Welded connections

Author: Welded connections in structures

Welding

In structural work, a variety of welded joints are used; these can all be made up from the basic configurations which are classified as follows:

• butt joints
• tee joints
• lap joints
• corner joints

 A welded joint is made by fusing (melting) the steel plates or sections (the parent metal) along the line of the joint. The metal melted from each member at the joint unites in a pool of molten metal which bridges the interface. As the pool cools, molten metal at the fusion boundary solidifies, forming a solid bond with the parent metal.

Two types of weld are in common use: butt welds and fillet welds. In the former the weld metal is generally contained within the profiles of the welded elements; in the latter, deposited weld metal is external to the profile of the welded elements. The solidified weld metal has a cast structure and has properties characteristic of cast steel, i.e. higher ratio of yield to ultimate strength than structural steel. The weld metal is a mixture of parent metal and steel melted from the electrode. In structural work the composition of the electrode is usually chosen so that the resultant weld metal is stronger than the connected elements.

Manual metal arc welding

This manual method is one of the most widely used arc welding processes. It requires considerable skill to produce good quality welds.

Metal active gas (MAG) welding

This process is sometimes referred to as Metal Inert Gas (MIG) Welding, although strictly speaking the term MIG should be limited
to the use of pure argon as a shielding gas, which is not used for carbon steel.

Submerged arc welding (SAW)

This is a fully mechanised process in which the welding head travels along the joint automatically. The electrode is a bare wire which is advanced by a governed motor.

Stud welding

This is a variation of arc welding in which studs are welded to plane surfaces automatically. The stud, which may be a plain or threaded bar (if plain it will have a head) is the electrode and it is held in the chuck of a welding gun which is connected to the power supply.

Welded connections

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.

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. In some cases, if the plate thickness is less than about 5mm, edge preparation can be avoided.

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. Slot and plug welds are seldom used in building structures. They principally prevent buckling or separation of lapped plates. Spot welds are seldom used in building structures. The parts to be joined, which must be thin, are clamped together by two electrodes.

Because a full penetration weld is more difficult to make than a full strength weld, full penetration welds should only be used where necessary, such as in connections where high fatigue stresses can occur. Good welding design reflects the economies and advantages of different types of weld by choosing types appropriate to the needs of the design. The full strength weld is easy to achieve using fillet welds; full penetration welds, however, without inclusions at the centre, can only be achieved by extensive back gouging prior to welding the reverse side. The risk of faults in full penetration welds is much greater and control of distortion more difficult.