Though RCC and pre-stressed concrete have become the unanimous rulers of construction world, steel structures keep finding their requirement and importance now and then. Many where, we find steel structures as the best option to choose. In many cases, the framework is chosen to be of structural steel while the filler and finishing items such as floors and walls are in masonry and concrete. That’s why structural steel work equipment is very much in demand along with good fabricators, welders and erection gangs.
Steel structures: Thermal power projects make extensive use of steel structures. So does railways. All the turbine, generator and boiler supporting structures in thermal plants are in structural steel and while the walls, floors, partitions and other such components are in concrete. To make speedy completion, a number of steel columns having heavy steel sections are erected on concrete foundations by embedding the anchor bolts in these foundations and bolting the column bases to them. While the foundations are completed, columns, beams and girder sections are fabricated at site and time is saved through parallel activities. Use of structural steel can thus be enlisted in following works:
1. Power Projects
2. Railway works
3. Roof trusses
4. Bridges
5. Steel chimneys for industries
6. Steel tanks
7. Multi Storey buildings
8. Site workshops
9. Bunkers and Silos
10. Industrial buildings.
11. Penstocks
Quality of fabrication and erection: Life of steel structures depends on the quality of steel used, its treatment and the quality of fabrication work. At the projects, the quantity of steel work in fabrication and erection is in the range of thousands of tonnes. While the major tonnage belongs to built up girders and columns, stairs, railings and gratings etc are other major items involved. Brackets, gusset plates, battens, lacing, stiffeners and braces are smaller but essential components required for steelwork. The welders have to be highly skilled, the electrodes used should be of radiographic quality and the equipment used for fabrication and erection should be safe and modern.
How to proceed with fabrication and erection work: At a fabrication work site, follow the following steps in sequence for systematic working—
1. Preparation of fabrication drawings.
2. Working out section wise steel quantities.
3. Procurement of structural steel.
4. Preparation of cutting plans.
5. Arrangement of fabrication equipment.
6. Arrangement of consumables such as electrodes and gas.
7. Recruitment of welders, their testing and issuing of identity cards for horizontal, vertical and overhead welding work.
8. Making site arrangements such as fabrication yard, positioning of winches, installation of gantry cranes.
9. Straightening and cutting of structural steel as per cutting plans.
10. Edge preparation and grinding of cut material.
11. Fabrication work with identification number of a welder punched near each weld.
12. Radiographic, ultrasonic and dye penetration testing of welding work.
13. Primer coat painting work.
14. Shifting of final fabricated members to erection area.
15. Making hoisting arrangements by fixing lifting points.
16. Erection of fabricated members strictly as per erection sequence.
17. Site welding and bolting work.
18. Erection work completion, checking and passing.
19. Final grinding and finishing work, if any.
20. Painting work.
Structural steel equipment: The equipment required for fabrication and erection of steel structures includes gantries, welding transformers, cutting machines, winches, grinders, drills, trailers, ovens, welder’s equipment. The quantity and type of each type of equipment varies from project to project. Some of the major equipment is as under:
Hoist cranes: Mobile hoist cranes having 5 to 10 tonne capacity are required in stores and fabrication yard for loading, unloading and shifting of material. Large size heavy plates are often used for plate girders or for bending them in shape of large sized pipes. These plates often need to be loaded and unloaded by these cranes. In addition, fabricated components are also handled by the mobile cranes. While the large sized and heavy material is shifted by using gantry cranes, small sized materials are shifted through these mobile cranes. In addition, these are useful in loading the fabricated components on the trailers and their unloading in the erection area for further use.
Gantries: During the fabrication of steel structures, availability of gantry cranes in the fabrication yard is essential and it facilitates to a great extent the shifting of steel materials and fabricated components from one place to the other. Large sized, 8 mm to 25 mm thick steel plates can’t be shifted manually. Movable cranes if used need to have sufficient boom lengths and lifting capacities. Here, gantry cranes with their span decided by keeping the area of fabrication yard in view, prove most useful. The span could be anywhere between 8 metres to 20 metres for optimum use of a gantry crane. Capacity of gantry cranes with large spans is kept as per requirement and in the range of 50 tonnes or so.
