Photo 1

In order to supply the required land for accommodating the huge population, and to provide ample infrastructure and community facilities to substantiate an acceptable standard of living, commercial operations and other necessary developments, many critical locations which may be unsuitable for development to most international yardsticks, are built with very large sized and high-rise buildings. The following situations are some of these examples. (Photo 1)

1. To build in close proximity to very steep, or sometimes quite unstable slopes.

   Photo 2

2. To build very tall building, often with deep basement, in extremely congested urban environment. (Photo 2)
3. To build in close proximity, or sometimes even within, very sensitive and congested underground facilities like the Mass Transit Railway subways, surcharged areas of building foundations, or layers of large-sized drains, gas and water pipes, and culverts etc. 
4. To build in close proximity of very large and complex traffic interchanges or busy transportation facilities.
5. To build in newly reclaimed or previous dump-filled areas.

Owing to the long period of experience working with such kind of harsh environment, practitioners in the construction field in Hong Kong have adopted their own practice to build, making full utilization of the technology and resources available locally. This paper tries to summary the local practices, common methods and choices employed in Hong Kong to construct high-rise buildings.

 2. COMMON STRUCTURAL FORMS FOR HIGH-RISE BUILDINGS IN HONG KONG  
Structural forms adopted to construct high-rise buildings in Hong Kong are in fact quite limited owing to some special local reasons, such as:

1. Regulations governing the land control, or planning and design of various kinds of buildings,
2. scale of development,
3. design and construction practices due to long years of tradition and custom,
4. efficient use of local labour and contractor expertise,
5. marketing trends to fulfil the needs of building consumers and the maximization of profit of the developers etc.,
6. design fashions and sales strategies.

As a summary, the popular structural forms employed for the construction of high-rise buildings in Hong Kong can be highlighted as follow:

1. In-situ Reinforced Concrete Frame - this is the most popularly used system in Hong Kong. Usual spans range from 4m to 10m depending on design and use of buildings. Recently, spans above 20m can also be seen but majority of which are tensioned in order to minimize the size of beams. Floor slab supported by beams is a major part of the horizontal stiffening member in framed structure. However, flat slab structure, often post-tensioned, is growing in popularity especially in commercial buildings for the benefit of providing a clear ceiling void to accommodate services.
2. Load Bearing or Shear Wall Structure - load bearing walls in this case are used to replace columns. The use of beams is often reduced due to the avoidance of complicated formwork detail between the junction of beams and walls. Usual spans range from 4m to 8m. Due to the limitation in the layout arrangement confined by the load bearing walls, buildings using this structural form are commonly limited to residential or apartment buildings. Panel type or large-sized shutter forms for walls and table forms for slab are often used for the highly repetitive nature of the building. However, detailing arrangement between walls and slabs, beams or staircases still imposes certain complexity to the design and erection of formwork.
3. Insitu RC Core Wall with RC External Frame - this structural form is used mainly in commercial buildings. The core wall which accommodates the lift shafts, staircases, toilet facilities and other building services provisions, is usually square or rectangular in section in order to make the forming process more efficient. Thickness of the wall ranges from 0.6m to 2.0m depending on height of building and loading requirements. Due to the lack of complicated architectural feature and highly repetitive nature, large-sized panel shutter forms, sometimes mechanically self-lifting, are used in the forming of the core structure. The external frame, in majority, is formed by the use of more traditional manually operated panel-type formwork, constructed in the same phase or separated phases with the core wall structure. Span ranging from 10m to 12m is most common.

   Photo 3

4. Insitu RC Core Wall with Structural Steel External Frame - similar to the above form but with the external frame constructed in structural steel. Almost without exception, the core wall in this case is constructed using self-lifting formwork system in an advanced phase, with the connection of the steel frame follows. Record of 3 days per floor cycle can be achieved with floor area up to 2000 sq m. Effective spans range from 12m to 15m. However, the incorporation of an anchor frame in the core wall, especially in floors where bracing members or the out-rigger frames are located, often complicates the construction process and retards the overall progress. (Photo 3)

   Photo 4

5. Mega-Structure using pure Structural Steel Frame - this kind of structure is not too common for it is not rigid enough to take up strong wind load under typhoon situation which occurs in Hong Kong during summer period. To strengthen the structure, often very complicated stiffening members in the forms of transfer trusses, sectional floor plates, out-riggers or heavy-sectioned bracing members, are required to add into the steel frame. This makes the design, fabrication, handling and erection become very difficult. However, certain forms of composite design such as the use of composite floor with reinforced concrete topping or concrete filled columns, are often used to increase the efficacy of the building. Recent high-rise examples in Hong Kong are the 70-storey China Bank Building and the 80-storey "The Center", which were completed in 1990 and 1998 respectively. (Photo 4)

