SUSTAINABLE CONSTRUCTION IN HONG KONG DEMONSTRATION PROJECT IN FU TEI

Transcription

1 Conference on Sustainable Building South East Asia, April 2005, Malaysia SUSTAINABLE CONSTRUCTION IN HONG KONG DEMONSTRATION PROJECT IN FU TEI D. K. L. SO 1, S. C. C. AU 2 1,2 Ove Arup & Partners H.K. Ltd. Level 5, Festival Walk, Tat Chee Avenue, Kowloon Tong, Hong Kong 1 derek.so@arup.com 2 steven.au@arup.com C. C. K. WONG 3, A. K. H. MOK 4, D. M. F. TAI 5 3 P.K.Ng & Associate (HK) Ltd. 25 th Fl., Eastern Commercial Centre, 83 Nam On Street, Shaukeiwan, Hong Kong ppkng@netvigator.com 4 Hip Hing Construction Co Ltd 29/F New World Tower, Queen s Road Central, Hong Kong kh_mok@hiphing.com.hk 5 Shunde Lunjiao Quon Hing Construction Material Co, Ltd 11/F, Tianjin Building, 167 Connaught Road West, Hong Kong mftai@quonhing.com.hk Abstract This paper presents a recent sustainable development demonstration project in Hong Kong which achieved system level solutions by evolving the nature of architecture system technology and innovating engineering solutions. The project is a thirty-three storey residential tower that was constructed using extensive structural precast components. It is the first of its kind in Hong Kong, and uses fully precast shear wall and columns, balconies, planking, kitchen modules and cladding, and volumetric structural precast bathrooms. This is one of the pioneering project in the use of joint construction of precast wall connections in a high-rise building in the region. This type of connection differs from monolithic cast-in-place concrete construction and exhibits an ability to deform inelastically. There were several objectives to the project: (i) promote wider use of prefabrication; (ii) minimize generated waste and on site wet trade work; (iii) achieve a better quality product for the end-user; and (iv) provide a safer construction environment. The project employed a partnering approach that brought together construction stakeholders in multi-disciplinary collaboration and thus attained excellent environmental and cost effectiveness results. The promotion of a whole-system strategy that managed the building system i.e. structure, building envelope, partitions, building services and equipment, resulted in new architecture configuration technology that was devised to harvest a spectrum of spatial variations inside the building mass. The successful completion of the project proved the achievement of the objectives are possible and that it is within current reach to erect projects using an efficient amount of site labour, fast construction, safer on site working environments, less waste, and better quality. Keywords : Sustainable development, Structural precast components, Joint construction, Partnering approach. 1. Introduction In the mid 80 s, the Hong Kong Housing Authority (HKHA) recognized a need to enhance the ease of constructing buildings. In so doing, the HKHA adopted prefabrication as an approach and since, prefabrication technology has become associated as a hallmark method in achieving cost effectiveness, improved durability, and enhanced build ability. On the contrary, the majority of current construction work 146

