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CONSTRUCTION WORLD
AUGUST
2015
PROJECTS AND CONTRACTS
>
Redefine Properties Limited,
the owner and developer of the
complex, set out to upgrade the
complex to an A-grade office
building by wrapping the envelope in high
performance architectural glazing, and adding
an additional 1 500 parking bays in the form
of a 13-storey parking structure between the
two towers, while keeping and upgrading the
public square.
Construction commenced in April 2013,
with the parking structure columns being
constructed in the existing basements, and the
project is due for completion in August 2015.
Aurecon was appointed as the structural
engineer of the project.
An interesting challenge on the project
was to maintain a column-free space in the
existing public square, with the top 10 levels of
the parking structure cantilevering 13 m over
this square for a distance of 55 m. To achieve
this, Aurecon conceived a 50 m span x 25 m
high A-frame Megatruss.
Dimensions of the Megatruss
The A-frame concrete truss encompasses an
area 55 m long x 25 m high. The main load
carrying members include a 600 mm wide x
2 750 mm deep bottom tension tie,
450 mm wide x 2 500 mm diagonal compres-
sion members and 450 mm x 1 800/1 500 mm
vertical hangers. The truss was temporarily
supported on 1 m square concrete columns
(approximately 20 m high) with precast
tapered column heads and packing plates.
How it works
The truss is designed to span 50 m clear,
carrying nine to ten suspended floors – it is
essentially a ‘mega’ transfer element, trans-
ferring the gravity load to the L-shaped shear
walls at the ends. It also plays a significant
role in the seismic resilience of the structure,
with ductile zones detailed within its critical
regions to absorb and dissipate energy during
strong ground motion excitation.
The A-frame form is a superbly rigid structure,
utilising the full depth of the front façade
(approx. 25 m). Long term deflections are
a critical design consideration and these
are controlled by utilising the stiffness of
the large concrete sections in compression
only – in other words, where tension would
exist in the final form, those members have
been prestressed so that the full, un-cracked
concrete area resists elongation stresses.
Why it is unique – and is it
unique to Africa?
The structure is unique in that it is an
entirely bespoke design, tailored to suit the
constraints of the site and the Cape Town City
Council requirement that the public square
remain as open (i.e. column-free) as possible.
In a way it is also unique to Africa in the
sense that concrete elements are largely
utilised to form the truss, whereas the more
conventional/classical methods in European
countries would be to employ mainly struc-
tural steel. In the African context, making use
of concrete in lieu of structural steel makes
sense in terms of employing more labour, and
improving cost-effectiveness.
What is especially unique is the fact that
an entirely separate, temporary concrete
structure was built (complete with piles) to
support the A-frame until the full frame was
completed, and then demolished afterwards.
This was required due to the large magnitude
of the temporary loads, which would overload
any conventional back-propping system. Once
the A-frame was completed, the tension ties
and hangers were prestressed (varying from
5 000 to 14 000 kN), and the bearing on the
temporary columns was released through
a phased downward-jacking process, using
INNOVATION
at Merriman Square
Merriman Square in Cape
Town’s Central Business
District consists of The
Towers, a 10 and 20-storey
building complex that was
built in the 1970s, with
an open public square
separating them.
Architect’s impression of the completed project.
Aurecon Revit model of the A-frame truss
resting on the temporary columns.
Megatruss bottom chord being jacked up, with shim plates separating; the total expected long term
deflection is 25 to 30 mm.