1. INTRODUCTION
Formwork systems are one of the key factors that govern the success of a construction project in terms of speed, quality, cost and safety of works.
Nowadays, most projects are required by the client to complete at the shortest time possible as a means to minimize cost of capital. For buildings of high-rise nature, the most effective way to expedite works is to achieve a very short floor cycle, that is, to have the structure of a typical floor completed in the shortest time. This can be done in Hong Kong within 3 to 4 days' time even for buildings with floor area up to 2,500 sq m. The key to achieve this, again, from the production point of views, is by the use of a set of efficient, appropriately designed formwork. Modern buildings in many occasions can be very complex, either in terms of scale or size of the building, no matter they are high-rise or horizontally spread, or in terms of architectural or structural design, or to fit for sophisticated building services or other facilities requirements. The design and use of the right formwork system, as well as the stipulation of an effective resource planning strategy to control and maximize the use of the formwork, are again crucial to the overall success of a project.
Aiming purely at speed often contradicts the achievement of other quality targets. Problems such as misalignment, misplacement, deflective concrete, or striking up of other works causing serious interruption to related activities can be resulted.
This paper, supplemented with several recent projects in Hong Kong, highlights the conditions and constraints in the application of formworks, and illustrate the practices and methods that local industry are used to construct complex buildings of various kinds.
2. CLASSIFICATION OF FORMWORK
Formwork can be classified according to a variety of ways, such as from the differences in sizes, from the location of use, materials of construction, nature of operation, or simply by the brand name of the products.
2.1 Classification according to sizes
Classification according to the sizes of formwork can be very straightforward. In reality, there are only two sizes for formwork, that is, small-sized and large-sized. Any size which is designed for the operation by workers manually is small size in nature. Very often, the erection process of formwork is preferably to be handled by a single worker only, for site work is best to be done independently to avoid possible waiting time. Due to the reasons of size and weight, the materials and construction of small-sized formwork are thus limited. At present, the commonest systems are made of timber and aluminium, and usually in the form of small panels.
Practically, there is no medium-sized formwork. In case of large-sized formwork is used, the size of the form can be designed as large as practical to reduce the number of jointing and to minimize the number of lift. The stiffness required by large-sized formwork can be taken care of by the introduction of more stiffening components such as studs and soldier. The increase in the weight of the formwork panels is insignificant for crane will be used in most cases.
2.2 Classification according to the location of use
Different elements in the structure of building have different design and performance requirements in the use of formwork. A number of formwork systems are particularly designed for constructing internal or external walls, vertical shafts, columns, beams and floor slabs. However, there is not much effective formwork system for stairs and staircases. The complicated 3-dimensional nature of the element which involved a number of suspended panels and riser boards, as well as the need to cope with very different spatial and dimensional variances as required by individual design situation, can hardly be achieved by an universally adaptable formwork system (Photo 1).
2.3 Classification according to materials of construction
Materials used for formwork are traditionally quite limited due to the difficult balance between cost and performance. Timber in general is still the most popular formwork material for its relative low initial cost and adaptability (Photo 2). Steel in the form of either hot-rolled or cold-formed sections and in combination with other sheeting materials, is another popular choice of formwork materials (Photo 3). In recent 2 to 3 years, full aluminium formwork system has been used in some cases but the performance is still being questioned by many of the users, especially in the concern of cost and labour control (Photo 4). Other type of metals including their alloys, are still uncommon in construction site, due to their cost and easy substitution by other common metals.
However, due to the huge amount of tropical wood being consumed each year as formwork purposes resulted to criticism from environmentalists, as well as the continual escalating in the price of wood products, there is a strong tendency to use other formwork materials or systems to replace timber. At the present moment in Hong Kong, steel form in general seems to get its popularity, especially with the incorporation of semi-fabricated construction techniques under design and built arrangement (Photo 5).
2.4 Classification according to nature of operation
Formwork can be operated manually, or by other power-lifted methods. Some systems are equipped with a certain degree of mobility to ease the erection and striking processes, or allow horizontal moment, using rollers, rails or tracks.
Timber and aluminium forms are the only manual-operateable type of formworks. They are so designed and constructed in a way that they can be completely work independently without the help of any lifting appliances. However, it is labour intensive, and is more appropriate to be used in simpler job; or to the extreme, used in very large-sized and horizontal-spread buildings with complicated layout design, to gain the benefit of flexibility.
