All You Want to Know About Welding And Structural Steelworks - lceted

In the construction of a building, we come across a number of items of steelworks. As welding is important in steel construction, we will first briefly deal with welding. We will also consider steel trusses and steel window grills that are commonly used in buildings.

 

WELDING AND INSPECTION OF WELDING OF STEELWORKS

 

There are three principal forms of welding of steel:

(a) Electric arc welding

 

(b) Oxyacetylene gas welding

 

(c) Flash butt welding

 

The first method uses an electric arc while the second uses an oxyacetylene flame for welding, and the third is carried out only in factories by special machines using electricity. For structural work, electric arc welding is nowadays invariably used. Oxyacetylene flame is used for cutting steel (gas cutting) and also for repair or other work involving small thicknesses of metal, which may burn off with an electric arc.

 

ELECTRIC ARC WELDING

There are many forms of arc welding but the most commonly used type is the Manual Metal Arc (MMA) welding. In this method, the metal is deposited manually by the operator who controls the direction of the weld and the build-up of the weld metal. The flux surrounding the core wire when melted by the heat of the arc gives gaseous protection for the molten metal from atmospheric contamination and controls the weld metal reaction. The temperature of the electric arc is of the order of 10,000°C.

 

Welding electrodes. The electrodes used for manual arc welding must comply with IS 814 (1991), the specification for "covered electrodes for manual metal arc welding of carbon and carbon-manganese steel". These should be stored properly in dry places. These electrodes consist of a core wire covered on its outside with a flux. The end that is to be connected to the electrode holder has usually no flux coating.

 

The size of the electrode to be used depends on the type of weld (fillet or butt), whether the work is to be done in one or more passes and also the position of welding as to whether it is flat, horizontal, vertical or overhead. The electrodes shall be of proper sizes to ensure thorough fusion and penetration, but, in any case, generally, it is to be not more than 4 mm (5/32 inch) in diameter.

 

Welding Transformer. The welding transformer should give the necessary current and voltage specified for the job.

 

Types of the weld. The two types of welds most commonly used are discussed below.

 

1. Butt welds. They are called single V, double V, single U, double U, single J., single bevel, or double bevel depending on the shape of the cut of joining surfaces

 

Butt welds: 1. Single V, 2. Double V, 3. Single U, 4. Single J.

 

2. Fillet welds. According to the welding position, fillet welds can be flat, horizontal, vertical or overhead.

 

Fillet welding: 1. Sides, diagonal and end weld, 2. Return weld (sides and end weld), 3. Single lap weld, 4. Double lap weld. (The overlaps as a rule should not be less than five times the thickness of the thinner part.)

 

Specifications of Butt Weld

The specifications of butt welds are briefly discussed below.

(a) Size of a butt weld is its effective throat thickness.

 

(b) Single V, single U, single J and single bevel butt welds are called incomplete penetration butt welds (also called unsealed single butt welds), where the weld metal is not deposited through the full thickness of the joint. Welds like double V welds are full penetration butt welds (the effective throat thickness of incomplete penetration butt weld is taken as 5/8th the thickness of the thinner part joined.)

 

(c) When structural members of unequal thicknesses are joined by butt welding and the difference in thickness is more than 25 per cent of the thinner part or 3 mm, it should be specially treated by tapering the thicker part or building the weld attached to the thinner part at the junction to 25 per cent greater than the thinner part or equal to the thicker part.

 

Reinforcement in butt welds. Generally, a surface convexity of 1.0 mm (but not to exceed 1.3 mm) is provided to ensure the full strength of the weld. If a flush surface is required, this reinforcement may be removed.

 

Specification for Fillet Welds

Fillet welds are of triangular cross sections. The standard weld is at 45 degrees (30 to 60 degrees are also used for special fillet welds).

