The most important factor when designing with any FRP composite product is to ensure that your supplier fully understands the materials and has a complete and broadly tested set of performance parameters. It is extremely important to ensure that the product you are incorporating in your design has been comprehensively evaluated by the manufacturer in both directional and localized stress analyses as well as full section testing. The capabilities of CMI's UltraComposite products have therefore been specified based on actual full section testing and include a design procedure that incorporates global deflections and insures real world performance will be accurately predicted.

Relying on an allowable moment alone can be disastrous for your FRP composite project

More often than not, the actual performance of an FRP composite product will be controlled by the localized strains and global deflections. Stress and moment limitations alone can overstate product capabilities and although theoretically possible are rarely seen and may not be obtainable in your specific application. FRP composites will normally suffer deflections beyond their limitations before they reach their bending load limits. It is often possible for an FRP product to fail long before a simple moment capacity is reached even with the incorporation of significant factors of safety. It is therefore standard throughout the FRP composites industry to use deflections as a primary design constraint. Even with the incorporation of a comprehensive Finite Element Analysis (FEA) design program relying on an allowable moment can result in overstating product capability and subsequent failure.

Simply put, a product may be strong enough to hold massive amounts of weight when calculated theoretically but it can only hold so much before it deflects beyond its capabilities.

Coupon test based properties are not sufficient for FRP composite design

The Strength Rating is theoretical only and is not based on full section testing of finished product. Certain competitors substantially overstate the capability of their FRP products by recklessly using this theoretical Strength Rating as an Allowable Moment. While this is common place in vinyl and steel sheet piling design, it is a recipe for disaster with FRP composites and here's why: unlike homogeneous and isotropic materials like vinyl and steel, FRP composites, are anisotropic in nature, meaning they have different strength and stiffness properties in all directions. Furthermore, due to the complex anisotropic nature of the of FRP materials, traditional design and analysis methods are no longer appropriate. Because of shear deformations, cross sectional stress transfer, and localized buckling, FRP composite products rarely perform to the bending performance predicted by traditional solid mechanics and beam theory.

Using an allowable moment for FRP composite products based only on coupon material properties is analogous to designing a concrete beam using all the same properties as steel, including tensile strength in all directions, just because it has some steel in it. It is as simple as looking at the specification for any FRP composite product, they all show drastically different properties in different directions. You can't just pick the most favorable property and base your entire design on it.

The Strength Rating for CMI's FRP products is simply a means of comparing competitive FRP products on an equal level. Strength Rating is a theoretical sheet piling characteristic based upon standard engineering practice where material flexural or tensile strengths are reduced by a predetermined reduction factor and then combined with cross-sectional shape properties to provide an indication of overall product bending performance. The standard calculation for Strength Rating is:

Where s F is flexural strength, FOS is the predetermined reduction factor or factor of safety and Z is section modulus.

Sample Strength Rating Calculation

For example the Strength Rating for GeoGuard 30 is calculated by:

When using FRP composite sheet piling the standard design procedure does not differ substantially from designing with vinyl or steel sheet piling. The main difference is for the designer to calculate and check deflections as a primary factor rather than secondary like with steel or vinyl. The deflection based design procedure itself is relatively simple: use deflection as a design limit rather than a serviceability limit. However, the designer must beware, using improper calculation inputs like a standard modulus rather than Apparent Modulus can result in significantly flawed results.