SUSTAINABLE PRECAST CONSTRUCTION METHODS FOR TALL OFFICE BUILDINGS IN HONG KONG

Corum K L Ip

Hsin Chong Construction (Asia) Ltd Abstract: According to an informal survey of the past 10 years records of tall office buildings, structural precasting construction method was adopted only once and it was one of the “Swire Properties” office buildings in Tai Koo place – Cambridge House completed in April of 2003. One of the major reasons for structural precasting not having been widely adopted for high rise office construction could be the lack of practice notes and relevant codes of practice (COP) providing the necessary guidance on development of such works. The Buildings Department did not publish a relevant COP to the practitioners in the industry as the formal guidance until October 2003. The absence of proven experience and track record in the market for successful adoption of precast concrete frame structures and construction methods might have further deterred the designer’s desire to explore and select this system as a cost-effective and efficient alternative to the common insitu concrete casting and steel method. A four day construction cycle was achieved on the Cambridge House project using structural precast construction and at a competitive price. Such speed of construction had only previously been achieved by insitu construction method in housing and could normally only have been considered using steel frame for commercial buildings with their clear and open span typical design. It is therefore hoped that the precast construction method can be promoted to the practitioners in the local construction industry for building sustainable developments. It is not necessary to compromise the primary concerns of the developers and designers, as capital expenditure levels, architectural requirements, and operation efficiency can be maintained whilst seeking to develop and construct sustainably. INTRODUCTION Background Tall buildings are an inevitable form and part of the contemporary landscape in metropolises nowadays. This has been sufficiently reflected in Hong Kong with immense population heavily concentrated in certain urban districts. Under the market driven force of supply and demand, super high rise residential buildings are abundantly visible in seafront areas, such as West Kowloon, while the “skyscrapers” of commercial developments are readily available in Central and neighbouring areas. With the tremendously high land acquisition cost in Hong Kong, construction time, in addition to the architecture, and functional requirements of the development become the primary concerns for determining its most desirable structural system. There are many ways to construct tall buildings and in practice it is the desired use of buildings which predominantly determines their design. The determination on adopting a certain construction method is always responsive to the selected structural design system, which is particularly significant in traditional “design-tender-then construct” form of procurement contract arrangement. Clients’ Desire for Internal Layout of Office Flexible types of structure are a requirement from commercial building developers in an increasingly fluid property market. More and more new buildings need to be designed to be adaptable to changes throughout their lifetime.

In high-rise structural engineering, there are essentially three building materials: structural steel, reinforced concrete, and a composite of the two. Within each material, there are a very large number of options that one could choose. According to Chris Luebkeman (1996)7, the design process is both a process of elimination as well as creation. Options must be eliminated so that the best value can be determined. At the same time, creative solutions to the specific site constraints should be considered. There are many cases in which both a complete structural steel design and reinforced concrete design will be developed. At that point, and only at that point, can the accurate cost of the final structure be analyzed? The evaluation criteria for the choice of a structural system can vary. However, the following items shall generally be included as the compiled list 7:

(1) Economics (2) Length of Construction (Time is Money) (3) Construction Risk (4) Convergence of architectural desires and structural needs (5) Convergence of structural and mechanical needs (6) Local Condition (such as site constraints)

Most structural design schemes for a development are generated from different use or various combinations of such materials and supplemented by other advancing techniques, like prestressing. The most desirable structural system for a development would be determined on the ultimate efficiencies of achievement to meet the clients and designers’ desires.

It is quite apparent to note in Hong Kong that the primary design concern for many tall buildings is their operational efficiency rather than their environmental impact. A balance needs to be struck between two factors. Inefficient energy use is a particular concern. Progressive developers and designers would seriously look at the environmental impacts together with the economic factors in the selection of structural systems. Speculative office developers have less interest in their buildings’ environmental performance than the companies that lease their offices. Whilst energy use is currently a relatively minor financial cost, it is associated with major environmental costs, particularly climate change. Life cycle assessment of buildings and construction materials is now gaining credence. Some 10-20 per cent of the energy used in buildings over their lifetime is in the form of embodied energy incorporated in materials and the process of building itself 10. Lifecycle analysis shows that much can be done to reduce the embodied energy of buildings, particularly in tall buildings with repetitive floor plans and large areas of façade. The potential for improving the sustainable development of new high rise buildings is immense. As there is quite a lot of study and research publications on the precast techniques adopted in residential building, this paper explores the use of different construction methods responding to some common structural design systems for tall office buildings in Hong Kong. COMMON STRUCTURAL SYSTEMS AND CONSTRUCTION METHODS FOR TALL OFFICE BUILDINGS IN HONG KONG Design for Flexible Use Required for Office Building The design life of buildings in Hong Kong is 50 years, whereas the average length of occupancy is about 6-9 years from discussions with some property agents of a renowned office buildings owner in Hong Kong. Similar to the situation in London, the vast majority of

