Construction World - Indian Edition | April 2007 Issue

Special Report - Equipment

No Tall Order!

Want to build high structures in a hurry? Slip-form equipment makes it possible, explains Engineer JAGVIR GOYAL

With increasing pressure on construction engineers to build structures in the shortest possible time, slip-form equipment has proved to be a boon. Ideally suited for the construction of high-rise reinforced concrete structures quickly, slip-form equipment builds a structure on a continuous basis, involves operations of a highly sophisticated nature and facilitates round-the-clock concreting. Slip forms primarily consist of shuttering panels that envelop the structure along its designed shape and are then moved upwards continuously, leaving below the structure in its final form.

The advantages
The main advantage of slip-form equipment is the considerable reduction in construction time. For a vertical structure like an RCC chimney, an average progress of 3 to 4 m per day can easily be achieved with slip-form equipment - such a rate of progress would be impossible to achieve with conventional formwork or jump-form equipment. A height of as much as 8 m can be achieved on a particular day if weather conditions and other factors permit, though it may not be recommendable to achieve such high progress everyday.
Another big advantage is that the construction joints get eliminated as slip-form equipment works on a round-the-clock basis. Only in exceptional cases are interruptions planned for extraction of jack rods. When conventional forms or jump forms are used, construction joints occur after every lift of 1.2 m to 1.5 m, but with slip forms, construction joints may be planned after every 50 m. For a 220-m-high RCC chimney, while jump forms will provide 140 to 150 joints, slip forms will give you only four to five joints.

The prerequisites
The construction of high-rise structures is a high-risk operation as speed of construction is very high while using slip-form equipment. The slip-forming process therefore demands meticulous planning to control each and every aspect during actual execution. All errors and deviations are to be corrected without any loss of time. Otherwise, by the time we prepare ourselves to rectify the errors, the slip forms would have moved much above.
All design and drawings, preparatory works, data charts, checklists, materials, manpower, staff, concrete properties, tolerances to be allowed and rate of slip forming should therefore be pre-planned and implemented at site before the operation starts. A detailed step-by-step programme is to be prepared. Nothing is to be left to the imagination. Even all contingencies are to be reviewed and preparations to deal with such situations are to be completed in advance. Factors such as human fatigue are also to be kept in mind.

The components
Slip-form equipment consists of forms attached to walers that stiffen the form panels and transfer the load to the yokes. Hanging scaffolds and working deck are also connected to the walers. The waler yoke connection is designed to transfer all loads of the decks to the yokes. The yokes transfer the loads to the hydraulic jacks that further transfer the load to the jack rods on which they ride and rise. The jack rods get embedded in hardened concrete and transfer the load to the base or the foundations.
The following are the major components of slip form equipment:
Shuttering/form panels.
Working decks
Hanging scaffolds
Truss system
Hydraulic jacks
Jack rods
Pumping units for jacks
Operation screws
Extraction jacks
Miscellaneous equipment.
The components and their setting are to be varied to suit the structure to be slip formed. In some structures, where wall thickness remains uniform, wall thickness screws are not required. For vertical structures with no variation in plan, radius screws become redundant. Chimneys require a separate top deck to receive steel reinforcement, concrete and other materials and a working deck to carry out all operations. Silos need only one deck to receive materials as well as to work. Therefore, slip-form equipment has to be planned with respect to the structure to be built. The number of yokes, their spacing and the number of jacks are to be decided by keeping in view the dimensions of the structure and quantity of concrete to be poured. More and more innovations are being made by various producers of these self-climbing forms to make the use of slip-form equipment easier.

Site arrangements
The following arrangements are to be made while using slip-form equipment:
Concrete production (installation of batching plant)
Concrete transport and placement (concrete pumps, placer booms or winch bucket)
Concrete compaction
Site testing
Access for supervisory staff and labour
Uninterrupted power supply
Deck lighting
Aviation warning lights
Lightning protection
Signal and communication
Material transportation to deck (tower crane or winch bucket)
Openings and block-outs
Precision instruments for dimensional checks
Safety arrangements, safety net, warning boards, etc
Water supply.
All these arrangements are to be made in a meticulous manner. Deficiency in any one area may hamper the progress of work. In a nutshell, no chances are to be taken while using slip-form equipment.