   Photo 5

6. Semi-fabricated Structure using Precast Concrete Components - it is not too practical for high-rise buildings to be constructed using totally precast methods for the relative flexible in nature especially under typhoon situations in Hong Kong. However, as a means of industrialization to minimize the intensive use of expensive labours, semi-fabricated structure using a certain number of precast concrete components are growing its popularity. In this case, the main structural members, such as for the core walls, columns and main beams, are cast insitu, often making use of some kinds of patented metal forms. While the secondary members like the flight of stairs, secondary beams, short span tie beams, slabs (or semi-slabs) and external facdes etc., are constructed using precast methods. In order to improve the rigidity of the joints, most precast elements are placed in position with build-in link bars and cast at the same time together with the main elements. Post-tensioning is sometimes employed to increase the overall performance of the structure. (Photo 5)

 3. FOUNDATION SYSTEMS AND METHODS  
In addition to the usual high-rise and heavy nature, buildings in Hong Kong are facing very severe typhoon environment which imposes very complication loading effect to the foundations of buildings. Wind speed above 200km/hr is not uncommon during the period of tropical typhoon. Furthermore, some other unique features such as the existence of shallow-laying hard rock over the territory, subsoil with large amount of boulders of volcanicous nature, or work sites are very close to developed areas with sensitive and congested underground or above-ground structure, or requiring to work under extremely tight schedule with complicated phasing arrangements, are quite usual under Hong Kong's experience. 

The following foundation methods are thus adopted after long years of practice and proven to be quite effective under local situations.

1. Steel H-pile - standard universal sections are used as pile with load taking up by end-bearing and skin friction. Driving operation and equipment requirement is relatively simple except that the noise and vibration so created is highly restricted especially under urban environment. In case of boulders exist, pre-drilling can be used before the insertion of the pile. This method is economical and effective for use in buildings sometimes up to 30 storeys or above.
2. Precast concrete pile - precast pile can be of square or circular sections. A prestressed hollow-section circular pile, modulated to 10m or 12m in length, is becoming more common in Hong Kong due to reasons of cost, convenience and reliability. However, problems such as occasion failures of concrete during the driving process, smoothness of pile surface reducing skin friction, as well as noise and vibration, are the major drawbacks.

   Photo 6

3. Mini-pile or Pipe-pile - by the use of compact drilling machines, steel pipes of size 150mm to 250mm are inserted into ground and grouted as pile. Various loading requirements can be obtained by controlling the numbers of piles used, or by additional reinforcing bars inserted into the pipe section before grouting. Due to the small diameter of pile, drilling can be done fairly easily and cause limited disturbance to neighborhood. This kind of foundation is suitable for use in congested areas with restricted working space or headroom. Besides, the pile can be tensioned and provide very good resistance to overturning due to wind load.
4. In-situ Concrete Pile, medium sized - generally refers to pile sizes ranging from 300mm to 900mm. The use of drilling rig of appropriate capacity does the forming of the bore conveniently. Drilling process can be facilitated by the use of steel casing or bentonite drilling fluid. Due to the rapid development in a wide range of highly effective mechanical drilling equipment, this kind of foundation choices is becoming quite popular in the construction of medium to high-rise buildings in Hong Kong.

   Photo 7

5. Large Diameter Concrete Bore Pile - boring process can be done manually or mechanically. In general, pile sizes ranging from 1m to 3m can be formed by the use of mechanical-dug methods. While pile of sizes 3m and above, manual-dug methods are to be employed. However, starting from the beginning of 1998, manual-dug method has been banned due to high accident rate in work, unless special approval is obtained satisfying certain safety requirements. Mechanical means for the forming of the bore can be achieved by grab-and-chisel methods or reverse circulation drilling methods. (Photo 6) Both methods require the use of steel casing as a means to stabilize the bore during excavation. Sometimes, super large-sized pile up to 6m to 8m can be constructed. In this case a cofferdam formed by sheet piles, soldier piles, in-situ concrete piles or diaphragm wall is to be provided for soil retaining purpose. (Photo 7)

Photo 7a

 4. BASEMENT AND SUBSTRUCTURE  
In addition to the extra building area obtained by the provision of a deep basement, substructure of this type can provide very good buoyancy effect to relieve the dead load of superstructure and to counter-balance the uplifting effect due to wind load acting on the building surfaces.

In Hong Kong, large basements can cover a site of more than 20,000 sq m in size, and 20m down into ground. A recent project case, with features briefly summarized below, can best illustrate the extremely complex construction environments that such a job may face.

One major element to facilitate the excavation and construction of basement is the provision of a cut-off wall. There are a wide variety of choices for cut-off walling design depending on the scale and depth of basement, period of work, neighborhood environment, mechanical equipment available, methods to construct the basement or cost planning requirements etc. Below is some common cut-off walling systems being used in Hong Kong.