tall buildings in Hong Kong are often initially financed by commercial developers and leased to the occupiers for a number of years. As the socio-economic drivers change throughout the lifespan of a building, so the demands on the building alter. A change of occupants often leads to sub-division of the building internally, with partitioning of floors and zoning of several floors together for a single lease-holder. Open plan offices are more common now. Designing new buildings for flexibility of use and the potential for future changes helps ensure their usefulness throughout their design life. The change in demand and the requirement for flexibility has led to an increase in span of the floor beams of offices. Whereas column grids were previously laid out with spacing of the order of 5-8m, new buildings today are constructed with clear floor spans of 10-15m 10. To improve the efficient use of materials to accommodate the increasing spans, construction methods have been adapted and new techniques developed in the last decade. There has been an obvious trend in using longer span steel beam floor systems in the past 10 years, particularly in those super high-rise buildings with post-modern architecture and extremely fast-tracked programmes. Advantages of the composite action between a concrete floor slab and its supporting steel beams has also led to a reduction in the depth of beams and hence weights of steel by up to 30 per cent 10. Efficiencies in the design and construction of office towers can make a significant difference to both economic and environmental burdens. Engineers strive to find savings in materials through efficient design, making best use of concrete and steel in floors and structural frames. However, the environmental impacts of their decisions are not always clearly understood. Popular Structural Systems for Tall Office Buildings The most common structural system for tall office buildings since the last decade is still reinforced concrete. This is despite everyone recognizing that structural steel enables designers to have a long span design with relatively smaller dead weight and member sizes which contribute to higher head rooms and clear spaces. Recent development in reliable production of high strength concrete and application of prestressing technology has made concrete design more versatile to suit the contemporary architecture and flexible client’s requirements. The inherently better resistance to fire also makes concrete more immediately compatible with design codes and less sensitive to quality of construction. In America and U.K, the early development of steel led to its use as the favoured material for high rise structures. In general, reinforced concrete systems are three to eight per cent more expensive than the steel options 4 in U.K. However, in other parts of the world, like Hong Kong, that situation is reversed. Steel is not so readily available and its material cost has been substantially higher than concrete. The difference has reached the peak at about 90% in 2005 (refer to an informal survey by “Arup” as attached in Appendix I). The total construction cost of using steel framed structures (taking into account of its smaller weight and resulting foundation saving) is 15% to 40% higher than that of concrete structures (from inquiries with some local cost consultants) disregarding the time cost derived from any difference in the construction programme. The wide range of variance is accounted by the project specific features/requirements, particular site constraints, and fluctuation of material cost, etc. Combinations of concrete and steel structures are often the most efficient form, utilizing the best characteristics of each material. In the UK, the most common form of structure in buildings up to 50 storeys comprises a reinforced concrete shear core, used to stabilize the

building against wind and for fire escape, with lighter composite concrete floor slabs on a steel frame, used to carry the building’s gravity loads to the foundations. The choice of materials for the structural frame is determined primarily to satisfy those requirements, with comparisons made of the most economical form that will do the job. The structural systems popularly adopted for most tall office buildings in Hong Kong are in form of rigid frames comprising a concrete core (to withstand high wind load, provide rigid lift cores, and fire restrained stair cores) and steel composite mega columns at the perimeter (carry the building’s gravity loads to the foundation). Concrete floor slabs on either steel beams and metal deck or concrete beams are then used to transfer lateral loads to the mega columns. It is very seldom in the market that a precast concrete structural system has been used. Construction methods for tall office buildings have also been developed according to the popular structural systems. A survey has been carried out for the structural design and construction method adopted by for tall office building development in the last 10 years. Over 80 % of the buildings have adopted the insitu concrete design and method while there has been a trend in the popularity of using steel frames recently. The result is indicated in Table 1. The details of the office developments included in the survey are shown in Appendix II. Table 1 Survey on the Structural Systems and Construction Methods for Tall Office

Buildings Completed from 1996 - 2005 in Hong Kong

Floor Schemes (2)

