Render versus reality. Photo credit: Sean Gibson

The Rubik stands tall in its context, straddling the transition between the old and newer cityscape of Cape Town’s central business district (CBD).  

Pierre Swanepoel, Partner and project lead at dhk Architects, discusses the challenges of designing a mixed-use 96m tower high structure on a relatively small site of only 28m x 29m. The project was particularly challenging in also having to accommodate a parking spiral 10 floors up the building.   

Using learnings from the 35 Lower Long Street tower, Swanepoel shares how The Rubik’s unique structure and core layout accommodates the limitation of the site, and the strategies employed to create an efficient form.  

The site 

A peninsula site located on the corner of Riebeek, Loop and Sea Streets, the site is on the transition strip of heritage CBD to the more corporate character with its taller buildings to the north.  

The juxtaposing characteristics of the site’s context include extreme variations in height, scale and facade typologies. These variations required city and streetscape impact modelling for the local statutory authorities, including key street views from Riebeek Street, Loop Street and Buitengracht, and more distant cityscape views towards Signal Hill and Table Mountain.   

The street-level frontage of the existing building was almost entirely inactive. An important design indicator was to maximise street level activation to improve the street level interface and pedestrian safety.  

dhk Architects’ The Rubik stands out for its innovative design and maximised efficiency

Design progression 

The initial proposal was to add four floors to an existing five-storey building on the site. However, Abland Property Developers acquired a second corner site, which doubled the site area. Concept viability studies after this acquisition resulted in a significant increase to the bulk and building height, leading to higher land values.   

In response, Abland briefed the architectural team at dhk Architects to design a unique, efficient, marketable multi-use tower that was suitably spectacular and fitting for a prominent Cape Town CBD location.  

Early concepts were discarded due to challenges with the centralised core, including insufficient lift numbers, a difficult parking layout and the loss of too much gross leasable area (GLA). Further iterations explored a more conventional orthogonal building typology.  

The design and client teams later opted for a more sculpted, dynamic and unique building form. This concept was developed into a series of stacked, twisting masses within the site boundary to fragment the tower building.   

Fragmentation of mass within The Rubik building. Image: dhk Architects

Maximised efficiency 

The glazed form is expressed as flush-glazed cubes containing reflective glazing that conceals the horizontal and vertical spandrels. Balconies are recessed into the façade with discrete openings to enhance the glazed cube effect, increasing usability and providing greater protection from wind. This in turn maximises the saleable floor area, since protruding balconies would pull the facade in further from the boundary.  

Stringent zoning regulations governing building against a common boundary presented complexities that needed to be negotiated and resolved with the local authority. These were mitigated by installing water drenchers to improve fire rating of the glazing and including in the deeds of sale the condition that future developments may be constructed on neighbouring sites, with office and residential windows facing the boundary line units.

Recessed balconies maximise the saleable floor area, since protruding balconies would pull the facade in further from the boundary. Photo credit: Sean Gibson

Walking and sloping columns accommodate the twisting form, as well as the layout changes. Image: dhk Architects

Other efficiency measures included:  

• Space-saving scissor escape stairs were placed along the common boundary 
• Plant was located on the roof and major plant in the basement to free up ground floor space 
• Common areas were minimised 
• Using the fire-fighting lift as a standard lift 
• The floor plate was deepened by positioning bedrooms in one-bedroom flats at the back of the unit, away from the façade. In these units, natural lighting is introduced by clerestory windows and into penthouses from lightwells set into the roof 

Structural interventions 

A detailed structural strategy was developed to accommodate the changing layouts of column positions from the parking levels to the office levels and to the residential levels above. This strategy included:  

Deep coupler beams are positioned to increased lateral stiffness in a massive portal framing action. Image: dhk Architects

To achieve a building height of 90m on a site footprint of 28m x 28m: 

• Strategic placement of the central lift and perimeter stair core, with deep beam connections on 11th and 18th overheight floors 
• Structural column dimensions were strategically placed and minimised to the absolute minimum to not impede or reduce parking bays 
• To enable this, high-strength, 70MPa concrete (usually 65MPa) and with a higher rebar percentage at 5% (usually maximum of 4%) was used 

Ground floor plan. Image: dhk Architects

A floor plate transition across different layouts: 

• Transfer beams were placed at level 18 between the offices and the residential floors above 
• Transfer slabs accommodate transitions of the column layout, while walking columns, offset from floor to floor, accommodate the twisting form and layout changes 

Parking floor plan. Image: dhk Architects

Ensuring structural integrity: 

• Design of cores provided an east-west sheer wall, but no north-south sheer walls. Deep coupler beams were positioned to increase lateral stiffness and mobilise cores in a massive portal framing action 
• The centralised core and stair core coupled effect to counter overturning moments 
• The building can withstand 1:50 year wind load, proven by withstanding 300km wind gusts in the April 2024 storms 
• The seismic response analysis was assisted by the stiff parking levels with large upstand beams and favourable hard-rock conditions 

Building data 

Bulk: 11 975m² 

Office: 5 170m² 

Retail: 393m² 

Residential: 6 447m² 

Plant: 110m² 

Office floors GCA vs GLA: 88% 

Residential floors GCA vs GLA: 90% 

For more information, contact dhk Architects: 

Tel: +27 21 421 6803  

Email: hello@dhk.co.za 

Website: https://www.dhk.co.za/