Geotechnical Engineers, Wind Experts and Architects. The primary concerns were safety (strength, stability, robustness), serviceability, economy, constructability and low maintenance. The design of the Grayston Pedestrian Bridge complied with TMH7 Parts 1, 2 and 3, SANRAL’s Code of Procedures for the Planning and Design of Highway and Road Structures in South Africa and the International Federation for Structural Concrete (FIB) Bulletin 32 – Guidelines for the Design of Footbridges. Building Information Modelling (BIM) was used for the design and drafting of the bridge. centric infrastructure, the ‘Great Walk’ project includes integrated bus rapid transit systems and provides unobstructed daily movement for the 10 000 people who make that journey via a 6 m wide sidewalk and bicycle lane with a 289 m long cable-stayed pedestrian bridge over the busy M1 freeway at Grayston Interchange. GRAYSTON PEDESTRIAN BRIDGE The Grayston Pedestrian Bridge forms part of a pedestrian and cycling corridor built to connect residents of Alexandra Township to Sandton in Johannesburg. Alexandra is one of the poorest urban areas in South Africa while Sandton is one of the most affluent. Previously separated by vehicle-

PROJECT INFORMATION • Company entering: Royal HaskoningDHV • Client: Johannesburg Development Agency • Main Contractor: Concor Infrastructure • Architect: Glenn Mills • Consulting Engineer: Royal HaskoningDHV • Wind loading expert: Dr. Adam Golliger • Geotechnical expert: Michael Pavlakis & Associates

The bridge has eight spans with a continuous post-tensioned concrete box girder deck. The main span over the M1 highway is supported along its centreline by cable stays. The cables were supported by a concrete pylon with a height of 54 m above deck level. The pylon was supported by backstay cables anchored into the ground. The length of the cable stays vary between 25 and 114 m, with the majority of the cables being in the range of around 60 m long. Redundancy was built into the design so that one cable, at any location, could be removed for maintenance. An ‘Iconic’ mandate for the bridge was set from the beginning by the client. Additionally, the structure needed to be accessible and to facilitate high volume pedestrian and non-motorised transport movements between Alexandra and Sandton on the M1 Highway. A Cable stay bridge was chosen as the preferred bridge type as it was the best fit within the local site constraints. Additionally, long-term lane reductions in the highway below were not allowed, thus a permanent pier in the M1 median could not be constructed. This indicated that a bridge with a long main span was required. The project was envisaged as a community focused project incorporating sustainable best-practices such as local labour and design approaches intended to minimise future maintenance needs. While the project has achieved the original design goals, there were many challenges. These required the project team to identify innovative solutions during construction without impacting the final cost. 

Construction WORLD

2 18 BEST PR O JECTS

57