Lecture 11.2.1: Generalities on Welded

Connections

NOTATION

A_s% Elongation of steel at failure
a Throat thickness of welds [mm]
f_y Nominal yield stress of parent metal [MPa]
f_u Nominal ultimate stress of parent metal [MPa]

1. INTRODUCTION

Where there are favourable working conditions, welding is the most economical way to make strong connections. Therefore, workshop connections are usually welded. Where site connections are necessary (erection) they are usually bolted, but the connections are often prepared in the workshop with welded plates, etc. necessary for the bolted joint.
Figures 1-3 show some examples on welded connections and the way they carry external loads.

2. TYPES OF WELDS

In welded construction for buildings approximately 80% of the welds are fillet welds and 15% are butt welds. The remaining 5% are plug, slot and spot welds.

2.1 Butt Welds

A butt weld is made within the cross-section of the abutting plates in a butt or tee joint. Normally, the plate edges have to be prepared before welding, see Figure 4b. In some cases, if the plate thickness is less than about 5mm, edge preparation can be avoided, see Figure 4a.

The bevelled plate edges in a butt weld may take various geometrical forms, see Figure 5.

For butt welds, a distinction is made between:

• full penetration butt weld in which there is a complete penetration and fusion of weld and parent metal throughout the thickness of the joint, see Figure 4.
• partial penetration butt weld, in which there is a weld penetration less than the full thickness of the joint, see Figure 6.

2.2 Fillet Welds

A fillet weld is a weld of approximately triangular cross-section applied to the surface profile of the plates. No edge preparation is needed. Therefore, fillet welds are usually cheaper than butt welds. According to the relative position of the parts to be welded, there are three types of fillet weld applications:

• lap joint, in which the parts welded are in parallel planes, see Figure 7a.
• tee or cruciform joint, in which the parts welded are more or less perpendicular to one another, see Figure 7b.
• corner joint, in which the parts are also more or less perpendicular to one another, see Figure 7c. To improve the strength and stiffness of the joint, the outer corner is normally butt welded, see Figure 8.

Fillet welds that can be laid in a single run are particularly economic; in the workshop 8mm welds are often possible but if site welding is to be used this figure may be reduced e.g. to 6mm.

2.3 Plug and Slot Welds

Slot and plug welds, see Figure 9, are seldom used in building structures. They principally prevent buckling or separation of lapped plates.

2.4 Spot Welds

Spot welds are seldom used in building structures. The parts to be joined, which must be thin, are clamped together by two electrodes, see Figure 10. A current passing through the electrodes melts the pieces locally, and the clamping pressure forces them together, forming a fused spot. A line of such spots constitutes a joint.

3. DESCRIPTION OF WELDS - DEFINITIONS

In this chapter some terms often used in weld design are defined.

• The parent metal or the base metal is the steel material which is welded.
• The filler metal or the weld metal is the electrode material used in the welding process.

Figure 11 shows the terminology used to describe the various parts of the weld:

• The root is the point of the joint line reached by the penetration of the filler metal.
• The face is the outside surface of the weld.
• The toe is the line on the weld face between the filler metal and the parent metal.
• The heat affected zone is a part of the parent metal which has not been melted with the filler metal, but which undergoes fast heating and cooling during the passage of the welding arc. In this zone, the parent metal is subject to a hardening treatment and can consequently become brittle.

The shape of a fillet welded cross-section can be convex or concave, see Figure 12.

For design, two geometrical weld parameters are used:

• The throat thickness, a, is the minimum distance from the root to the weld face ignoring the convex zone. Figure 13 shows the throat thickness a of a butt weld and a fillet weld.
• The length is the longitudinal weld dimension in the direction of the welding arc displacement. Figure 13 shows welds for which the length l is the full length of the plates; for the lap joint of Figure 2 l would be the sum of twice the overlapped length plus the width of the smaller plate.

4. MECHANICAL PROPERTIES OF MATERIALS

4.1 Parent Metal

The parent metal must have the weldability properties defined by the steel qualities of each steel grade. The weldability of a steel depends on its chemical composition and metallurgical characteristics. These aspects are described in Lecture 2.6.
In accordance with Eurocode 3 [1] and EN 10025 [2], hot-rolled steel grades S235, S275 and S355 with quality B, C or D are suitable for all welding processes.
The mechanical characteristics adopted in calculations are the yield strength f_y and the ultimate tensile strength f_u. Further, in the case of plastic analysis, steel must have an ultimate strain at least 20 times the yield strain and have a minimum elongation A_s at failure of at least 15%. The values of these characteristics are given in Table 1.

Nominal steel grade	Thickness t
	t < 40mm			40 mm < t < 100mm
	fy MPa	fu MPa	As %	fy MPa	fu MPa	As %
Fe 360 Fe 430 Fe 510	235 275 355	360 430 510	26 22 22	215 255 335	340 410 490	24 20 20

Table 1 - Mechanical properties of hot rolled steels
The steel quality B, C, or D reflects the impact resistance determined by a Charpy-V impact test in which the impact energy needed to fracture a V-notch specimen with a certain temperature is measured. The quality class indicates if the steel is suitable for the application. For example, steel quality B, corresponding to an impact test at +20°C, is used for structures with normal working conditions. The steel quality D (impact test at -20°C) is suitable when factors such as, for example, low service temperature of the structure, thick steel material and/or impact loading are present.

