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Structural info-
Project
Name Grosvenor Place
Project Team
- Architect Harry Seidler & Associates
Davis Heather & Dysart
- Structural engineer Ove Arup & Partners
- Service engineers D.S. Thomas, Weatherall & Associates
- Builder Concrete Construction
Function Commercial office building
Year 1988
Location Sydney, NSW
Cost $350m
Building
Type office building with parking
Form
- Plan shape two crescents with elliptical central core
- Number of stories 44 levels above ground (including a 3 storey lobby), 4
levels of basement
- Typical floor area 3 000 sq m
- Net rentable floor area 2 000 sq m
- Number of zones 4 - offices, plant equipment, lobby and car parks
Relationship to ground ground level pedestrian entrance with underground
parking
Primary Structure
floor system
-material composite structural steel/concrete
- type beams, composite metal deck/concrete floor
- pattern radial beams, edge beams and one-way floor slab
- beam clear span 14 m
- floor slab span 2.2 - 3.25m
core
- material reinforced concrete
- type shear walls
- shape elliptical
- position in plan central
support structures
- material composite structural steel/concrete
- types external columns, triangulated piloti and core walls
- external column spacing 6.5 m
- external column height 3.5m
footings
- material reinforced concrete
- types raft slab for core and pads for columns
Design requirements
Grosvenor Place occupies one of the finest locations in Sydney, forming a
transition between the CBD and The Rocks area, and has a site area of
7,192 square metres. The Sydney Cove Redevelopment Authority exercises
development control over this area and required the site to become the
gateway to The Rocks area by allowing diagonal pedestrian through
traffic from George Street to Harrington Street. In addition, a height
restriction of 176m above sea level was placed by requiring the office
tower structure to form part of a stepping envelope between Qantas and
the Regent Hotel.
The brief called for a large energy efficient office building, with total
rental office space of 90,000 square metres and each floor containing up
to 2000 square metres of flexible floor space, and designed to meet the
needs of a rapidly evolving office technology and changing user
requirements. Provisions for parking 600 cars, truck docks and
engineering services, in addition to food services for the anticipated
7000 occupants, were also required.
The building designed to meet the above requirements had a diagonal siting
of the tower with a low site coverage of 30% and consisted of 43 levels
of offices, a three storey ground floor lobby and four levels of
basements. The tower floor plan consists of two quadrants, offset but
with a common axis on each side of a sharp ended elliptical service
core, with each quadrant having a clear width of 14m. The plant and
equipment floors are at levels 10/11 and 33/34 respectively. The ground
floor lobby/reception area is to be more open than the office levels.
The structural requirements arising from the above decisions are a span of
14m for the office floor system, a floor to floor height of 3.5m and
supports at ground level around 19.5m spacing. Smaller functional
modules, with dimensions varying from 8 to 8.5m are, however, permitted
at the car park levels.
In addition, office floor slabs are required to carry an applied load of
4.5 kPa (3 kPa for general, 1kPa for partitions, and 0.5 kPa for
services and ceiling) for general areas and 10 kPa for areas - a zone
approximately 4.8m around the core - where compactus type loading occur.
Plant room slabs and the roof are required to carry a general loading of
5 kPa or specific plant loads where known. For car park slabs and
loading dock areas the design live load is 5 kPa. The general design
wind load, based on a 50 year return period, was considered to be 1.5 -
1.8 kPa for overall stability of building and 2.5 kPa for facade
design.
The site is underlain by Hawkesbury sandstone of Triassic age, which is
generally of medium to high strength apart from a few near horizontal
clay seams. The bearing pressure on the foundation should thus be
limited to 3 MPa. The basement level is approximately 3.5m below mean
sea level, and maximum water level of 2m above sea level is considered
for the design of structural elements at basement level.
Fire resistant level for structural adequacy is to be 120 min for all
elements. Speed of construction was an important requirement due to high
interest rates and holding charges. Vibration control requirements for
different parts of the structure to keep vibration effects -
particularly those arising from sway movements of the building and
vibration of long span floors during normal use - below the level of
human perception.
Structural Solutions
The key requirements that influenced the selection of structural solutions
were (a) an efficient floor system to span 14m, (b) floor system that
minimizes the floor to floor height and allows integration of structure
and services, (c) speed of construction to enable early tenant occupancy
and (d) cost.
Structural Alternatives and System Selection
The following structural alternatives were considered for the floor system
based on the above key requirements.
composite steel/concrete
banded pre-stressed concrete beams
reinforced concrete slab and beams
pre-cast flooring systems between beams
Each of the above systems was fully designed for a typical bay and
comparisons were made between systems based on the above key
requirements. Even though the initial material and fabrication cost is
higher, the composite steel and concrete floor system was selected as
overall economies can be achieved, including early return on
investment.
