What external forces act on deck cargo?

Annex 13 of the CSS Code defines three external forces: transverse force Fy (caused by rolling and steady heel, dominant for most sea areas), longitudinal force Fx (caused by pitching and slamming, typically 0.3–0.5g), and vertical force Fz (reducing the effective weight of the cargo, typically 0.5g at sea). Fy is the largest and most critical, reaching 0.8g transverse in the North Atlantic for ships under 100 m LOA.

What is the Maximum Securing Load (MSL)?

MSL is the maximum load that a securing device may carry. For chain lashings, MSL = 50% of the Breaking Load (BL). For wire rope, MSL = 70% of BL. For fibre rope, MSL = 50% of BL. For webbing straps, MSL = 70% of BL. For shackles and turnbuckles, MSL is stamped or given in the manufacturer's certificate. Total lashing capacity used in the balance-of-forces method is based on the sum of MSL values of all lashing devices, adjusted for their geometry.

What is the balance-of-forces method?

The balance-of-forces method (Annex 13, Section 7) checks that the sum of lashing forces in each direction (transverse, longitudinal, and vertical) plus friction is sufficient to balance the external forces. The calculation is: CS (calculated securing number) ≥ MSL × cos(α) × cos(β) for each lashing, where α is the horizontal angle and β is the vertical angle from the axis of the force. A safety factor μ (friction coefficient, 0.3 for timber, 0.5 for steel-on-rubber) reduces the required lashing force.

Does Annex 13 apply to all cargo?

Annex 13 applies to general cargo and units not covered by specific IMO guidelines (e.g. containers, vehicles, timber). It is the residual calculation method used in the Cargo Securing Manual for unusual or project cargo. The method is conservative: the advanced method (Annex 13, Section 8) considers more parameters and typically yields a smaller required MSL. Classification society software (e.g. DNV Safecargo) implements both methods.

Lashing forces

Annex 13 of the IMO Code of Safe Practice for Cargo Stowage and Securing (CSS Code, Resolution A.714(17) as amended in 2011) provides a standardised method for calculating the forces on deck cargo and verifying that the securing arrangement is adequate. It defines external accelerations, equipment strength limits (MSL), and two calculation methods — balance-of-forces (basic) and the advanced method.

External forces (Fx, Fy, Fz)

· Transverse (Fy): function of sea area (North Atlantic, North Pacific, Indian Ocean, or sheltered) and ship length (LOA). North Atlantic, LOA < 100 m: Fy = 0.8g; LOA 300 m: Fy = 0.5g. Values taken from Annex 13 Table 2.
· Longitudinal (Fx): typically 0.3g forward, 0.5g aft for standard sea areas; Table 2 also provides these values.
· Vertical (Fz): upward, reducing the cargo's effective weight. Typically 0.5g vertically in North Atlantic conditions. Combined with the cargo mass this gives the effective compressive force on friction surfaces.
· Annex 13 Table 1 classifies sea areas (A = North Atlantic/North Pacific, B = other open seas, C = sheltered); the sea area used must match the intended voyage — not the most favourable.

Maximum securing load (MSL) by equipment type

· Chain lashings: MSL = 0.50 × BL. BL from IACS/Class certificate or stamped on the chain.
· Wire rope lashings: MSL = 0.70 × BL (higher because wire is more elastic and shares load better).
· Fibre rope / synthetic strops: MSL = 0.50 × BL; must have documented BL; untested rope not accepted.
· Webbing straps: MSL = 0.70 × BL when tested to EN 12195-2; check label on strap or certificate.
· Shackles, turnbuckles, D-rings: MSL stamped or from SWL certificate; SWL as marked may equal MSL if so stated.

Balance-of-forces method

· For transverse balance: ΣCS(transverse) + μ × (m × g − Fz × m) ≥ Fy × m, where m is the cargo mass, μ the friction coefficient.
· CS contribution of each lashing = MSL × cos α × cos β (α = horizontal deviation from transverse axis, β = vertical angle from horizontal).
· Friction coefficients: 0.3 (steel on steel, wet), 0.3 (timber on steel), 0.4 (rubber on steel), 0.5 (timber on timber).
· Tipping check: lashings must also prevent tipping about the cargo's lower outer edge; this is a moment calculation about that tipping axis.

Advanced calculation method

· Takes into account the elasticity of lashing equipment (stiffer materials carry more load), pre-tension, and the simultaneous action of transverse and longitudinal forces.
· Produces a Calculated Securing Number (CS number) for the arrangement; CS ≥ 1.5 × external force requirement.
· Requires software or skilled calculation; used for project cargo, heavy lifts, and non-standard configurations.
· P&I clubs and hull underwriters may require an advanced-method calculation or a marine surveyor's endorsement for cargo above 100 t per unit.

Cargo securing manual requirements

· Every cargo ship must have an approved Cargo Securing Manual (CSM) — IMO MSC/Circ.745 and SOLAS VI/5.6 requirement.
· The CSM lists lashing points, MSL ratings, and standard securing patterns approved by the flag State administration.
· Chief Officer must verify that actual lashing equipment matches the CSM specifications; substitute equipment must be rated equivalent or higher in MSL.
· PSC inspectors check that the securing arrangement matches the CSM and that lashing equipment is certified and in good condition.

Sources: IMO CSS Code (Resolution A.714(17)) + 2011 amendments incorporating Annex 13; IMO MSC/Circ.745 — Guidelines for the preparation of the Cargo Securing Manual; SOLAS Chapter VI Reg. 5 — Stowage and securing. See also breaking load, cargo securing manual, hatch cover testing.

Last verified 4 months ago