Aluminum offers the familiarity and comfort of a structural metal, and corrosion and weatherability characteristics superior to those of steel or wood. Aluminum has a very high strength-to-weight ratio, higher than most other metals and 3-5 times that of steel, making it cost effective and easy to handle on the job site. These characteristics explain why aluminum is the material of choice in the aviation and automotive industries. In addition, aluminum exhibits excellent corrosion resistance, illustrated by its dominance in food packaging and marine applications. The marine environment delivers excessively harsh and corrosive conditions, making aluminum a top performer as a sheet pile material.

As for all load bearing structures, it is important that proper techniques are followed when designing an aluminum sheet pile wall. This paper will aid the designer in establishing the proper criteria for aluminum sheet pile selection and application.

There are over 100 variations of aluminum alloys and tempers commercially available, each with its unique strengths and weaknesses. In its pure form, aluminum is a soft and relatively weak material that would struggle to perform in modern industrial applications. The first step towards achieving high structural performance is the addition of small amounts of other elements, which dramatically impact aluminum's strength. This process is known as alloying, and the resulting blend is designated with a 4-digit identifier based on the alloying elements. Although aluminum is renowned for its excellent corrosion resistance, many alloys have been developed for specific applications where corrosion is a secondary or nonissue and have correspondingly poor corrosion fighting abilities. Alloy selection can drastically affect corrosion characteristics and it is imperative that the appropriate alloy be selected to perform as expected in the field. For marine applications, the 6000 series provides the best combination of low cost, excellent corrosion resistance, and strength. The second step in achieving higher structural performance is a process known as tempering. For the most common structural alloys, this is a heat treatment that further increases the material's strength. This process is designated by a 2-digit identifier that follows the alloy designation based on the type and number of tempering stages. CMI selects the most suitable alloy and temper for load bearing structures in the marine environment, 6061-T6, for use in all its aluminum sheet piling.

When designing a sheet pile wall, two characteristics, shape properties and material properties, are used to calculate an appropriate sheet for a project. First, it is necessary to establish the values needed for design.

Establishing the Moment of Inertia (I)

A body's moment of inertia is a geometric property of its cross section, used to predict its resistance to bending and deflection. Depending on the specific shape of this cross section, calculation can be very complicated and difficult. Most CAD programs today include a feature that can calculate this value for a given profile, but it is important to understand what is involved and about which axis the shape's moment of inertia is calculated. Since sheet piling varies in width, the moment of inertia is generally described per running foot of wall for an apples-to-apples comparison.

Establishing Section Modulus (Z)

For bending moment calculations, a sheet's shape properties are determined solely by the geometry of the profile and combined into one design value, section modulus. Section modulus is defined below, where Z is section modulus, I is the shape's moment of inertia, and y is the distance from the outer face to the neutral axis (typically approximated by ½ the section depth for sheet piling).

A section modulus of 17.0 (in3/ft) is predicted for PZH-159, a heavy duty sheet offered by CMI.

Establishing Design Stress (s)

Material properties used in bending moment calculations can be more challenging to quantify due to their inevitable inconsistencies and differing applications. Unlike Section Modulus, which is entirely objective, design stress is determined by taking the maximum stress a material can withstand before failure and applying a chosen level of safety, making it much more subjective. Fortunately, The Aluminum Association gives guidelines for determining the design stress for a given structural component.

The Aluminum Association is a non-profit organization that represents the leading producers of aluminum and aluminum products in the U.S. and abroad. They publish the U.S. standard for aluminum structural design, Specification & Guidelines for Aluminum Structures. Only methods endorsed by The Aluminum Association should be used for structural aluminum design. At CMI, we follow their methodology to establish allowable stress for design and ultimately an allowable bending strength or bending moment.

For typical applications involving sheet piling made from 6061-T6 aluminum, a factor of safety of 1.95 is applied to the material's maximum rated or ultimate tensile strength, 38,000 psi, to arrive at the design stress value of 19,500 psi.

Establishing Modulus of Elasticity (E)

Modulus of elasticity is a material property that describes a substance's tendency to deform under an applied load. Like design stress, this design value is published in The Aluminum Association's Specification & Guidelines for Aluminum Structures. They recommend using an E value of 10,000,000 psi for design with all 6000 series aluminum.