The Towers [Image Source: Flickr – Andre Wessels]
The Towers building on the Foreshore in Cape Town’s CBD has undergone substantial refurbishment over the past 13 months and is set to be completed on 01 September 2015. One of the most dramatic visual features of the refurbishment is the expansive three-story open courtyard facing Hertzog Boulevard, which required unique building technology to construct.
According to Daryl Sher, Development Manager responsible for The Towers redevelopment at Redefine Properties, this open courtyard is created by means of a massive A-frame concrete truss, the likes of which has never before been constructed in Africa. “The location of the building, which is in a public area declared by the City of Cape Town as “open”, column-free space, and the labour and economic conditions specific to Africa, required an entirely bespoke method of construction.
“The structure we have used is unique to Africa in the sense that concrete elements are largely used to form the truss, where the more conventional or classical methods in European countries would employ mainly structural steel. In the African context, making use of concrete instead of structural steel makes sense in terms of employing more labour, and improving cost-effectiveness,” he says.
While meeting the City’s requirements and achieving cost-effectiveness were primary concerns in the development of the mega-truss, the design has other benefits. “Long-term structural performance in terms of creep deflections are minimized and, because the A-frame truss projected horizontal area is minimized, it allows sufficient fresh air and smoke ventilation so that mechanical ventilation systems for the parking slabs are not required,” says Sher.
What is particularly different about this building technology is the fact that an entirely separate temporary concrete structure is built, complete with piles, to support the A-frame until the full frame is completed, after which it is demolished. “This is required due to the magnitude of the temporary loads which would overload any conventional back-propping system,” explains Sher.
“Once the A-frame is completed, the tension ties and hangers are pre-stressed (varying from 5000 to 14000 kN), and the bearing on the temporary columns is released through a phased downward-jacking process, using 8000 kN flat-jacks on top of the temporary columns. Once the A-truss is free of the temporary supports, the temporary columns will be demolished.”
Sher says this pre-stressing method of construction is an adaptation from techniques often used in bridge design. “It has been really exciting to watch the refurbishment of this building – and to know that is has been made possible due to inventive and innovative building technology. I am confident that the success of this build has illustrated the value of new building technique in Africa and I’m sure we will see promising knock-on effects in the industry,” says Sher.
The Towers building remained operational throughout the refurbishment, which created some substantive challenges. In simple terms, a new external envelope needed to be created before the existing exterior could be punctured or removed. In order to achieve this, the new glass and aluminium skin (approximately 42 260m2 of glass or enough glass to cover 6 rugby fields), needed to be installed top down.
“However, unitised curtain walls, the term used to describe the type of glass and aluminium skin on the Towers, are by their nature designed to be installed bottom up. This meant that the façades design team had to come up with a new curtain wall system design from scratch. This is the only building in the country to use this kind of system as its weather line,” says Sher.
A positive by-product of this process has been the reduction in damage to the new façade because work is being carried out below the finished work rather than above it. The process requires strong communication with tenants to keep them informed of the process and to manage their ongoing business activities with the programme of the works.