Building Design and Construction

Load Capacity of Welds

For welds joining structural steel elements, the load capacity depends on type of weld, strength of electrode material, and strength of the base metal. Fillet or groove  welds (Fig. 7.43) are commonly used for steel connections. Groove welds are classified as complete or partial penetration. (See Art. 7.3.5.)

A significant characteristic of fillet-welded joints is that all forces, regardless of the direction in which they act, are resolved as shear on the effective throat of the weld. For instance, when joining elements such as a girder flange to a web, fillet welds are designed to carry the horizontal shear without regard to the tensile or compressive stresses in the elements.
For computation of load capacity, the effective area of groove and fillet welds is the effective length times the effective throat thickness. The effective area for a plug or slot weld is the nominal cross-sectional area of the hole or slot in the plane of the faying surface.
Except for fillet welds in holes or slots, the effective length of a fillet weld is the overall length of weld, including the return. For a groove weld, the effective length should be taken as the width of the part joined.

The effective throat thickness of a fillet weld is the shortest distance from the root of the joint to the nominal face of the weld (Fig. 7.3). For fillet welds made by the submerged-arc process, however, the effective throat should be taken as the leg size for welds 3⁄8 in and smaller but as the theoretical throat plus 0.11 in for larger fillet welds.

For a complete-penetration groove weld, the effective throat is the thickness of the thinnest part joined. For partial-penetration groove welds, the effective throat thickness depends on the included angle at the root of the groove. For all J or U joints and for bevel or V joints with an included angle of 60 or more, the effective throat thickness may be taken as the depth of the chamfer. When the included angle for bevel or V joints is between 45 and 60, the effective throat thickness should be the depth of chamfer minus 1⁄8 in. For flare bevel and flare V-groove welds when flush to the surface of a bar or a 90 bend in a formed section, the effective throat thickness is 5⁄6 and 1⁄2 the radius of the bar or bend, respectively. When the radius is 1 in or more, for gas metal arc welding, the effective thickness is 1⁄4 the radius.
Welds subject to static loads should be proportioned by ASD for the allowable stresses and by LRFD for the design strengths in Table 7.27. If connections will  be subject to fatigue from stress fluctuations, load capacity should be reduced as provided in the AISC ‘‘Specification for Structural Steel for Buildings.’’


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