CONCRETE BUILDINGS: ACHIEVING ECONOMICAL STRUCTURAL SOLUTIONS BY INTEGRATING ADVANCED CONSTRUCTION METHODS AND HIGHLY EFFECTIVE STRUCTURES

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1 CONCRETE BUILDINGS: ACHIEVING ECONOMICAL STRUCTURAL SOLUTIONS BY INTEGRATING ADVANCED CONSTRUCTION METHODS AND HIGHLY EFFECTIVE STRUCTURES J Webb*, Connell Mott MacDonald, Australia S Giblett, Connell Mott MacDonald, Australia G Fyvie, Mott Connell, Hong Kong SAR China N Lal, Connell Mott MacDonald, Australia 30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2005, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

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3 Figure 1 - Floor plan showing elements of lateral load resisting system The construction systems used by the structure contractor included separate self-climbing formwork jump systems for the core and perimeter columns. This enabled construction to occur simultaneously on several fronts as described below: The core was constructed some 25 to 30 floors ahead of the floors with a two-storey jumpform system. This allowed earlier installation of lift services for occupation of the low-rise floors. The lobby slabs and fire stairs within the core were constructed immediately below the core walls by a separate team with access obtained via a trailing stair hanging off the core jump system. The perimeter columns were constructed by an independent team two storeys ahead of the floors using a self-climbing formwork system. The perimeter jump also incorporated safety screens providing additional safety and further time savings. A further advantage from this construction approach was that it eliminated the need to mushroom the floors with high strength concrete at column locations as traditionally required. Key elements of the wind stability system, such as the outrigger walls, were constructed independently of the floors providing substantial time saving in the floor construction cycle times. Simultaneous construction on several fronts and construction of key structural elements off the critical path was successfully carried out with the structure design team working closely with the structure contractor. This included analysis and design for temporary stability of the tower and detailing of complex connections between different elements using proprietary reinforcement connection systems. Figure 2 World Tower Jumpform Systems

4 Figure 3 World Tower Figure 4 - Section showing elements of lateral load resisting system World Tower 3. Eureka Tower, Melbourne Eureka Tower is a 88 storey residential building under construction in Melbourne, shown in Figure 5. Grocon, the contractor responsible for World Tower in Sydney is also constructing this building. This building is described in more detail in Reference 2. Similar methods are being used with the perimeter being constructed in a separate jumpform advancing ahead of the floors. In this case, the perimeter includes both columns and spandrel beams. The main core, as is common practice in Australia, is also constructed with a jumpform. 4. Civic Tower Sydney Civic Tower is the 26 level office building recently constructed on top of the existing Masonic Centre in Sydney. The 150m high building has been constructed on an existing nine storey high central core located within the existing building. The existing central core supports the entire tower, with no perimeter columns continuing down to footing level. This is described further in Reference 1. The vertical structure of the tower consists of a central reinforced concrete core and an exposed facade which was constructed with an innovative jumpform system developed by the structure contractor to form an orthogonal lattice tube of reinforced concrete spandrel beams and 82 architecturally expressed columns. While a precast concrete façade would have been a traditional construction solution, detailing of the connections in a precast concrete façade resulted in larger elements being required to resist the design actions. Larger elements would have meant increased loads on the transfer structure. The core was also jumpformed some five floors ahead of the floors, with construction of the floors, central core and the façade able to proceed contemporarily. The construction of the transfer is described in detail in Reference 1. Figure 5 Eureka Tower

5 The structural steel space frame allowed construction to occur some 35 metres above street level and this was designed to support the weight of the wet concrete to the lowest tower floor without any propping. The tension component of the full design loads imposed on the transfer structure is resisted by the posttensioned concrete tie beams acting compositely with the structural steelwork. To ensure that the transfer system was structurally adequate during construction, the tie tendons were stage stressed to limit stresses in the tie floor structure to within acceptable limits. Figure 6 Civic Tower Figure 7 - East West Section, Civic Tower 5. Chevron Renaissance Tower 3 Gold Coast This building (Figures 8 and 9) was completed in It was the third in the development, with the developer, Raptis Group, with construction subsidiary, Rapcivic Contractors progressively refining the design of the blocks to simplify construction as the project evolved. Connell Mott MacDonald was involved in the latter stages of the two earlier developments, providing advice on potential cost savings. Rapcivic wanted to speed up construction by completely eliminating all masonry on the outside of the building, meaning that the vertical elements on the perimeter were constructed entirely from concrete, with infill glazing, either windows or curtain wall. The concrete was painted from swing scaffolds, eliminating the need for perimeter scaffolding, common on buildings of this type in Australia. Rapcivic worked with the suppliers of climbing formwork systems, Cantilever, to develop a jumpforming system which constructed the columns and core at the same time, up to three levels ahead of the floor construction. This separation of the two workfaces meant that Rapcivic regularly achieved a three day floor cycle on the project. The conceptual design of this project is described further in Reference 1.

