'It puzzles me why post-tensioned construction is not a prime method of construction for buildings in the uk, when it is a competitive medium to design in. We have designed as many prestressed concrete structures as we have reinforced concrete buildings in the Asia Pacific market,' said Ian Thompson, director of Sinclair Knight Merz's uk operations.
The use of 100,000m3 of concrete and a lot of prestressing in the structure of the new Olympic stadium in Sydney - named Australia Stadium by its owners - suggests that Thompson knows what he is talking about.
From a hot-air balloon the structure looks like a giant ladybird about to fly, with its two roof canopies 'folded' on each side. Four massive cylinders of concrete rise 30m out of the sun-scorched earth, flanking the corners of the stadium to disgorge spectators to the upper tiers, private boxes, food halls, press boxes and hospitality suites.
The 165m long concrete grandstands on the east and west are divided into three sections - the main structure and lower seating bowl, a 16m cantilever mid-tier section and a smaller upper tier. At each of the 'goal ends' to the north and south, the lower seating bowl is continued.
The capacity of 115,000 seats is only for the four weeks during the Olympics. 'The stadium will then be reconfigured to an 80,000 seater stadium by demolishing the upper tiers of the north and south stands. The additional seats for the Olympics provide much-needed revenue to pay for the stadium construction,' said Thompson. 'This meant a significant reduction on the government's funding contribution.'
The stadium is essentially a reinforced and prestressed concrete structure supported on bored concrete piles with a steel roof over the east and west stands and a 6m high reinforced-earth, blockwork wall in the basement.
The lower bowl creates a circumference of nearly 600m. However it was the requirement to provide a mobile structure which could slide in and out to create the different playing areas of field games that presented the toughest design and construction challenge.
The lower bowl must slide inwards 15.6m on each side to cater for the pitch sizes of Australian rules football and rugby - 'our major revenue earning sporting events', Thompson adds.
Most international stadia designed this way, for example the Stade de France, are built as steel-framed structures with steel decking to reduce the dead weight. Not so Australia Stadium, which is the biggest in the world. The moving decks were constructed using post-tensioned concrete.
At 100m long by 31m wide and 12m high, the lower bowl structure of the east and west grandstand had to be built as a stiff diaphragm, a monolithic construction with no movement joints that could twist, warp or deflect the structure during sliding operations.
'To adjust the shape of the pitch from the required oval shape for Australian rules to the rectangular pitch of rugby and football, the lower bowl is winched forward on rails constructed under the pitch,' explains Thompson. 'The final seating arrangement to the front of the concrete bowl is created using steel framing.'
The lower bowl structure had to be built in such a way that there would be no planes of weakness nor any movement at planned construction joints. It was impractical to pour the whole structure - columns, beams and slab - in one hit, so it had to be cast piecemeal.
That is where post-tensioning comes into its own, pulling the whole structure together as individual strands are tensioned, and slab pours are drawn together, just like tightening shoelaces to get the shoe to fit snugly around the foot.
'Post-tensioning has been used extensively by bridge engineers to slide bridge replacement structures into position,' said Thompson, 'so we thought we should seriously consider it for the lower bowl, which we wanted to slide'. The value- engineering appraisal confirmed that the post-tension option not only satisfied the cost plan, but met the stringent criteria on structural integrity.
The beams on the east and west lower bowl were poured to the underside of the floor slab, then the slab and steppings of the tiered seating were cast in a series of pours, before they were post-tensioned and tied together structurally. The inner concourse floors were generally 320mm deep reinforced- concrete flat slabs with spans ranging from 7.6m to 15.2m between columns, which incorporate drop panels or column capitals for shear requirement. Towards the front of the stands, the column centres reduce to 7.6m to reduce tension forces induced on the slabs from the mid-tier cantilever.
