One Airport Square is set to redefine the Accra skyline and raise the bar on design and construction processes throughout West Africa. It is the first commercial building to be certified through the Ghana Green Building Council.
One Airport Square is a 17 000m 2 multifunctional building with nine floors of A-Grade office space above a ground-floor retail piazza. The retail floor is arranged as a ring of shops that shield the commercial floor and encircle a landscaped public square that is open on the north-east side.
Each floor has terraces that cantilever out of the building. Set back from the slab, the terraces are supported by a network of pillars that, when viewed from the street, visually reference the patterns of the palm-tree bark and the cultural practice in the country’s north-western areas of decorating facades.
The concept of cultural celebration was carried through the public square, designed as an exhibition space for permanently installed works of art.
The project’s green features include a rainwater harvesting system that uses non-potable water for flushing toilets and irrigation; energy and water consumption metering for ongoing monitoring; an upgrade to the site ecology from contaminated land to indigenous landscapes; and a state-of-the-art air-conditioning system, complete with refrigerant leak-detection.
But what is most special about this building is not its list of add-on green devices – it is the way in which the team worked together to produce integrated responses to the environmental mandate.
As a green building, the form was carefully considered to maximise the latent environmental benefits afforded by good design. The offices are arranged around a central atrium that facilitates natural ventilation through the building and brings daylight to the inner office space.
The electrical engineers in turn designed a lighting system with smart controls that responds to occupancy and light levels.
The site is located five degrees north of the equator and experiences solar radiation on the northern facade in winter and the southern facade in summer. PJ D’Cruz, MEP engineer of Chapman BDSP says of the local context: “The upper floor terraces provide overhangs for reducing solar gains and offer an opportunity for working in comfort outside.”
Projecting terraces were included along the perimeter of each floor to shield the office windows. Chapman BDSP used parametric modelling to understand the requirement for shading. The analysis showed that an overhang of at least 3m to the perimeter of the building could reduce the peak cooling load by as much as 60%.
The structure posed some interesting technical challenges, including the inclined column network across the facades and the cantilevers.
Most critically, Accra is a seismically active area. Seating the building on rubber bearings called seismic isolators reduced the requirement for additional reinforcing and bulk. On top of this, U-boot formwork was used to cast hollow-core slabs, which resulted in an overall lightweight structure without compromising the load-bearing capacity of the slabs.
Positive aspects and achievements
“It is the first building in Ghana to be constructed on seismic isolators; the first building in Ghana to utilise the U-boot slab system to reduce the massing of the superstructure; it is the first building in Ghana to utilise Halfen coupler systems for rebar connections and the first building in Ghana to be constructed without grid systems,” says Mustapha Wahabi, general manager of Micheletti & Co Construction.
“Aside from the Halfen couplers and the seismic isolators, all the materials required for the structural superstructure were sourced within a 50km radius.”
The project team targeted all the Indoor Environmental Quality (IEQ) credits. The strategy for achieving individual control, thermal comfort, flexibility and energy efficiency of the ventilation system included the specification of an AET system that uses pressurised flooring voids to distribute air through fan tiles placed in the access flooring.
The fan tiles can easily be moved to any desired location within an office environment to suit particular office partitioning or desk layouts. Local thermostatic control is provided to each fan tile, which affords users local control.
Aside from flexibility, the system was economical and it did not require specialist skills for installation – an important consideration that enabled the project to contract local installers.
The HVAC’s energy efficiency was achieved through selecting an efficient chiller; heat-recovery wheels in the air handling units; night flushing settings controlled by the BMS that use cool night air to reduce the internal temperature of the building in the morning; and a hybrid system that partially relies on natural ventilation through opening windows.
“The amount of time natural ventilation can and will be adopted will be very much down to the end users but it is hoped that this option will be adopted as often as possible,” says D’Cruz.
”We set out to build the best office building in Ghana, not only in terms of design, but also in terms efficiency, flexibility and the quality of the space for the tenant,” says Carlo Matta of Laurus Development Partners.
However, delivering a green building was easier said than done – and delivering a green building within budget was a challenge. Upon completion, the green extra-over was about 3% on the real building costs.