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Designing and erecting the ‘Megatruss’ of Merriman Square

An artist

An artist's impression of Merriman Square, Cape Town

12 June 2014 - 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. 

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 compression 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, transferring 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 structural 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 8 000 kN flat-jacks on top of the temporary columns.  

De-propping of the Megatruss
The de-propping of the Megatruss was carried out successfully in the second week of April 2015, with the final measured deflection of 15 mm correlating well with the calculated theoretical short-term elastic deflection of 14 mm. No tension cracking was observed during the process.

“The successful de-propping of the mega-truss was a major milestone during the project and is worth celebrating,” commented Aurecon’s Jaco de Villiers, professional engineer on the project.

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