What is the CSS Code Annex 13 method?

Annex 13 of the IMO Code of Safe Practice for Cargo Stowage and Securing is the standard simplified method for checking that securing arrangements can resist the forces from ship motion. It uses acceleration factors as multiples of g that already include roll, pitch, heave, and gravity components.

Why does the transverse case usually govern?

Rolling produces the largest sideways accelerations on a ship, and the gravity component when heeled adds to them. For deck containers the transverse force is therefore normally the critical direction, so this tool sizes the lashings against it.

MSL is the maximum securing load, the highest load a lashing component may carry. The calculation strength CS is MSL divided by a safety factor, commonly 1.5 for lashing rods, and the required tension must stay below CS for the arrangement to pass.

How does friction help?

Friction between the container base and the deck or the stack below resists sliding before the lashings take any load. The restraint is the friction coefficient times the net downward force, weight minus any upward vertical force, and only the force beyond that needs to be carried by lashings.

Can I use this to sign off a real lashing plan?

No. This is a simplified balance for understanding and first-pass checks. A real plan must use the vessel's approved Cargo Securing Manual, the exact position-dependent accelerations, stack-weight and stack-height limits, and corner-casting strengths. Always follow the manual.

Container Lashing Force Calculator

When a ship rolls and pitches, every deck container is thrown sideways, forward, and up by forces that can run to hundreds of kilonewtons. This calculator follows the simplified CSS Code Annex 13 method to work out those forces, credit the friction that resists them, and size the lashing tension that the rods must provide.

How it works

The external forces come from the unit mass and the acceleration factors, then friction is subtracted to find what the lashings must restrain:

W   = m·g                      (weight)
F_y = m·(a_y/g)·g              (transverse, usually governing)
F_x = m·(a_x/g)·g              (longitudinal)
F_z = m·(a_z/g)·g              (vertical)
friction = μ·(W − F_z)
net force = max(0, F_y − friction)
required tension per lashing = net force / (n · cos α)
allowable = CS = MSL / 1.5

If the required tension stays below the calculation strength CS, the arrangement is adequate; if not, you need more lashings, a higher-MSL rod, or better friction.

Example and notes

A 20-tonne container at a transverse factor of 0.7 g sees about 137 kN sideways. With a friction coefficient of 0.3 and no upward vertical force, friction holds about 59 kN, leaving roughly 78 kN for the lashings. Spread over two rods at 45° that is about 55 kN each — within a 100 kN MSL rod’s calculation strength of 67 kN, so it passes. Lower the friction (a wet or icy deck) or raise the acceleration toward the ship’s ends and the same stow can quickly fail. Always verify against the vessel’s Cargo Securing Manual before securing real cargo.