Sydney Architecture Images- Central Business District

Capita Centre


Harry Seidler


Castlereagh Street  




Late 20th-Century Late Modern


reinforced concrete


Office Building


The thinking man's skyscraper. Ingenious structural system allowed for multiple sky-gardens. 

- Architect Harry Seidler & Associates 
- Structural engineer Miller, Milston & Ferris 
- Service engineers Addicoat Hogarth Wilson 
- Builder Civil & Civic 
Function Commercial office building 
Year 1989 
Location Sydney, NSW 
Cost $200m 

Type office building with parking 
- Plan shape rectangular 
- Number of stories 34 levels above ground (including a 3 storey lobby), 2 levels of basement 
- Total floor area 32,155 sq m 
- Net rentable floor area 24,450 sq m 
- Number of modules 6 rectangles, 1 core 
Relationship to ground ground level pedestrian entrance, most of ground floor devoted to pedestrian plaza, underground parking 

Primary Structure 
Material composite structural steel/concrete 
Floor system 
- type steel beams, composite metal deck/concrete floor 

- pattern one-way floor slab 

- beam clear span 12.5-13 m 

- floor slab span 4 m 
Core structure 
- material reinforced concrete 
- type core 
- shape rectangular 
Support structures 
- types external columns, lateral bracing truss 
- material composite structural steel/concrete 
Footings piles, pad footings 

Design requirements 
The Capita Centre occupies a tight, landlocked site on Castlereagh Street in the centre of Sydney city. Measuring only 1,537 square metres, and open only to the East, the restricted nature of the site demanded a very different design from the architect - Harry Seidler's other works in the city. In fact, the problem was so difficult and unusual that Seidler nearly abandoned the commission before putting pen to paper. It compelled Seidler to address the issue of the building's relationship with neighboring buildings. 

The building bears the hallmarks of a Seidler building; maximizing public open space at the ground floor, with a proposed link through a neighboring building to Pitt Street, controlling sunlight with external louvres to the North and East and a bold, expressed structural system, characterized by the dramatic vertical truss to the Eastern facade on Castlereagh Street. 

The irregular shape of the site allowed the core, fire escape stairs and main lift shaft to be located at the perimeter of the site to the West, North and South, leaving a clear rectangular area measuring 25 x 40 metres in the middle of the site. This allowed the floor areas to be more open and flexible than would be allowed with a centrally located core. This rectangle was then subdivided into six column-free segments of roughly 12 x 13 metres, which were manipulated to form the changing position of voids through the building so that roughly one-third of each floor was void. The depth of the floor in plan was then narrow enough to provide all parts of the floor with some level of natural light from the atrium. 

The structural requirements arising from this strategy were for each of the six segments to be supported over a span of 12 x 13 metres. The building lacked adequate lateral stiffness as the voids through the building restrict the opportunity to gain continuity within the structure. However the only area where extra lateral load resisting elements could be used was on the East facing street facade. This created a dichotomy whereby the street elevation was required to act as a shear wall. The clients required a flagpole to mark the building's location on the skyline. This was added to the top of the truss giving the building a greater status, making best advantage of a poor location. 

The office floors are required to carry an applied load of 4.5 kPa. The pedestrian plaza is required to carry an applied load of 16.7 kPa - equivalent to that of a highway. This was because of the need to carry the weight of the planting areas and to resist the hydrostatic pressure build in the soil. The footings were based on a bearing pressure of 200kPa. 

Structural Solutions 

The major requirements that determined the structural systems selected were: an efficient floor system to span 12 metres; maximum integration of services to minimize ceiling depth to allow for a higher ceiling appropriate to a prestige office building; fast-track construction; and cost. 

Structural alternatives and structural selection 
A prototype building was constructed off-site to evaluate the use of steel structure. A composite system was chosen as it allowed faster construction, services penetrations through the beams and efficient spanning of the large grid. Open web beams were used to maximize flexibility in services arrangements, also allowing for future changes. The floor system used Bondek permanent form work which provided a working platform during construction and allowed work to continue in the floors above and below during a concrete pour. This resulted in a considerable time saving over a conventional form work system. 

The secondary beams were evaluated as open web beams to facilitate services distribution and minimize ceiling space, allowing for higher ceilings; the additional cost of these members was reasonable when considering the advantages they offered. 

Final structural solution 
A solution comprising cast in situ concrete floor slab on permanent form work of Bondek with steel universal beams in both directions. The floor system allows the office areas and glazed facades to be free of columns. 

Vertical load resisting system 
Structural types: Columns, core, external truss 
Materials: Composite structural steel/concrete external truss, reinforced concrete core and columns. 

Lateral load resisting system 
Structural types: Core, external truss. 
Material: Composite structural steel/concrete, reinforced concrete. 

Foundation and footings 
Structural types: Piles and pads footings.
Materials: insitu reinforced concrete. 


Design Decisions 

The design is very much in keeping with the architects modernist philosophies. The structure is clearly expressed in the external truss which is set away from the building enclosure to articulate its structural action. Also in keeping with Seidlers ideas the ground floor is open to the public, providing a small oasis in the middle of the CBD. This was also favourable to the client because the open space provided enabled them to build an additional five floors, under the FSR regulations at the time. 

The odd shape of the site as well as the desire to have a pedestrian through link determined the position of the core; removed from the main body of the site, where the rectangular floor plan is placed. 

Seidler's desire to get natural light into a narrow and constrained site drove the placement of the voids and the section and also required that the core be placed outside the footprint of the floor areas. The placing of the core in this location freed up more floor space, where a centrally or side placed core would have restricted the use of the floor area. This also caused lateral weakness in the frame, therefore a lateral stiffening device was required to the East, which was satisfied by the external truss. 


HARRY SEIDLER AND ASSOCIATES, Harry Seidler and associates - Towers in the city. Milan ; Melbourne : Edizioni Tecno, c1988. 
HARRY SEIDLER -Four Decades of Architecture
by Kenneth Frampton, 1992
Thames and Hudson Inc
pp 282-295 
E. M. FARRELLY, Capita Centre, Architectural Review, August 1991, Vol. 189 No 1134 pp. 49-54. 
Interview with the engineer - Bob Eckhart. 
Interview with the building manager - Alistair Wishart. 


Capita Centre design - Vertical load resisting system - Lateral load resisting system - Foundations and footings - External truss - Core -Floor slab 

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