 

The fillet weld can be of three types:

(a) Side fillet weld

(b) End fillet weld

(c) Diagonal fillet weld

 

The size of a fillet weld is measured as the minimum 'leg length' (1.414 times the effective throat for a 45° fillet). The leg length of a fillet weld is the distance from the root to the toe of the weld measured along the fusion face. The minimum size of a single-run fillet weld is to be as follows:

 

(a) Thickness of thicker part of welded members up to 9.5 mm = 3 mm

(b) Thickness of thicker part of welded members up to 19.0 mm = 5 mm

(c) Thickness of thicker part of welded members up to 32 to 50 mm = 8 mm

 

The effective lap length of a weld measured along the weld is taken as the actual length minus twice the size of the weld, as the specified size and throat thickness may not exist at the ends. This should not be less than four times the size of the weld.

 

When the welds are 'returned, it should be carried around the corners not less than twice the size of the weld. This is especially applicable to side and top fillet welds in tension.

 

INSPECTION OF WELDING OF STRUCTURAL STEELWORKS

The primary concerns of inspection of welds are to see that

(a) The welds produced are fully in accordance with the sizes and shapes shown in the drawings,

(b) There are no defects in the welding process and

(c) There are no defects in the finished welds.

 

Firstly, the steel that is welded should be weldable steel. The presence of increasing carbon can cause cracking of the weld. Hence, weldable steels are those whose carbon content is sufficiently low so as not to cause cracking. Cast iron, which contains a large amount of carbon, for example, cannot be joined by ordinary welding.

 

of the two types of welds used to join up members, namely the butt weld and the fillet weld, butt welding is more difficult. In butt welding, the two surfaces to be welded need much more preparation before welding than in fillet welding. The first step in inspection is to see whether the preparation of the surfaces is satisfactory.

 

In manual butt welding, when welding has to be done on both sides of the joint, the first side is completed first. For welding from the second side, it is first 'back gauged' at the root to get full penetration.

 

PROCESS OF WELDING

When a weld is made, some of the parent material melts and combines to form the deposited metal. On cooling, the first to cool is the material adjacent to the parent material and the last to cool is the throat area. The quality of the weld depends on the way the weld material is placed. A skilled welder will be able to make good welds, and an inspector should be able to check each weld for defects and ensure that the welding is satisfactory.

 

DEFECTS IN WELDING

In this section, we will briefly discuss various common defects found in welding.

Undercut. This occurs when a grove is melted into the parent material adjacent to the arc formed by the arc action and is not subsequently included in the weld metal.

 

Porosity. Porosity is' the presence of cavities in the weld metal caused by the gas entrapment, usually spherical in shape. It is caused by the moisture, scales, oil and other contaminations on the plates, or from damp electrodes. Also, if the arc length is too long, porosity will almost certainly occur when using basic electrodes. Since it takes some time to establish necessary stable conditions after the start of welding work porosity is often associated with 'start porosity'. It is usually overcome by striking the arc and travelling for 15 mm and then going back over the initial arc to melt out the start porosity.

 

Incomplete penetration. In fillet welds, this occurs invariably when larger diameter electrodes than necessary are used to fuse the root. In butt welding, incorrect 'back gauging' can result in a lack of penetration. Lack of fusion at edges. This usually occurs in fillet welds and is due to the failure to fuse together adjacent surfaces of the weld and parent materials.

 

Slag inclusions. The slag is derived from the flux used in the welding process and if it is entrapped in the weld, it is harmful. This can be prevented by correct plate preparation, the correct diameter of electrodes and good technique of welding.

 

Hot crack. The typical hot crack in a fillet weld is a longitudinal crack characterized by a blue appearance along the crack due to surface oxidation at high temperatures. They are due to bad material composition, weld strain and bad bead shape.

 

Gas pores. Gases are formed inside the weld.

 

Surface imperfections. The main surface imperfections that can happen are as follows:

(a) Edge of the plate melted off

(b) Overlap

 

METHOD OF INSPECTION | WELDING INSPECTION

The inspector should be in full attendance supervising the work in case of important works. The following is generally used as a checklist:

 

(a) Whether the steel is properly cleaned and is free from corrosion, water, oil, scale, dirt, paint, etc.

 

(b) Whether proper methods were employed when setting up work to ensure a tight fit without displacement of competent parts.

 

(c) When the work is completed, it should be examined for defects and irregularities described in Section 37.6. For this purpose, it may be necessary to hammer the weld or clean it with a wire brush as required.