Year of completion Insitu Steel Precast Total

2005 1 1 0 2 2004 5 0 0 5 2003 1 1 1 3 2002 1 0 0 1 2001 3 0 0 3 2000 2 0 0 2 1999 7 1 0 8 1998 5 2 0 7 1997 4 0 0 4 1996 2 0 0 2

Total (%) 31 (83.8%) 5 (13.5%) 1 (2.7%) 37 Remarks: (1) Tall Office buildings are defined as those office buildings with 30 storeys or more (2) The floor schemes are referring to the main beams involved in typical floors (3) Buildings completed within 1996 - 2005 but with unknown structural systems adopted is

not included in the above table

COMPARISON OF DIFFERENT STRUCTURAL SYSTEMS AND CONSTRUCTION METHODS Each structural system and construction method brings its own particular merits to a development in favour of the client and designer. There is no definite conclusion that one system and method is totally outweighing the other. The performance of insitu concrete, precast concrete, and steel design is evaluated in the following aspects: (1) Cost (2) Time (3) Environmental Impact (4) Logistic Constraints (5) Flexibility for Design/Use The detail comparison is shown in Table 2. Insitu concrete design, with its significant merits of easy availability, relatively cheaper cost and flexibility for design changes, is still the major structural system and construction method favoured for most tall office buildings. With the increasing building height or number of floors required for skyscrapers, structural steel frame system, which can result in less sophisticated foundation design and lower foundation cost, and faster construction speed, becomes more popular. Most practitioners in the industry believe that the steel construction method is more environmental friendly in particular with off-site prefabrication in efficient factory conditions, and bolt and nut assembly design minimizing on-site welding and cutting activities. However, when we take into account the embodied energy and the extra fire proofing and decorative encasement required for the structural steel frames, the environmental impact from using steel is not obviously reduced in comparison with the concrete systems. Among the major building materials including steel, concrete and timber, the embodied energy for steel is the highest while timber is the least and concrete is in between 10. Where architecture requires extensive use of conventional timber formwork, insitu concrete begins to loose its environmental benefits and therefore use of steel/metal system forms with highly repetitive and recycled use has been more and more desirable. The precast concrete structural system is rarely adopted in office buildings because of the typical open plan design and long spans which give rise to deep beams and heavy weight of each individual member. The feasibility or constructability of using precast concrete structural system and construction method, especially for office buildings located in high value urban districts usually with highly constrained site areas, is not well ascertained by most designers. As a result, the precast concrete system and method, especially one that involves structural precasting, is not popular in Hong Kong notwithstanding its merits in minimizing construction waste and environmental impacts.

Table 2 Comparison of Different Structural Schemes and Associated Construction Methods

Evaluation on the Relative Performance of Different Structural System and Associated Construction Method

Aspects of Concerns Relative Performance of Different Structural System and Associated Construction Method

Categories Items Insitu Steel Precast Foundation Cost High Medium High Material Cost Low High Low Transportation Cost Medium High High Installation Cost High Low Medium Protection Cost Low High Low Decoration Cost Low High Low Maintenance Cost Low High Low

Cost

Overall Low High Low Design Finalization Lead Time Short Long(1) Medium Procurement Lead Time Short Long Medium Fabrication Time Short Long Medium

Procurement and Preparation Time

Overall Short Long Medium Installation Time Long Short Medium - Short(2)

Fire Protection Application Time Nil Long Nil Finishes Application Time Medium Long Short

Construction Time

Overall Medium Medium Medium-Short Embodied Energy Low High Low Insitu Formwork Requirements High Low Low Noise Pollution High Low Low Air Pollution High Low Low Water Requirements High Low Medium Wastage Generations High Low Low Difficulties in Recycling High Medium(3) High

Environmental Impact

Overall High Low Low Off-Site Storage Low High High On-Site Storage Low High High Site Access Requirements Low High High Effect from Site Surroundings Low High High Off-site Transportation Low High High Vertical Transportation on site Low Medium High Just In time Requirements Low High High

Logistic Requirements

Overall Low High High Resistance to Design Change Low High Medium Difficulties in Finishes Application Low High Low Constraint on Headroom Requirements High Low High Constraint on Column Spacings High Low High Constraint on Services Penetration Low High Low

Flexibility

Overall Medium Medium Medium Remarks: (1) For bolt and nut connections (2) Depends on the area of floor plans (3) Normally with fire protection paint

