Effective length of a fillet weld for strength calculations For a straight fillet weld, the effective length used in design is taken as the actual overall length:

Introduction / Context:
Design strength of fillet welds uses the effective throat over the effective length. End portions of fillet welds do not develop full throat due to start/stop craters and stress distribution, so a length reduction is applied.

Given Data / Assumptions:

• Straight fillet weld of uniform size.
• Design per traditional working stress approach with length reduction for end returns.
• “Size of weld” means leg size (e.g., a).

Concept / Approach:
Standard practice takes the effective length as the actual length minus two times the weld size to discount the under-strength at the two ends. This gives a conservative net length participating at full capacity.

Step-by-Step Solution:

Verification / Alternative check:
Code commentaries and steel design handbooks present the same reduction for isolated fillet weld segments.

Why Other Options Are Wrong:
Subtracting one size underestimates the end effects; adding sizes is unconservative; taking full actual length ignores known end weaknesses.

Common Pitfalls:
For overlapping or returned welds, details may restore some end capacity; however, unless detailed otherwise, the standard 2a reduction applies.

Final Answer:
minus twice the size of weld