Technical Information > Floor Frame (Bottom Rail and Cross Members)

Floor Frame (Bottom Rail and Cross Members)

The floor frame of a shipping container, often referred to by its English terms the “understructure” or “floor frame,” is a fundamental structural component that significantly affects the strength, durability, and safety of the entire container unit. Although largely hidden beneath the floor, its proper construction and condition determine whether the container can reliably transport dozens of tons of cargo across continents and oceans without damage. This article will detail the individual parts of the floor frame, their function, design, standards, and recommendations for operation and maintenance.

Main Definitions and Functions of the Floor Frame

What is the floor frame of a shipping container?

The floor frame is a welded steel structure forming the base of the container, to which four lower corner castings are attached. Its main roles include:

  • Load-bearing function – carries and distributes the entire weight of the cargo and floor.
  • Load distribution – transfers forces generated during handling, transport, and storage to the vertical corner posts and then to the entire container structure.
  • Structural rigidity – prevents deformation, sagging, and twisting of the floor and the entire bottom of the container.
  • Protection – protects the floor from moisture, mechanical damage, and allows air circulation under the container.

Key Standards and Certifications

Floor frames must meet international standards, in particular:

  • ISO 668 – Dimensions and classification of containers.
  • ISO 1496-1 – Structural and testing requirements for general containers.
  • ISO 1161 – Specifications for corner castings.
  • CSC (International Convention for Safe Containers) – Safety certification for transport.

All containers are certified by an independent classification society (e.g., GL, BV, ABS, LR, RINA) and are subject to regular inspections.

Detailed Analysis of Floor Frame Components

The floor frame consists mainly of two types of profiles:

  • Bottom Rails/Bottom Side Rails
  • Cross Members

Bottom Rails

Description and Function

  • Two main steel profiles running the entire length of the container along its edges.
  • Provide longitudinal stability to the structure, transferring forces to the corner posts and corner castings.
  • Serve as a base for welding cross members and define the width of the container.

Material, Dimensions, and Profile

  • Material: High-strength Corten steel (COR-TEN A or B), which creates a protective rust layer and is extremely resistant to atmospheric corrosion.
  • Standard profile dimension: C-profile or box-profile, e.g., 100 × 100 × 15 mm (source: CASSSC Structure Sheet), length 6,058 mm for a 20’ container.
  • Wall thickness: Typically 4–6 mm, depending on the type and size of the container.
  • Surface treatment: Anti-corrosion coating, often a combination of primer and topcoat according to ISO 12944.

Construction Details

  • Profiles are welded to the lower corner castings and corner posts.
  • Often contain openings for so-called “forklift pockets” (see below).

Cross Members

Description and Function

  • Steel profiles running across between the bottom rails along the entire width of the container.
  • Provide direct support for the floor and distribute point loads (e.g., from forklift wheels).
  • Ensure transverse rigidity of the frame and prevent expansion or contraction of the rails.

Material, Dimensions, and Spacing

  • Material: Corten steel of the same properties as the rails.
  • Profile dimensions: Most often C- or I-profile with dimensions of approximately 80 × 45 × 4 mm (source: CASSSC Sheet, Containi technical sheets).
  • Spacing (distance between beams): Standard 300–400 mm, even less for heavy-duty containers.
  • Wall thickness: 3–5 mm, depending on the required load capacity.

Fastening and Construction

  • Welded to the inner surfaces of the bottom rails.
  • In the areas of “forklift pockets,” some beams are omitted or reinforced.
  • The ends of the beams often have drainage holes for water drainage and corrosion prevention.

Table: Technical parameters of the floor frame (example of a 20′ ISO container)

ComponentMaterialProfileDimension (mm)Thickness (mm)Spacing (mm)
Bottom railCorten steelC/box100 × 1004–6–
Cross memberCorten steelC/I80 × 453–5300–400
Forklift pocketCorten steelC-profile360 × 1155–

Floor and its connection to the frame

  • Floor material: 28–30 mm thick, multi-layer marine plywood (certified against pests according to ISPM 15).
  • Assembly: The plywood is screwed with self-tapping screws to the upper edge of the cross members (every approximately 250 mm).
  • This creates a composite structure, combining the strength of the steel frame with the surface rigidity and durability of the plywood.

