105 Corlett Drive

Location
 The building is located at 105 Corlett Drive, Birnam in Johannesburg, adjacent to the on-ramp to the M1 North [26.128412S,28.070786E]. Conceived before the Green Star rating tool implementation found traction, the building is not specifically Green Star rated but does incorporate some sustainable qualities. The budget for the building was R74 million rand. Documentation commenced during April 2010 with the construction phase starting on 25 January 2012 and occupation commenced on 25 June 2013.

 Design Concept
 Carla says the design is based on a glazed box which appears to float, supported by the three conical forms. The product on the cones is Caïman cladding by Arcelor Mittal. Product selection was based on an aesthetic requirement as well as being competitively priced. The cladding provides a glimmering effect that is achieved with changing light qualities. Additionally it could hold the required form while other materials could not. On the north and south elevations a reflective laminated Solarshield S30 glass has been specified. On the east and west elevations a tinted Eclipse Grey Advantage glass has been specified. Ivan Lin assisted with glass specification vis à vis sound insulation, advising that laminated glass could achieve the insulation performance for the allocated budget.

General description
This is a two storey office building sitting atop a 3 storey parking podium, separated by double volume metal clad cone forms. The office space is an elongated flattened concrete and glass form enclosed by wrapped ends, which are punctuated with vertical slits, with the east elevation champfered in plan to follow the site boundary. The conical forms are double volume and include the reception and training spaces. The parking provides for 190 bays.

Building Elements
Base Podium
This rectilinear box is constructed from a brickwork and concrete enclosing element with a coated aluminium infill panel. Below this are panels of African Blue Slate Riven Walling as a contrasting ‘band’ to break up the scale of the basement’s bulk.

External floor finishes
Polished concrete walkways with a saw-cut pattern and inlays of slate tiles are placed at the entrances, in a pattern similar to the riven walling on the basement façade. Beyond this lies the infill landscaping which is xeriscaped using indigenous, low maintenance plants with tall grass-like planting on the perimeter to soften the edge of the building and the balustrades. The roof landscaping matches the ground floor podium with a combination of polished concrete walkways and similar planting.

 Concrete and glazed office box
The concrete box form accommodates two levels of offices and is characterised by its wrapped ends with striking slot windows making vertical ‘slashes’ in the form. The dark glass in the slot windows contrasts with the matt texture of the concrete box.

The main glazed façades are single glazed in a slightly reflective glass, in an effort to move away from a very black glass which complies with heating, cooling and glare requirements, but without compromising on performance.

 The Cones
A network of steel columns is clad in a layer of galvanized sheeting which forms the weather line with the final cladding of the silver CaÏman on a special sub structure made up of ‘omega rails’, which forms a gap between the cladding layers for ventilation. The steel structure was workshopped between structural  engineering, a 3D model of the steel structure submitted by the steel sub-contractor and overlaid onto the Revit model for co-ordination and then approved. This proved to be challenging to co-ordinate the process between steel, cladding and glazing subcontractors, with each junction being unique because of the unusual forms.

The glazing in the cone forms is a highly reflective glass Solarshield S10 Silver, for the narrow strip windows and dark glass at the entrance shopfronts to accentuate the folding forms of the cones. The roof cones have a lightweight sheet roof on a steel structure with a flat concrete roof over the stairs and lift shafts.

The internal walls of the cones are skimmed and painted dry walling, in sculptural forms which appear to be folding and pull away from the structure in places to make dramatic envelopes of space. The windows are punctured in the walls, with slanted sills and tapered reveals, with an almost Gothic feel.

Lighting
Each cone has a band of lighting at the base and head to create a sensation of floating, so that at night they appear to be detached from the concrete forms above and below. The ground floor podium has winding strips of cold cathode lighting in the soffit which will lead the user to the entrance doors and enhance the playful forms of the cone structures. The reception cone has a striking barrisol light insert in the ceiling bulkhead to create a ‘spaceship-like’ glow.

Construction Experience
Contractor’s Comment
Mario Ferreira from Tiber Bonvec commented on the construction phase: “There was a lot of rock on the site and this forced us to change our methodology for the preparation of our bases,” says Ferreira. “For between 60 and 70% of our bases we had to dowel into the rock to commence with the columns. We sub-contracted Gauteng Piling to undertake the drilling into the rock between 2.8m and 3.5m deep. We then doweled and grouted using W32 and Y40s. Rock removal was a major challenge because of the hardness of the material. Overall, this unexpected geological issue added about two weeks to site preparation.”

He said the high water table resulted in a large volume of water flowing into the site and additional sumps have been constructed. A pumping system designed by JKJ Pumps were installed to manage the water flow.

“The challenge lay in constructing off-shutter slab recesses without damaging the soffit while other work was proceeding above that level. The curve itself, along with the recesses for the strip windows as well as the gable ends, forms part of the entire cast in order to achieve a seamless finish. This work-type took twice as long to complete as the rest of the job. ”

He said the lower slab of the podium was cast in two parts with the curved gable ends being part of the slab. “Special steel shutters were fabricated by PERI for the construction of the curved gables. The shutters were split into 2.470m lengths and comprised an external shutter with a floating internal shutter. We used a crane to install the shutters for the construction of the lower curved gable ends, and then removed the shutters, reversed them and fixed them into place for the construction of the upper curved gable ends.

“Our methodology was to fix the rebar and cast the bulk of the podium lower flat section initially and then tie in the curved sections. Once those were cast, we built the vertical walls of the podium as separate sections before tying the upper curved gable ends into the walls and starting on the upper slab of the podium.”

Ferreira said a total of 2 000 m³ of concrete was required for the entire podium. In addition, 600 m³ of concrete was used for the first floor slab, and the same amount for the second floor slab.

‘Our initial intention was to erect a batch plant adjoining the site but since there is a council park directly opposite we opted to have Readymix deliver concrete on demand.He said the construction of the cones was also an unusual aspect of the project, undertaken by fixing vertical steel vertical posts between the ground floor and the first floor. Galvanised steel material and waterproof membranes were fixed to the steel frames prior to the cladding being applied on the exterior.

Accommodation schedule
• Three levels of basement parking
• Ground floor with a landscaped podium at highway level and three pods which are used for reception and waiting area, separate training room and a separate office
• Upper ground with a mezzanine in one of the pods
• First floor offices
• Second floor offices
• Roof top: entertainment area with executive dining.

Construction Materials
Total quantities used in the construction of the building comprise:
concrete – 6 400 m³, rebar –  687 tons and stock bricks – 392 000
A total of 250 people, including sub contractors, was employed on the construction site at the height of the build.