The control of shrinkage, thermal contraction and the restraints to movement was an important consideration. Tension forces that build up in the floor slabs due to restraint against thermal contraction of the concrete, and to some extent early drying shrinkage, exert a horizontal thrust on supporting columns and other vertical load-bearing structures. These effects can be significant at the perimeter of the slab and can add significant costs if they have to be catered for in the design.
Introducing infill pour strips to allow the slab to contract would have disrupted continuity of work and delayed the waterproofing to the slabs which was on the critical path. 'In the end a low-heat cement and low water-cement concrete were specified to minimise thermal contraction and additional reinforcement, was detailed for the column heads,' said Thompson.
Waterproofing the floor slabs was a critical issue, especially for level 1 which acts as the roof to level 0 as well as serving as the main concourse for the various food halls.
Following a series of tests, a capillary-pore blocking sealer was chosen to produce a watertight concrete, a water-based resin was selected for the curing compound, sodium fluoro- silicate was preferred as a surface sealing coat, and Rockite for applying a permanent colour to the concrete surface. For the other floor slabs, the decision to apply the full waterproofing specification depended on the location, wear resistance and degree of exposure to the elements.
For the lower bowl at the north-south ends, the steppings were poured after casting and stressing the floor slab, creating a large horizontal joint covering the entire surface of the slab. Preparation was simplified because the joint did not have to transmit longitudinal shear, and because the steppings were designed as a non-structural topping. 'The surface was left with a tamped finish with the curing compound doubling as a bonding agent,' said Thompson. There was also provision for dowels and reinforcement to prevent delamination and curling of the topping slab.
The cantilever mid-section tier
Besides the sliding movement of the lower tier stands, and the dramatic structure of the roof canopies, the smart part of the structural engineering on Australia Stadium lies in the way the 16m cantilever tiers were designed. They are essentially a series of steel trusses cantilevering from the main grandstand and connected to the floor slabs at levels 3, 4 and 5. The trusses are spaced at 7.6m centres and braced laterally to one another. The trusses house the corporate boxes as well as supporting the concrete seating platforms projecting from the mid-tier floors.
The cantilevers impose significant compression and tension forces on the inter- connecting floors. The two upper floors were post- tensioned with high-tensile bars to clamp the steel trusses to the slab and to transfer the force to the back of the floor slab and to the body of the core structures.
During construction the cantilever trusses were propped by large hollow steel columns, until the high-yield bars were connected by couplers to the main cores and then stressed. Each prop was bedded on a sand-filled, steel annulus. As the sand was slowly released, the props were able to settle smoothly. This allowed the mid-tier section to cantilever and engage the stadium structure and floor slabs in a controlled way. 'This kept the deflection between adjacent truss segments to less than 7mm, with total deflection of the mid-tier section not greater than 20mm, which is what we anticipated,' said Thompson.
Four cores brace each half of the east and west grandstands. At the ends of the grandstand the cores are circular, with external ramps for mass pedestrian movement from all around the ground - while the two rectangular centrally-located cores contain lifts, staircases, service ducts and natural- ventilation shafts.
In addition to providing lateral restraint from wind and seismic forces, the cores are subject to large and sustained overturning moments from the mid-tier cantilever. 'Because of this, vertical prestressing has been installed to anchor the mid-tier section to the core and to control the long-term deflection at the leading edge of the cantilever,' said Thompson. This required all the core structures to be complete to full height and prestressed before the props supporting the mid-tier cantilevers could be released. The stressing of the floor slabs at level 3, 4 and 5 and the cores was carried out in three sequential stages, to match the transfer of load from the props to the cantilever section.
The central cores were slipformed, while the body of the spiral ramp core was jump-formed to enable starter bars to be positioned accurately.
Construction was complicated by having to install both the vertical and horizontal post-tensioning ducts within the wall thickness, and threading them between the rebar layers. The horizontal stressing used 32mm diameter high-tensile bars fed through the ducts and anchored in the core walls. Couplers connected the horizontal stressing bars in the core to those in the tension slabs at levels 4 and 5. The cores were then concreted, and prestress applied in stages before grouting up the ducts, to match the release of the cantilever props.