USE OF PRECAST CONCRETE STRUCTURAL SYSTEM IN “CAMBRIDGE HOUSE PROJECT” Background To support the development of sustainability in construction industry, a renowned progressive property developer in Hong Kong – “Swire Properties” explored the use of structural precasting system in one of their tall office buildings in Tai Koo Place – Cambridge House, in 2001. Same as the general design feasibility and optimization process, the engineer had investigated different structural systems with various combinations of concrete and erection system. Ultimately, a prestressed concrete structural frame system, with composite steel and concrete columns at the perimeter, was found to be the most efficient scheme to meet the client’s requirement. The client had considered a preference for a precast concrete system but since the market lacked proven record as well as experience in precast construction, the engineer prepared two design schemes – insitu and precast concrete, and a construction advisor was employed to evaluate the constructability and construction programme for these two options at the pretender stage for the superstructure contract. On pretender stage, it was believed that the precast design and construction could be of about 2 months faster than the insitu scheme. When the tenders returned, the results indicated that the cost to client for adopting the precast method was very close to that for insitu one and the anticipated programme gains of 2 months time could be achieved. The precast concrete structure and construction method was therefore eventually adopted. Project Particulars Cambridge House is a Grade “A” office building erected in Tai Koo Place. The building, with total construction floor area of about 31,000m2 , consists of 36-storeys of offices sitting on two levels of podium floor. The floor plate of the tower is designed in an octagonal shape, and the columns are about 11 m apart. The longest primary beams are 16 m span. The primary beams, secondary beams and slabs were designed as prestressed and semi-precast. The staircases in typical floors were also designed in precast concrete. Each typical floor from 3/F to 21/F has 11 precast beams, 44 precast planks and 3 stair modules (see Figure 1). On the floors of 22/F to 36/F, 1 more beam and 4 more planks were involved because larger office floor area was obtained from omission of the low zone lift cores.

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Sketch - SK-1 : Semi- Precast Elements at Typical Floors (3-21/F) of Cambridge House Figure 1 Semi-Precast Elements at Typical Floors (3-21/F) of Cambridge House

The superstructure contract mainly consisted of the structural works from the pile caps upwards and included all internal finishing and M&E installation. The scope also included the construction of link bridges and alteration of some building services connecting to an existing building – Devon House adjoining to the site. To enable proper planning and sufficient preparation to use the precast construction method, the superstructure contract was awarded with lead-in period of about two and a half months before the actual required works commencement. This allowed the contractor sufficient lead time to design and fabricate the system forms and moulds for precast elements and carry out trial assembly before the actual installation works started on site. Special Design for Semi-precast Beams To reduce the weight of the semi-precast beams, the cross-section of the beams was designed with a prestressed U-shape (see Figure 2) rather than the traditional half-beam. As a result, the heaviest member of 16m long, weighed about 13 tonnes. With the conventional half beam design, the dead weight of the heaviest member would be over 21 tones, i.e. about 61% heavier. This would have required the use of a rare and expensive heavy duty tower crane for the installation of the precast elements.

Figure 2 Two Design Section of Semi- Precast Beam Construction method The building was situated in an urban district location surrounded three sides by existing property and with, on its fourth side, a heavily-traffic carriageway – Kings Road (see Figure 3). The podium floor footprint occupied nearly the whole site area, and no open area could be allowed for any precast element storage. Only one site entrance was allowed from the King’s Road. To cope with such site constraints, an off-site storage was designated at a nearby location, at about 10 minutes travel distance from the site. The local off-site storage was sufficient for holding the required precast elements for three storeys construction. In addition, on site, a temporary unloading area was provided at a late cast portion of the podium structure outside the tower footprint. A luffing jib tower crane with lifting capacity of 16 tones at 25m was provided for installation of the precast elements.

Figure 3 Location of Cambridge House To minimize the environmental impacts arising from construction activities upon the neighbouring areas, the construction works were carried out within a full perimeter enclosure. No transportation plant was erected outside the tower footprint and a three-storey high external climbing metal platform was provided at the working floors (see Figure 4). In addition, the number of construction joints was minimized as far as possible.

