Central Park
INTRODUCTION
Built in the early 1990s the 248m tall Central Park development is Perth’s tallest commercial development at the time of writing (2020).
THE STRUCTURAL DESIGN & CONSTRUCTION TEAM
- Structural Engineers: Bruechle, Gilchrist & Evans Pty Ltd
- Architects: Forbes & Fitzhardinge. Design Architect – Peter Wilkes
- Cost Consultants: Ralph & Beattie Bosworth Pty. Ltd
- Head Contractor: Multiplex Constructions Pty. Ltd
- Precast Concrete Sub contractors: Delta Corporation and Unit Construction Pty Ltd
- Steel Fabricator: Kewdale Structural Engineering
- Reinforcement Sub contractors: Pictoria Nominees
- In Situ Concrete Sub contractors: The New Cement Co
- Formwork Sub contractors: CASC Formwork Pty Ltd
- Slip Forming and Prestressing Sub contractors: Structural Systems Ltd
- Piling Sub contractors: Frankipile (Aust) Pty Ltd
FEATURES
Features of the development, other than the structure, are:
- It is set in a parkland with an area of 5000 square metres that has been established over a three storey, underground car park with a capacity for 1,050 cars. This park area, in the commercial heart of Perth, provides breathing space for the city. The car park has been sunk into the perched water table of the area.
- It is set in a parkland with an area of 5000 square metres that has been established over a three storey, underground car park with a capacity for 1,050 cars. This park area, in the commercial heart of Perth, provides breathing space for the city. The car park has been sunk into the perched water table of the area
- Each floor has air conditioning plant rooms, kitchen and toilet areas.
- There are commercial areas and pedestrian access at ground floor level.
- There are covered pedestrian access ways, entrances to elevators, food outlets and a small shopping mall at the ground floor level.
- The building is located in an area of interbedded sands and clays, approximately 30m deep, overlying a siltstone known as “Kings Park Shale” or more recently as the “Kings Park Formation” (KPF). There are two water tables – a natural water table which is confined and is under pressure, and a perched water table.
- The entire basement area is isolated from the water tables by diaphragm walls that were temporarily ground anchored until the car park floors were in place to support them and by lower basement slabs designed to resist the water pressure uplift.
THE BUILDING STRUCTURE
The structure was developed by Bruechle, Gilchrist and Evans with input from the design architects Forbes and Fitzhardinge, and with further input from other members of the design and construction teams, on the following principles:
- Convention was to be challenged and conventional approaches to the structure employed only if they proved to be optimum
- Close liaison was maintained with the head contractor, Multiplex, and with the major structural sub contractors during the period of design development. Comments on any innovations proposed were asked for. Several full size load tests were carried out by sub contractors to prove that proposed innovations were effective.
- Given the industrial climate of the time it was considered that there were advantages to be gained by constructing elements off site. The site labour force was therefore small for such a large building
- There was to be an attempt at achieving a balance between construction cost and construction time. Low initial costs were not necessarily the more economical answer if the use of the building was delayed or the building’s life cycle costs were required to be greater. Shortening the floor to floor cycle time had positive financial advantages.
- Floor to floor heights were critical. How to achieve a minimum that met all practical requirements was studied and alternatives checked.
- If possible the service core was to provide the necessary resistance to lateral loads, even though the building was taller than normal core stiffened buildings, so that expensive moment carrying joints in the structural steel framework could be avoided.
- The core was to be designed to be slip formed so that it was not on the critical path.
There were other considerations that affected the design of the structure:
- It was recognised that the building had to be supported on a system, probably by large diameter piles, which penetrated through the site’s interbedded sands and clays and were socketed into the King’s Park Shale under them. A design and construct package was prepared. Alternatives were called for and the optimum package, tendered by Frankipile, was selected. Frankipile’s winning scheme was for large diameter concrete piles poured into steel sleeves that were forced down as an auger penetrated through the interbedded sands and clays and into the King’s Park Shale to depths agreed by Frankipile and the design team’s geotechnical experts. As the piles were poured, the steel sleeves were withdrawn for reuse. The sockets of all piles into the King’s Park Shale were inspected in situ by engineers prior to the piles being poured to ensure that there were no faults in the shale.
- To lower the cost of what needed to be a substantial pile cap and to overcome the possibility of the heat of hydration causing problems, it was decided to design a voided raft that could be poured in stages. (See the section headed Foundations).
- Because the car park needed to extend out under Hay Street, it was necessary to arrive at a structural solution that shortened the period of disruption to traffic. A top down solution for the section of the basement that was under Hay Street, that proved successful, was adopted. This area was covered by precast concrete elements that were integrated with in situ concrete and by prestressing beams between the inverted “bathtub” units that were designed to carry the traffic loading.
THE MAIN STRUCTURAL ELEMENTS
The main structural elements were the core, the columns and the suspended floors, and so it was on these elements that design thinking and detail design and documentation was concentrated.
Foundations
Given that the site was composed of interbedded sands and clays of limited bearing capacity overlying King’s Park Shale, it proved necessary to place such a tall building on piles socketed into the bedrock. The most heavily loaded element, the 30m by 30m tower with its elevators and services shafts, needed to be placed on a united pile cap. Normally this would have been a raft slab. To avoid the necessity of a continuous pour, the difficulties of heat of hydration problems and to minimise costs it was decided, instead, to use a voided pile cap structure. A 600mm thick layer of heavily reinforced concrete was poured, in which anchors for the core prestressing were located. Then heavily reinforced ribs were poured and made integral with the base slab. When these had cured the voids between the ribs were filled with backfill material from the site and the heavily reinforced 600 mm thick capping slab poured. The more lightly loaded columns were supported by small individual pile caps.
