Calculate section modulus, elastic stress and inverse sizes for one rectangular weld-throat strip. The stated axis and simplified model do not establish weld-group capacity or code compliance.

Single rectangular throat strip: Z = L × t² / 6, bending across throat t about an axis parallel to L. Not a weld-group or connection design check.

Related Calculators

Weld Section Modulus Formula

For a single idealized rectangular strip of width L and effective throat depth t, bending across t gives Z = L × t² / 6. This follows from I = L × t³ / 12 and c = t/2. The other rectangular axis uses t × L² / 6; real weld groups require their own geometry and loading analysis.

WSM = t² × L / 6

Variables:

  • WSM is the Weld Section Modulus (mm^3)
  • t is the effective throat depth (mm), not automatically plate thickness
  • L is the length of weld (mm) 

Square the effective throat depth, multiply by effective length and divide by 6 for this axis. An ideal equal-leg 90° fillet has throat t = a/√2. Elastic bending stress magnitude is M/Z. The optional electrode reference 0.30 × FEXX is a weld-metal ASD reference, not an automatic allowable normal bending stress or a complete connection check.

How to Calculate Weld Section Modulus?

The following steps outline how to calculate the Weld Section Modulus.

  1. First, determine the effective throat depth for the stated rectangular-strip model (mm).
  2. Next, determine the length of weld (mm). 
  3. Next, use WSM = t^2*L / 6 for the stated axis.
  4. Finally, calculate the Weld Section Modulus.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

effective throat depth (mm) = 3

length of weld (mm) = 10

WSM = t^2*L / 6 = 3² × 10 / 6 = 15 mm³