2 in the private sector continues to engage antiquated techniques that date back thirty years. This traditional construction approach demands a high level of craftsmanship and labour intensity, and involves extensive wet trades work on site. Ensuring wet work results are of quality further necessitates excellent on site logistics management and supervision, which typically drives up costs significantly. Indeed due to the priorities of cost minimization, fast construction time, early revenue generation, and maximum profit, the important concepts of sustainable construction and precautionary principals are often sidelined. Like most industrial processes, traditional construction practices are linear in the exploit of energy and natural resources while discharging relatively large amounts of waste. As Hong Kong continues to run out of both reclamation sites and landfill space for the disposal of construction and demolition waste, it is a matter of time before these issues will result in an impact on areas beyond construction sites and the overall environment. As alluded to earlier, on site wet trade work is lessened through the adoption of a manufacturing approach that uses metal formwork, prefabrication, and standardised modular components. This impacts positively on the cleanliness of construction zones and contributes to better site safety and less waste generation. However, there are only a few cases where track records of consequent and significant successes have been achieved. This is particularly true in the private sector and the reasons for this are as follows: 1. Private developers continue to maintain the perception that precast construction cannot provide an agile design. 2. The design of the building is market driven and the developers look for distinctive styles on each project, creating difficulty in standardizing designs. 3. Most buildings in Hong Kong are high-rises that must endure significant wind loads, posing difficulty in using precast construction. 4. Many liability-conscious designers and contractors are reluctant to try innovative construction methods. 5. Developers and contractors need to see clear business benefits before deciding to adopt sustainable construction. The Hong Kong economic downturn of 1998 marked an impact in the local construction industry. With the local property market in a slump and a decline in demand for domestic property, private sector developers became more cost-conscious. In 1999, the Hong Kong Government completed a 40 million dollars study titled Sustainable Development for the 21 st Century (SUSDEV21) that independently researched and identified that continuing economic growth and a commitment to sustainability was vital to Hong Kong. The Hong Kong SAR Government then announced a Greener Construction campaign with the objectives of creating a better quality of life through sustainable development. In January 2001, the Government published a Construction Industry Review Committee Report relating to the overall improvement of the construction industry. With a change in the market environment and the Government s desire to move forward, the industry began to take note of methods and good practices that improved efficiency, cost-effectiveness, quality, and delivery timeliness. These factors are the main drivers for in-depth studies of existing housing construction methods, with particular emphasis on reducing the generation of construction waste and the improvement of overall efficiency while meeting specific requirements on time, cost and quality. This demonstration project sets an example of holistic changes in the design and construction process for the purpose of sustainable construction and showcases a way towards local industry improvement. The sustainability goal of this project was to unite segments of the building industry that typically work independent of one another. Bringing these segments together in a team of architects, engineers, and builder manufacturers was an innovation to the engineering process that ultimately yielded resource efficient, environmentally sensitive, affordable, and adaptable residences on a single project scale. 147

3 2. The Project The demonstration project is a thirty-three storey residential building in Fu Tei, Tuen Mun, which is the current tallest precast building in Hong Kong that involved extensive use of concrete prefabrication technology. The project was a collaborative effort between developers New World Development Group and main contractors Hip Hing Construction Company (a subsidiary of New World Development Group). Architects were brought onboard from P.K. Ng and Associates with Ove Arup & Partners providing structural and geotechnical engineering consultancy services. The site layout plan and elevation for the development is shown in Fig.1. Being the subsidiary of New World Group, Hip Hing teamed up with PK Ng and Arup to find innovative solutions and radical improvements in construction quality and efficiency. An increased use of fabricating building components off-site and the partnering of the construction stakeholders played a major role in achieving success, particularly in light of the fluid and fragmented nature of the construction industry. Fig.1 Site layout plan and elevation for the development Conventionally, there is often little collaboration between architects, engineers, and contractors in the design and construction process. Normally architects prepare designs based on client requirements and issue instructions to other consultants for action. The typical design-bid-build procurement process sees designers prepare plans and specification for contractor bidding, who in turn submit low bids to maximize their chance of securing contracts. This tends to have a negative effect on build quality and the approach is inflexible to the dynamic changes in materials and methods of fabrication. At times, the contractor may also be forced into a solution that is inefficient. In other words, a typical procurement process is hardly conducive to achieving innovation in design and construction. The adoption of a partnering approach (So D.K.L.,2003) alleviates many of these issues by having the contractor, precaster, and subcontractors form part of the design team early on. In so doing, the knowledge and expertise of each group is factored in from the onset, allowing parties to share equally in decision making responsibility. Further, parties can also then have their operational constraints and concerns represented at the design development stage, which is simply the best time for such concerns to be considered. 3. Design 3.1 Architectural Design The project was devised through several key design and construction strategies that were sustainable and friendly to environmental integration. By adopting an Integrated Design Process, multiple benefits were achieved for the developer, builder, occupants, and environment. Identifiable elements of the process are: Whole-systems thinking End-user considerations Front-loaded design Teamwork 148