Integrated and Special Elements of the Floor Frame

Forklift Pockets

  • Construction: Steel tunnels reinforced at the point of passage of the cross members (often reinforced sheets, box profiles).
  • Dimensions according to ISO: Width 360 mm, height 115 mm, distance between the centers of the pockets 2,080 mm.
  • Function: Allow safe lifting of an empty container by a forklift. They are not intended for handling a fully loaded container!
  • Standard: Location and dimensions of pockets defined by ISO 1496-1.

Gooseneck tunnel

  • Specific to 40′ and 45′ containers: Tunnel in the front of the frame for lower seating on special semi-trailers (gooseneck chassis).
  • Construction: Reinforcement and reduction of the profile in the middle of the frame, modification of the cross members in the area.

Load, Force Transfer, and Structural Significance of the Frame

Load Transfer Scheme

  1. The weight of the cargo acts on the plywood floor.
  2. The force is transferred to the cross members.
  3. The cross members distribute the load to the bottom rails.
  4. The bottom rails transfer the force to the corner castings and posts.
  5. The entire frame allows safe lifting, stacking, and transport even under extreme loads (7 containers on top of each other, each up to 30,480 kg).

Strength and Testing Requirements

  • Static floor test: According to ISO 1496-1, the floor must withstand a point load from a forklift wheel of at least 5,460 kg on an area of 142 cm².
  • Stacking load: The container must withstand 192,000 kg transferred through the corner castings (corresponding to 7 containers in a stack).
  • Watertightness test: The frame must not show any cracks or deformations.

Inspection, Common Damage, and Repairs

Typical Damage

  • Corrosion: Most often in places with water accumulation, dirt, or mechanical damage to the coating.
  • Deformation and cracks: Result of overloading, improper handling, or material fatigue.
  • Mechanical damage: Impacts from forklift forks, falling heavy cargo, slipping of the container from the ramp.

Inspection

  • Visual inspection: Regularly before and after each deployment, especially for rented containers.
  • Corrosion measurement: According to IICL, the maximum permissible loss of profile thickness is approximately 15%.
  • Joint inspection: Welded joints are checked for cracks, peeling of the coating, and signs of overheating.

Repairs

  • Standards: All repairs must comply with IICL, CSC, and ISO 1496. Each intervention must be documented.
  • Cross member repair: In case of damage, only the damaged part is cut off, the upper flange in contact with the floor is left to maintain strength.
  • Surface treatment: After repair, it is always necessary to restore the anti-corrosion protection.

Related Terms and Components

  • Corner Castings: The only points by which the container can be lifted, stacked, and secured.
  • Corner Posts: Main vertical load-bearing elements, connecting the upper and lower frame.
  • Top Side Rails: Analogous to the bottom rails, forming the upper frame of the container.
  • Door Sill and Front Bottom Rail: Cross members closing the frame on the rear (door) and front sides.
  • Corten Steel: Special low-alloy steel with high resistance to weathering.
  • ISO Standards: Standards specifying construction requirements, dimensions, testing, and marking of shipping containers.

Practical Advice for Operators and Owners of Containers

  • Regular inspection of the frame condition is essential for maintaining CSC certification.
  • Corrosion prevention: Regularly clean the bottom of the container, renew coatings.
  • Observe the maximum payload and never handle a loaded container via forklift pockets.
  • Entrust repairs only to certified workshops with experience in container technology.

Conclusion

The floor frame with cross beams and bottom rails is a key element on which the safety, load capacity and service life of every shipping container depend. Its high-quality construction, regular maintenance and professional repairs are essential not only for the reliable transport of goods, but also for meeting all international certification requirements. Understanding this part of the container is essential for everyone who works with containers – from logistics to owners to technicians performing repairs and inspections.