Power-lifted formworks can be of self-climbing type and crane-lifted type. Self-climbing type can be done by the use of built-in hydraulic (Photo 6) or screw jack systems (Photo 7), under full-form or sectioned arrangement. Since the lifting power of the jacks is enormous, therefore, a supporting gantry system for the erection of the formwork panels, as well as an enclosed scaffold system with inner and outer work platforms, are usually provided in the system to form a self-sufficed work station for the casting operation.
Crane-lifted type is usually in the form of large panels (sometimes called the gang form). It is fabricated either in steel sections and sheeting, or using plywood sheeting and stiffened by metal studs and soldiers. These large panels can be stood on solid slab or fixed on brackets in case they are used for external walls or shafts (Photo 8).
Other forms of operation include the gantry-type, traveling or tunnel form. They are more suitable for use in long repeated sections such as for railway stations, terminal buildings or other large-sized horizontal-spread structures. Examples in Hong Kong in the recent years can be found in the construction of the 600m-long elevated expressway for the Lantau Link (Photo 9), the Ground Transportation Centre of the Airport Railway (Photo 10), and the structure of the Shui Hong Station of the West Rail projects (Photo 11).
2.5 Classification according to brand name of the product
Several patented or branded formwork systems have been successfully penetrated into the local construction market in the past decade. Brand names such as SGB, RMD, VSL, MIVAN, Thyssen, or Cantilever. Each of these names has its own specialized products, some can even provide a very wide range of services including design support or tender estimating advises. As the use of innovative methods to construct is gaining more attention by various sectors in the community, advanced formwork systems are obvious outlets in this respect. The inputs in research and development by the well-established formwork manufacturers are of no doubt contributing their effort in these areas.
3. TECHNICAL CONSIDERATIONS WHEN USING FORMWORK
The selection and application of formwork is not a single-issued matters, a number of attributing factors have to be considered before a selected formwork system can successfully fulfill its performance targets. The followings are the major factors to be considered when using formwork, particularly in large-scaled and complex projects.
3.1 Design related factors
3.1.1 The shape of the building
Simple block-shaped buildings are of no doubt much easier to construct than buildings in awkward shape such as with curved, inclined, stepped, undefined or sculptured features. As a general rule, awkwardly shaped buildings can be much easier to deal with using more traditional, labour- intensive formwork systems for their better adaptability nature.
3.1.2 Design of external wall
Some buildings may have a lot of architectural features on the building exterior such as in the forms of fins or ribs, shading blades, planter box, deep rebate windows, hoods for air-conditioner units or alike. This may limit the choice of using system-type formwork due to the provision or interruption of the features during the casting process.
3.1.3 Internal layout
Some buildings may have very simple layout with a few in-situ walls and framed with regularly located columns like most of the commercial and office buildings. While some are having very complicated internal walls, most are of load bearing nature, that make the casting process very difficult.
3.1.4 Structural forms
Bearing certain similarity to internal layout, the structural form of buildings will also affect the options of formwork. For example, buildings with a structural core in the form of a vertical shaft will limit the use of other formwork systems other than those of self-climbing nature. Buildings in flat slab design, similarly, leaving table form or flying form as the most optimistic choice of formwork. For buildings in regularly arranged shear wall design, the natural selection will be large-panel type steel forms or other type of gang forms.
3.1.5 Consistence in building dimensions
The design of some buildings may involve a lot of non-standardized dimensions to cope with the architectural design and layout; or to fulfill other structural requirements like the regular reduction of sizes for beams, columns and walls in particular for those high-rise buildings as the structure ascends. Some formwork systems, like the climb form or steel form, may be quite difficult to use in such situation, for the frequent adjustments of the form to meet the change in dimensions may eventually incur extra cost and time when amendments are to be carried out during the course of construction.
3.1.6 Headroom
Higher headroom will obviously increase the amount of falsework, and create additional problems in accessibility and safety; and make the erection of formwork, ensuring formwork stability as well as in the placing of concrete more difficult. Working headroom of more than 5m is frequently encountered in buildings such as for the construction of the transfer structures (Photo 12), entrance foyers (Photo 13), or interior spaces inside shopping malls, and many other functional, institutional and public buildings.
