Architect and Builder Magazine South Africa November/December 2014 | Page 28

The majority of staff
and visitors will take
the bank of shuttle lifts
located on the Grayston
edge of the site up to
the atrium level on top
of the parking structure

Landscaping
A combination of internal planting to pause
areas, atrium trees and roof terrace planting
at the atrium level ensure that although this is
a building in the heart of the business heart of
Africa, nature is never far away.
Trees in the atrium on the floor and the
feature balcony overlooking the atrium
provide human scale and soften the acoustics
of the tall space.
Sustainability
As part of the best-practice good business
journey, the 90 Grayston building is a
sustainable one which achieves a minimum
of a 4-star GBCSA green building rating.
A combination of sensible passive design
principles and selected active technologies
were used to minimise the impact of
the building and its ongoing use on the
environment. These include the use of a highefficiency ammonia chiller HVAC system and
the harvesting of rainwater to flush toilets.

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Construction & Engineering
The considerable challenge for the main
contractor, WBHO, was to construct a 21
storey building on a relatively small site of
±45m by 83m with various restrictions due to
the three neighbouring buildings and a busy
Grayston Drive in central Sandton on the 4th
boundary. Due to the restricted site access
and the variable rock on site the tight program
deadlines was always going to be challenging,
even though the main contractor managed to
erect 3 tower cranes on the small site.
The lateral support for the five level deep
basement was constructed using 600mm
diameter piles with 200mm gunite arches
that extended 17m in depth at the South
face into the typical Sandton soil profile. This
soil profile was a combination of soft sand
and soft weathered granite rock with a hard
Diabase intrusion requiring blasting lower
down slowing down the program. Due to the
variable nature of the founding conditions a
combination of piles and pad footings were
chosen for the foundations and these could
only be finally determined as the excavation
depth was near completion allowing piling
contractor, Franki Africa, to probe the
rock level.
The main structure has five basements
and six levels of parking above ground with
ten levels of office space above that on a
grid of 8.4m by 7.6m. The parking slabs were
constructed with 230mm post-tensioned
slabs and the office slabs were 270mm postTensioned. The main stability elements were
the central core and two smaller stair shafts
at either end providing lateral stability to the
long building.
On the north side of the building the floor
plates from office level 3 upwards to level 9
project out gradually in increments of 1.48m
into the Atrium space with raking columns
90 Grayston