gardless of the loading conditions. Rotation can be prevented by diaphragms or stiffeners, or in some cases, simply by the overall configuration of the member. Inspection of fillet welds with radiographic (RT) and ultrasonic (UT) inspection is not practical, for multiple reasons. The triangular cross-section alone is problematic for these inspection techniques, and the naturally occurring lack of fusion plane will always confuse interpretation of the results. For all fillet welds, regardless of direction or type of loading, matching or undermatching weld metal may be used (see Section 3.11 of this Guide). 3.5.1
Minimum Size of Fillet Welds
AISC Specification Table J2.4 specifies minimum weld sizes that are a function of plate thickness. These are not designrelated requirements but are used to address welding-related concerns that involve fusion and cracking. In order to make a good arc weld, there is a minimum amount of thermal energy that must be introduced into the joint. Failure to do so may result in the deposition of weld metal, albeit without fusion to the base metal. Alternatively, when insufficient thermal energy is delivered to the joint, the cooling rate experienced by the weld and the heat-affected zone may be such that cracking occurs. To ensure that a reasonable level of thermal energy is introduced into the joint, AISC Specification Table J2.4 requires that a certain minimum weld size be applied, regardless of design loads. This is possible because there is a direct
Figure 3–11. Determining effective throat dimensions.
relationship between the thermal energy introduced into the joint and the weld size applied to the joint. This assumes, however, that the weld size is applied in a single pass. Heat input is typically used to directly estimate the amount of thermal energy that is introduced into the joint, using the following equation: H=
60EI 1000SW
where E = arc volts I = amperage SW = travel speed (in./min) H = heat input (kilojoules/in., or KJ/in.) In order to create a larger weld in one pass, two approaches may be used: higher amperages (I), or slower travel speeds (SW) must be employed. Notice that either procedure modification results in a higher heat input. Table J2.4 specifies minimum acceptable weld sizes with the primary purpose of dictating minimum heat input levels. For example, almost independent of the welding process used, a 4 in. fillet weld will require a heat input of approximately 20-30 KJ/in. By prescribing a minimum fillet weld size, these specifications, in essence, stipulate a minimum heat input. The minimum fillet weld size need never exceed the thickness of the thinner part being joined. In some extreme circumstances, the connection might involve an extremely thick plate being joined to a very thin plate. The AISC Specification dictates that the weld need not exceed the size of the thinner part. However, under these circumstances, additional preheat based on the thicker material may be justified.
Figure 3–12. Welds on edges.
DESIGN GUIDE 21 / WELDED CONNECTIONS—A PRIMER FOR ENGINEERS/ 37