4 As buildings are site-related and technology is normally factory-related, three families of building systems were employed in the whole-systems thinking process: Site intensive kit of parts; Factory-made 3D modules and 2D panels; Hybrid solutions, limiting transportation and minimizing site operations. A whole-system thinking strategy in a sustainability context organizes architectural functional systems based on flexible modular structures and mechanic components. These are basic functional skeletons from which it is possible to create a rich variety of livable designs where the logic of production resolves any discord between man intervention and the environment. Considering the needs of end-users is another important element for achieving sustainability. The considerations included: Site issues and planning Resource efficiency Indoor air quality Construction materials and methods The resulting minimized development footprint further reduced the urban heating effect and the incorporation of a cross-ventilation lobby takes to heart occupant comfort and health needs. Through respecting the natural environment during the selection of building placement, the net benefits of good views, natural light, and excellent ventilation were further realized. This was possible only because the project s building system allowed for such thinking in design and construction. Waste is again reduced since the effective planning process coordinates dimensions and positions properly, instead of improvising on site changes to the size later on. Construction and material use was also a careful consideration in devising the project. The issues considered were: Intensified usage of spaces Simple walling schemes Minimized the number of components Standardized services and circulation Uniform openings with regulated positions Structured clarity and efficiency Suitability for adoption in hoisting and erection Ease of manufacturing, storing and transportation Speed and ease of creation Simple on-site jointing system Working within the constraints of module size to simplify production and keep cost down Capturing potential savings associated with economies of scale and adoption of standardized module from manufacturers 3.2. Standardization and Modularization Standardization of components (i.e. standardisation of the dimensions of components such as doors and windows for certain common materials such as steel and concrete, etc.) brought many benefits. These include: Reducing manufacturing costs Having fewer interface and tolerance problems Achieving better end product quality Enjoying greater certainty over outcomes More efficient research and development of components Reducing maintenance cost for end-users Reducing waste and offering more scope for recycling. 149

5 To maximise these benefits, the design team factored in the use of standardised components early in the design stage to ensure compatibility between design and the manufacturing process. As covered earlier, industry professionals will need to act together in order to achieve the necessary economy of scale. Resulting from the intention of standardisation and modular design, the layout of the typical floor consisted of three standard types of units that met the client s marketing strategy. Provided in a standard form were bay windows, balcony, and kitchen designs (Fig.2). This approach minimised the number of casting forms required. Fig.2 Standardization and modularization of precast components 3.3 Structural Design The high wind load typical of Hong Kong buildings posed a significant challenge to the engineering and design. Current building codes in Hong Kong do not cover the design of precast high-rise buildings. A literature review was required of existing guidelines and codes of practices on precast buildings. Issues of concrete, current construction practice, unique experiences, and latest technologies were researched in foreign countries prior to the commencement of the structural design. Eventually British Standard BS8110 was used as the design standard for the project with references also made to standards from Singapore, USA, New Zealand, and the Netherlands. The structure was designed for wind loading in accordance with the Code of Practice on Wind Effects Hong Kong

6 \ From a marketing perspective, the team advised no changes to the layout of the residential unit given the design had already been widely accepted in the market for some time. The building was thus designed as a central core wall and shear wall structure which provided lateral stability to resist wind load (Fig.3). The flooring structure is a beam-slab system and the central core was constructed by a cast-in-place method to avoid complicated connection of precast elements. The building envelope adopted precast construction. Fig.3 Building lateral system Precast Shear Wall Hong Kong rarely sees the use of precast walls for lateral load resistingelements in high-rise buildings (Freedman,1999). In order to adopt precast construction of the external building envelop, engineers decided on the use of joint-connected precast shear wall for the external shear wall. This allows the precast shear wall to deform inelastically. Unlike traditional design methods commonly used in other countries, the precast systems are designed to have load resisting characteristics that are equal or superior to those of equivalent cast-in-place concrete systems (Cleland,1997). This cast-in-place emulation has the tendency to undermine the cost-effectiveness of the precast concrete systems. Instead the joint-connected shear wall approach adopted in this project allowed the precast wall to have its stiffness tuned by detailing and connection definition of load path. The joint-connected shear wall and the in-situ central core wall provided lateral load resistance to the building. ETABS was used to model the building and the analysis was reiterated by adjusting the stiffness of the precast walls to simulate the actual behavior of the lateral system. Fig.4 Typical horizontal joint detail for precast structural wall 151

