Over 1 000m² of precast concrete panelling has been used to clad a section of the external façade of
No.1 Silo, South Africa’s latest and largest green office building. Completed in September at Cape Town’s V&A Waterfront and spanning 18 000m², it is also one of Africa’s most advanced sustainable buildings.
Incorporating numerous green-design attributes, recycled waste and heat, the use of seawater in the cooling plant, and a roof garden, to name a few, it is one of only two buildings to have been awarded a six-star design rating by the Green Building Council of South Africa.
Developed by the V&A Waterfront, No.1 Silo’s architectural team comprised Van der Merwe Miszewski Architects (VDMMA) in association with Rick Brown + Associates. Other members of the consulting team included Mace (Project Managers), MLC (Quantity Surveyors), Sutherland Consulting Engineers (Structural Engineers), ARUP (Façade, Wet Services and Rational Fire Engineers) Collaboration (Interiors Architects)
Most of the development was clad with double glazing, automatically controlled blinds which track the sun, and an external single glazed skin. The blinds prevent unwanted heat gain and control solar glare and the double-glazing prevents radiant heat loss or gain and maximises views and natural light throughout the building.
The precast concrete panelling was supplied and installed by Concrete Manufacturers Association (CMA) member, Concrete Units.
According to architect Karien Trengove, of VDMMA, the panelling was specified to dress 280mm masonry cavity walls on the building’s two service cores and western façade, and to distinguish them from the glazed façade of the atrium and office floors. Both textured and flat panels were specified for aesthetic reasons and to give different functions unique identities.
One hundred and eighty concrete panel sections, 120mm thick, were supplied by Concrete Units, 132 with textured (Reckli Tigris) façades and 48 in flat smooth-face finishes. The former were used on the service cores – the east core measuring ± 128m² and the west core 738m². They were manufactured in 17 dimensions, the smallest spanning 1 590 x 1 750mm and the largest extending to 1 705 x 4 295mm. The smooth-face panels were supplied in eight sizes and clad a ± 320m² directional wall on the western façade. The largest measured 2 650 x 3 760mm and weighed 3.4 tons. Smaller smooth panels were also used at the base of the west core to create a plinth line at ground level.
Structural engineer, Justin Arendse, of Sutherland Consulting Engineers, said a combination of steel sub-frames, corbels and dowels were used to anchor the individual panels to either the façade sub-structure or the protruding concrete slab edges.
“Each panel was lowered from the roof into position using a purpose made six ton crane and a four-man cage. The reason for this was that the project’s tower crane had a maximum lifting capacity of only one ton at the end of its reach. Lifting and then lowering the sections into position required meticulous care and coordination between the cage team and the crane operator and on average only three panels could be installed each working day,” commented Arendse.
Two 12mm galvanised steel dowels were cast into the bottom end of each panel. Designed to be epoxied into the guides of the panels below they provided a vertical connection between each panel and facilitated continuity through the panel joints, thereby transferring wind loads to the corbels at each level.
Two further dowels, 20mm in diameter and also galvanised, were cast into the supporting ledge and epoxied into the panel corbels. Their prime function was to lock the corbels permanently into position after the grouting had been completed and they also acted as guiding pins during installation. High density polyethylene (HDPE) shims, wedged between the floor ledge and the corbels, were used to ensure that the panels were perfectly aligned before grouting took place.
Trengove added that in keeping with the design ethos of the building, the overall carbon footprints of the panels were reduced through the manufacturing process.
“They were produced using recycled reinforcing and a slag-waste cement extender sourced from the Saldanha steel mills. Once the moulds were stripped the panels were tilted into a vertical position and then stored on an A-frame steel rack before delivery to site.
“Far from being dull and boring this panelling demonstrates how, when correctly designed and applied, concrete facades can be extremely attractive,” says Trengove.