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Guest Column | September 2010

Chimney Construction: Structures that Soar

In the second and concluding part of this column, Er Jagvir Goyal discusses the equipment required for the construction of the shell of an RCC chimney.


 

When the chimney shell reaches a height where its tapering is gentle and suitable for operation of slip forms, conventional forms are discontinued and slip forms are installed. Many designers prefer to keep uniform shell tapers from the ground itself, to avoid the trouble of using both types of equipment. They intend to gain the advantage of saving on conventional formwork and the time involved in construction as slip form equipment works much faster than conventional formwork. Both alternatives are carefully studied for each site by keeping the size of power units, size of flues and the allotted period of construction in mind, and accordingly, the chimney shell design is finalised. Slip form equipment is required to be operated round-the-clock for continuous concreting work. Only then does it serve its real purpose. Therefore, the engineers and workers are required to work in shifts. Availability of slip form equipment, also called climbing form equipment, has made it possible to construct chimneys with heights of more than three to four times that of the Qutab Minar, in a period of a few months.

 

The design factor

 

During the design of the chimney shell, care must be taken that its construction is done using slip form equipment. While using slip forms, the shell gets exposed to many different types of loadings within a few hours of its construction. Therefore, the low strength achieved by the shell concrete exposed to loads and its modulus of elasticity are to be kept in view. Additional loads such as that of slip form equipment, crane loads, wind loads on slip forms and decks are also to be counted. It has to be further seen that the thickness of the shell is not less than 200 mm and preferably not more than 600 mm. The quantity of concrete to be poured as per metre height of shell has also to be kept within limits, preferably not exceeding 200 cu m, to avoid extra arrangements for material and concrete transportation to the top.

 

Truss framework: Keeping the diameter of chimney in view, a truss framework is to be specially designed. Broadly, it has a central triangular frame and ring from which the radials emanate to all directions. The radials are equi-spaced. After every few radials, main trusses are provided at equal intervals, numbering six to twelve. The trusses are built up of joists, channels and angles while in between radials, also known as spider beams, are made of back-to-back channels. All the outer ends of radials and trusses are connected by distance bars.

 

Form panels: These are made of 6 mm thick MS plates, normally having a size of 1 m depth and 2 m length along the curvature of shell. Plates of this size are easy to handle. As the diameter of chimney reduces with the increase in height, these plates overlap one another at the ends. Care needs to be taken that the overlapping of form panels is vertical, not inclined. Inclined overlapping results in a horizontal drag component, further resulting in torsional effects on the slip form assembly.

 

Walers: Walers are those components of slip form equipment which stiffen the form panels and transfer the loads to yokes. These must be made strong enough to bear the lateral pressure of concrete laid inside the form panels. Walers are made of steel and are braced suitably.

 

Yokes: An yoke consists of yoke legs and yoke beams; they are primarily used to transfer the load of the forms to the hydraulic jacks. These also support working decks and prevent form panels from bulging. Yokes are the most important components of slip form equipment. These are made of steel sections. All the loads on slip form and truss assembly get transferred to the hydraulic jacks through the yokes. A yoke assembly has one outer yoke leg, one inner yoke leg, one lower yoke beam on which the hydraulic jack rests and one upper yoke beam.

 

Working decks: This part of slip form equipment enables the performance of all operations of slip form equipment. For chimney construction, two such decks are created. The upper deck receives all the materials including concrete to be poured, reinforcement steel, inserts, block- outs and any other materials. The lower deck allows the movement of engineers and workers for checking the jacks, providing force to them to lift the system, checking the parameters of chimneys, tying reinforcement bars and multiple other works.

 

Hanging scaffolds: These are the lower decks provided along the inner and outer periphery of the chimney shell, and are also called mason’s platforms. These consist of tubular steel pipes suspended from the yokes and are connected by laying timber planks bridging the tubular frames. These are used to apply final finish to the chimney shell.

 

Lifting equipment: The lifting equipment consists of a number of jacks, one jack installed on each yoke beam. Hydraulically operated jacks have a central hole in them through which the jack rod passes. These jacks work on the ball grip principle. When the hydraulic pressure is applied to the jack, the piston inside the jack gets pressed down, pulling along with it the spring provided inside the jack. The spring under pressure makes the balls of the top catcher squeeze making them grip the jack rod. The lower ball catcher gets released as soon as the upper ball catcher grips the rod. On release of hydraulic pressure, the spring gets released upwards, taking along with it the top sleeve provided inside the jack. The lower ball catcher again gets loaded, clamps the jack rod and the process continues. Therefore, one of the two ball catchers is always gripping the jack rod.