 

In all important cases of welding work, as in truss work, it is better to keep a record of the work, the inspection, erection etc. so that during and after the work, systematic checks can be made on the quality and completeness of the work that has been carried out. Such evaluation will ensure a satisfactory quality of the construction work. In very important works, it is possible to make a radiographic examination of the weld joints in steel up to 50 mm in size.

 

TACK WELDING

What is Tack welding?

In many cases, as in welded structural work, the structure is divided into parts for easiness of transport or erection and these parts are shop welded, transported to the site and erected at the site first by temporary tack welding. They are finally site welded carefully to form the actual structure. Thus, a long truss may be divided into three parts—the two sides and the central portion—or into halves.

 

These are then aligned and erected at the site first by tack welding and after all the trusses are erected and fully aligned, the joints are field welded to form good and perfect joints, strong enough to take the imposed weights. Under these circumstances, utmost care should be taken to see that records are kept to ensure that these field welds have been actually done and the weldwork is satisfactory in all respects. Cases have occurred where the field staff had forgotten to field weld the joints after tack welding and upon loading, the structural collapse took place. This should never happen at a site supervised by competent staff.

 

GAS WELDING

What is Gas welding?

In gas welding, the flames can be adjusted to three types of flames—reducing, oxidizing and natural. With more acetylene, we get a reducing flame, with more oxygen, an oxidizing flame and in between, we get a neutral flame. A neutral flame is generally used in welding. The temperature varies from 3150 to 3230°C only. Compared to electric arc welding of 10,000°C, it is only one-third the temperature. Hence, it can be used for welding thin plates which otherwise will be burnt off by electric arc.

 

GAS CUTTING

What is Gas Cutting?

Oxyacetylene flame can also be used for gas cutting. This process makes use of the property of oxygen's affinity to ferrous metals at ignition temperatures. The surface to be cut is first heated by an oxyacetylene flame. After heating, a jet of high purity oxygen is directed to the heated surface when combustion takes place with the generation of heat. This principle is used in the cutting of steel with oxygen. Since only the metal in direct contact with the jet is acted on, a very accurate line of cut can be made by gas cutting.

 

TUBULAR ROOFS AND COLUMNS

In steel tubular truss work made of steel tubes, the recommendations to be followed are discussed below.

Nature of structural steel tubes. The tubes shall conform to IS 1161 – 1998, "Steel tubes for structural purposes – Specifications", and shall be one of the following types:

·       Hot finished welded (HFW) type

·       Hot finished seamless (HFS) type

·       Electric resistance welded (ERW) type

 

The minimum wall thickness of the tube. Structural tubes shall have the minimum wall Thickness as indicated below depending upon the exposure:       

·       Construction not exposed to weather: 3.2 mm

·       Construction exposed to weather: 4.0 mm

·       Structures not readily accessible for maintenance: 5.0 mm

 

Caps and base for ends in columns. The ends of all tubes for columns, transmitting loads through the ends, shall be true and square to the axis of the tube and shall be provided with a cap or base accurately fitted to the end of the tube and screwed, welded or shrunk on. The cap or base plate shall be true and square to the axis of the column.

 

Sealing of tubes. When the end of a tube is not automatically scaled by virtue of its connection by welding to another member, the end shall be properly and completely sealed. Before sealing, the inside of the tube shall be dry and free from loose scale.

 

Flattened ends. In tubular construction, the ends of tubes are allowed to be flattened or otherwise formed to provide for welded, riveted or bolted connections. These ends are connected to gusset plates. However, the methods adopted for such flattening should not injure the material. The change of sections shall also be gradual.

 

These tubular trusses are light and are very popular for moderate spans.

 

GUIDE OF ERECTION OF STEEL TRUSSES

Fabrication and erection of steel trusses are important items of work in building construction. For erection, trusses shall be lifted only at nodes. The trusses below 10 m in span are usually slung at the apex. However, as this will develop compression stresses in the bottom tie member, it is better if trusses are lifted by slinging at two mid-points on rafters, which are temporarily braced by a wooden member of u suitable section. After the trusses are placed in position, the purlins and wind bracings must be fixed as soon as possible.