1. steel sheet pile wall - most efficient for excavation up to 8m to 10m deep. However, complicated horizontal support in the form of strut or bracing frame may be required which restrict onward excavating operation. It is not suitable for use in areas with large amount of scattered boulders.

   Photo 8

2. Soldier pile - similar to the principle of sheet pile wall but H-piles are inserted into ground at intervals with lagging structures to seal up soil surfaces between the piles. This is particularly suitable in areas with boulders for pre-drilling can be carried out in a fairly convenient manner. The H-piles can also be inserted into concrete bored pile to produce a 2-stage retaining design. (Photo 8)
3. Insitu concrete pile wall - concrete bored piles ranging from 0.9m to 1.5m are often used. The piles can be arranged in secant, contiguous or at spaced interval. Effective retaining depth can be up to 12m or above and is suitable for use in more sensitive ground due to basically vibration-free drilling operations.
4. Pipe-pile wall - similar to in-situ concrete pile wall but smaller pipe piles are used. This system is most effective for use in very delicate environment where disturbance can be kept to minimum, or for site which is small in size where large-sized machines are inconvenient to operate.

   Photo 9

5. Diaphragm wall - panels of trench wall are first formed by grab and chisel or by semi-automatic trench cutting machines. Wall thickness ranges form 0.9m to 1.2m usually. The forming of the diaphragm wall unavoidably requires significant amount of plant facilities and may not be economical for job of smaller scale. However, for larger site and deeper retaining requirement, the cost effectiveness increases due to the elimination of too complicated shoring supports during the onward excavation process. In addition, the diaphragm wall can be used as permanent wall to save up the fixing of formwork within confined space inside the excavated pit. (Photo 9)

In addition to the use of an appropriate cut-off wall to facilitate the excavation, grouting is often introduced as a means of subsoil strengthening and to improve water-tightness of ground. Besides, as the excavation proceeds, horizontal support using strut frame or other shoring and bracing systems are to be installed in order to counteract the lateral pressure due to the newly exposed cut. Sometimes when situation allows, ground anchors can be used as lateral support to eliminate the strut system in order to gain more working space inside the work pit. Finally, appropriate dewatering provisions to suit specific geological or neighbouring environment should be incorporated to keep the pit safe and free from the entering of ground water.

Construction of basement can be done by traditional bottom-up method, or on the contrary, using the top-down method. The sequence of works for the two methods can be summarized as follow:

Photo 10

Using Bottom-up method (Photo 10)

1. Construct the cut-off wall.
2. Start excavation within the basement parameter.
3. Erect lateral support, layer by layer, as excavation proceeds until the required depth is reached.
4. When reaching the formation level, construct the foundation rafts, pile caps or ground beams.
5. Construct the basement slab and other internal structure starting from the lowest basement level. The works usually done in carefully scheduled sections to avoid the disturbance of the strut members.
6. Repeat the basement works until it reaches the ground level.
7. Release and dismantle the strut members at suitable stages as the basement structure is completed.

Photo 11

Using Top-down method (Photo 11)

1. Construct the appropriate cut-off walling system, usually, diaphragm wall is used for this method is more effective in handling project of larger scale.
2. Erect temporary columns, usually at the same position of the permanent columns and in the form of steel stanchions, as support to the basement structure that is construct from the top level downward.
3. Construct the first basement floor slab, usually starts from the ground level, which will be used also as the horizontal support to the cut-off wall as excavation proceeds.
4. Excavate downward and construct the second level of basement slab similarly. Erect intermediate temporary shoring support where required.
5. Repeat the excavation and basement slab construction until the required depth is reached.
6. Construct the foundation caps, rafts, ground beams or sub-soil drains as required.
7. Construct other internal structure where required.
8. Encase the temporary columns in concrete to transfer them into permanent columns.

The employing of bottom-up method has the benefit of requiring fewer plant facilities to operate and thus it is more suitable for use in smaller site where the depth of cut is relatively shallow. However, disadvantages will appear as the size of site getting bigger, basement deeper, or project under tighter schedule. Under this situation, very complicated horizontal support is to be erect. This will increase the working time and cost of the project, and at the same time limited the working space within the basement pit. Besides, the superstructure can only be commenced until the completion of the basement structure, unlike employing the top-down method that the superstructure and the basement can be carried out at the same time making use of the first basement slab as a separating plate. Hence, basement projects of larger scale at recent years are, almost without exception, all constructed using top-down method.

