Minimum Weld Size for Thicker Members up to 20 mm

Determining the correct weld size is crucial in structural fabrication to ensure the joint's strength and integrity. The minimum size of a fillet weld is generally specified based on the thickness of the parts being joined, particularly the thickness of the thicker member.

Understanding Weld Size Requirements

Weld size typically refers to the leg length of a fillet weld. Design codes provide guidelines for minimum weld sizes to prevent issues like inadequate fusion, lamellar tearing, or excessive stress concentrations. These minimums often depend on the thickness of the base metal.

Fillet Weld Size Rule for Thickness Range

According to common practices and standards like the American Welding Society (AWS) D1.1 code for structural welding, the minimum size requirement for fillet welds changes based on material thickness. For situations where the thicker material has a thickness greater than 1/2 inch (approximately 13 mm) and up to 1 inch (approximately 25 mm), the minimum fillet weld size (leg size) required is typically 3/16 inch.

Converting 3/16 inch to millimeters:

$$ \frac{3}{16} \text{ inches} \times 25.4 \frac{\text{mm}}{\text{inch}} \approx 4.76 \text{ mm} $$

This value is generally rounded up or specified as 5 mm in metric units for practical application.

Applying the Rule to the Question

The question asks for the minimum weld size when the thickness of the thicker member is up to 20 mm.

• 20 mm is approximately 0.79 inches.
• This thickness falls within the range specified by standards like AWS D1.1 for thicknesses greater than 13 mm (1/2 inch) up to 25 mm (1 inch).
• In this thickness range, the minimum fillet weld leg size stipulated is 5 mm.

Evaluating the Options

Based on the standard requirements for the given thickness range:

• 3 mm: Generally considered too small for members thicker than 10 mm.
• 5 mm: This aligns with the minimum requirement for thicker members up to 20 mm (approx. 3/16 inch).
• 6 mm: This might be a minimum for slightly larger thickness ranges (e.g., >20 mm or >25 mm) depending on the specific code.
• 10 mm: This is a larger weld size and not the minimum requirement for this thickness range.

Therefore, the minimum size of the weld for the thickness of the thicker member up to 20 mm is 5 mm.