6 Figure 8 Building under construction Figure 9 View showing Jump Form 6. Proximity Apartments, Arncliffe Sydney This project contains 296 apartments in four buildings sharing a common podium/basement, constructed by Multiplex Constructions for their own development company. Multiplex carried out extensive option analysis on this project, looking at various floor schemes, including precast and prestressed floors, and a range of wall and façade systems. Eventually, a hybrid system was adopted taking advantage of a number of vertical and horizontal structural systems. For the tallest building, a 21 storey tower, precast load bearing walls were used for the vertical elements with post-tensioned flat plates adopted for the floors. A 15 storey tower used load bearing precast walls and Ultrafloor for the floors. Ultrafloor is a proprietary floor system that involves pre-cast concrete ribs spanning between the walls. A topping slab is then poured. The slab spans between the Ultrafloor ribs which are typically spaced at 450mm centres. The wet concrete is supported on fibre cement formwork which is supported by the ribs. This system requires little to no propping, dependent on the length of the span. Two smaller, 4 storey buildings adopted loadbearing masonry for the vertical elements with Ultrafloor floors. While the original cost analysis of this scheme did not show significant savings, Multiplex chose the methods described because they perceived it would be quicker to construct than the insitu methods. The end result was a much cheaper structure for a range of reasons not fully appreciated at the time the decision was made. Essentially, reduced waste and rework and less preliminary costs meant that substantial additional savings were made. Figure 10 Proximity Apartments, Sydney

7 Figure 13 ESP Project, Sydney 7. ESP Project, Victoria Park, Sydney The ESP project consists of four buildings and 220 apartments. The structural system for this development was loadbearing precast concrete walls with post-tensioned flat-plate floors. The foundation of the three lowrise buildings (up to 10 storeys) was a raft founded on sand, while the 20 storey tower building was founded on piles to rock 30m below ground. There were significant advantages in providing precast concrete loadbearing elements for the buildings at this development. The most significant of these was the utilisation of the spanning capability of precast concrete walls to eliminate transfer beams. The development consisted of residential towers above retail and basement parking levels below. For typical framed construction, transfer beams would be necessary to enable vertical load paths from the residential levels to the footings below. By carefully coordinating the residential apartment walls with the basement parking layout, the precast walls could be used to span onto the basement columns and therefore eliminate transfer beams. This eliminated the need for costly and time consuming transfer slab construction. The use of precast walls also enabled long cantilevers to be achieved without requiring slab thickenings by hanging the slabs from the walls which acted as deep beams. This enabled dramatic architectural effects to be achieved without adding cost to the development. Other advantages of the precast concrete system included the ability to minimise the thickness of the raft foundation. Originally, the design of the lowrise buildings was based on a piled foundation system with piles extending 30m below ground to rock level. This was changed to a simple raft at basement level, further reducing the raft thickness by utilising the spanning capability of the precast concrete walls to evenly spread the load over the raft, rather than have high peak point loads. This saved over 200mm from the raft thickness and enabled the raft design to be significantly less expensive than the original piled scheme. The final solution for the basement was to replan the carparking to eliminate any parking under the piled 20 storey tower. This enabled an early start to the tower construction to be possible. Under the remainder of the buildings, there were two parking levels. To avoid difficult basement construction, these were raised to enable the lowest basement floor level to be above the ground water table level. This resulted in one basement being above ground and one below. By raising the basements, the below ground structure could be constructed with simple sheetpiled site retention which was later shotcreted to give a final basement wall of 250mm thickness. Only minimal dewatering was necessary to enable the lift pits and raft to be constructed. Overall, the precast concrete wall system provided a quality finished building with efficient structure with few transfer beams, an optimised raft foundation and cost effectively achieved cantilevered features.

