Container ships have very large cargo hatch openings, which remove a large part of the deck and the upper side structure, so the hull girder is an open section that is weak in torsion. When the ship is loaded asymmetrically (containers of different weights on the two sides) or when it is in a seaway with the wave acting unevenly, the open section twists, producing torsional (warping) stresses in the structure. The structure is designed to combat torsion by:

• Providing a strong closed section at the ends (the fore and aft peaks and the machinery space) which resist the torsion.
• Using a strong double-bottom and a strong side structure, with the hatch coamings acting as longitudinal girders.
• Fitting strong transverse (web) frames and a strong deck structure, and using the hatch coamings and the side shell to form a torsion box.
• Using high-tensile steel and adequate scantlings in the deck and side to resist the warping stresses.
• Ensuring the cargo is loaded symmetrically to minimise the torsional moment.

Discontinuities in the longitudinal structure (e.g. at hatch corners, openings, changes in section, the ends of the superstructure, the break of the deck) create stress concentrations. At these points the longitudinal stress is concentrated, and the local stresses can be several times the average stress, leading to fatigue cracking and failure. The problem is that the longitudinal material is interrupted, so the load must be transferred around the opening, producing high local stresses at the corners and edges.

1. Hatch corners: the corners of large hatch openings are stress raisers. They are overcome by making the corners rounded (large radius), fitting doubling plates and soft patches, and providing adequate local reinforcement.
2. The break of the deck / ends of the superstructure: the sudden change in section at the ends of the superstructure or the break of the deck creates stress concentrations. They are overcome by tapering the structure, fitting doubling plates, and providing adequate local reinforcement and smooth transitions.
3. Openings in the shell or deck (e.g. doors, scuttles, air pipes, sea chests): these interrupt the longitudinal material. They are overcome by fitting compensating (doubling) plates around the openings, rounding the corners, and providing adequate local reinforcement.