Contract documents always include information about the quality of the finished concrete and the time needed to finish the project. A description about how this information can affect the selection of the formwork system is provided below.
Category CONCRETE FORMWORK
A building’s structural layout is either uniform ‘‘modular’’ or irregular. Uniform ‘‘modular’’ design is characterized by regular spacing between columns and walls, equal story heights, and regular spacing of cantilevers and openings. Irregular design is characterized by irregular positions of the different structural elements and broken lines or irregular curves in architectural plans.
This system is a combination of the framed-shear wall (type III structure) which acts as an interior tube, and the framed tube (type IV structure), which acts as an exterior tube. The floor structure ties the interior and exterior tubes together to allow them to act as a unit to resist lateral loads.
Table 9.1 indicates the optimum height of each lateral support system. It should be noted that Table 9.1 serves only as a rough guide for determining the suitable lateral supporting system for each building height. Judgment, experience, and the personal pref
erence of the designer are all important factors in choosing the proper lateral support system.
This system consists of a combination of frames which utilize beams and columns with the shear wall designed to resist lateral loads.
Type IV Structure (Framed Tube)
This tube is a structural system in which the perimeter of the building, consisting of vertical, closely spaced supports, connected by beams or bracing members, acts as a giant vertical internally stiffened tube.
Rigid frame systems consist of rectangles of vertical columns and horizontal beams connected together in the same plane. It should be noted that a bearing wall is a special case of the rigid frame system.
Type II Structure (Shear Walls)
The shear wall formation is a thin slender beam cantilevered vertically to resist lateral forces. It can take the form of a rectangle or box-shaped core, which can be used to gather vertical transportation and energy distribution systems (e. g., stairs, elevators, toilets, mechanical shafts).
Rigid Frame System (a)
Shear Walls (b>
Framed Tuba (d)
Structural system Optimum height
Rigid frame Shear wall Framed shear wall Framed tube Tube in tube
Buildings are classified as being tall when their height is between two and three times as great as their breadth. For example, a building with a minimum dimension of 50 ft (15.2 m) in plan, and a height of 100 ft (30.5 m) or more is considered a high-rise building.
One of the major characteristics of high-rise building design is the need to resist the lateral forces due to winds, earthquakes, and any other horizontal forces. As a result special structural elements must be provided to resist lateral forces and prevent or minimize the building sway. In the following sections, a brief description of lateral load resisting systems, along with their corresponding height limitations are presented in Figure 9.1 and Table 9.1.
Vertical supporting structures such as buildings’ cores in high-rise construction, towers, or silos are typically a critical activity that control the pace of the project progress. Consequently, when selecting a core forming system, time is a critical factor. Duration that must be considered are the movement of the form from floor to floor, original assembly, time to set rebar and inserts within the form, stripping time, close-in time, and final disassembly.
Other factors to consider in formwork selection are the amount of labor required to strip, set, pour, and control the system; the amount of precision needed as far as plumbness and corner tolerances, ease of lifting, and the designer’s intent when developing the structural system. Other methods substitute the manual labor with valuable crane time. The decision between labor or crane time requires careful financial analysis. As explained in Chapter 8, crane-independent systems have been developed that
do not require crane time and require considerably less labor than other systems. However, these systems are generally proprietary and require a significant investment.
Precision requirements make some systems better than others. On huge towers, cores must remain very plumb due to elevator tolerance requirements. The formwork must have a method of remaining plumb and level. If the formwork is moved piece by piece, each piece must be checked for being plumb and level, which leads to a gross amount of field engineering. Most systems become increasingly more difficult to keep plumb and level as wear and tear loosens corners and the form deteriorates in general. Also, wind loads at higher elevations tend to deform the system.
The architect usually doesn’t design a building with any particular form system in mind. In some cases, such as slipforming, the design must reflect the method of forming. However, it is usually assumed the building will be conventionally formed.
Many other factors that affect the selection of vertical formwork systems for buildings are similar to those factors affecting the selection of horizontal formwork systems. However, there are some factors that are particularly important to the selection of vertical formwork systems. Among these are:
1. Factors related to building architectural and structural design, including lateral supporting system(s) and building shape and size
2. Factors related to project (job) specification, including concrete appearance and speed of construction
3. Factors related to local conditions, including area practices, weather conditions, and site characteristics
4. Factors related to the supporting organizations, including available capital, hoisting equipment, home-office support, and availability of local or regional yard supporting facilities
An overview of all the factors affecting the selection of vertical formwork systems is shown in Figure 5.1 that included the factors
that are critical to selection of both horizontal and vertical formwork systems. The following sections are focused on those factors that are related solely to the selection of vertical formwork systems.
9.1 Factors Affecting the Selection of Vertical Formwork System
Choosing the Proper Formwork System Using the Comparison Tables
A large portion of the cost of the structural frame is the cost of formwork for columns and walls. Typically, the selection of a formwork system is made by a senior member of the contractor’s organization or by a consultant from a formwork company. The decision is heavily based on that individual’s experience. This chapter presents a criteria for selecting vertical formwork system for buildings. The chapter is concluded by presenting a table to assist the contractor in selecting the appropriate vertical formwork system for walls and columns.
With the advancement of technology in the construction field, limitations of a system or technique are often reduced. The result is increased efficiency in costs and time, versatility, quality, and
safety. Every formwork system, including the self-raising forms, has its limitations and range of uses.
• Members of the manufacturer’s staff experienced in operating the forms are needed for several weeks for field service and to train workers, which may introduce added costs.
• Site must be fairly accessible since forms are preassembled and may be large in size.
• An accurate wall footprint must be built as a starter wall using conventional wall forming.
• The initial cost of the self-raising system is much larger than other formwork systems.
• The formwork system is economically used only for structures 20 stories or higher.
Changes in wall size and/or location during construction are expensive and impact schedule.
Safety on the construction site plays an important role in the productivity of the project. It should begin in the planning and management of a project continuing on down to the equipment and the workers. The self-raising forms have a number of safety features.
• The self-raising forms are preassembled in a factory-type setting, which provides predictable strengths and quality assurance of the system.
• The working platforms on the formwork system include guard rails and toe boards, thus improving safety and productivity at great heights.
• A safety factor of 3 is usually used when designing hydraulic rams.
• Forms not dismantled between raises result in fewer injuries from stripping.
• The raising of the forms are completely controlled by workers at the form, instead of crane operators and crane attendants a distance away. Therefore, self-raising forms can be stopped almost immediately in case of emergency.