In aluminum's case, like most homogeneous materials, shape and material characteristics (established above as design stress and section modulus) are used specifically to calculate a sheet pile's bending strength or bending moment capacity (see Detailed Engineering Analysis of a Sheet Pile Structure). Since bending will typically be the mode of failure with aluminum sheet piling, this should be the primary design criterion. Using the Euler-Bernoulli beam equation, shown below, we can combine material and shape properties to predict bending capacity, where M is the maximum allowable bending moment, Z is the section modulus, and s is design stress.

A maximum allowable bending moment of 27,625 (ft-lb/ft) is calculated for PZH-159

The maximum allowable bending moment should typically be used to select the appropriate sheet according to the requirements of the wall design.

Although deflection is not typically a structural constraint for homogeneous materials, and there are no current standard design limits for these type applications, it is commonly checked as a secondary measure to ensure that unwanted or excessive deflection does not occur and the aesthetics of the wall are maintained. Much like in bending strength, the combination of shape and material properties allow the prediction of how the structure will deflect under load. There are many different deflection equations, each depending on conditions specific to the application, but they are all a function of a sheet pile's stiffness, shown below, where E is the material's modulus of elasticity, and I is the shape's moment of inertia.

When aluminum is exposed to the atmosphere, a thin, invisible oxide layer forms instantly on its surface. This oxide layer gives aluminum its superior resistance to the damaging effects of corrosion. The United States Army Corps of Engineers states that as a material for sheet piling applications, "Aluminum has excellent corrosion resistance in a wide range of water and soil conditions because of the tough oxide film that forms on its surface. Although aluminum is an active metal in the galvanic series, this film affords excellent protection except in several special cases."1

The Aluminum Association adds "Unless exposed to some substance or condition which destroys this protective oxide coating, the metal remains resistant to corrosion. Aluminum is highly resistant to weathering, even in many industrial atmospheres which often corrode other metals. It is also resistant to many acids."2

If a "special case" is encountered, eliminating areas of concern in the design phase of the project can be much more effective and convenient than waiting for problems to arise in the field.

While 6000 series aluminum is the best metal for use in waterfront construction, there are several design options that may be considered for added protection (see Corrosion Resistance of Aluminum a CMI white paper). If possible, dissimilar metal contact should be avoided to limit galvanic action. If non-aluminum fasteners will be used, stainless steel is preferred, but hot dipped galvanized fasteners are also recommended by The Aluminum Association. To further limit galvanic action, it is also recommended that you isolate dissimilar metals with a non-absorbent insulator like paints or non-porous synthetics. Some less obvious dissimilar metal contact may result from the use of pressure treated wood (CCA, ACQ, etc.), and re-enforced concrete.Aluminum's tough oxide layer is generally stable in the 4.5 to 8.5 pH range, making it suitable in the most common marine environments, however checks should be made to ensure that native soils and backfill materials are not extremely acidic (high chloride content, etc.) or basic (High clay or organic content, most concretes, etc.). If economical to do so, it is recommended that areas that require contact with extreme pH levels also be isolated with a resistant, nonabsorbent insulator. Due to the typically high pH of most aggregates, isolation is typical when forming a concrete cap. In the unusual event of soils with extreme pH levels, to further reduce the chance of damage due to corrosion, cathodic protection or a sacrificial anode systems may be applied. These tools enhance the corrosion resisting ability of aluminum.

· There are several key steps to consider when designing with aluminum sheet piling.

· Ensure that an appropriate alloy is being used in regards to both strength requirements and environment.

· Establish an appropriate design stress.

· Determine the section modulus from the cross section of the selected sheet pile.

· Determine the maximum bending moment to be carried by the selected sheet pile, and ensure that this value is greater than that calculated for your wall design requirements.

· Check for undesirable deflection.

It is our hope that, with the steps laid out and information presented here, the designer or end user will be able to make a more informed and responsible decision regarding the selection and application of aluminum sheet piling

1. United States Army Corps of Engineers. EM 1110-2-1614 Design of Seawalls and Bulkheads. Washington DC: USACE, 1995

2. The Aluminum Association. Specifications & Guidelines for Aluminum Structures. 8th ed. Arlington, VA: The Aluminum Association, 2005