However, no matter which method is used to construct the basement, some very fundamental considerations such as the detail arrangement for the erection or later dismantling of the temporary support; sub-division of the basement structure to workable sections and the provision of construction joints; the co-relationship of the phased sections to cope with the access of labour, plant and materials; arrangement to remove spoil; or how basement work is to be merged to match with the overall progress of the superstructure, are some very critical factors in the planning and execution of a successful basement project.

 5. CONSTRUCTION OF SUPERSTRUCTURE  
The construction of the superstructure involved 3 major activities, that is, the provision and erection of a suitable formwork system for work, steel fixing and the placing of concrete. And of course, an efficient site layout with the required plant facilities properly set-up is a "Must" condition.

Photo 12

There are a wide variety of formwork choices suitable for use in high-rise construction. However, under Hong Kong practice, the more traditional manually operated timber form is still the most frequently used system due to its flexibility to meet difficult shapes, non-standard layout requirement and less demand on other plant facilities or logistic supports. (Photo 12) Though obvious drawbacks can easily be observed, such as more labour intensive, slower speed in work, environmental unfriendly or unsatisfactory concrete quality may be resulted, it is still readily received in many projects of smaller scale for its simplicity and independence in the planning and coordination of works.

For projects of larger scale with tighter schedule and higher performance requirements, the following formwork systems are often employed:

1. Large-sized panel shutter, often in mild steel or coated timber/metal combination.
2. Manual operable alumimium panel forms, various options to use as wall or slab formwork.
   (Photo 13)
3. Table form and flying form for slab construction. (Photo 14)
4. Self-lifting formwork system such as the slip-form, jump-form and climb-form, often under special patented design. (Photo 15)
5. Other patented formwork systems such as the SGB, RMD or PERI systems etc.

Photo 13	Photo 16
Photo 14	Photo 15

The practice in fabricating and fixing of steel reinforcement in Hong Kong is still very traditional. The current way, in majority, is to cut and bend the steel bars on site, transport the bars to the work spots by the use of tower crane, then fix the bars into position. The use of prefabricated components in workshop and transport to spot for erection is uncommon. This can be explained by the usual non-standardization in design, using of slim structural elements and congested reinforcement condition in order to minimize the size of structure. (Photo 16)

Concrete with strength ranging from 20 to 35 N/mm2 (Grade 20 to 35) is used most commonly in construction due to easier in performance control under the built environment of Hong Kong where supervision and quality of labour standard is not consistent. However, in order to produce higher and slimmer structure, as well as to reduce dead weight of building, grade 40 or even up to Grade 60 concrete is occasionally used in some projects. Typical examples of use are for core wall, transfer structure, and other large-span or tensioned structures etc. Most of the concrete used are supplied by specialist suppliers and delivered to site using concrete mixer trucks.

Photo 17

The placing of concrete for high-rise building is mainly by the help of mechanical equipment such as using booster pumps, placing boom; or using skip, hopper or bucket lifted by tower crane or hoist rack etc. The placing process may still be quite labour intensive for horizontal movement of concrete on the floor slab level, as well as to render satisfactory placing and compaction result within the congested placing environment, greater amount of human labours is still unavoidable. (Photo 17)

In term of speed of work, fast track schedule is almost a "Must" under most contract requirements. In general, for a building of about 800 to 1000 m2 in size and with the arrangement and set-up as mentioned, it should be of no difficult to complete a typical floor cycle within 5 to 6 days' time. For building of larger size and with more difficult layout, progress may be up to 10 days or above for a typical floor.

Other planning options can also be made such as to sub-divide the floor area into two convenient phases, or to cast the wall first and the floor/beam follows, or even work with staggered floor arrangement, so as to obtain the best result in scheduling and efficient use of resources to suit the layout and other design constraints.

 6. CONCLUSION  
The construction of high-rise building is a very broad subject by itself. The author tries to make use of a few pages to discuss on this subject matter can only highlight a few of the local features. Besides the application of mere technology in construction, the overall structure and common practices in the local industry, in fact, form the most powerful drive to shape the actual outcomes.

In recent years, due to the drift from sellers' market to buyers' market after the economical crisis occurred after 1997, there is a very great tendency for developers to produce really high quality buildings to satisfy the genuine need of the public. Hence, some forward-looking developers begin to invest more in investigating possibility and options to employ advanced technology in order to make building more reliable and better perform. At the same time, thanks to the change in attitude by the government to become more answerable and accountable to public, tighter control in the building development process is adopted. As a result, smaller firms which operate under traditional managerial ideology with limited capital and resource can hardly survive. Construction firms of the new generation are more ready to establish a work organization with stronger professional capability, more proactive to input more resources in research and development and to restructure themselves by setting up more efficient administration system in the running of projects. The gradual change in this direction is obvious and it is expected that a modernization process will eventually lift the overall professional standard of the Hong Kong construction industry from the backbone within this decade or so.