Figure 4 External Climbing Working Platform

The precast concrete frame system was used for the open office area (front of house) but the services core (back of house), including lift shaft, lift lobby, stairway and other plant rooms, was in insitu concrete design. The construction of a typical floor was split into two portions, namely insitu and precast, accordingly (see Figure 5). For the insitu portion, contrary to the traditional method of using self-climbing slip or jump forms for construction of the lift shafts

only, the contractor built the whole services core on a complete floor basis, i.e. walls, beams, and slabs together. This method introduced the merit of providing safe worker access to the working levels and also eliminated any unnecessary construction joints from late cast elements which would always gave rise to risk of quality and environmental problems. To implement such method, the contractor adopted large panel metal forms for the external walls of the services core and aluminum handset panel forms for the internal walls, beams and slabs. For the precast portion, special modular metal scaffold (falsework) was used to support the precast elements until they had gained sufficient strength after concreting. This type of scaffold can be pre-assembled in a modular form (see Figure 6) which allows easy and fast erection and transportation. The contractor specially designed an internal small gantry hoisting device (see Figure 7) for moving the scaffolds up from the lower floors to the working level. The mobile scaffolds could be efficiently moved to the required location and assembled together with fixing of the additional bracing and ties among different modular units.

Figure 5 Typical Floors Construction (4 Days Cycle) Split into 2 Portions

Figure 6 Modular Metal Scaffold for

Supporting Precast Elements Figure 7 Gantry Hoisting Device for

Modular Metal Scaffold With optimized utilization of the craneage time, the luffing jib was the only mechanical plant used for the execution of the structural works activities, including installation of the precast elements, fixing of the steel forms as well as the concreting. No placing boom and pumping concrete was used and hence least concrete wastage has been achieved. As no mechanical plant was erected at the external face of the building, the curtain wall panels could be completed on an entire floor basis in line with the superstructure progress so that there were no left out openings at the façade throughout the whole construction stage. Early achievement of watertightness enabled the early commencement and completion of internal finishing as well as building services installation. 4-Days Construction Cycle Achieved for Typical Office Floors To meet the 18 months construction period requirement for the project, a 4-day construction cycle must be achieved for the structure of each typical office floor. The structural construction activities for respective insitu and precast portion should be well scheduled to enable completion within the cycle time. To validate that the planned schedule for each activity was practical, an off-site trial assembly was carried out before the production of the precast units and the system forms. In addition, any necessary modification of rebar details and minor dimensions of the precast units enabling the achievement of the tight construction cycle time should be identified through the trial assembly. The detailed schedule for carrying out the construction activities is explained as below (see Table 3). Photos showing the activities carried out in the 4 –day cycle is attached in Appendix III. This construction speed was same as that given by the use of structural steel method.

Table 3 Detail Schedule of 4-Day Cycle Activities

The insitu portion was constructed two floors ahead of the precast portion in order to allow the appropriate airspace for operation of the large panel metal system forms to the core walls and also installation of the steel stanchions for the composite columns. Since the largest concrete pour to the typical floor only involved about 230 m 3 and the heavy duty tower crane had some spare craneage time, crane and skip method was adopted for placing concrete. A large skip of 2.2m 3 capacity aided the process with a total weight of 6.5 tonnes with fresh concrete per crane lift. “Just on time” delivery of the precast elements was the critical factor for achieving the tight construction cycle where on-site storage was highly restricted. Success on the logistic arrangements has direct bearing on the achievement of the required fast track programme. Other Environmental Friendly Measures Adopted associated with the Construction Method Two jump lifts were used, being an internal transportation system making use of the permanent lift shafts. This provision can help to avoid the use of temporary hoisting facilities normally erected at the external face of a building or inside a building. Jump lifts avoid the need for numbers of left-out floor and wall openings and therefore cause the least environmental impacts to the surrounding regions. With the contractor’s method of constructing the whole service core, including the lift shaft, lobby, plant rooms and staircase, at the same time, maximum benefit was demonstrated by the use of jump lifts which could be installed and put in use for vertical transportation purpose up to only a few floors below the working levels with structural works in progress. Two jump lifts were used in this project for both building materials and passengers.

Area ActivitiesSemi-precast Concreting to Semi-precast Portion (9:00am to 4:00pm)Portion Delivery of Semi-Precast Beams to site (9:00am to 4:00pm)

Delivery of Semi-Precast Planks (9:00am to 4:00pm)Erection of False-work Support (8:00am to 6:00pm)Raise Up External Working Platform (4:00am to 6:00pm)Steel Fixing of Columns (8:00am to 12:00pm)Installation Steel Column Formworks (1:00pm to 4:00pm)Concreting to Columns (4:00pm to 7:00pm)Hoisting and Installation of Semi-Precast Beams (8:00am to 12:00am)Hoisting and Installation of Semi-Precast Planks (1:00am to 6:00pm)Steel Fixing to Beams and Slabs (7:00am to 5:00pm)Installation Steel Stanchions (4:00pm to 6:00pm)

