Joint Design and Fillet Welds
Fillet weld leg size, throat dimensions, groove preparation, backing bars, and weld sizing.
- Calculate fillet weld leg size and theoretical throat dimension
- Describe groove joint preparation including root gap and root face
- Explain backing bar use and weld sizing requirements
Leçon 1
Fillet Weld Leg Size and Throat
Fillet Weld Geometry
A fillet weld joins two surfaces at an angle (typically 90 degrees). It is the most common weld type in structural steel fabrication. Understanding leg size and throat is essential for sizing and inspection.
Calculating Throat Size
For an equal-leg fillet weld, the theoretical throat is calculated as:
Throat = Leg Size x 0.707
For example, a 10 mm fillet weld has a theoretical throat of 10 x 0.707 = 7.07 mm. The throat is the critical dimension for strength calculations because it represents the minimum cross-section of the weld.
Remember 0.707
The number 0.707 appears frequently on exams. It is the ratio of the throat to the leg on an equal-leg fillet weld. Throat = Leg x 0.707. If the leg is 8 mm, the throat is 8 x 0.707 = 5.66 mm.
Fillet Weld Profile
The ideal fillet weld has:
- Flat to slightly convex face - not excessively convex (waste of material) or concave (undersized throat)
- Equal leg sizes unless specified otherwise
- Smooth transition at the toes (no undercut)
- Full fusion at the root
The theoretical throat of an equal-leg fillet weld is 0.707 times the leg size. The throat is the critical strength dimension. A proper fillet weld has equal legs, a flat to slightly convex face, and smooth toe transitions without undercut.