Structures Design

The deterioration of carbon-steel reinforced/prestressed (RC/PC) concrete and structural steel to is one of the prime causes for increasing maintenance costs and structurally deficient structures. In addition to being exposed to weather effects, transportation structures in Florida are also commonly located in aggressive environments such as chloride ion-rich coastal locations and inland water crossings with low pH (acidic) or high sulfate content (SO4). Structural steel is not permitted for use in the splash-zone as defined by the FDOT Structures Manual, and RC/PC structures with the splash-zone are typically required to utilize corrosion-resistant materials. Another innovative approach to combat this major issue is to utilize Fiber Reinforced Polymer (FRP) structures, members and/or components. FRP members are made from filaments or fibers bound in a polymeric resin matrix. FRP members of current interest are be made from various in-organic fibers such as glass (GFRP), basalt (BFRP) or carbon (CFRP). A surface coating is often provided for exposed elements to provide UV protection, or alternatively surface treatment (aggregate coating, deformations, or grooving) may be required at an interface to improve shear transfer to composite concrete surfaces.

Beneficial characteristics of FRP reinforcing include:

• It is highly resistant to chloride ion and chemical attack
• Its tensile strength is greater than that of steel yet it weighs only one quarter as much
• It is transparent to magnetic fields and radar frequencies
• GFRP and BFRP have has low electrical and thermal conductivity

Like any construction material, there are pros and cons to the use of FRP members:

• Due to its higher flexibility, typically elastic behavior, and emerging findings from ongoing research, current applicable design codes significantly reduce the allowable stress capacity that can be assumed when designing with FRP. Engineers must take into consideration the more stringent strength reduction factors and environmental reduction factors in the applicable design guidelines when designing with FRP reinforcing.
• Due to the manufacturing processes currently in use and the progressive standardization that they are undergoing, requirements for project specific acceptance testing of FRP can be more extensive compared to those which are required for traditional steel and concrete materials.
• Storage and handling requirements for FRP reinforcing on the construction site can be more restrictive due to FRP's susceptibility to damage from improper cutting, drilling, lifting techniques or aggressive handling.
• The initial cost of the FRP reinforcing is considerably higher than traditional materials. However, this higher initial cost may be partially offset by a reduction in the loads and the elimination of expensive corrosion-resistant prestressing or corrosion inhibiting admixtures typically used for steel reinforced concrete construction in extremely aggressive environments. A longer service life of the component may also be expected if FRP is used by reducing the need for repairs and eliminating cathodic protection or sacrificial anodes.

Due diligence must be performed by the designer to ensure FRP benefits outweigh the costs of implementation for each component or system.

Traditionally, composite materials like FRP have been used extensively in aerospace and consumer sporting goods applications where the material's high strength to weight ratios were first exploited. In the 1960s US Government agencies recognized the potential benefits that composites can provide to society's infrastructure and thus begin funding significant amounts of research in the field of FRPs. Since then advances in the field of polymers, advancements in production techniques and implementation of authoritative design guidelines have resulted in a rapid increase in usage of FRP materials, especially in the last 10 years. Because of these advances, the FDOT Structures Design Office has implemented specifications and design criteria to support the use of FRP materials in major bridge components. The use of these innovative material in certain Florida bridge components will keep Florida on the leading edge in the design of state-of-the-art transportation facilities.

Volume 4 of the Structures Manual (FRPG) provides guidance on where the members and structure can be used with and without prior approval from the SSDE:

• Fender Systems
• Boardwalks
• Maintenance Platforms
• Pedestrian Bridges
• Vehicular Bridge Beams

These usage restrictions take into consideration the following items:

• The criticality of the components and/or structural connections proposed
• The desirable service life of these components and/or structures
• The historical in-service performance of these components and/or structures that were designed, detailed and constructed using the conventional reinforcing steel, prestressing steel and concretes that are currently required.

See the following references for the application of FRP members:

• FDOT Structures Manual, Volume 4 - Fiber Reinforced Polymer Guidelines.
• AASHTO LRFD Guide Specifications for Design of Concrete-Filled FRP Tubes for Flexural and Axial Members, 1st Edition
• ASCE Pre-Standard for Load & Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures
• AASHTO Guide Specifications for Design of FRP Pedestrian Bridges.

The potential use of FRP members for a given application will be evaluated on a project by project basis. Extensive coordination with the Structures Design Office may be required in order to develop acceptable final designs. See Structures Manual, Volume 4, Fiber Reinforced Polymer Guidelines for more information.

Specifications 471 and 973 are available on the Specifications webpage for the use of FRP Fender Systems and similar components. Technical Special Provisions have been developed for several member types (such as Sheet Piles) which are available on request and will be converted to Developmental Specifications for broader application. Other specifications for structural components will be written and made available on an as-needed basis.

The following Standard Plans and associated Instructions are available on the Standards webpage for the standardized applications:

• Index 471-001 - Fender System - Prestressed Concrete Piles and FRP Wales

Future Developmental Standard Plans and associated Instructions will be published as needed on the Developmental Standard Plans webpage:

FRP producers seeking to be included on the FRP Production Facility Listing may find guidance for material acceptance on the State Materials Office Fiber Reinforced Polymer Composites webpage. Listing is only available for products specifically addressed in the FDOT Standards and Specifications.

Fast-Facts sheets for selected projects are listed below:

• Halls-River Bridge - Hybrid Composite Beams
• Skyplex - Composite Arch Bridge
• Ocala Water-Recharge Park Boardwalk

Technology Transfer (T²)

The following links to FDOT meetings, seminars and workshops are provide as background information for potential users and industry partners:

• 2018 ASCE-Florida Section Annual Conference - FDOT FRP Initiatives (July 12-13)
• 2019 TRB 2019 Workshop 1023: FRP Activities and Experiences Using FRP Composites (Jan 13)
• 2019 CAMX 2019: Florida Bridges and Structures for 100+ Years' Service with FRP Composites (Sept 24)
• 2019 FDOT-FRPM&S Industry Implementation Meetings - Outline & Summary Notes (Oct. 3)
• 2020 TRB Annual Meeting – FRP Workshop 1063 (Jan. 12)
• 2020 FTBA Construction Conference - FRP Composite Bridge Beam Implementation (Feb. 7)
• 2020 CAMX 2020 - Infrastructure Feature Speaker and Panel Presentations (Sept 23)
• 2021 TRB Annual Meeting - Session 1055 - "Forecasting the FRP Future for FDOT Highway Bridges and Structures" (Jan 25)

AASHTO Innovation Initiative (A.I.I.)

• FRP - New Bridge Material Design Options

FHWA FRP Composite Technology

• Current Practices and Design Information
• Seminars, Training, and Research
• Other Resources