In-situ Portion Steel Fixing to Core Wall (7:00am to 3:00pm)Striking of Steel / Aluminium Formwork (8:00am to 12:00pm) Raise up lift shaft wall form and working platform (by Tower Crane) (8:00am to 9:00am)Erection Aluminium Wall formwork (1:00pm to 6:00pm)Hoisting and Install of precast staircase (4:00pm to 6:00pm)Erection Aluminium Slab Panel (8:00am to 5:00pm)Raise up external steel working platform (2:00pm to 4:00pm)Installation of Steel wall mould for external Lift Shaft and Staircase (5:00pm to 7:00pm)Hoisting Reinforcement from Storage yard to working floor from (7:00am to 8:00pm)Fix Beams and Slabs Reinforcement from (8:00am to 5:00pm)Cast in Curtain Wall embeds from (3:00pm to 6:00pm)Concreting to Core Walls beams and Slabs from (8:00am to 4:00pm)

Day 4Day 3Day 2Day 1

As mentioned above, a 3-storey high external climbing platform was adopted for the structural works progress. After the structural works had been finished to certain upper floors, the curtain wall panels were installed by means of gondola starting from the lower floors upwards. The use of conventional scaffold to the external face of building has therefore been omitted. Benefits of Using Precast Construction for Cambridge House The construction cost to the client for using precast construction was almost the same as that for total insitu concrete system. The 2 months saving in the shorter construction programme achieved by using precast method (derived from 4-day construction cycle of precast method against 6-days cycle of insitu method) has also enabled the client to take over and lease out the buildings earlier with extra financial gain. The use of precasting methodology in Cambridge House enabled the contractor to deliver a better quality product than using insitu casting. As the concrete elements were manufactured in an off-site precast yard with well planned and equipped factory conditions, the production process was closely controlled and monitored to achieve the required standard satisfactorily. Concrete tolerances and surface finishes were improved, providing savings in follow on trades for the shell and core finishes and MEP installations. Material wastage and rubbish, with their associated safety concerns, were minimized in avoiding site cutting and insitu concreting activities. The client was able to review and accept the quality of the products before their delivery and incorporation in the project, and ahead of any risk of abortive works or programme delays. The prefabrication method also helped to reduce the demand for skilled trades labour on site for the project during the construction stage because significant parts of the works had been executed off-site. Reducing the range and skill level of workers required on site, the contractor not only reduced the impact of fluctuation in the labour market availability and price upon the tight erection programme, but was also able to be more selective with his site team, developing a disciplined work force for optimum safety and quality. In summary, the quality and environmental performance of the construction has been effectively raised to a higher standard. The project has gained an excellent grade award by the HKBEAM and a number of other environmental related awards, such as “Innovative Practice” and “Green Office” of Eco-Business from the Environmental Protection Department, and “Good Housekeeping” commendation award from the Labour Department. CONCLUSIONS AND RECOMMENDATIONS In current practice, economic factors tend to outweigh the social and environmental factors in commercial developments. Very rarely do commercial buildings developers and designers place serious thoughts and attention on determining a structural system with high environmental concern, particularly in absence of incentive schemes or legislation of relevant standards, or guidance for sustainability. However, pressures to redress that balance are increasing and new developments are going to be more sustainable in future. For sustainable development to become common practice, legislation is needed to ensure further measures are taken to safeguard the environment. While best practice and guidelines are helpful in raising awareness of opportunities for improvements, “the bottom” line is the

dominant factor in procuring buildings. Property development is a market driven business and tall buildings are financial instruments to most developers and clients The use of precast concrete construction in private developments and other projects requiring approval by the Buildings Department (BD) was limited. One of the main reasons was the lack of practice notes and relevant codes of practice providing the necessary guidance on such works 1. The use of precast concrete construction can significantly reduce the amount of construction waste generated on construction sites, reduce adverse environmental impact on sites, enhance quality control of concreting work, and reduce the amount of site labour. To assist in promoting precast construction, the BD commissioned a consultancy study on precast concrete construction and the drafting of the Code of Practice. The Code was first published by BD in October 2003. The code gives recommendations and guidelines on the design, construction and quality control of precast structural and non-structural elements. It was drafted following an extensive review of international standards and other published literature and based, where applicable, on local experience and practices. It is intended to be used in conjunction with the new code of practice for reinforced concrete covering the following areas1: - Design including stability, durability and water-tightness of joints; - Construction of precast elements ranging from factory production, transportation and

