Frequently Asked Questions

FRP offers several advantages over traditional building materials: 1. High strength-to-weight ratio 2. Corrosion Resistance 3. Design Flexibility 4. Electrical and Thermal Insulation 5. Reduced Maintenance 6. Fast Installation

Fiber Reinforced Plastic can be manufactured in many ways; the two most popular methods for manufacturing structural FRP components are vacuum infusion molding and pultrusion. Both processes produce parts with high strength-to-weight ratios, durability, and corrosion resistance. Although similar there are a few key differences to note when designing a molded FRP part versus a pultruded part. Pultruded FRP is limited to the forms and dyes that are available by the manufacturer. However, pultruded parts can be produced faster and in greater quantities with a tighter tolerance with most shapes available to purchase off the shelf. Molded FRP is not limited to the tooling readily available and can be formed into custom dimensions and shapes desired but will take longer to produce and will require the engineer to calculate properties instead of finding the data published by the manufacturer. Molded FRP excels in lower quantity and complex parts, while also producing the same shape with varying levels of strength and stiffness to meet the demands of the project. When designing FRP the two manufacturing methods follow different design approaches. Pultruded FRP parts have an LRFD (load and resistance factor design) standard called “Pre-Standard for Load & Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures” that was put out by ASCE. When designing a Molded FRP part the typical method is to use ASD (allowable stress design) approach since no LRFD standard has been made as of publishing this article.

ASTM D7290 is formally titled “Standard practice for Evaluating Material Property Characteristic Values for Polymeric Composites for Civil Engineering Structural Applications”. It outlines the process for determining the characteristic properties of FRP for engineers to use in their design. It states in the scope “The characteristic value is a statistically based material property representing the 80% lower confidence bound on the 5th-percentile value of a specified population.” In other words, the property determined from this ASTM practice is on the low end of all tested values so that we have a high confidence that the stated value will be correct for design of structural members and can be used for qualification and acceptance criteria for designs. The practice outlined in ASTM D7290 includes exposing the material to conditions of moisture, temperature, and UV radiation to see how the properties of FRP change over time of exposure. The material is then tested to evaluate changes in mechanical properties such as tensile strength, flexural strength, modulus of elasticity, and impact resistance.

The design process for Fiber Reinforced Plastic (FRP) components differs from traditional materials like steel or concrete due to several factors. Here are some key differences in the design process: Material Properties: FRP materials have different mechanical properties compared to traditional materials. FRP has comparable strength to steel but a lower stiffness which means that geometries will differ when comparing FRP to steel. FRP is also anisotropic (meaning the mechanical properties vary with fiber orientation). This requires a thorough understanding of FRP material behavior and the consideration of anisotropy during the design process. Design Codes and Standards: Traditional materials like steel and concrete have well-established design codes and standards that provide guidelines for structural design. However, FRP materials have relatively few design codes and standards specific to their unique characteristics. Failure Modes: FRP components exhibit different failure modes compared to steel or concrete. FRP has lower ductility and will rupture at ultimate capacity with no yielding beforehand. The design process for FRP requires considering these distinct behaviors, such as progressive fiber failure or delamination, to ensure appropriate safety factors and design margins. Life Cycle Considerations: Life cycle is an important aspect of structural design. Components may be exposed to environmental factors like UV radiation, moisture, and chemicals. The design process for FRP requires considering the long-term durability and degradation of the material due to these factors, as well as selecting appropriate surface coatings or protective measures. Manufacturing and Installation: FRP components often require different manufacturing and installation techniques compared to traditional materials. Molded or pultruded processes are commonly used for FRP fabrication, and the design process needs to account for these manufacturing methods. Additionally, installation considerations such as specialized connections, adhesive bonding, or fastening techniques specific to FRP need to be incorporated into the design process. Analysis and Modeling: Analytical and numerical modeling techniques for FRP differ from those used for steel or concrete. FRP's anisotropic behavior and the need to consider failure modes like fiber breakage or delamination require specialized modeling approaches. Advanced modeling techniques, such as finite element analysis (FEA), are often employed to accurately predict the behavior of FRP components under various loading conditions. Quality Control and Inspection: Quality control and inspection practices for FRP may vary from those for traditional materials. The design process for FRP components should include considerations for quality control during manufacturing and field inspection to ensure compliance with design requirements and industry standards.

When designing with molded FRP (Fiber Reinforced Polymer), allowable stress design (ASD) approach is used to ensure structural integrity. In the case of bending stress and shear stress, it is common to use safety factors that are 20% of the ultimate load (SF=5). These safety factors serve as a margin of safety and account for uncertainties in material properties, fabrication, and service conditions. By employing conservative factors, the design can withstand loads well beyond the anticipated working conditions. Although bending and shear will almost always be limiting factors in the design, it is important not to overlook other safety factors such as bearing capacity and embed pull out strength. These factors should be carefully considered to ensure a comprehensive and robust design, guaranteeing the safety and reliability of FRP structures. Pultruded FRP components take a different approach for design and use the LRFD approach. Pultruded components should follow the ASCE “Standard for Load & Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures” to apply the correct factored loads and knockdown values to determine the proper part to use in a design.

