According to ASME B30.26, the design factor for shackles over 150 ton rated load shall be a minimum of:

• A. 3
• B. 4
• C. 5
• D. 6

Answer: (B)

The key idea is the safety margin applied to shackles based on their size. ASME B30.26 defines a design factor (safety factor) as the ratio of the minimum breaking load (MBL) to the working load limit (WLL). For shackles rated above 150 tons, the standard requires this design factor to be at least four. In other words, the MBL must be at least four times the WLL, providing a substantial reserve against overloads or unexpected stresses.

So, if a shackle has a WLL of 100 tons, its MBL must be at least 400 tons. The fourfold margin is chosen to ensure safety without imposing an unnecessarily large or impractical size for large shackles. The other numbers would either not provide enough margin (3) or exceed what's required (5 or 6).