 

The end of the truss which faces the prevailing winds shall be fixed by holding down bolts to the bed plate as shown in Fig. 22.5, and the other end is kept free to move with the bolts in an oblong slot. For this purpose, in case of trusses of spans up to 10 m, the free end of the truss shall be laid on lead sheet or steel plate as per design, and the holes for holding down bolts shall be made in the form of oblong slots, so as to permit the free movement of the truss end. For larger spans, the truss shall be provided with suitable bearing as per the design.

 

GUIDE OF FIXING OF MILD STEEL BARS AND GRILLS IN WOODEN FRAMES OF WINDOWS AND VENTILATORS

Grills are provided for safety on windows. The addition of grill doors may also be provided as external doors. They are usually made from mild steel bars, squares, flats, etc. according to the required design. In a simple design round (12 mm diameter) or square bars can be used. Steel flats of approximately 4 mm can be used to make attractive designs for windows. Floral designs with thinner sections are also very popular.

 

Bars fixed through window frames give us an economical construction. For fixing these steel bars in wooden window frames (instead of separate grills), holes can be drilled on one side or both sides of the frame. The bars are then passed into the frame and they shall be of the correct length as to end flush with the outside of the frame. If the windows come side by side, the steel rods should continuously be passed through all the frames so that it is difficult to bend. Attractive designs can be made by varying the spacing of these bars.

 

In the case of grills fabricated as per the design, they are fixed to the wooden window frame using round headed bolts and nuts running through the frames in new work before windows are fixed in the wall. If the grills are to be installed in frames that are already attached to walls, they are fixed with countersunk wood screws with heads painted to conceal their position.

 

FIXING ROLLING SHUTTERS | GUIDE ON FIXING ROLLING SHUTTERS

The rolling shutters consist of mild steel laths, 1.21 mm thick (18 G) and 80 mm wide or as specified. The laths shall be machine rolled from a continuous strip into an easy curve free from crimps and sharp bends and with an effective bridge depth of 16 mm. These shall be interlocked together throughout their entire length and jointed at the ends in such a way as to maintain alignment and protect the slats against abrasion in the guides. All joints shall be completely air- and weather-tight

 

The shutter is supported by means of a 'spring barrel' which in turn is supported by cast iron or steel brackets. The shutter slats shall coil around the spring barrel. A galvanized steel sheet hood (not lighter than 18 G reinforced as required for rigidity) shall be provided at the fixing level. The brackets on either side also form the end closures for the hood. The spring shall be preferable of coiled type and shall be manufactured from high tensile spring steel wire or strip of adequate strength to balance the shutters in all positions.

 

The guide channels shall be a mild steel deep channel section rolled, pressed or built up (fabricated) construction. The thickness of the sheet used shall not be less than 3 mm. The minimum depth for guide channels shall be as follows:

 

Clear width of shutter	Depth of guide channel
Under 3.5 m	60 mm
3.5 m and above	75 mm

 

The gap between the two legs of the guide channel shall be sufficient to allow the free movement of the shutter and at the same time, close enough to prevent the rattling of the shutter due to wind. Each guide channel shall be provided with a minimum of three fixing cleats or supports for attachment to walls or columns by means of bolts and screws. The spacing of cleats shall not exceed 750 mm. Alternately, the guide channels may be provided with suitable dowels, hooks, or pins for embedding in the walls.

 

The installation shall be mounted plumb, square and true on the vertical surface of lintels and/or masonry on each side of the opening. When completed, the door shall completely fill the opening for which it was designed and shall not obstruct the opening when in the open position. The shutters should operate easily and smoothly under all conditions.

 

FIXING COLLAPSIBLE STEEL GATES | GUIDE TO FIXING COLLAPSIBLE STEEL GATES

This is one of the works commonly carried out in the construction of buildings. They are fixed under a lintel or outside the opening or inside the opening. When it is fixed outside or inside, it can be folded clear of the opening, if necessary. The height of the gate should be 150 mm more than that of the opening. The gates are made to roll on the bottom or top runners

 

CONCLUSION

The amount of steelwork to be carried out in the construction of a building will depend on the type of the building. Some of these have been briefly discussed in this article.