4.2 Filler metal

According to Eurocode 3 the filler metal must have mechanical properties (yield strength, ultimate tensile strength, elongation at failure and minimum Charpy V-notch energy value) equal to or better than the values specified for the steel grade being welded.
The selection of welding consumable is to a great extent governed by the welding process, see Lectures 3.3 and 3.4. The basic principles are:

• Welding consumables shall be appropriate to the welding process used, the parent metal welded and the welding procedure adopted.
• Welding consumables must be stored and handled with care and in conformity with the recommendations of the manufacturer.
• Electrodes for manual arc welding shall be stored in their original containers in a dry and warm place, efficiently protected from adverse effects of weather.
• Flux shall be stored and transported in containers which protect it from absorbing moisture.

5. EDGE PREPARATION AND FIT-UP

Some important aspects, which are described in detail in Lectures 3.3, 3.4 and 3.5, are summarised here.
One of the following arc welding processes may be used:

• metal arc-welding with covered electrodes
• flux-cored arc welding
• submerged arc welding
• MIG (metal inert gas) welding
• MAG (metal active gas) welding.

All these processes can be used in the workshop. Usually, only bolting or metal arc welding with covered electrodes is employed in the erection phase on the building site. With the metal arc welding process, welds can be made in all positions. The various weld positions are shown in Figure 14, where the arrows give the arc direction during the welding operation. It is clear that welding in the flat position is easily carried out, allowing a greater rate of metal deposition than the other positions;by welding in this favourable position, the maximum size of weld run can be obtained. With ordinary welding consumables and favourable welding conditions, a fillet weld with a throat thickness of 6mm can be produced with only one run. For welds of greater thickness, more than one run is necessary. In this case, the welding sequence must be carefully planned, see Figure 15.

The welding conditions, particularly the current limitation of the welding equipment, constitute a limit to the depth of penetration into the parent plate. For example, if a closed butt joint (no gap between the two plates) is welded with one run on each side, the penetration may not be complete and the central part of the joint will remain unfused (Figure 16a). With a gap between the two parts of the joint, full penetration can be achieved with the same welding equipment. The limitation is then set by the thickness of the plates to be joined. In practice, the limit for butt welds with square edges, i.e. without preparation, is 10mm plate thickness with a 5mm gap. When the plate thickness exceeds this value, bevelled edges permit full penetration by several runs, see Figure 16b.

Edge preparation consists essentially of cutting and bevelling the edges of the plates which are to be welded. These operations can be done by thermal cutting, by machining or by chipping or grinding. The resulting surfaces of the bevelled edges should be smooth, uniform, free from cracks and without rust. If thermal cutting or another process which hardens the material is used, the approved welding procedure must take account of this weld preparation process. Different bevel geometries are shown in Figure 3. Practical recommendations, for example Eurocode 3, Annex W (1), give some tolerance values for various weld types. Finally, before welding, surfaces and edges adjacent to the weld location must be cleaned to remove oil, grease, paint or any other contaminants, which can affect the quality of the weld and the weld strength.
The appropriate welding method and procedure are defined in a project specification provided by the designer, see Lecture 11.5.

6. WELD QUALITY - DISCONTINUITIES

Quality control has been for some time an important part of industrial activity. The term quality includes all the characteristics of a product which affect its ability to serve its purpose. In the lectures of Working Group 3, and particularly in Lecture 3.4, attention is drawn to quality control applied to welding, including the qualification of firms and the procedure qualification for welding tests.
Normally, all welded structures undergo some weld inspection. The type and the extent for the inspection, as well as the choice of welds to be inspected are selected in accordance with the project specification.
The principal purpose of weld inspection is to discover possible weld defects. Examples on weld defects are, see Figure 17:

• Undercut. The thickness of the parent metal is reduced near the weld toe.
• Porosity or gas inclusions. Air or gas bubbles are incorporated in the melted metal, where they remain after cooling.
• Insufficient throat. The throat thickness is smaller than the design thickness. The resistance of the joint might be insufficient.
• Incomplete penetration. The throat thickness is smaller than the design thickness. The resistance of the joint might be insufficient.

All these defects can be measured. Codes of Practice specify the allowable tolerances for each defect, see for example Eurocode 3, Annex W [1].

7. CONCLUDING SUMMARY

• Providing the right conditions with respect to welding - particularly fillet welding - normally represents the most economical way to join two pieces of steel.
• Two types of structural weld - fillet welds and butt welds - are in common use. Wherever possible fillet welds should be used.
• A number of standard terms are used when referring to welds. Throat thickness and length are the two most important descriptions for determining load carrying capacity.
• Weld metal (electrodes) and parent metal (the parts to be joined) must be properly matched in terms of their physical properties.
• For butt welds suitable edge preparations will normally be required.
• Welds must be inspected for possible defects so as to ensure adequate structural performance.