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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
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.
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.