 

Jack rods: Solid jack rods are used to transfer the loads from the hydraulic jacks to the structure below. These jack rods either get embedded in concrete and are left there or are extracted on completion of slip form equipment’s work.

 

Pumping station: All the hydraulic jacks of slip form equipment are connected to a centrally placed pumping station. This pumping station is electrically run and applies oil pressure to the jacks for their upward movement, thereby lifting the whole assembly. The upward movement increments to be given to the jacks are pre-calculated. This pumping station itself provides oil pressure to the perimeter jacks also which rotate the spindles to adjust the perimeter of slip form assembly to match the theoretical requirements. A manifold outlet system is attached to the pumping station to release oil under pressure to various hydraulic circuits which include lifting jacks, inner perimeter jacks and outer perimeter jacks. In addition to the electrically operated pumping station, a petrol driven pumping station is kept as standby arrangement to continue working during power failures. The pumping station is able to operate the jacks collectively as well as individually. This helps in adjusting all lifting jacks to one level in case a jack has moved less or underperformed.

 

Extraction jack equipment: These jacks help in extraction of jack rods on completion of slip form work. These are hydraulically operated and applied on the jack rods, one by one, in sequence. Whenever the extraction jacks are to be used, sleeves must be provided around the jack rods to avoid their bonding with concrete and facilitate their easy extraction.

 

Miscellaneous equipment: Bracings, both diagonal and lateral for form panels, adjustment screws for wall thickness, radius adjustment and yoke inclination, stretching screws to counter any twist of slip form assembly, safety nets to catch the slipping men and materials and grouting pumps to grout the holes created in the shell on extraction of jack rods, are other miscellaneous equipment that form part of slip form equipment.

 

Concrete equipment

 

During the construction of the chimney shell using slip form equipment, no mass concreting work is involved. The daily requirement of concrete for the shell is worked out. It is better to create a concrete production area where the required number of concrete mixers is installed to produce the desired quantity. The required quantity of concrete keeps reducing as more height is gained. Concrete transportation arrangements are similar to those provided for construction of the shell using conventional formwork. For compaction of concrete, needle vibrators are to be provided on the working deck at five or six locations to cover the full periphery of the shell.

 

Internal platforms of chimneys: Multi-flue chimneys have internal RCC platforms provided at regular height intervals as an essential component. It is not possible for a chimney to serve its purpose unless these platforms are provided. These platforms are required to support the internal flues carrying flue gases. The internal flues are built-in, fire-resistant refractory brick masonry. It is not possible to run this masonry from the ground up to the full height of chimney as it is unable to support itself. Therefore, the provision of internal platforms is essential. In case steel liners are provided in the chimneys instead of masonry flues, the height interval of these platforms is increased.

 

Time constraint: Internal platforms of a chimney require a significant amount of time for construction. Generally, these are provided at height intervals of about 10 m. A 220 m high chimney therefore, has around 22 such platforms while a 275 m high chimney has 25-28 such platforms. In the lower parts of the chimney, the diameter is quite large. Therefore, these platforms contain heavy RCC beams, of depths in the range of 7-10 feet and width ranging from two to four feet. There are a number of such beams, inserts and openings to be provided in these platforms. Heavy quantity of concrete is required for each such platform. As the height of the chimney increases, its diameter decreases, and the size of beams and slab also decreases. Comparatively, less quantity of concrete is required for higher platforms. However, the height involved acts as a constraint here. While the bulk quantity of concrete and supporting arrangements pose a challenge in lower platforms, the height does so for higher level platforms. Another unique feature of these platforms is that these are one above the other, within the internal diameter of the chimney. Therefore, no two platforms can be taken up at a time unless some innovative steps are devised. Space constraints, too, add to the problem. Building so many platforms in a sequence, from bottom to top, thus becomes an endless activity.

 

Innovative steps: One method of cutting short the overall construction period of activity of internal platforms is to replace the RCC platforms at certain levels by steel platforms. The overall height of the chimney can be divided into five equal parts by providing steel platforms at the bottom of each part. This step opens five fronts for the construction of platforms. Now, the platform activity need not go up in sequence but work can continue on each front independently. The completion period of the platforms gets reduced to somewhere between one-fourth to one-fifth. However, the innovation demands five-fold arrangements of scaffoldings, shuttering, equipment and manpower. Also, extreme level safety measures are required as the work is going overhead for the workers deployed on the lower platforms. Any small object falling from the upper platforms gains high acceleration since such a height is involved and hits the workers at lower levels like a bullet. The writer himself has had narrow escapes while working under such conditions.

