Glossary > #BL

BL – Breaking Load

In the context of cargo securing, Breaking Load (BL) refers to the maximum tensile force or stress that a securing element or material is capable of withstanding before it breaks or fails. This key property is a fundamental indicator of the strength of securing devices, such as lashing straps, chains, or ropes, which are used to secure cargo during transport. Breaking Load (BL) is typically expressed in kilonewtons (kN) or tonnes and is determined under controlled laboratory conditions.

Understanding the BL value is essential for safe and efficient cargo transport. By knowing the strength of securing materials, it is possible to calculate what force they are capable of withstanding during transport. The performance of securing equipment under stress directly affects the safety of the cargo, the transport vehicle, and the personnel involved.

For example, the unit kilonewton (kN) is commonly used to describe breaking load. In the International System of Units (SI), it replaced the previously used metric tonne. The conversion is as follows:

1 kN = 0.1 t or 100 kg

Why is Breaking Load Important in Cargo Securing?

The safety and stability of cargo during transport depends on the ability of securing devices to withstand forces generated by ship movements, vibrations on the road, or external influences such as wind or waves. Breaking Load helps determine the maximum capacity of securing elements and ensures they can withstand these forces without failure.

Examples of transport methods:

  • Maritime transport: In the maritime environment, cargo is exposed to forces caused by pitching, rolling, and yawing of the ship. Securing devices with sufficient BL value are key to preventing cargo from shifting or falling. The CSS Code requires that BL and its derivatives, such as Maximum Securing Load (MSL), are correctly calculated and observed.
  • Road transport: Sudden braking or sharp turns during road transport can exert considerable force on cargo. BL ensures that securing systems keep cargo in place even under these conditions. For road vehicles, securing equipment is often marked with a Lashing Capacity (LC) value according to the European standard EN 12195-2.

Key Related Terms

1. Maximum Securing Load (MSL)

Maximum Securing Load is the highest permissible load that a securing device is capable of withstanding during normal operation. It takes into account safety factors and is calculated as a percentage of the BL value. For example:

  • MSL for chains is typically 50% of their BL.
  • For straps, MSL is often 70% of their BL.

Formula for calculating MSL:

MSL = BL × Safety Factor

MSL values are determined based on material properties and their ability to withstand wear. For example, steel chains with a BL of 15 tonnes (150 kN) have an MSL of 7.5 tonnes (75 kN) if the safety factor is 50%.

2. Safety Factor

A safety margin is applied to account for wear, damage, and unforeseen stress. The International Maritime Organization (IMO) recommends using a safety factor of 2:1 or 3:1 for most securing equipment. The CSS Code provides detailed guidance on this matter.

3. Pre-tension (STF)

Standard Tension Force (STF) refers to the force applied to securing equipment during its initial tightening. This force, often measured during manual tensioning, helps ensure secure fastening of cargo. STF typically amounts to approximately 10% of the BL value.

4. Working Load Limit (WLL)

WLL is the maximum weight or force that a device is capable of safely withstanding during normal operating conditions. It contains an additional safety margin compared to BL.

How is Breaking Load Determined?

Breaking Load is determined by testing the tensile strength under controlled conditions. A tensile testing machine applies force to the securing device or material until it reaches the breaking point. The maximum force recorded before failure is the BL value.

Example:

  • A steel chain with a BL of 15 tonnes (150 kN) is tested. The MSL is calculated as 7.5 tonnes (75 kN) if the safety factor is 50%.

Materials and Their Breaking Load Characteristics

Different materials have different BL values, making them suitable for specific cargo securing applications. Below are common types of securing materials and their BL properties:

1. Chains

  • Material: High-strength steel.
  • BL Calculation: Determined according to diameter and strength class of the material.
  • Use: Often used for heavy cargo in maritime transport.
  • MSL: Typically 50% of the BL value.

2. Synthetic Ropes

  • Types: Polypropylene (PP), Polyester (PES), and Polyamide (PA).
  • BL Calculation: Proportional to the square of the rope diameter.
  • Advantages: Low weight and corrosion resistance.
  • MSL: Varies (e.g., 33% for natural fibres, 70% for straps).

3. Lashing Straps

  • Material: Synthetic fibres.
  • BL Marking: Often indicated by coloured stripes (e.g., 1 stripe = 1 tonne BL).
  • Use: Suitable for securing general cargo.
  • MSL: 70% of the BL value.

4. Steel Ropes

  • Material: Steel.
  • BL Calculation: Proportional to the square of the rope diameter.
  • Use: Used for securing large and heavy cargo.
  • MSL: Between 30% and 80% of the BL value, depending on use.

Practical Applications of Breaking Load in Cargo Securing

1. Direct Lashing

Direct lashing connects cargo directly to fixed securing points. BL determines the number and arrangement of lashings used.

Example calculation:

  • Cargo weight: 12,000 kg (120 kN).
  • Expected forces: 0.8 g lateral acceleration (96 kN).
  • Device MSL: 2,400 daN (24 kN).
  • Number of required lashings: 96 kN / 24 kN = 4 lashings (minimum).

Compliance with BL ensures the safety of cargo, the transport vehicle, and personnel.