3.1.7 Building span
Working with large building span usually encounters the same problems similar to that of high headroom. In addition, long span structure in general will have larger sections in beam size, heavier reinforcing situations, or with the introduction of post-tensioned works (Photo 14). This will further complicate the formwork design and erection process.
3.1.8 Repetitive nature
High-rise block-shaped structures usually create highly repetitive cycle and this is a favour to the working of formwork. However, for some horizontal-spread buildings of very large construction area, or for most underground structures such as basements, the level of repetitiveness will be limited and the use of formwork as an expensive resource, will become very critical.
3.1.9 Finishes of the surfaces
It is well-understood that fair-face concrete demands very high quality formwork in terms of surface treatment of the panels, tightness of the formwork joints and in dimensional accuracy. Requirements are a bit relaxed where concrete surface will later be finished.
3.2 Construction related factors
3.2.1 Complexity of the built environment
Situations like exceptionally small or large site (Photo 15), sloped (photo 16) or very crowded site (Photo 17), working in close proximity to sensitive structures, site with other major activities working at the same time (Photo 18), or site with lots of physical or contractual restrictions, will increase difficulties in working with formwork. There is no specific solution to improve the situation and problems are to be tackled according to individual circumstances.
3.2.2 Speed of work
When working with buildings of large construction area and horizontal-spread nature, project can be expedited by the introduction of more sets of formwork in order to create more independent work fronts. This will, of course, increase the cost of production. For high-rise buildings, merely by the increase of formwork input cannot often solve the question of speed, for the critical path still depends on the floor cycle. However, a properly selected, designed and arranged formwork system will for sure increase the efficacy of work for each typical cycle. In some cases, adding in of half or one full set of formwork, especially for floor form, may help to expedite the cycle for the additional set of formwork can provide more flexibility when consider striking the form at an earlier time.
3.2.3 Number of possible reuse
The number of reuse for traditional timber formwork will usually be limited due to the durability of the plywood sheeting. The optimum reuses of timber form is usually around 12 to 14 times, and thus, it is still economical enough to use timber formwork for high-rise buildings of 25 to 30-storey (or its multiple) in height. Though the reusability of metal form can be greatly increased, the high initial cost of providing the form will often discourage the choice especially when there is no need for large number of reusing times in low-rise buildings. Careful balance between cost, speed, performance and quality of outputs should be properly maintained when making the selection.
3.2.4 Construction planning and arrangement
Constructing planning such as the phasing or sectioning arrangement, integration of the structure, site-layout and setting up arrangement, or the provisions of hoisting and concrete placing facilities etc., are influencing factors when considering matters related to formwork selection and application.
3.2.5 Area or volume of cast per pour
The optimum volume of cast per pour will be different for different types of formwork used, for particular elements of structure to be placed, for specific scale of work, or for different level of provisions of facilities. Usually volume of concrete ranging from 60 to 200 cu metres per pour can be comfortably handled in most site environment. At the same time, whether the volume of concrete will be placed for the vertical elements at a time only, or to include also the beams and slab to save up an additional phase to the overall work arrangement per cycle, should be considered in advance when planning for the formwork.
3.2.6 Involvement of other construction technique
The applications of tensioning and prefabrication techniques are often involved in the construction of high-rise buildings. This may create certain impact to the use of formwork especially where precast elements are to be incorporated during the casting process (Photo 19). The additional provisions of temporary supports, or slot space and boxing out positions in the formwork for the precast elements, or extra working spaces for the placing of stressing tendons and the onward jacking, should be allowed in such cases.
3.2.7 Dependence of work
Whether there will be lifting appliances provided for the erection of formwork, or will those appliances be able to access into the work spot to assist in the operation as the structural works proceed, or will any special equipment be required for the striking of the forms, how can the removed formwork panels be transported to other spot to continue work? These are factors that should be considered before the selection of a right formwork system.
3.2.8 Provision of construction joint
Sometimes it will be inevitable to introduce a large number of construction joints in a large structure in order to subdivide the works into effectively workable sizes. The provision of construction joint can deficient the output and affect the quality of the concrete (Photo 20). Careful selection have to be made to ensure a particular formwork system can satisfactorily cater for such arrangement.
