Report of ISSC 2015 Committee III.1 Ultimate Strength

Ultimate strength is a critical and fundamental assessment in the design of a ship or offshore structure. The global ultimate strength of a structure is usually first assessed in the early phases of design. Evaluation of the local ultimate strength of scantlings is also an important part of structural design and analysis. Buckling and elasto-plastic collapse dominate the ultimate strength of slender members in compression while yielding dominates the ultimate strength of members in tension.

This report concerns ductile behaviour of structural and components. Due to the improvements of toughness of the material used in ships and offshore structures, brittle fracture is now a rare occurrence. For plates greater than 50 mm in thickness, such as may be used in the upper deck plating of a container ship, brittle fracture can developed as the result of fatigue crack propagation from an initial defect. As this can be avoided through adequate inspection of defects, the limit state of brittle fracture is excluded from discussion in this report.

It is now common to design ship and offshore structures to withstand buckling or yielding under the design load. However, until the middle of the 19th century, the design criterion was the breaking strength of the material. This is partly because brittle iron of low tensile strength was used for ship structures at that time, and partly because buckling was not then well understood. It was after (Bryan, 1891) that panel buckling was theoretically understood and calculated.

From the beginning of the 20th century, it had become common to consider the buckling strength as a design criterion. Moving into the 21st century, this has evolved into the ultimate strength criteria.

The first attempt to evaluate the ultimate strength of ship structure was performed by Caldwell (1965). He applied “Rigid Plastic Mechanism Analysis” to evaluate the ultimate hull girder strength. The influence of buckling was considered by reducing the yield stress of the material in the buckled part.

The finite element method (FEM) was introduced in 1956 by Turner et al. (1956). At first FEM was used only for the analysis of elastic behaviour of structures. To evaluate the ultimate strength of structural members and systems, it is necessary to consider the influence of both buckling of structural members and yielding of materials. Since the early 1970’s, such analysis, so called elasto-plastic large deflection analysis, has become possible to perform using FEM. However, computations were time consuming and it was two decades before commercial codes were commonly used for such analysis.

As with previous ISSC reports, a literature survey is performed related to assessment procedures for ultimate strength (see Chapter 3) and ultimate strength of various structures, such as tubular members, plates and stiffened plates, shells, ship structures, offshore structures, composite structures, and aluminum structures (see Chapter 4). It was from the 10th ISSC that benchmark calculation using different nonlinear codes were completed by this committee (Oliveira, 1988), and benchmark calculations have been performed in each committee since 1988.

For this report two benchmark calculations are performed (see Chapter 5). Both benchmarks show the application of nonlinear finite element analysis to the ultimate strength problem. The first investigates a box girder ultimate strength under four point bending loads and compares results with the experimental results by Gordo et al. (2009). The second benchmark investigates the hull girder ultimate strength of a bulk carrier considering the effect of initial imperfections and lateral loadings.

To complete a rational based design, it is usual to consider the ultimate strength as the strength standard instead of yielding and buckling strength. Recent developments in design standards for the marine industry have led to the Goal-Based New Ship Construction Standards (GBS) by International Maritime Organization (IMO), and Common Structural Rules (CSR) by International Association of Classification Societies (IACS). The GBS consists of five tiers as indicated in Figure 1. In Tier I, goals are specified for design and construction of new ships. In Tier II, functional requirements are specified to achieve the goals. Tier III, is verification of Tier IV, which is an existing framework of regulations, IMO conventions and rules of recognized organization such as classification societies. CSR are closely related to GBS though Tier IV. An important aspect of CSR is the requirement to evaluate ultimate hull girder strength as well as the ultimate strength of plates and stiffened plates.

In the Common Structure Rules (CSR), which came into force in April 2006, the multi-step procedure (Smith’s method) is applied for calculating hull girder ultimate strength of bulk carriers (CSR-BC). In the Harmonized Common Structure Rules (CSR-H), which combines CSR-BC(for bulk carriers) and CSR-T (for double hull tankers) and comes into force in 2016, a double bottom factor, γdb will be introduced to take account of the effect of lateral loading on the hull girder ultimate strength. Structural redundancy is also taken into account in CSR-H according to SOLAS regulations.

With these developments ultimate strength assessment is now becoming a more important issue to ensure the safety of ship structures. From this point of view the role of this committee continues to be very important.