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INTRODUCTION

With the arrival of much awaited infrastructure boom in India, ‘time’ has become the most precious element in maximizing success and profits. He, who can deliver in shortest possible time, is the winner. Funds are no more a constraint. The government and the clients are willing to pay, provided the projects assigned to the firms are completed and commissioned in time. Of course, accuracy in construction is equally desired as the structures that are completed in time but don’t behave properly are of no use.

For high rise structures, fast moving slip form equipment and jump form equipment have become an essential requirement. Conventional shuttering methods are no more in use as these sacrifice accuracy and consume extra time. Some structures can be conveniently raised by use of slip form equipment while on others, it is not possible to use slip forms and jump form equipment has to be deployed.

JUMP FORM EQUIPMENT

Latest Jump forms are a climbing equipment that take support for climbing from the structure itself and no scaffolding pipe network is required to support them from the ground. Nor any layers of brackets are required as used in conventional formwork. Access towers, one or two in number as required, are raised to facilitate concrete transportation to the top and to allow upward passage to the workers. A small passenger hoist can also be accommodated in these scaffolding towers. Anchor bolts are provided in the walls of the structure, removed and re-fixed as the equipment rides up. Vertical Rails are fixed all around the perimeter of the structure and the equipment keeps riding up on these rails. Fixing up of these rails is a continuous process and has to be ahead of the upward movement of the equipment. The task seems to be simple but needs extra vigil for no problems or bottlenecks.

DECK SYSTEM

In general, there is a five deck system in jump formwork in comparison to three deck system in slip forms. The top deck facilitates concrete placement and reinforcement binding work. The second deck is the main working deck for fixing shutters, anchor bolts, form supports and sliding shutters. The work of alignment to the designed profile of the structure, checking of parameters such as height, radius, curvature and thickness is also carried out from this deck. The third deck allows rail extension for upward movement of equipment. Fourth deck is for operation of hydraulic jacks and to ensure that the equipment rigs are well locked in position. The fifth and last deck provides platforms for smooth finishing and painting of the structure. This five tier system is provided both along the inside as well as outside surfaces of the structure.

STRUCTURAL FRAMEWORK

Depending upon the peripheral area of the structure to be built, the required number of rig units can be worked out, keeping the centre to centre spacing of units ranging from three metres to eight metres. Each unit has to consist of a structural formwork required to support the five deck system, the shuttering formwork and the hydraulic circuit. The structural framework units hang on to the rail tracks by means of cradles. Rail tracks are fastened to the structure being built by use of anchor bolts. Cradles ride up along the rails as and when the hydraulic circuit is operated, taking along with them the whole of structural framework.

Each structural framework carries two separate hydraulic jacks that can be operated independently or jointly. In one stroke, the equipment rides up by 30 cm or say one feet. Each stroke hardly takes three to four minutes to operate. Power packs to operate the hydraulic jacks are stationed on the lower most deck. The number of jacks to be operated by a power pack depend upon its capacity. However, it is feasible to operate 18 to 20 jacks through one power pack.

VERSATILITY

Jump form equipment can be used on structures with uniform as well as variable dimensions. The dimensions can increase or decrease as the structure is built and the jump formwork has the facility for adjustment to suit the required parameters. Telescopic railings are provided on each deck to accommodate the reduction or increase in perimeter. Similarly, the deck planks can also be adjusted. Top portion of the equipment is kept in traveling form so that the concrete-pouring deck could be adjusted and locked in a position near the periphery of the structure. Diagonally installed spindles can adjust the formwork to suit the actual profile of the structure.

SPEED IN CONSTRUCTION

Jump form equipment can work with a fast speed. The daily output depends upon the dimensions of the structure, quantity of concrete to be poured and the tonnage of reinforcement bars. To have an idea of the speed of construction, let us take example of a natural drought cooling tower to be constructed with jump form equipment.

For such a structure having diameter as large as 90 metres, Jump form equipment can provide an initial progress of 0.75 metre height per day and this may ultimately be increased to 1.5 metre per day. For ideal utilization of jump formwork, the following chart should lay an ideal schedule:

Sr. Activity Time period (days)
1. Shell Construction 0—4 metres 35
2. Erection of jump forms equipment 28
3. Shell height 4—25 m 30
4. Shell height 25—35 m 15
5. Shell height 35—60 m 15
6. Shell height 60—100 m 30
7. Shell height 100—120 m 12

Overall time for 120 m high cooling tower shell = 165 days
Say 5 ½ months.

This schedule is easily achievable if required material and manpower have been arranged at site and work is not allowed to suffer due to shortage of these. Thus a cooling tower shell that usually takes about 8 months for construction with conventional forms can be raised in 5 ½ months including the period of erection if optimum use of jump formwork is ensured.