The core
The core, which houses four banks of lifts, the main vertical service ducts and escape stairs, was developed to be the primary stabiliser of the tower with help from the tower’s perimeter columns. At the very top of the building, the “flying shores” form part of the core bracing system. They carry tension and compression out to the corner columns, which are aided by parallel chord trusses that shed load to the central columns on each elevation. The core’s cruciform shape was developed by the design architect, Peter Wilkes, of Forbes and Fitzhardinge, in conjunction with Bruechle, Gilchrist and Evans to provide a core that not only was almost able to stabilise such a tall building, but that provided an efficient and unique disposition of the elevators. Because the structure of the core would, theoretically, go partly into tension under design wind loads, it is aided by the columns of the tower in the same way as yacht masts are stabilised by their rigging. To further counter any possibility of tension being created in the structure of the core, to reduce the amount of reinforcement required and to help alleviate long term creep differentials between the normally lightly stressed core and the permanently, heavily stressed columns, the lower levels of the core are prestressed vertically. The diagonal braces at the top of the building are part of the stabilising system. The similar tubular members at lower levels (“the flying buttresses”) have been designed only to carry a roof over garden areas that were not implemented because of fire regulations and cost.
The columns
An architectural requirement was that the columns were to be no greater than 1200mm in diameter. That is extremely slender for columns supporting a building of the height of Central Park. A structural requirement that came out of a study of alternatives was that some columns had to accept beams from eight different directions. Another desirable function of the columns was that they could carry steel framing and precast concrete floor units prior to being concreted.
After considering the possible options, the decision was taken to use composite steel/concrete columns with the reinforcing steel being billets and not normal reinforcing bars. To carry the necessary loads at lower levels, the most heavily loaded columns had to have steel percentages of approximately 20% of the total column areas, while at upper levels only 1%. The columns were fabricated in two storey lengths. They were each reinforced with eight steel sections capable of carrying steel beams from eight different directions. They were delivered to site with all necessary subsidiary reinforcements and beam cleats in place and with their metal cladding already attached. To ensure that there were no voids in the columns at cold joints, (which were to be just above floor levels), the bottom sections of all columns had removable formwork so they could be examined. The ends of the steel sections were accurately machined so that they abutted. To even up crane usage, half the columns were spliced at alternate floors. Placing the columns took only minutes of crane time as erection bolts, which also acted as the necessary tension capability, were placed. The columns were concreted after the precast floors were placed and provided safe working decks.
The floors
Floors are the part of a structure that is most expensive and that determine the speed at which the structure can be built. Eight different approaches, from conventional reinforced flat plates to different prefabricated approaches and integral steel decks, were designed, and costed. Multiplex assessed the floor cycle times of each. The system that was used was selected on the basis of balance between floor cycle time and cost. Once an optimum method was selected, it was prototyped and full scale test loads were carried out on the triangular precast panels in both topped and un topped states. The floor panels were made to be as light as possible and the concrete toppings, which had several functions to fulfil, (including being economical), were placed when most convenient. The functions of the toppings were:
- To provide the necessary fire barriers between floors
- Once the topping had gained strength and bonded to the roughened surfaces of the precast deck panels, the floors were capable of carrying the live loads in different areas.
- The toppings also combined with the steel support beams through shear studs that had been pre welded to them, to form composite T beams, also capable of carrying live loads.
AWARDS
Central Park has received several awards. These include an award from the Master Builders Association, a special award from the Western Australian branch of the Association of Consulting Engineers Australia (the ACEA) and an award from the Australian ACEA. For one award the judge’s citation said:
“The building, which is the largest and tallest in Perth, bristles with innovative and unusual structural concepts. The submission faithfully recounts the arguments in favour of a number of novel concepts because conventional approaches were questioned and alternatives researched. Central Park is considered a structure in advance of most of the world’s high rise buildings and is a tribute to the consulting engineer’s technical and innovative effort”.
OUTCOMES
The main building contractor, Multiplex, the head contractor for Central Park, looked for new markets that could exploit the successes of Central Park and Exchange Plaza (another high rise tower that utilised similar thinking but in a different form) in Dubai. It was successful in winning the tender for the taller of the Emirates Towers using a re design of the structure, prepared by Bruechle, Gilchrist and Evans, that incorporated the structural and construction principles developed for Central Park. That building, and others, have been successfully built in what is a very competitive market.
During the period in which Central Park was being constructed, the internationally renowned, design engineer, Mark Fintel, visited Western Australia and looked at what was being done. He already had an interest in the building because he had developed a computer program that assessed long term vertical movements in high rise buildings and Bruechle, Gilchrist and Evans had provided him with the salient information for Central Park and asked him to assess how it would perform. His suggestions were followed. Mark, whose work with Fazlur Khan for high rise structures, was pre eminent at that time, was impressed with what was being done for Central Park and said it was in advance of what was being done in America at that time.
Author:
Peter Bruechle. 2020