handling through to site erection; - Quality control during both production and erection Another major reason for the lowest popularity of precast concrete design and construction in tall office buildings is lack of proven experience and track record in the industry. The logistic constraints, including the use of heavy duty mechanical plant required for the installation of the precast elements, transportation facilities for vehicular access to site, and the construction speed, create lots of uncertainties to the designers and developers. There is no conclusion that the merits of a concrete structural system and construction absolutely outweigh those of the steel system or vice versa. The determination is based on the evaluation on the efficiencies of the system in meeting the design and client’s requirements. In general, steel with significantly higher construction cost could be adopted in circumstances where lighter structural dead weight was required to suit the practical foundation design, and where longer span of structural frame with smaller member sizes was required to achieve the fundamental architectural requirements or client’s desires. A lot of research papers suggest that precast concrete planks should be used instead of metal decks on the steel framed structures for both better functional efficiency and environmental performance. If speed of construction is the predominant factor for the determination, the successful use of precast construction technology achieving a 4-day cycle in Cambridge House project should give an indication to the Hong Kong industry that precast design can be a competitive alternative to steel system. When a concrete structural system is chosen, precast design and construction can be really considered as economical alternative to the insitu method with its potential to achieve faster programmes. Although the production and transportation cost for precast concrete elements is generally higher than that for insitu casting, this can be offset by the resultant smaller site preliminaries from a shorter construction period and reduced waste, rubbish clearance, and housekeeping. As indicated in the Cambridge House case, the contractor’s price or the construction cost to the client was almost the same in using either insitu or precast method.

In addition, the client has gained an extra financial benefit in taking over the building earlier. Of course, the critical transportation as well as logistic issues, which are prerequisites for adopting the precast method, should be properly addressed in addition to its efficiency of meeting the client’s desire. It also worth noting that a relatively longer preconstruction lead time is generally required for the precast method if fast-track programme and smooth progress is to be the returned. Modular architecture, dimensional co-ordination and standardized concepts should be widely adopted for the architecture to facilitate the re-use of precasting moulds and for fabrication of the elements to be cost effective. This concept is significantly important for the use of a precast design where such as progressive changing floor plates or irregular structures are involved. It cannot be concluded that the precast construction method should prevail over other systems in all cases with different project specific requirements and site constraints, particularly where the current market has no clear standards and guidance for the relevant design and practice in structural precasting. However, as well as generating a return on capital expenditure, primary concerns of designers include the need for new buildings to enhance their environment, to be aesthetically pleasing to the eyes of observers, and to be effective and comfortable to the senses of their occupants. But these main aims need not be compromised in seeking to develop and construct sustainably. With the successful completion of the Cambridge House project, we believe a desirable balance between economic, social, and environmental effects can be achieved by a holistic approach to the whole building design. Increased awareness of the issues is needed throughout the industry to give impetus to improvements and potential savings in practice.

Appendix I Material Cost Trend for Concrete, Reinforcement and Structural Steel

Appendix II Particulars of the Office Buildings Completed from 1996 to 2005 of the Survey

No. Building Name Year of Completion Storey Structural Floor Scheme

1 AIG Tower 2005 40 Steel

2 8 Fleming Road 2005 30 R.C.

3 Langham Place Office Tower 2004 59 R.C.

4 May House (Police Headquarter) 2004 47 R.C.

5 Enterprise Square 3 (Kowloon Bay) 2004 41 R.C.

6 Three Pacific Place 2004 40 R.C.

7 Skyline Tower (Kowloon Bay) 2004 38 R.C.

8 Two International Finance Centre 2003 88 Steel

9 Cambridge House 2003 36 Precast

10 One Peking Road 2003 30 R.C.

11 Chater House 2002 30 R.C.

12 The Centrium (Wyndham Street) 2001 41 R.C.

13 148 Electric Road 2001 41 R.C.

14 Enterprise Square 2 (Kowloon Bay) 2001 33 R.C.

15 333 Lockhart Road 2000 52 R.C.

16 88 Hing Fat Street (Gordon Road N.P.) 2000 37 R.C.

17 Cheung Kong Centre 1999 62 Steel

18 AIA Tower 1999 44 R.C.

19 The Westpoint (Connaught Rd. W) 1999 41 R.C.

20 Oxford House 1999 41 R.C.

21 Dah Chong Hong Commercial Building 1999 31 R.C.

22 Man Yee Building 1999 31 R.C.

23 MLC Millennia Plaza (King's Road) 1999 30 R.C.

24 1063 King's Road 1999 30 R.C.