7 The horizontal joints of the precast wall were the weakest link in the overall performance of the building. Joint design and treatment are critical to the performance of the finished structure. It was therefore important that the performance and durability of the joint precast wall construction are adequately provided to prevent water leakages and air penetration. The design team spent considerable effort designing and detailing the joint of the precast structural wall to ensure optimum performance (Fig.4). The three-line defense against water and air penetration that was devised include the joint sealant between the tiles, the inorganic compression seal which also serves as the grout seal during grouting of the horizontal joint, and finally the joggled joint of the wall panel Semi-precast Planking Semi-precast slab was used to minimise the use of false work and formwork. The overall thickness of the slab is 150mm which consisted of precast slab planking 50mm thick with lattice trusses and 100mm thick in-situ topping. The lattice trusses give strength and stiffness for handling and transport and also allow panels to support construction loads with a minimum of temporary propping. The precast plank is of variable width (up to 2.5m) and length due to transportation constraints. Part of the slab next to the precast balcony was in-situ construction in order to provide better tieback to the cantilever precast balcony Precast Balcony The project marked the first its kind in terms of fully precast balcony use in Hong Kong that also fully complies with Government regulations on cantilever structures. Part of the edge beam (100mm width) was designed to attach to the precast balcony making the construction joint within the internal environment (Fig.5). The floor drain was concealed in the precast balcony so that the contractor needed only to install the vertical storm drainpipe on site under very safe conditions. Fig.5 Precast balcony Precast Bay Window/Column The precast bay window was integrated with the precast column on one side (Fig.6). The bay window is designed as a spandrel beam spanning between the precast column and the precast structural wall. The precast columns are connected by using mechanical splice sleeves to transfer the axial loads. This is the first project to use mechanical splice sleeves in a precast building project in Hong Kong. 152

8 Fig.6 Precast bay window / column Volumetric Precast Components Volumetric precast bathroom, kitchen and stair core modules were also employed in this project (Fig.7). The benefit producing in factories entire rooms such as kitchens and bathrooms (which are particularly service-intensive building elements) is clearly evident here, since on site wet tray work and building services are greatly reduced. With the use of structural precast wall in this project, the design team decided to integrate the precast modules and precast structural wall into one module taking into account the transportation restriction and lifting capacity of the tower crane. The service inside the bathroom module and the connection detail with the external services was carefully planned during the design development phase together with the M&E subcontractor. All the concealed drainpipes and waterproofing work was done in the factory with satisactory testinivery to site. Fig.7 Volumetric precast components 153

9 4. Cyclic Load Test and Full Scale Mock-up Full scale testing for the precast shear wall was performed in order to understand the behavior of the horizontal joint between precast wall panels (Fig.8). The aim of the study was to determine the performance and durability of the horizontal joint of the structural precast wall panel when subject to lateral wind load. The test specimen is selected to simulate the horizontal connection near the 28/F of the typical external wall of the building where maximum inter-storey drift is estimated to occur. The result of the test demonstrated that the horizontal joint of the precast wall performed satisfactorily in terms of load carrying capacity and long-term durability. After the completion of the design, a full scale mock-up for the precast components (Fig.9) were fabricated and erected in the precaster s yard in Shunde China to examine the connection detail and the build ability of the precast system. With the extensive use of structural precast components in this project, the connections for the precast elements were an essential part of the overall structural support system. In general, grouted connection was used for the horizontal and vertical joint of the precast wall panel. As a basic requirement, the grouted connection had to meet all strength, stiffness, and other performance criteria for serviceability and ultimate limits states. Mock-up trial grouting for the horizontal and vertical joint was carried out in Shunde to confirm the configuration, constructability, grouting performance and grouting procedure for the joint. After finalizing the precasting and connection details for components, a two storey mock-up flat was then erected to check the adequacy of the erection procedure and other connection details. Fig.8 Cyclic load test for precast wall panel Fig.9 Full scale mock up During this process, the main contractor and the precaster advised the size and weight of the precast components within the limitation of transportation, handling and erection capacity of the proposed lifting crane. A detailed method statement for handling, transportation and erection was also prepared for engineering considerations. Based on the proposed installation sequence of precast components and site limitations, the contractor had established a detailed logistics plan of precast component production and delivery schedule to fulfill requirements in the master construction program. 5. Construction Highlights The entire construction comprised a twenty-two month schedule. The foundations of the building started on February 2003 with the installation of structural steel H-piles. The sub-structure work was completed in August A comprehensive site planning study at the design stage was carried out to fully appreciate the tower crane s location, storage area for precast elements, and lifting capacity at various reaches (Fig.10). 154