 

Scaffolding pipes and clamps: Huge quantities of scaffolding pipes and clamps are required for the construction of RCC platforms. The height interval between the platforms is around 8-10 m while the length of scaffolding pipes is generally 6 m. Sometimes, the engineers keep in view the length of staging pipes as the cutting of these pipes results in wastage. They must take care that at least below the main beams, the arrangement can be made by using a single pipe and that the balance difference of height interval is met by beam depth and adjustable joist supporters. The pipes are also required to support the platform slab. As soon as the concreting for a platform is completed, staging work for the next platform begins over it, after the lapse of final setting time of the concrete. It is always better to design the support system for slabs and beams. The designer should issue a proper drawing for this, showing the exact location of verticals. Main beams should be supported on at least two or three rows of verticals. Another very important point to be kept in view before dismantling the staging and shuttering below a platform is to check whether that platform is able to support not only its own load but also the load transferred by the staging for the next higher platform.

 

Additional scaffolding towers are required to run material and concrete hoists to the platforms. Vertical scaffolding pipes must be load-bearing and therefore, C class, heavy duty, 48-60 mm diameter pipes in true plumb. Horizontal scaffolding pipes and diagonal bracing pipes are also required, as are rigid and swivel clamps, a few lakh in number. Base plates for verticals are required to give them wider area for dispersion of load, as also end-to-end pipe couplers to extend the verticals with the increase in height of the shell. Adjustable trough pins, cross battens, wooden planks are all required, too, for laying over the verticals.

 

Friction hoists: Three to four friction hoists are required to run the concrete and material buckets to the platforms. Generally, two such hoists are put on job to transport concrete carrying buckets and one or two additional friction hoists are put on carrying steel reinforcement and other materials to the platforms if ducts or openings to run the same are available. The hoists are often 5-7 tonne capacity, single drum type, equipped with magnetic brakes and using 16 mm diameter ropes. 16 - 22 mm diameter wire ropes are additionally required as guide ropes for the concrete carrying buckets. D-shackles, U-clamps, two sheave pulleys are also required to complete the running arrangement. While the flue ducts can be used to run the concrete buckets, some more openings are to be normally left in the platforms for ventilation purposes. It is not possible to use a tower crane inside a chimney for construction of platforms as its erection and movement interferes with the platforms to be constructed. Therefore, friction hoists prove to be the best equipment for the purpose.

 

Passenger hoist: One passenger hoist is required to transport the engineers and workers to the platforms and back to the ground level. The passenger hoist should be a double drum type with magnetic brakes, overload alarm and limit switches as safety guards. 22 mm diameter wire ropes should be used to run the passenger cage. Guide ropes should keep the cage in position to avoid its extra swing or accidents.

 

Concrete mixers: The quantity of concrete required for concreting of lower level platforms is quite large while that for higher level platforms is small. Making arrangements by erecting the staging, shuttering, laying reinforcement, fixing inserts and block-outs takes more time while laying of concrete in each platform is one or two day job. If a chimney has 20 platforms, the overall time required to complete the platforms may be six months while concreting days will be just 20-25. Therefore, it is not suitable to have a batching plant installed for the concreting of platforms. A tie-up with an existing batching plant or RMC plant that could supply concrete of required mix design proves ideal. Otherwise, concrete mixers should be used to produce concrete at ground level. Six concrete mixers, three mixers feeding each concrete bucket for the lower level platforms and four mixers for the higher level platforms can and will do the job.

 

Bucket trolleys: Similar to the concreting of the shell, the best arrangement to carry the concrete filled buckets from the mixers shed to the hoist point is to lay rails and carry the buckets over trolleys running along the rails. This involves minimum effort on the part of workers who keep sliding the trolleys between the points of discharging of mixers and bucket lifting points.

 

Concrete unloading arrangements: Space constraints on the platforms make it very difficult to unload the concrete there as the concrete is to be unloaded on a temporary platform built over the platform to be concreted. Secondly, these platforms have to be near the hoisting point of concrete carrying buckets. In a way, there are no options in deciding the location of temporary platforms over which concrete has to be unloaded. A swinging type chute should be installed near the concrete bucket position. When the bucket is to be emptied, the chute is swung and adjusted below it. After the unloading of the bucket, the support below the chute is removed to hang it vertically and to give way to the bucket to travel down.

 

Concrete vibrators: Concrete vibrators are required to be stationed at each platform to fully compact the concrete laid in the beams and slab. Plenty of spare needles need to be kept available as the needles often need repair and replacement during construction. Plate or form vibrators are to be avoided as the work is carried out at very high levels and any chances of accidents due to high vibrations caused by the form vibrators in the forms and support system need to be avoided.

 

(The writer is an author of technical books; Columnist in a technical journal and Deputy Chief Engineer, (Civil).

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