Steel has a high strength to weight ratio and is thus a more efficient
material for spanning 14m and for minimizing the structural depth
required for the floor system. The composite steel option allowed the
building to have 2 more floors than the reinforced concrete option. With
the height restriction on the building as one of the constraints,
minimizing the floor to floor height maximizes the number of floors, and
therefore the rentable floor area and hence the return on investment.
There is also a reduction in the overall cost of the cladding relative
to the usable space enclosed.
Use of steel beams for the floor systems also permits the integration of
structure and services, with the services zone being within the
horizontal zone for the structure. There is thus no need to increase the
floor to floor height to accommodate the service ducts. The air-handling
ducts penetrate the webs of the beams at two locations, where the shear
forces to be resisted by the web are not critical.
The composite steel/concrete construction has a number of advantages which
results in reduced construction time. The steel deck for the composite
construction provides a working platform during construction, and
eliminates the need to either prop or strip form work and the attendant
delays resulting from these operations. By designing the steel columns
to support the dead load of the floors, the upper frames can be
assembled before concrete casing for the columns are in place, thus
taking this operation off the critical path. With the traditional
reinforced concrete systems, the construction time cycle per floor was
12 to 15 days (at the time of design), whereas the composite
construction had a time cycle per floor of 4 to 6 days. It was estimated
that the saving in construction time for the building, as a result of
using composite construction, would be around 8 months, resulting in
early return on investment, particularly at a time of high interest
rates and holding charges.
The exposed steel in the concrete construction, however, requires fire
protection. In this project the cost of fire rating the structure was
considered to be small in comparison to other costs, and did not
influence the selection of this system. The penetration of steel beams
by service ducts reduces the flexibility available for future changes,
and may be considered as one of the disadvantages of the selected
system.
Final Structural Solution
The floor system for the tower is of composite construction and consists
of radially arranged universal steel beams - spaced a 2.2m at the core
and at 3.25m (half column centres) at the perimeter - supporting
concrete floor slabs cast on permanent steel form work. Composite steel
beams span the 14m between the core and the outer columns. The steel
beams placed between columns are supported at the perimeter by steel
spandrel beams, that span between the columns.
The central elliptical core is of reinforced concrete, with the two
elliptical portions connected together by a number of concrete cross
walls. The floor slab and the core are cast monolithically.
The columns on the perimeter of the building are at 6.5m spacing and of
composite construction, having high tensile steel fabricated steel
sections encased in concrete, with strength varying with elevation. The
column spacing at ground floor is increased by gathering sets of three
columns via a triangulated piloti into single column supported on
concrete caisson. Each piloti is fabricated from high tensile steel
plate with post-tensioned plate girder tying the top of the piloti legs
at the first floor level.
The car park has in general a 8m column grid and flat slab floor system.
The eight columns from the tower and the core are integrated with the
column grid of the car park to provide vertical supports for the
basement floor slabs.
The footing for the core is a raft slab and for the columns are pads.
The structural elements that contribute to the different functional
systems are: Structural types: composite steel deck/ concrete floor ,
external columns and piloti , and core wall
material: composite steel/concrete
Structural type: shear wall
material - reinforced concrete
Structural types: - raft slab and pad footings
materials - reinforced concrete
Design Decisions
The decision to choose a plan configuration of a double curve and counter
curve was to maximize the full sweep of the best views and open space
outlook. The shape offers opportunities for long span, column free
system of construction, where every structural span and beam are
identical, and results in every column, its space, and the floor load it
carries are the same, just as is every facade element. This decision
also results in a core shape which is structurally more efficient for
resisting lateral load and in reducing lateral load effects.
The spacing of the column, of 6.5m, at the perimeter of the building was
determined to keep the size of the column to within acceptable limits.
The spacing of the radial beams was selected to (a) reduce the amount of
concrete to be lifted, (b) eliminate propping of metal deck during
construction and (c) keep to a minimum the floor to ceiling dimensions
thus maximizing the number of floors possible within the building
envelope.
The car park grid of 8 - 8.5m was selected to accommodate three cars
between the columns. Flat slab construction was selected as it is an
efficient and cost effective system for this span and minimizes floor to
floor heights, thus reducing the depth of excavation into the
sandstone.
References
Grosvenor Place, Promotional brochure.
Grosvenor Place, Consultants report.
Interview with Bill Thomas, Ove Arup and Partners.
Architecture in Steel, Alan Ogg
Harry Seidler-Four Decades of Architecture by Kenneth Frampton, 1992
Thanks to
http://www.arch.usyd.edu.au/kcdc/caut/html/GPT/front.htm
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