10 Fig.10 Site planning logistics Production of the precast components, including the fabrication of steel moulds in Shunde, began two months before the completion of the foundation work. The precast components were shipped by barge to Tuen Mun jetty. This jetty is located close to the site and allowed for further transportation of components by truck trailer. Storage of precast components was done on site. Superstructure work began immediately after the completion of the foundation and pile cap work. Fig.11 shown the precast elements layout on typical floor. Fig.11 Precast elements layout on typical floor From the pile cap to the first floor, cast-in-place construction was used. This was necessary because of the non-repetitive elements involved for the first floor. Above the first floor, precast construction was used. Setting the precast members onto the first floor was complicated by the surface profile of the first floor. Considerable time was spent adjusting the level of the surface to ensure all pieces aligned properly between the cast-in-place first floor and the precast members. Initially, erection of the precast components was slower, but as work progressed an average rate of two and a half floors completion per month was attained. Fig.12 shows various erection phases of the 155

11 building. Fig.13 is a completed view of the building amidst other residential high-rises in Fu Tei. The sequence of erection followed the original sequence prepared in the planning stage. A major time benefit was the rapid enclosure of each floor with significant wet trade work being completed in the factory. Fig.12 Progress photos of building Fig.13 Completed building elevation The floor construction cycle took 5 days to complete and started with the placement of the precast concrete up to the sixth floor. The floor was divided into two portions and detailed work breakdown of the typical construction cycle is shown as below. The contractor spent a good deal of effort in engineering the design of a temporary bracing system for the precast members. Day 1 Installation of upper stair flights for the floor below Installation of precast bay windows with column Installation of precast facades Installation of 200 and 300 thick precast walls Installation of staircase cells & lower stair flights Day 2 Final adjustment of installed precast elements Grouting of horizontal joints Filling of vertical joints with grade 45/10 concrete Fixing of wall reinforcement and service conduits Day 3 Installation of aluminum formworks for in-situ walls Erection of falsework for precast balcony and semi-precast slab Installation of precast balcony Installation of semi-precast slabs Day 4 Installation of precast bathroom units Installation of precast kitchens Fixing of topping slab reinforcements and service conduits Day 5 Placing concrete of topping slabs and in-situ walls. 156

12 The 300 ton-meter tower crane (a common type for Hong Kong) was used to erect the precast members and placement of steel mould. Despite being the first structural precast project, the contractor reported that the erection and assembly process was smooth and that noise and pollution were minimal. The construction of the structure frame was completed within nine months and the project finished by December Conclusions The demonstration project is evidence of several promising achievements: i. As a result of better control over materials and workmanship, precast elements generally have better durability compared to cast in-place structures. Defects, such as tile fall and water leakage in windows and bathrooms, are reduced significantly. ii. Wastage of tile, reinforcement, and concrete are found to be 40%, 75% and 65% respectively less as compared with a traditional construction method. iii. On site skilled labour requirements were reduced by 30%. iv. There was a two-thirds reduction in on site construction waste. v. Cost for miscellaneous work such as make good work, rectification work, and site clearance was reduced by 90%. This innovative project paves the way for a wider use of sustainable construction practices in the local housing industry. The essential factors for this project s success include enthusiasm by the project team, a willingness to share with and learn from others, and careful planning. From a design perspective, the performance and assembly advantage associated with the development of precast concrete system is clear and promising. This project is evidence of the benefits of a true partnering approach in finding innovations that meet client, architect, engineer and contractor requirements, despite also improved construction from the view of costs, productivity, quality, safety and sustainability. Acknowledgements The authors gratefully acknowledge New World Development Group for their support to this project. The authors would also like to thank Mr. T C Chu of Hip Hing Construction Company for his visionary contribution in promoting the use of innovative construction technology for this project. References So, D.K.L, Au S.C.C., Mok A.K.H., Wong C.C.K., Tai D.M.F (2003), Partnering and Total Solution of Precast Concrete Technique in High-Rise Apartments: Green Building Proceedings, HKIE Ned M. Cleland (1997), PCI Ad Hoc Committee on Precast Walls, Design for Lateral Force Resistance with Precast Concrete Shear Walls Sidney Freedman (1999), Loadbearing Architectural Precast Concrete Wall Panels 157