Determining proper fiber orientation and ply thickness involves considering several factors such as loading conditions, mechanical properties, and manufacturing constraints. Below are some tips for guiding the process: Find the design requirements for the FRP component such as structural loads, deflection limits, environmental conditions, and required mechanical properties. Identify the type of reinforcing fibers and the polymer matrix to be used in the design. Since FRP is anisotropic it is important to know the mechanical properties in each direction of the materials that will be used in the design. Perform mechanical analysis of the load conditions to determine the design stresses and determine if there are any critical zones that will need additional reinforcement. Check in different directions to make sure that the fiber lay up properties work for all loading conditions and directions. Optimize the ply orientation and thickness based on the mechanical analysis to make sure it will resist the applied loads while still being feasible to manufacture. If applicable testing is recommended to validate the design of the component. It's worth noting that designing FRP components can be a complex process that may require expertise in FRP materials and structural engineering. Engaging with experienced professionals or consultants specializing in FRP design can provide valuable guidance and ensure the successful development of the FRP component.

When looking at the cost of FRP compared to traditional materials, the life cycle cost of the project needs to be considered. The initial comparison on cost of materials is always fluctuating as supply chains ebb and flow, but FRP typically costs more from a material standpoint. However, the cost of the project tips in FRP’s favor when looking at production and installation timeline. Currently, the lead time for complex steel structures is over a year to receive the part while FRP can be procured in a few months. FRP requires less trucking to transport the parts, smaller equipment to install on site, and less time to complete than concrete or steel construction. It is also far cheaper to maintain FRP products due to the ability to withstand corrosion. A recent United States Army Corp of Engineers life cycle cost analysis revealed total cost of a project when using FRP can be as much as 90% cheaper than using traditional materials.

Fiber Reinforced Polymer can have a negative connotation due to its association with other resin and plastic materials. FRP is thought to be brittle and weak compared to the standard materials of steel and concrete. However, FRP can be a highly durable material with the right fiber and resin matrix. It can withstand collisions with large marine vessels like barges, submarines, and aircraft carries. It also holds up well under small, localized impacts like floating logs down the river. One of our favorite examples is to set out a sample of FRP and let customers take turns hitting it with a sledgehammer so that we can show that no structural damage is done to the part. If FRP needs to be repaired there are maintenance guides on how to handle the repair process depending on the manufacturing method originally used to make the part. Most of the time a localized repair is possible and can be done in a matter of hours. It is simple to train technicians on the repair processes and how to use the equipment necessary to complete the job, like how a welder will need the right training and equipment for steel maintenance or repairs.

Unfortunately, there is nothing as encompassing as the steel manual (AISC) or Building code requirements for structural concrete (ACI) for FRP materials. There are multiple specialized specifications for various components made of FRP. Below is a list of specifications that Axcess has used in their design work: The most popular standard is for pultruded structures by ASCE called “Standard for Load & Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures”. AASHTO has published a design for pedestrian bridges constructed from FRP called “Guide Specification for Design of FRP Pedestrian Bridges”. FDOT Structures Design Guidelines along with Section 471 & 973 discuss the design of Fiber Reinforced Polymer Fender Systems. The Army Corp of Engineers (USACE) has published multiple documents on FRP including various testing and results of FRP material.

FRP material does well in many environmental conditions. It withstands highly corrosive environments from marine infrastructure to toxic chemical containers. There is minimal impact on FRP in freeze thaw cycles. FRP does degrade under UV exposure, however the rate of degradation is about a thousandth of an inch per year. This can be prevented by applying coatings to the part exposed to the sun or designing the FRP to allow material degradation that will not impact the structural integrity of the part by making it thicker. The design properties do receive a reduction factor to account for environmental conditions during the service life of the part. Tensile, compression and shear strength receive approximately 10% reduction. Likewise, the bending and shear modulus also receive a knockdown that is approximately <5%. These knockdowns are based on the type of material used, so it is best to work with a manufacturer or design specialist when determining what values to use for environmental knockdowns.

Vinyl-ester based FRP parts do not pass the building fire codes for most building structures. However, traditional materials also struggle with fire and high heat, recent history has shown steel bridges that had to be closed due to fires on or under them. In those instances, the steel was not designed to handle that type of condition, however materials in buildings and critical structures need to be able to perform a certain way when exposed to fire. The solution for traditional material is to add coatings or surround the material with a more fire-resistant material to protect it. FRP follows the same approach to solve fire resistance, a different material is used instead of the vinyl-ester matrix. The panel uses a phenolic resin which allows the FRP to pass building code requirements for fire situations.

FRP has been in structural designs consistently for the past 20 years. State DOTs have used it successfully for vehicle and pedestrian bridge decking and fender systems. The United States Navy has used it for their camel design to dock submarines and aircraft carriers. The Army Corp of Engineers has used it for wicket gates along the Illinois river. FRP is even moving more into the private markets for foundation piles and curtain walls. FRP has proven it is durable, strong, and capable of handling structural applications.