The Civil Engineering Contractor January 2019 | Page 35

PROFILE
ground. The length of the cable stays
vary between 25m and 114m, with
the majority of the cables around
60m in length. Redundancy was built
into the design so that one cable at
any location could be removed for
maintenance.
At the west end of the bridge, the
poor quality of the fill required piled
foundations socketed into competent
rock to resist uplift forces from the
cable stays. Although less extreme
conditions were encountered at
the other founding locations, piled
foundations were used for the entire
bridge structure.
The pile type used was an
augured cast-in situ concrete pile
with minimum diameter of 900mm.
These piles were socketed 1m into
the hard granite or diabase rock. For
the cable-stay anchor blocks and main
pylon, ground anchors in the form
of 32mm-diameter DYWIDAG bars
www.civilsonline.co.za
were installed and grouted to a depth
of 10m below the base of the piles to
resist uplift forces.
Records indicate that this is the
first cable-stayed bridge in South
Africa supported by back-stay anchor
blocks instead of being supported
within the deck itself. The main deck
is supported by a single row of cables
in the middle of the deck, which
blends in well with the garden planter
box area.
Meeting client expectations
For aesthetic reasons, steel railings
and lighting posts were used along
the edges of the deck. Additionally,
steel anchor plates were used in the
pylon for attaching the cable stays.
Because of increased durability and
long-term appearance, stainless steel
was used in the construction of these
components. The cable-stay anchor
plates were constructed using grade
316 stainless steel. Grade 316L
stainless steel was used for the railings
and lighting posts.
A garden area dividing cyclist and
pedestrian lanes was incorporated
to enhance the user experience and
incorporate a park-like theme. Seating
benches were also incorporated in the
garden area allowing people to rest
and enjoy the surroundings.
“Overall, this was a challenging yet
tremendously rewarding project. The
completed bridge is both functional
and aesthetically pleasing. The fact
that it serves such an important socio-
economic purpose is fundamental
to the sense of achievement the
whole team gained from the work.
As a company, Royal HaskoningDHV
states its vision as ‘enhancing society’
— and we have certainly achieved
that with the Grayston Pedestrian
Bridge,” says Klassen. nn
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