8 Figure 11 Construction of ESP Building Tower without scaffolding Figure 12 Long cantilevers using precast concrete, ESP Project 8. Form Project, Victoria Park, Sydney This development, adjacent to the ESP project described above, adopted the same structural system based on the excellent results achieved at ESP. This development again consisted of four residential buildings of up to 16 storeys with over 230 apartments. One significant difference between this and the ESP project was the requirement for the basement to extend partially below the piled tower. The piles were constructed from ground level in this zone and top down construction was employed to again enable an early construction start on the tower. Generally, precast concrete walls were again utilised to maximum effect to eliminate transfer structures, enable long dramatic cantilevers and minimise foundation thickness. Figure 14 Form Project, Sydney 9. Application of Precast Concrete to Asian Residential Projects The application of precast to Asian residential projects will have many of the advantages common with those found in the Australian market. The lower cost of site labour will tend to make the economic advantages less obvious than in a high labour cost environment like Australia. However, the large scale of many projects and the resulting potential repetition will tend to favour precast concrete. Shortages of skilled labour in overheated economies can often result in reduction in quality. Precast concrete can help control this problem because activities occur in a more controlled factory environment. The environmental benefits also need to be considered. There are special factors in Hong Kong and Macau which make precast concrete of interest at the moment.

9 10. Residential Projects in Hong Kong and Macau Residential projects in Hong Kong can now take advantage of a floor space bonus by using a non load bearing precast concrete façade. This has driven a rush to use this material in the SAR, as developers seek to take advantage of the concession. It is understood that this bonus has been made available by the authorities to encourage the use of precast concrete as an environmental measure to reduce the waste associated with masonry construction of facades. Precast concrete can provide many advantages. The rules in Hong Kong only encourage a limited take-up of the full environmental and other benefits of the use of precast concrete. These rules basically allow the area of non-load bearing precast panels to be excluded from the gross floor area calculation. The following points should be considered: Over the years in Hong Kong there have been a large number of problems associated with mosaic tiles falling off residential buildings resulting in significant maintenance effort often quite early in the building s life. Higher quality adhesion of the tiles would be possible by fixing them in the factory under controlled conditions. To take full advantage of this, all external surfaces would need to be constructed from precast concrete. Better durability is a positive sustainable outcome. If all external faces were prefinished, the tiling trade along with external scaffolding could potentially be eliminated. This would be another positive sustainable outcome. The precast concrete has inherent load-bearing capacity, which is unable to be used under the rules as currently in place. Although it may not be possible to carry all the load on the precast items, especially where large tensions are present due to wind loads, there is considerable redundant use of materials particularly at the top of the buildings. More efficient use of materials would be a positive sustainable outcome. The further advantage of precast Hong Kong is that it can be constructed over the border in the People s Republic of China, where labour is substantially cheaper than in Hong Kong. All the reasons that apply to the use of precast in Australia also apply to precast in Hong Kong, although the precise dollar figures will be different. The floor area bonus currently in place provides further encouragement, but a more sophisticated approach by both the law makers and the developers will result in further environmental and other advantages. In Macau, the imperatives are slightly different. The current construction boom has resulted in skilled labour shortages. The abundance of available nearby cheap labour in the PRC means that precast is an obvious choice to deliver economic structures. Like Hong Kong it has an aggressive environment, making durability important. Well designed and constructed precast concrete will assist in maintaining quality in this difficult economic and external environment. We are currently investigating a number of projects in both centres to take best advantage of precast concrete construction. Conclusions A variety of different construction systems have been presented which improve the speed of construction of insitu structures. The use of precast in recent multi-storey construction has also been examined. Recent projects in Australia in precast concrete have been extremely effective, both in cost and constructability terms. Precast concrete is also an extremely effective material for producing positive environmental outcomes. The more extensive application of precast concrete to Asia, particularly Hong Kong and Macau is beginning but more work needs to be done to develop the full potential of the material in these cities. References: 1. Conceptual Design of Tall Buildings: Creatively Integrating Architectural, Structural and Construction Requirements J. Webb, G. Fyvie J Lenac O Martin. 3rd Specialty Conference on the Conceptual Approach to Structural Design, Singapore August Innovative Tall Building Design B.Dean, D.Emery, P.Chancellor, I. Calderone. IABSE Symposium Melbourne High Density Residential Development Implications for the Concrete Industry G.Fyvie, J.Lenac, J. Webb Concrete Institute of Australia Conference 2003 Brisbane.