Construction World March 2015

STEEL CONSTRUCTION

Pylon Pile Cap An unusual aspect of the project is that the pile cap for the two main pylons has to straddle over a bank of high voltage elec- trical cables. These services were installed during the expansion of the OR Tambo Inter- national Airport and did not appear on any service drawings. Critically they could not be moved. Rather than redesign the bridge a bespoke pile cap was designed using a detailed finite element model. Aesthetics of engineering elements The shaping of the towers was carefully considered as they are a visually dominant feature of the footbridge. To create a taper that appeared intuitively correct the section tapers in accordance with the golden ratio. The golden ratio is where two quantities have the same ratio to each other as the ratio of their sum is to the larger of the two quantities. Mathematically, the ratio is 1 to 1,618. The lower tower section is 13,225 m long and has a constant diameter. The next section is a tapering section with a length of 8,175 m, which is 13,225 m divided by 1,618. The taper is then continued with the same pattern as shown. Ingenuity originality and innovation It is hoped that the bridge is a testimony

The urban planning inputs to link the rail, taxi and pedestrian transport modes played a large part in the structure’s final form. Significant effort was invested in concep- tualising the functionality of the bridge to ensure it can be used and accessed by all. Safety and security concerns were also mitigated by providing multiple access and egress points. Planning and technical design planning From previous experience the design team was aware of the importance of trans- lating the original design intent through the detailed design stage and onto the engineering drawings. The innovative step of producing a design intent booklet with sketches illustrating the look and feel of components and swatches of the pre- ferred materials was developed and main- tained. This helped team members focus of the desired outcomes during the various design iterations. Meeting the client’s deadlines for readiness The project experienced delays due to the discovery of underground services. However, the carefully considered construc- tion sequence and temporary works designs meant that pedestrian access across the R21 was maintained throughout the construction period.

to an engineer’s ability to create original structures that combine economy with aesthetics. Unseen ingenuity is perhaps the integral abutments and connections that minimise the number of expansion joints required in the ramps or the complex interaction between the structure and its piled foundations. The deck’s torsion box is also a key structural element in dealing with the unbal- anced cable forces at each pair of outriggers. Innovation within the context of a rational structural system drove the final form of the outriggers, which went through many design iterations. Responding to needs of client and community The most satisfying aspect of the project is to witness the daily use of the bridge by thousands of people and the improved space that commuters now have. It is fulfilling to see the structural form that is conventional yet unconventional as such a functional piece of infrastructure. The bridge sits in an industrial zone next to the airport and stands apart from its surrounding to some degree. In doing so it acts as a visual marker for pedestrians heading to and from work. The intent with the leaning towers was to give the structure a dynamic feel that reflects the urgency of the commuters who hurry across it. The architecture of the bridge also responds to the adjacent international airport.

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CONSTRUCTION WORLD MARCH 2015