25 The Center 1998 73 Steel

26 Cosco Tower (Queen's Road) 1998 53 R.C.

27 Manulife Plaza (Hysan Avenue) 1998 52 R.C.

28 One International Finance Centre 1998 38 Steel

29 Standard Chartered Tower (Millennium City) 1998 38 R.C.

30 Grand Millennium Plaza 1998 30 R.C.

31 28 Marble Road 1998 30 R.C.

32 Henley Building (Queen's Road Central) 1997 36 R.C.

33 Citic Tower 1997 33 R.C.

34 China United Plaza 1997 30 R.C.

35 Western Harbour Centre (Connaught Rd. W) 1997 29 R.C.

36 Laws Commercial Plaza 1996 34 R.C.

37 Bank of Communication Tower 1996 33 R.C.

Appendix III 4-Day Construction Cycle for ‘Cambridge House’

DAY 1 In-situ Portion

Precast Portion

Striking of Steel/Aluminium Formwork (8:00am – 12:00pm)

Rebar Fixing for Corewall (7:00am – 3:00pm)

Concreting for Topping Semi-precast Beams and Slabs (120 m3)

(9:00am – 4:00pm)

DAY 2 In-situ Portion Precast Portion

Hoisting and Installation of Precast Staircase (5:00pm – 7:00pm)

Hoisting and Installation of Steel/ Aluminium Formwork and Working

Platform (2:00pm– 4:00pm)

Delivery of Semi-precast Planks (9:00am – 4:00pm)

Delivery of Semi-precast Beams (9:00am – 4:00pm)

Day 2 (Cont’d) Precast Portion

Precast Portion

Lifting & Installation of Working Platform (2:00pm – 6:00pm)

Erection of Modular Metal Scaffold (8:00am – 6:00pm)

Rebar Fixing for Columns (8:00am – 12:00pm)

Installation of Column Formwork (1:00pm – 4:00pm)

Concreting Columns (22 m3) (4:00pm – 7:00pm)

DAY 3 In-situ Portion

Precast Portion

Rebar Fixing for Beams and Slabs (8:00am – 5:00pm)

Hoisting and Installation of Semi-precast Beams (8:00am – 12:00pm)

Hoisting and Installation of Semi-precast Planks (1:00pm – 6:00pm)

DAY 4 In-situ Portion

Precast Portion

Concreting Core Wall, Beams & Slabs (220 m3) (8:00am – 4:00pm)

Rebar Fixing for Beams and Slabs (7:00am – 5:00pm)

Installation of Steel Stanchions (Max. 8 m High, Max. 11 Tons) (4:00pm –

6:00pm)

REFERENCES 1. Albert W.K. Leung & Allen Spring: “The new concrete and precast concrete

codes – their development and essential features”, The Structural Engineer – 18 October 2005, P.30-33

2. Angela Tam: “Sailing into the gap of the Furama”, Hong Kong Engineer

October 2005 3. Angela Tam: “Structural Precasting gets thumbs up in Tuen Mun”, Hong Kong

Engineer – Cover Story August 2005 4. Corus Construction & Industrial: “Supporting the Commercial decision –

Comparing the cost of steel and concrete framing options for commercial buildings”, 2004

5. C.S. Poon, Lara Jaillon (Department of Civil and Structural Engineering, The

Hong Kong Polytechnic University): “A Guide for Minimizing Construction and Demolition Waste at the Design Stage”, The Hong Kong Polytechnic University, February 2002 ISBN: 962-367-334-5

6. Environment, Transport and Works Bureau: “Progress Report on the

Management of Construction and Demolition Materials” – Legislative Council Panel on Environmental Affairs, January 2005

7. Prof. Dr. Chris H.Luebkeman: “Structural Systems Selection - Resources on the

course of special studies in Building” , Lecture in University of Oregon, 1996 8. Thomas K.C. Wan, William Leung, K.F. Tam: “A Commercial/Office

Development at 1 Peking Road, Tsimshatsui (The One Peking)”, HKIE Transactions Volume 11 number 4, December 2004

9. US Army Corps of Engineers: “Technical Institutions – Metal Building

Systems”, T1 809-30, 1 August 1998 10. Will Pank (Maunsell Ltd.), Herbert Girardet (Urban Futures), Greg Cox (Oscar

Faber Ltd.): “Tall Buildings, Sustainability and the City” – The Corporation of London, December 2001