Technical Information
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Steel Cross-members of a Shipping Container
Steel Cross-members of a Shipping Container
What are steel cross-members of a shipping container?
Steel cross-members of a shipping container (in English, cross members) are the basic supporting elements of the floor structure of shipping containers, which are indispensable for their strength, durability, and full functionality in demanding logistics processes around the world. These crossbeams are made of highly durable steel, usually Corten (COR-TEN), and are located under the floor of the container, where they run perpendicular to the bottom side rails.
Functions of cross-members:
- Ensure even distribution of cargo weight across the entire floor area.
- Support the marine plywood floor and prevent it from sagging or cracking under point loads (forklifts, heavy machinery).
- Separate the floor from the ground and minimize the risk of moisture ingress.
- Ensure structural rigidity and stability of the bottom of the container, which is exposed to dynamic forces during transport and handling.
Cross-members are absolutely essential not only for the load-bearing capacity itself, but also for the long-term life and safety of the shipping container.
Anatomy and placement of cross-members in a container
Placement and orientation
Cross-members form the “skeleton” of the floor of every standardized ISO container. They are permanently welded to the bottom side rails, which form the long edges of the bottom frame of the container. The cross-members run across the container (from one side to the other) at regular intervals and are visible when looking at the container from below.
Typical layout example:
| Container type | Cross-member spacing | Number of cross-members (20’/40′) | Height of space under the floor |
|---|---|---|---|
| 20′ Standard | 25–30 cm | 16–23 | 15–17 cm |
| 40′ Standard | 25–30 cm | 34–45 | 15–17 cm |
| 40′ High Cube | 25–30 cm | 34–45 | 15–17 cm (lower in the gooseneck area) |
Space created by cross-members
- Floor protection: The gap (approx. 15 cm) allows air to flow under the floor, preventing moisture from being retained.
- Durability: Separation from the substrate protects the floor from direct contact with water, snow or mud.
- Foundation without concrete slab: The container can be placed on panels, gravel or pads without the need for massive foundations.
- Forklift Pockets: Some types have forklift pockets integrated in the space between the cross-members.
Connection to other components
Cross-members are a load-bearing element in the force transmission system:
- Floor → Cross-members → Bottom side rails → Corner posts → Corner castings
Key functions and importance of cross-members
Table of main functions
| Function | Description |
|---|---|
| Floor support | Ensure that the floor can withstand point and distributed loads according to ISO 1496-1 (e.g. up to 28 tonnes in a 20′ container) |
| Force transmission | Distribute and transfer loads to the frame and corner posts, allowing safe stacking and crane handling |
| Moisture protection | Prevent direct contact of the floor with the ground, extending its life and reducing the risk of rot and corrosion |
| Ensuring rigidity and stability | Protect the container from twisting, deformation and ensure permanent rectangular geometry of the frame even with repeated handling |
| Enabling mobile deployment | Thanks to the robust frame without foundations, containers can be used as warehouses, offices or construction units |
Strength and standards
Standards:
- ISO 1496-1: Defines the minimum requirements for load capacity and strength of cross-members.
- IICL: Sets inspection and repair limits for cross-members.
- CSC (Convention for Safe Containers): Specifies safety limits and periodic inspections.
Materials, construction and manufacturing process
Cross-member material
- Corten Steel (COR-TEN): Alloyed steel with high resistance to atmospheric corrosion. Creates a protective patina that prevents rust from penetrating deep.
- Alternatives: In some applications, high-strength steel with anti-corrosion treatment may be used.
Chemical composition of Corten steel:
| Element | Content (%) |
|---|---|
| Carbon (C) | 0.12 |
| Silicon (Si) | 0.25–0.75 |
| Manganese (Mn) | 0.20–0.50 |
| Chromium (Cr) | 0.40–0.65 |
| Nickel (Ni) | 0.65–1.20 |
| Copper (Cu) | 0.25–0.40 |
Cross-member profiles
- C-profile (most common): Cold-formed, high bending resistance.
- I/T/Z profile: Used in exposed locations or special constructions.
- Dimensions: Profile height approx. 100–120 mm, flange width approx. 50 mm, wall thickness 3–5 mm.
Manufacturing process
- Cutting and bending: Steel strips are cut and formed into a profile.
- Surface treatment: Application of a basic anti-corrosion coating (mostly grey) before welding.
- Welding: Robotically controlled welds with high quality control.
- Assembly: Precise placement and welding to the side rails.
Cross-member variants
- Standard: For normal loads and applications.
- Reinforced: For heavy-duty containers (e.g. transport of heavy machinery, storage).
- Gooseneck cross members: Reduced height in the “gooseneck” area for special chassis (mainly for 40′ High Cube containers).
