Structural Steel

Types of Welds

The main types of welds used for structural steel are fillet, groove, plug, and slot. The most commonly used weld is the fillet. For light loads, it is the most economical, because little preparation of material is required. For heavy loads, groove welds are the most efficient, because the full strength of the base metal can be obtained easily. Use of plug and slot welds generally is limited to special conditions where fillet or groove welds are not practical.
More than one type of weld may be used in a connection. If so, the allowable capacity of the connection is the sum of the effective capacities of each type of weld used, separately computed with respect to the axis of the group.
Tack welds may be used for assembly or shipping. They are not assigned any stresscarrying capacity in the final structure. In some cases, these welds must be removed after final assembly or erection.
Fillet welds have the general shape of an isosceles right triangle (Fig. 5.12). The size of the weld is given by the length of leg. The strength is determined by the throat thickness, the shortest distance from the root (intersection of legs) to the face of the weld. If the two legs are unequal, the nominal size of the weld is given by the shorter of the legs. If welds are concave, the throat is diminished accordingly, and so is the strength.
Fillet welds are used to join two surfaces approximately at right angles to each other. The joints may be lap (Fig. 5.13) or tee or corner (Fig. 5.14). Fillet welds also may be used with groove welds to reinforce corner joints. In a skewed tee joint, the included angle of weld deposit may vary up to 30 from the perpendicular, and one corner of the edge to be connected may be raised, up to 3⁄16 in. If the separation is greater than 1⁄16 in, the weld leg should be increased by the amount of the root opening.
Groove welds are made in a groove between the edges of two parts to be joined. These welds generally are used to connect two plates lying in the same plane (butt joint), but they also may be used for tee and corner joints.
Standard types of groove welds are named in accordance with the shape given the edges to be welded: square, single vee, double vee, single bevel, double bevel, single U, double U, single J, and double J (Fig. 5.15). Edges may be shaped by flame cutting, arc-air gouging, or edge planing. Material up to 5⁄8 in thick, however, may be groove-welded with squarecut edges, depending on the welding process used.
Groove welds should extend the full width of parts joined. Intermittent groove welds and butt joints not fully welded throughout the cross section are prohibited.
Groove welds also are classified as complete-penetration and partial-penetration welds.
In a complete-penetration weld, the weld material and the base metal are fused throughout the depth of the joint. This type of weld is made by welding from both sides of the joint or from one side to a backing bar or backing weld. When the joint is made by welding from both sides, the root of the first-pass weld is chipped or gouged to sound metal before the weld on the opposite side, or back pass is made. The throat dimension of a completepenetration groove weld, for stress computations, is the full thickness of the thinner part
joined, exclusive of weld reinforcement.
Partial-penetration welds generally are used when forces to be transferred are small.
The edges may not be shaped over the full joint thickness, and the depth of weld may be less than the joint thickness (Fig. 5.15). But even if edges are fully shaped, groove welds made from one side without a backing strip or made from both sides without back gouging are considered partial-penetration welds. They often are used for splices in building columns carrying axial loads only. In bridges, such welds should not be used where tension may be applied normal to the axis of the welds.
Plug and slot welds are used to transmit shear in lap joints and to prevent buckling of lapped parts. In buildings, they also may be used to join components of built-up members.
(Plug or slot welds, however, are not permitted on A514 steel.) The welds are made, with lapped parts in contact, by depositing weld metal in circular or slotted holes in one part.
The openings may be partly or completely filled, depending on their depth. Load capacity of a plug or slot completely welded equals the product of hole area and allowable stress.
Unless appearance is a main consideration, a fillet weld in holes or slots is preferable.
Economy in Selection. In selecting a weld, designers should consider not only the type of joint but also the type of weld that would require a minimum amount of metal. This would yield a saving in both material and time.
While strength of a fillet weld varies with size, volume of metal varies with the square of the size. For example, a 1⁄2-in fillet weld contains four times as much metal per inch of length as a 1⁄4-in weld but is only twice as strong. In general, a smaller but longer fillet weld costs less than a larger but shorter weld of the same capacity.
Furthermore, small welds can be deposited in a single pass. Large welds require multiple passes. They take longer, absorb more weld metal, and cost more. As a guide in selecting welds, Table 5.12 lists the number of passes required for some frequently used types of welds.
Double-V and double-bevel groove welds contain about half as much weld metal as
single-V and single-bevel groove welds, respectively (deducting effects of root spacing). Cost of edge preparation and added labor of gouging for the back pass, however, should be considered. Also, for thin material, for which a single weld pass may be sufficient, it is uneconomical to use smaller electrodes to weld from two sides. Furthermore, poor accessibility or less favorable welding position (Art. 5.18) may make an unsymmetrical groove weld more economical, because it can be welded from only one side.
When bevel or V grooves can be flame-cut, they cost less than J and U grooves, which require planning or arc-air gouging.


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