POURING OF CONCRETE

The load of the formwork, the live load and all working loads get transferred to concrete that is 3 to 5 days old. Therefore the calculations are made carefully to work out the load carrying capacity of the anchor bolts and their number is kept safe considering that the load on each bolt does not exceed the calculated limits. In the initial stages, the quantity of concrete to be poured is more, falling in the range of 150 cum. to 200 cum because of larger diameter and thickness of shell. With the increase in height of shell, these parameters decrease gradually and quantity of concrete also reduces to 60-70 cum. Keeping in view the concrete transportation capacity of hoists (when a tower crane is not used), ambient temperature and area of shell to be covered, each of the initial few lifts can be split into two halves. After five or six lifts, the concrete quantity falls within manageable limits and a complete lift can be poured every day.

While constructing one such cooling tower with jump form equipment, concrete was being poured by running one concrete carrying trolley (capacity 0.25 cum) from each of the two feeding points. Most of the shell structure was being built by concreting half lift per day and each half required 10 to 15 hours for pouring the concrete. To save time, the number of concrete carrying trolleys was doubled and accordingly four concrete mixers were used instead of two mixers used earlier. The concreting time for half a lift was reduced to just four hours. By increasing concrete placement capacity to double, the concreting time was cut short to one third. A close study at site of work revealed that this arrangement had a positive effect on the efficiency of workers also, thus causing a significant reduction in time. This resulted in construction of most of the shell by pouring one lift a day.

ARRANGEMENTS AT SITE

Whenever the work progresses with jump form equipment in a speedy manner, immaculate planning has to be done, arrangements to be made are to be identified to microscopic level and stringent safety rules are to be applied. Serious attention needs to be paid to the safety factor. One fatal accident may lower the morale of the workers and the whole purpose of deploying a new equipment to achieve faster progress may get defeated. There should never be any compromise on safety issues and arrangements made should be repeatedly checked to keep them fool-proof.

The arrangements include concrete mixing, transportation, placing and compaction arrangement, reinforcement bar transportation and binding arrangement, curing arrangements, signal and communication system, lighting system and illumination arrangement, aviation warning lighting system, protection from lightening and earthing arrangement, access-ladder system, passenger hoist arrangement, parameter checking arrangements, painting arrangements, hydraulic operation arrangement, surface finishing arrangement and quality control arrangements as major items while a few more minor arrangements are to be made as per site conditions. Engineering staff is to be controlled through duty rosters by assigning specific duties to each and every engineer. Manpower gangs for different activities are to be worked out in a precise manner. A time cycle to cover the start and finish of each activity has to be framed for a smooth and round the clock working.

MAINTAINING STRUCTURE PROFILE

For each structure, certain dimensional data needs to be worked out from the drawings and kept handy to perform on-spot checks. This data shall vary with each job. Again taking example of most typical structure like a cooling tower which has a hyperbolic profile that is most cumbersome to maintain at site, the following shell parameters shall have to be worked out for each lift to maintain complete accuracy:-

1) Top level of inner face of each lift with respect to 0.00 level.

2) Radius of inner face of each lift at the top of forms.

3) Slope of each lift to the horizontal.

4) Thickness of shell at the top of each lift.

5) Vertical and circumferential reinforcement details for each lift.

6) Offset of each shuttering plate with respect to the chord for each rig unit.


Maintaining required offsets at each shuttering plate will be extremely important to provide true curvature to the shell. If the offsets are not corrected, the shell will look like a polygon and not circular in plan. Though the offset checking procedure will demand sincere efforts and devotion on the part of site engineers yet a look at the final product will tell them that their effort was worth trying.

7) At site of work, Radial lines will have to be established at ground level all along the circumference in such a manner that the central joint of each set of main inner shuttering plates is exactly above the corresponding radial line. The sequence of checking will include checking the top level of inner face of forms, working out the corresponding inner radius of the shell, adjusting each unit to the required slope, checking inner radius of each unit and adjusting it to the required figure without disturbing the slope of the unit, checking the thickness of shell at the top of each form panel, checking the offsets for each shuttering plate before release of concrete for placement.

INDIAN SCENE

Though Indian Construction Industry is highly labour intensive, it is time now to induct more of machinery and equipment for better progress, quality and expeditious completion of various projects. Jump form equipment helps in speedy construction of high-rise structures provided awareness is generated among engineers for better safety precautions and against mishandling of sensitive machinery and instruments.

A number of jump form equipments such as Doka, Heitcon, Bierrum and Strief are being used in India for construction of high-rise structures. Yet full potential of jump form equipment is not being realized and utilized. Its more frequent use will only bring economy, speed and accuracy in high rise construction, thus saving a lot of time and money of construction industry.
(concluded)





 
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Ensure that the ground around the basement area does not slope towards the basement. Ensure that the back filling in foundations has been compacted very well.
 

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