Construction World August 2015

PROJECTS AND CONTRACTS

INNOVATION at Merriman Square Merriman Square in Cape Town’s Central Business District consists of The

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

> 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 Towers, a 10 and 20-storey building complex that was built in the 1970s, with an open public square separating them.

Aurecon Revit model of the A-frame truss resting on the temporary columns.

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.

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Architect’s impression of the completed project.

Megatruss bottom chord being jacked up, with shim plates separating; the total expected long term deflection is 25 to 30 mm.

CONSTRUCTION WORLD AUGUST 2015