Electric winches: Electric winches are most suitable for erection of fabricated components. Single drum electric winches with 16 mm thick standard quality wire ropes, equipped with magnetic brakes for use in case of electricity failure and having 5 to 10 tonne capacity mostly do the job well. Depending upon the spread of the area of erection, electric winches can be located in winch sheds planned strategically for full coverage of erection area. A winch shed may have 3 to 4 winches and 3 to 4 such sheds can be planned. Structural steelwork for a power project may require 15 to 20 electric winches located in 5 to 6 sheds. The erection and lifting points for different winches can be created by using sheaved pulleys at different distances from the winches.
Hand winches: Often, the provision of guide ropes with electric winches may not be found suitable due to space constraints. Sometimes, it is not possible to create the required pulley system at the top point for main wire rope as well as guide ropes. In some cases, a single guide rope can be accommodated. In all such circumstances, hand winches prove very useful in controlling the movement of large sized fabricated structural steel members suspended in the air by the main winches during their erection. Manila rope can be used on hand winches instead of wire ropes to guide the steel components being erected or to control their sidewise sway. Hand winches can be of just 1 to 2 tonne capacity as these are not taking any load but only guiding the components. Their number can also be restricted to 3 to 4 as these can be easily moved from one place to other.
Long Trailers: Long Trailers are required to be ever available in fabrication yard for shifting of fabricated material to the erection area. These also fetch the raw material like plates, joists, channels and angles from the storage area to the fabrication yard. The trailers have a capacity of 12 t to 30 tonnes.
Pug cutting machines: These machines are required to cut the steel plates to required lengths and widths. The size of cutting nozzles is kept strictly as per the table provided with the pug cutting machine. These prove very useful in accurate and right angled cutting of plates and other members. Normally, these machines are quite light in weight and can do straight line or circular or curved cuttings. Both, Acetylene gas or LPG can be used in them. However, for each gas, selection of appropriate nozzle is necessary. Plates of even 300 mm thickness can be cut by these simple machines. The tracks supplied with them are generally in Aluminium and extendable. These machines can cut 100 mm to 800 mm of plate length per minute. Some of the producers are Weldstar, Kolkata, Atlas, Mumbai, Thomax, Bharat Udyog, Metasafe in India.
Grinders: Grinders are required to grind and finish the steel sections where required. These help in taking the rust off and particularly prove useful in best fitting of stiffeners in plate girders, smooth seating and fitting of fabricated components with one another by grinding their edges. Non removal of rust destroys the painting work. Actual thickness of paint film is given by the Alcometer only if the rust is removed by using a grinder. Otherwise, thickness of rust gets counted as paint thickness.
Drilling machines: The fabricated sections need to be provided with smoothly finished bolt holes of accurate diameters for making bolt joints. Drilling machines are required to drill these holes. Simple and magnetic drilling machines are usable. Drilling bits of different diameters are also required for use in drilling machines. It is wise to keep in stock some spare bits at site.
Gas cutting sets: Though gas cutting is kept to minimum at fabrication and erection sites due to the irregular and rough cutting done by the gas cutters, there are certain points inaccessible to pug cutting machines and electric cutters. Besides, certain enabling works also demand gas cutting sets for quick arrangements. Sometimes, holes are to be drawn or some cutting work is found necessary after erecting a large sized fabricated component. Here also, gas cutters come handy. However, there use in cutting work should be minimized if smooth finishing and fine fabrication work are to be achieved.
Welding sets: Scores of welding sets are required for fabrication and erection of steel structures. These are used at different locations all over the site of work. The welding sets are mostly manual type. Automatic welding machines are also added at locations where continuous welding work is to be done. These automatically control the fusion of the two components being welded. Equipped with automatic welding heads, these machines run on a track and are mounted on the steel components to be welded to do the required welding. Manually operated welding sets are generator or transformer type. The transformer type welding machines are cheaper in cost and use Alternating Current (AC) for their running. The generator type machines use Direct Current (DC) and are suitable for welding sheets or thin plates and components. For such plates and sheets, it is necessary to use low currents of 30 ampere range for proper welding. Such low current availability is possible in DC welding equipment.
Welding accessories: A range of welding accessories is required during fabrication and erection of steel structures. These include the cables required to connect the welding sets to the electrode holders. These cables are chosen to be heat, fire and oil resistant and are categorized as HOFR cables. Their sizes depend upon the current to be flown through them. Cables are always preferred to be single core and rubber sheathed. Wherever the length of cable is to be increased, it should be done by using cable connectors to join two lengths instead of making local joints. Copper conductor welding cables are preferred. The electrode holders are fully insulated type. Partially insulated electrodes are also available in the market but should be discouraged. The cable enters the electrode holder from the rear while the electrode is held by the holder through spring action. Earthing wire holders are required to connect the earthing leads from the welding equipment to the welder’s seats.