Area of application
All vertically oriented structures that have non-varying plans (plans that remain unaltered as we go higher or that reduce gradually and uniformly with height) can be constructed using vertical slip-form equipment. The reductions in the plan area must be gradual and not drastic as slip forms are not suitable for acute slopes.
Vertical slip-form equipment is most suitable for:
Vertical and tapered tall RCC chimneys
Square or rectangular towers with uniform plans
Bridge piers
Water tanks
Storage bins
Any other shaft-type building
Columns of greater heights
TV towers
Skyscrapers with non-varying plans.

Design requirements
A structure to be built with slip-form equipment should be designed keeping in view that its construction is to be done through slip-forming as this fact will affect the design of the structure. During slip forming, the structure shall be exposed to a number of loadings within a few hours of its construction. The low strength of concrete exposed to loads and its modulus of elasticity are to be kept in view while designing structures to be built with slip-form equipment.
Further, the structures to be built with slip-form equipment should be designed keeping in view the following loads:
a)Vertical loads
1. Dead loads
2. Live loads
3. Loads during construction due to slip-form equipment and crane loads.
b) Horizontal loads
1. Wind loads on the structure
2. Horizontal loads caused by cranes
(as advised by crane supplier)
3. Wind loads on slip forms, working deck and top deck during construction.
c) Other loads
1. Stresses caused by temperature
2. Stresses caused by creep and shrinkage in concrete
3. Erection loads
4. Earthquake forces.
In addition, the analysis of all the horizontal and vertical forces should be
done keeping in view designed openings in the structure.

Dimensional requirements
The use of slip-form equipment demands the following dimensional requirements:
The wall thickness of structures to be slip- formed should not be less than 200 mm. Otherwise it may become difficult to operate the slip-form equipment smoothly. In exceptional cases only, the minimum thickness can be reduced to
150 mm. However, this figure is quite conservative and normally the designed thickness is much more than 200 mm. So this dimensional requirement causes no problem or extra expenditure.
The thickness of a wall may be uniform throughout the height of the structure or may vary gradually and uniformly.
The area to be slip formed should require
a concrete quantity of not more than
200 cu m per metre height of the structure as it may become difficult to manage large quantities of concrete.
Adjoining structures or existing structures should be at least 5 m away from the outer edge of the structure to be slip formed, as the slip-form framework may protrude over the outer edges of the structure and enough space around the structure to be slip formed is required for movement of cranes and other necessary arrangements.

Other factors
Once a structure is found suitable for slip forming by fulfilling the above technical requirements, the following factors should further be examined:
Whether the cost of execution by slip-form equipment is less than expenses to be incurred in using any other equipment.
Whether height of the structure to be slip formed is more than 50 m.
Whether saving in construction time is an essential requirement.
Whether arrangements for transportation of materials, concrete, labour and staff are available.
Whether the inflow of materials and manpower can be managed to match the speed of construction, which shall be threefold or even more than normal requirements and that too in a much shorter period.

The Indian scene
In India, the use of slip-form equipment is becoming increasingly popular. It has been found suitable for the construction of vertical structures such as single flue and multi flue, vertical and tapered chimneys, silos, bridge piers, shaft type buildings, water tanks, storage bins and even some cooling towers. Horizontal slip-form equipment has been used for tunnel inverts, drainage channels, canal linings and highway pavements.
A number of power plants, thermal or nuclear, are built in India every year. All of them require chimneys of greater heights to satisfy the minimum height norms fixed by pollution-control bodies. In India, Pollution Control Boards have prescribed a minimum height of 220 m for chimneys constructed for units up to 210 mw; while for 500 mw units, the minimum height of chimneys has been prescribed as 275 m. Construction of chimneys of such considerable height with conventional formwork seems to be an impossible task. Moreover, the use of conventional formwork for such high structures is not safe, let alone the high cost and considerable time period involved. At this juncture, slip-form equipment for vertical construction has come to the rescue.
Doka, the renowned producer of slip-form equipment, has tied up with Larsen & Toubro for handling jobs in India and Asia. Gammon, HCC, and Unitech are some other Indian companies making frequent use of slip-form equipment. India is likely to see a wider use of this wonderful equipment in days to come.

At a glance

Slip-form equipment builds a structure on a continuous basis.
It can be used to construct all vertically oriented structures that have non-varying plans.
For a vertical structure an average progress of 3 to 4 m per day can easily be achieved.
The equipment has to be planned with respect to the structure being built.
Construction joints get eliminated.
All errors and deviations are to be corrected without any loss of time.
The wall thickness of structures to be slip formed should not be less than 200 mm.
Adjoining structures or existing structures should be at least 5 m away from the outer edge of the structure to be slip formed.


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