Thrust block and roof truss
The unusually high thrust blocks that support the main roof truss have to resist significant shear and bending forces. 'It would have been structurally more efficient to have continued the arch truss directly to the ground, but that was unacceptable as it reduces pedestrian movement around the precinct area,' said Thompson.
The final design solution located the arch joint 16m above precinct level. The arch force of 22,400kN was transferred to the thrust block via a 265mm thrust-plate connecting the pin joint to the roof truss. Large 1.5m diameter raking piles and 3m diameter vertical piles were installed for the thrust- block foundations .
Due to the size of the thrust block, aesthetics were given careful consideration. A bush-hammered concrete finish was chosen following a series of mock- ups with different concrete mixes, forming systems, release agents and surface treatments. The large mass of the thrust block could have created high thermal gradients within the concrete core and caused cracking if precautions were not taken to limit the peak temperature. 'The formwork and concrete surface were covered with foil-coated insulation bats and a low-heat cement with a 65 per cent slag replacement was specified' said Thompson. This kept the temperature differential to below 22degreesC which was acceptable. When the formwork was removed, the concrete showed no signs of surface cracking.
The spectacular hyperbolic paraboloid roofs sail across the stadium with a clear span of 300m to form nimble umbrellas sheltering 50,000 people from sunstroke and a good soaking in the rain. The roof structure is suspended from a prismatic truss arch on the leading edge and a peripheral truss section on the rear face, collecting the loads through the upper-tier seating and its supporting V struts and transferring them on to the columns below.
The body of the roof canopy is a pinned jointed diagrid made up of a top and bottom layer of tubular steel, braced in a tetrahedral form on a 10m grid module. The diagrid structure spans between front and back trusses. The top surface of the diagrid is clad in polycarbonate sheeting of graduating translucency for optimum screening and transmission of natural lighting.
Multiplex Construction, its design consultants and subcontractors have clearly demonstrated the ability to work together, to mastermind the design and construction engineering of this massive undertaking in record time. The accolades are coming in thick and fast as the international community begins to recognise the quality, scale and integrity of the architectural and engineering achievements of this world-class stadium.
The design-construct team was appointed in September 1996 with the detailed design and contract documentation overlapping construction which started during that month. 'We had a close partnership and good working relationship with the architects on this project and this is how we were able to maintain construction momentum. We hope to continue our successful partnership with hok Lobb on the Wembley Stadium design which we are currently working on,' said Thompson.
By May 1997 a massive amount of work had been completed: all 2600 bored piles; the cut and fill work; site remediation on this former abattoir and industrial wasteland; and, construction of an innovative 6m high, reinforced-earth concrete block wall 1km long. And by the end of 1998, 100,000m3 of concrete had been poured and 15,000 tonnes of rebar fixed; the grandstands, the end tiers, the seating plats and the roof of the stadium were all finished.
Construction was speeded by: a 'one column - one pile' design and elimination of pile caps; high early-strength concrete for quicker formwork release; slipforming and jump forming to build the main cores; the use of permanent formwork to shorten the floor cycling times, and the use of prestressed, precast composite construction for the upper-tier section.
The official opening ceremony scheduled for June 1999 went ahead without a glitch, after a series of 'soft' opening events including a 100,000 gate in March 1999 for the first public event - a double-header rugby league match. Multiplex will continue to run and operate the Olympic Stadium until 2031, when it will be handed back to the government.
Olympic Co-ordination Committee
Stadium Australia Trust
PRINCIPAL CONSULTING ENGINEER (STRUCTURE & SERVICES)
Sinclair Knight Merz
Bligh Lobb Sports Architecture ROOF DESIGN ENGINEER
Modus Sinclair Knight Merz (UK)
Mahaffey Bemac Group