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3.2.9 Accessibility to work
During the course of construction, no matter it is high-rise or horizontal-spread structures, accessibility problems may be created due to the segregation, temporary discontinuation, or blocking of the layout by the partially completed building (Photo 21). Or in cases like the construction of a shaft-type core wall in an advanced manner, access can hardly be provided for the shaft will be independently stood for a long period of time before connecting into the horizontal elements. This should be avoided or specially arranged for when carrying out the construction planning.
3.2.10 Feasibility of introducing alternative design
Under traditional way of design and construction detached procurement system, designers often design a building that is incapable of using more advanced and efficient formwork system to build. It is quite common nowadays for builder to submit alternative design proposal for client to consider with minor structural or architectural amendments so that more effective formwork methods can be applied. Examples such as to minimize the number of beams or to rearrange the alignments of the shear walls, such that table form or semi-slab method can be used in the construction. Very often, the cost reduction will be shared between the builder and the client in order to achieve a win-win situation.
4. EXAMPLES OF APPLICATION
To make the explanation more concrete, the speaker wish to present a number of recent construction cases in Hong Kong to illustrate the use of some formwork systems that are of special significance. These project cases include:
4.1 Festival Walk - using traditional manual-type timber formwork
This project locates in a 21,000 sq m site. The development is used as shopping mall and leisure centre, composing of 4-level basement and 7-level upper structure. Features in design and construction consist:
- A 48m-span skating rink constructed of 8 numbers of cast-in-situ and post-tensioned beams, supported on the sides by bearers (Photo 22).
- Other large span structures up to 32m in length, cast-in-situ, some are post-tensioned.
- 3 atrium spaces, averaged 35m in span, and with 25m headroom (Photo 23).
- Average headroom for each floor is about 4.5m.
- Basement is constructed using top-down approach.
Due to the specific function of use in the building design, the 160,000 sq m interior spaces are practically non-repeated in the layout that makes the application of system formwork not feasible. As a result, traditional way of formwork method using manual-type timber panels is employed throughout the project (Photo 24). Since the overall building areas are extremely large, the structure is thus subdivided into 6 main phases with further sub-sectioning in each phase. The main phases are constructed in a progressive and staggered manner, each with a lapse of about 1 to 2 months' time. A great number of construction jointing have to be provided during the construction, that make the casting, coordination and quality assurance process fairly difficult (Photo 25).
4.2 Belcher's Garden - using traditional manual-type timber form
The development situates on a sloped site with area around 24,000 sq m. A 10-level podium, used as carpark, shopping mall and recreational facilities, is built on top of the sloped site in order to provide a terrace to seat the residential towers. The upper and lower level of the slope has a difference of about 65m. Six 48-storey residential towers are built on top of the podium, providing about 2200 residential units each of about 80 sq m in size. To allow the podium be able to construct on the formed slope, complicated falsework are to be erected, that create a lot of elevated work positions and significantly retarded the progress of work. The huge size of the podium together with the sophisticated typographic environment of site, demands very complicated phasing and sectioning arrangements (Photo 26).
For the superstructure, due to the irregularity in the layout and the incorporation of a lot of architectural features in the external envelops, as well as the use of large amount of short-span slabs and shear walls in the structure, the use of timber panel form, operated manually, is again being adopted (Photo 27). In fact, as most structural forms for high-rise residential buildings are quite typical in the design, the Belcher's can be regarded as typical representation of similar works in Hong Kong.
4.3 Lee Garden Hotel Redevelopment - Climb form for core, composite slab and structural steel outer frame
This is a 50-storey office building in the form of composite structure with the inner core constructed in reinforced concrete and the outer frame in structural steel. The RC core is constructed using the climb form of VSL, which is a self-lifting formwork system making use of hydraulic jack (Photo 28). The panel shutters that formed the walls are operated on rail to allow open-and-shut action during erecting and striking process. The shutters and rail tracks, together with the scaffold systems, are hung onto a set of steel gantry frame which further articulate to the jacks that make the entire system lifts (Photo 29).
4.4 Cheung Kong Center - Jump form system for core, composite slab and concrete-filled steel tube as the outer frame
Cheung Kong Center is a 62-storey office building in the form of a composite structure similar to the Lee Garden Hotel Redevelopment project but in a bigger scale. Instead of using the climb form, this project employed a jump form system patented under the product name of "Cantilever" (Photo 30). Besides using the form to construct the core wall in an advanced phase with necessary provisions of starter bars for connection to the future composite floor slab, the other difficult work required in the formwork is to allow for the placing in of 3 sets of anchor frame inside the core for the connection of the 550-tons out-rigger frame at 3 prescribed levels (Photo 31).