- Compact with higher profile: For special containers with increased stiffness requirements.
Cross-members in the context of the overall container construction
Floor frame structure
| Component | Function | Material |
|---|---|---|
| Bottom Side Rail | Carries the load from the cross-members, forms the long sides of the frame | Corten steel, I-profile |
| Cross Member | Transfers forces from the floor to the side rails, forms the grid | Corten steel, C-profile |
| Door Sill | Provides stiffness and protection to the entrance | Corten steel, reinforced profile |
| Corner Post | Transfers vertical load during stacking | Corten steel, closed profile |
| Corner Casting | Allows handling, joining and stacking | Steel casting |
Importance for structural integrity
Cross-members are the connecting link between the floor and the frame. Their damage or significant corrosion leads to:
- Loss of floor load capacity (risk of floor cracking or collapsing).
- Endangering the safety of work and the transported cargo.
- Potential degradation of the entire container (fails CSC/IICL inspection).
Maintenance, inspection and repair of cross-members
Typical faults and damage
- Corrosion: The most common problem, especially in exposed areas (ends, door area, around welds).
- Deformation: Bending or sagging due to overloading, falling heavy objects or impact from forklift forks.
- Cracks: Caused by material fatigue or poor handling.
- Loosening of welds: Can lead to detachment of the cross-member from the frame.
Inspection and repair standards
- IICL (Institute of International Container Lessors): Sets limits for corrosion (e.g. max. 10% of profile cross-section), permissible deformation, tolerated cracks.
- ISO 1496-1: Defines the necessary properties and procedures for repairs.
Inspection procedure
- Visual inspection of the underside of the container (looking for rust, sagging, holes).
- Checking the condition of welds and connections to the side rails.
- Checking the floor from the inside (soft spots indicate cross-member damage).
- Measuring dimensions and determining material loss.
Repairs
- Local repair (patch repair): Cutting out the damaged section, inserting a new segment with overlap, quality welding.
- Replacement of the entire cross-member: In case of extensive corrosion or deformation.
- Anti-corrosion protection: After repair, always apply a primer and possibly additional protective layers.
CAUTION:
Inexpert interventions (welding a flat strip, covering the corroded part with sheet metal) are unacceptable and endanger safety.
Innovation, sustainability and recycling
Development and innovation in cross-member construction
- Higher strength at lower weight: Manufacturers are introducing new profiles and alloys with higher strength.
- Production automation: Robotic welding and precise coating thickness control.
- Anti-corrosion treatments: Use of advanced epoxy-based coatings that extend the life of cross-members in aggressive environments.
Sustainability and recycling
- Steel recycling: After the container is retired from service, the cross-members can be recycled up to 98%.
- Ecological coatings: Growing trend of using coatings that are environmentally friendly.
- Reuse of containers: Old containers serve as building modules, where cross-members are often retained and used in a new function.
Technical tables and comparisons
Typical dimensions and parameters of cross-members
| Parameter | 20′ Standard | 40′ Standard | 40′ High Cube |
|---|---|---|---|
| Number of cross-members | 18–23 | 38–45 | 38–45 |
| Spacing | 25–30 cm | 25–30 cm | 25–30 cm |
| Profile height | 100–120 mm | 100–120 mm | 100–120 mm (gooseneck: 60–80 mm) |
| Wall thickness | 3–5 mm | 3–5 mm | 3–5 mm |
| Material | Corten Steel | Corten Steel | Corten Steel |
Comparison with other floor support systems
| System | Advantages | Disadvantages | Use |
|---|---|---|---|
| Steel cross-members | High load capacity, simple production, easy replacement | Susceptibility to corrosion if not maintained | Standard ISO containers, storage containers |
| Wooden beams | Lower price, easy availability | Low life, susceptibility to rot | Special containers, limited use |
| Aluminum cross-members | Low weight, higher corrosion resistance | Higher price, lower load capacity | Special applications (air transport) |
Related terms and terminology
- Corten Steel: Alloyed steel with a patina that protects against corrosion.
- Bottom Side Rail: Longitudinal frame beam.
- Corner Casting: Casting in each corner for handling and stacking.
- Corner Post: Vertical beam carrying the load.
- Marine Plywood: Water-resistant plywood for container floors.
- IICL (Institute of International Container Lessors): Organization for inspection and repair standards.
Conclusion
Steel cross-members of a shipping container are the backbone of the entire floor system, without which safe, efficient and long-term transport of goods in containers would not be possible. Their design, material, placement and quality of welds determine the service life and value of the container on the market. Regular inspection, professional repairs and the use of quality materials are the key to the safety and long-term usability of every container. Cross-members are an example of the “hidden heroes” of global logistics, who do not need to be seen to be absolutely indispensable.