Er. Jagvir Goyal trains his thoughts on structural steel work equipment which is much in demand.
Though RCC and pre-stressed concrete have become the unanimous rulers of the construction world, steel structures are equally vital. 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 much in demand, along with good fabricators, welders and erection gangs.
Thermal power projects make extensive use of steel structures. So does the 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 for 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 the site and time is saved through parallel activities.
Quality of fabrication and erection
The life of steel structures depends on the quality of steel used, its treatment and the quality of fabrication work. At the project site, 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 the other major items involved. Brackets, gusset plates, battens, lacing, stiffeners and braces are smaller but essential components required for steel work. 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.
Structural steel equipment
The equipment required for fabrication and erection of steel structures includes gantries, welding transformers, cutting machines, winches, grinders, drills, trailers, ovens, and welder’s equipment. The quantity and type of each type of equipment varies from project to project.
Mobile hoist cranes having 5-10 tonne capacity are required in the stores and fabrication yard for loading, unloading and shifting of material. Large-sized 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.
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 m to 20 m 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 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.
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.
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 the 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 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, the thickness of rust gets counted as paint thickness.
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 some spare bits in stock at the site.
Gas cutting sets
Though gas cutting is kept to a 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.
The welding sets are mostly of a manual type. Automatic welding machines are also added at locations where continuous welding work is to be done. 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.
The welder’s equipment 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.
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 of the 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 the site. Portable electric ovens should be of the single- phase type with at least a twin tray provision so that a welder has sufficient number of electrodes in stock for accomplishing his job efficiently.
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 are 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. D-shackles and U-clamps are required wherever a material lifting arrangement is to be made. Also, a punching machine is required to punch- mark the fabricated components.
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 used. A number of such jacks are fitted around the fabricated structure to apply pressures and bring it to true shape or alignment.
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 tested from a reputed laboratory. They should be issued identity cards as per their capability. The 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.
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 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 minimising 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, thus minimising steel wastage.