The IMO Code of Safe Practice for Cargo Stowage and Securing (Resolution A.714(17)) Annex 13 'Methods to assess the efficiency of securing arrangements for non-standardised cargo' provides a deterministic method for verifying that lashings can withstand the forces from rolling, pitching, and ship motion. Forces depend on cargo mass, ship motion accelerations (latitude, GM, season), and the friction at the cargo / deck interface. Lashings must satisfy a force balance at every angle of heel up to 30°.

Maximum Securing Load — for any securing element (lashing chain, web strap, wire, twistlock, dunnage friction), the load that a single piece can withstand under operational use. CSS Code Table 2 gives MSL = 50% of MBL (Minimum Breaking Load) for fibre rope and web lashing, 70% of MBL for wire and chain, and the manufacturer's stamped value for fittings (twistlocks, D-rings, eye plates). Always quote MSL on the cargo securing manual, not MBL.

Friction at the cargo / deck interface contributes to longitudinal and transverse restraint. Coefficients in CSS Code Table 5: steel-on-steel 0.10, steel-on-wood 0.30, rubber-mat-faced friction stoppers 0.60-0.70, soft-wood dunnage on wood deck 0.40. Multiplying by the cargo weight gives the friction force; the lashings make up the remainder. Friction-only stowage is not permitted for non-standardised cargo on deck above 30° beam wind exposure.

The accelerations the cargo experiences are functions of vessel size, speed, GM, and operating area. CSS Code provides standard transverse, longitudinal, and vertical accelerations as fractions of g. For a typical container ship under 100 m: transverse 0.8 g, longitudinal 0.4 g, vertical ±0.5 g. Larger or specialised vessels may use vessel-specific values from the cargo securing manual.