Sydney Architecture Images- Central Business District

Grosvenor Place

architect

Harry Seidler

location

George Street at the Quay.

date

1988

style

Late 20th-Century Late Modern

construction

see below

type

Office Building
 
 
 
 
  Towering over St. Patrick’s RC Church
 
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 

   

 

www.sydneyarchitecture.com 

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