4.5 Gateway - Climb form for core and table form for slab
There are 3 detached building towers in the project each of 38-storey high and used as office or service apartment purposes. The structure of the towers comprises a central core with 12m span RC columns around to form the outer envelop, and a flat slab system, post-tensioned, used as floor plate and horizontal restraint. The core wall is constructed using the VSL climb form, columns in gang forms faced with plastic-coated sheeting, and the slab is cast using an aluminium-strutted flying form system (Photo 32). The slab is cast in two separated sections in a staggered manner with a lapse of 2 to 3 storeys (Photo 33). This arrangement can have the flexibility of gaining one more work front so that the floor area can be split into easy-handle smaller portions with better access to facilitate the tensioning works (Photo 34). The drawback is that the integrity of the structure is broken and quite a number of construction jointing provisions are thus required in the structure.
4.6 Waterfront - Steel panel form for shear walls and table form for slab
This is a 72-storey high residential development with 3 attached towers on a 6,500 sq m site. The structure of the buildings consists of a series of shear walls to form the compartment units, centered with a core structure. A full steel form system is used for all the walls (Photo 35). While for the slabs, an aluminium-strutted flying form system is used for the majority areas except for a small portion at the rear serving as kitchen and minor lobby, at which traditional timber/plywood formwork is used. The central cores are constructed in the form of a vertical shaft. The inner structures such as the slab for the lift lobby, the lift walls and landings for the stair, are cast-in-situ at a later phase (Photo 36). The stair flights are prefabricated and erected inside the pre-arranged position inside the core.
The construction concept looks quite typical, and similar methods have been broadly used in the construction of a large amount of public houses in Hong Kong. This project is in fact one of the pioneers of this kind used in private developments, especially when the complicated external shape of the building is concerned. The project is not run under design and built contract, yet certain design alternatives have been introduced. Under the relative conservative culture in the construction industry of Hong Kong, this may serve as a prototype in the application of more innovative technology in building construction in future for the private sector.
4.7 Park Avenue - Aluminium form
The project consists the construction of four 46-storey high residential towers (Photo 37). full aluminium formwork known as the MIVAN system is used in the construction (Photo 38). Typical to similar design for residential buildings, the structure consist of a large number of shear wall forming the external walls, staircases and lift walls, as well as majority of the party walls between apartment units. Certain number of beams serving as tie elements, external architectural features such as planter boxes and air-conditioner hoods, are also incorporated in the design.
The first five storeys of typical floor take an average of 15 days each to complete. It takes about 8.5 days per cycle to complete the remaining storeys. This is much too slow when compare to similar construction which can be done in 5 to 6 days' time. The problems seem to come from the large amount of walls to be formed, complicated layout and other architectural features, as well as the inconsistence in the sizes of the major elements. As a result, labour input is unexpectedly high and causes significant delay in works.
5. CONCLUSION
There is no simple, ready-available solution for most formwork situations for complex buildings, especially when the construction is to be done in a fast-track manner, like most of the projects in Hong Kong.
The local industries have for long lack the motivation to introduce highly innovative methods to construct due to a lot of understandable reasons. Short-sightedness in putting money in research and development both by the developers and the contractors, extremely severe competitive environment through the lowest-bid arrangement, very high labour costs, or relative conservative culture within the related professions and industries, may be some of the reasons, if not used as the excuses.
The good old days have indeed passed away. This may not be too bad a thing for at least it gives a very strong motivation to changes. Hong Kong in this respect is improving. As mentioned before, the use of formwork in construction contributes a critical part in the technological improvement process, in this connection, the pace of change is rather slow and the coverage is still not board enough as a whole. Sometimes, very fundamental matters as straight-forward as the training and attitude of the work team from the management down to individual labours, sense of loyalty and belonging, housekeeping issues on site (Photo 39), or safety and quality consciousness, in fact share the same driving impact with the introduction of advanced technology alone. In this respect, Hong Kong is still having quite a long journal to improve.
6. REFERENCE
| 6.1 | M K HURD, Formwork for Concrete, American Concrete Institute, 1995, 6th edition |
| 6.2 | A W IRWIN & W I SIBBALD, Falsework - Handbook of Design and Practice, London, Granada, 1983. |