Welder’s equipment: It includes the gloves that all welders are supposed to wear, the aprons for protecting their clothes from heat and sparks, the shields required to protect the welder’s eyes, the chipping hammer to chip off the slag from the welding joints, the wire brush to clean the welded portion after removal of slag and the tongs to handle hot pieces. The welding shields are either helmet mounted head shields or hand held or eye shields. The shield screens are to be fitted with expensive dark colored glasses.
Electrode ovens: When ever the ambient temperature falls below 25 degree C, welding electrodes should be stored in ovens. The temperature in the ovens should be maintained at 110 degree C. The electrodes should always be saved from ingress of moisture by them from the atmosphere. Care has therefore to be taken in winters. The ovens can be stationary or portable type. A stationary oven has multiple trays to store the electrodes at required temperature. The portable ovens can be carried by the welder with him and conveniently kept at site. Portable electric ovens should be single phase type with at least twin tray provision so that a welder has sufficient number of electrodes in stock for accomplishing his job efficiently. A big job of fabrication and erection may require 3 to 4 stationary ovens and 8 to 12 portable ovens.
Tools and tackles: As required at all sites, these include wire ropes, pulleys, punching machines, D shackles, U clamps etc. A large number of two sheave or single sheave pulleys is required to make erection arrangements as per site conditions. Wire ropes of 22 mm and 16 mm are required for lifting of beams and for guide ropes. The wire ropes are always to be free of splices or broken strands. No risk should be taken in this regard. D-shackles and U-clamps are required wherever a material lifting arrangement is to be made. Punching machine is required to punch mark the fabricated components. Steel section lots created by cutting the full lengths to sizes as per cutting plans can also be punch marked for identification by keeping a separate identity code for them.
Hydraulic jacks: Hydraulic jacks of different capacities are sometimes required to straighten the plates and other members and to drive out the minor kinks and bends in them. For bend test of test pieces also, these jacks are made use of. A number of such jacks are fitted around the fabricated structure to apply pressures and bring it to true shape or alignment.
Skilled welders hold the key: All the equipment arranged for fabrication work loses its meaning if the welders employed for fabrication and erection work are not skilled. The welders should be assigned jobs only after testing their capability. IS 817 lays down the procedure for the testing of welders. They should be tested for different positions i.e. flat, horizontal, vertical and overhead welding. The test jobs prepared by the welders under different positions should be got tested from a reputed laboratory. They should be issued identity cards as per their capability. Most difficult job is overhead welding. A welder skilled in overhead welding can do all kinds of welding jobs. They should be assigned work as per their tested capability only for best fabrication and erection results.
A word about the welding electrodes: The welding electrodes are of many types, each type suitable for a specific purpose. There are general purpose electrodes, low hydrogen electrodes, submerged arc welding electrodes, low alloy electrodes and many other types. For best welding results, only radiographic quality electrodes should be used. The date of manufacturing should always be checked while buying or using these electrodes. The date of manufacturing should be within 6 months of their use. The size of lot being purchased should therefore be decided carefully, depending upon the monthly consumption of electrodes. Further, only a reputed and ISI marked make should be chosen. ESAB, Advani Oerlikon’s Overcord S, D & H, IOL (Indian Oxygen Limited), ESAB, Philips are some of the good brands of electrodes. All electrode boxes should be purchased in sealed form only. The electrodes are produced in different sizes such as 2.5 mm, 3.15 mm, 4.0 mm, 5.0 mm, 6.3 mm and so on. 3.15 mm and 4.0 mm are the most common sizes of electrodes used at sites. The size of electrode to be used depends on the size of weld to be provided. Poor quality electrodes if used eliminate the advantage of using high strength steel.
Saving steel through cutting plans: It is always wise to work out all the required cut-sizes of plates, angles, channels, joists, flats and other members from the fabrication drawings before proceeding with the fabrication work. A list of all such sizes with numbers required for each size should be prepared. These should now be compared with the available sizes of plates and lengths of other members. Keeping the two charts in view, cutting plans of plates and other members should be prepared. This exercise helps in minimizing the wastage of steel. Once the cutting plans are ready, actual cutting and identification marking of cut sizes can proceed. A lot of saving in steel is made through properly prepared cutting plans by minimizing steel wastage.
