Advanced Airport Pavement Design Procedures
FAARFIELD - 3D Finite Element Based Design Procedure
Introduction
The last decade has witnessed an unprecedented increase in the speed and power of personal computers. Because of this revolution in personal computing power, PC-based programs can now perform designs in minutes that not long ago would have required hours or even days of processing on the most common office computers. As computer technology continues to advance, even more sophisticated models will become available to the pavement designer. The FAA is currently developing a new generation of PC-based airport pavement design tools that employ advanced computer programs based on the three-dimensional finite element method (3D-FEM). These procedures will be capable of designing the future airport pavements to serve new aircraft types, including new large aircraft with 6 or more wheels per gear now in the conceptual stages
The 3D-FEM can handle greater detail and more complex characterizations of construction materials than can layered elastic analysis (LEA). It is particularly useful for modeling rigid pavements, since the slab edges and joints that are often the critical components in rigid pavements can be modeled directly, something not possible in LEA. In addition, 3D-FEM can incorporate nonlinear and non-elastic material models not available in LEA.
In finite element modeling, the structure is divided into a large number of smaller parts, or “elements.” Elements may be of different sizes, and may be assigned different properties depending on their location within the structure. By breaking down the structure into elements, the pavement structural problem is transformed into a finite set of equations that can be solved using a computer. Because a structure can be broken down into finite elements, or “discretized,” in any number of ways, it is essential that any design standard incorporating finite elements include rules for systematic three-dimensional meshing. To be a true standard, the model must have specified element formulations, material properties and element sizes for each of the layers, since all of these factors can affect the predicted response and the design engineer will expect consistent behavior from the design tool. At the same time, the design program should be as “user-friendly” as possible, with all modeling decisions such as element sizes and shapes made at the programming level transparently to the end user.
The FAA has developed a new airport pavement thickness design procedure called FAARFIELD (for FAA Rigid and Flexible Iterative Elastic Layered Design) that incorporates 3D-FEM for rigid pavements and overlays. FAARFIELD combines the FAA’s layered elastic analysis program LEAF, and a 3D-FEM structural model, using each structural model where it is most appropriate. For flexible pavements and flexible overlays, the FAARFIELD design procedure is essentially the same as LEDFAA 1.3. The thickness design is based on strain computed using the LEAF model. For rigid pavements and overlays, however, there are significant differences from LEDFAA 1.3. The design stress is now computed using a 3D-FEM model of an edge-loaded slab on a multiple-layer support structure. In addition, the rigid pavement failure model, which relates the computed 3D-FEM stress to the predicted pavement life, has been completely updated based on full-scale tests conducted at the NAPTF in 2004.
In September 2009, the FAA issued AC 150/5320-6E Airport Pavement Design and Evaluation, which adopted FAARFIELD version 1.302 as the approved design standard for airport pavement thickness. FAARFIELD 1.302 is available for download. FAARFIELD 1.302 replaces the previous standard, LEDFAA 1.3.
Major changes from LEDFAA 1.3 that have been incorporated into FAARFIELD include the following:
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Implement new rigid pavement failure models based on NAPTF full-scale tests.
- Completely rewrite the overlay design subroutines.
- Expand and update the aircraft library to include latest data and models from aircraft manufacturers.
- Provide additional run-time user guidance keyed to the Advisory Circular.
- Migrate the program to the Microsoft Visual Studio 2005™ programming environment (to ensure compatibility with current and future computer operating systems).
Three-Dimensional Finite Element Method (3-D FEM)
Essential to the Advanced Design Procedures for airport pavements is the continued development of a practical, three-dimensional finite element structural response model. FAA has successfully developed a 3D finite element model for multi-layer, jointed rigid pavements loaded by multiple-wheel aircraft gears. The model incorporates existing 3D finite element software (NIKE3D, originally developed by the U.S. Dept. of Energy's Lawrence Livermore National Laboratory) modified by the FAA specifically for pavement analysis. The modified NIKE3D and INGRID software programs are distributed under a software sharing agreement between FAA and the Lawrence Livermore National Laboratory, developer of the original programs.
Key concepts in the FAA's 3D-FEM modeling effort are:
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Capability of modeling key components of rigid pavement systems: joints, finite-size slabs, bonded and unbonded layer interfaces, multiple layered systems, stabilized base layers
- Rapid, systematic generation of three-dimensional finite element meshes.
- Compatibility with a PC-based, user-friendly design system.
The 3D-FEM model for rigid pavements has been field-verified using in-service data from the Instrumented Pavement Project at Denver International Airport. Key pavement responses from the model were compared to sensor data from the DIA project. A technical report summarizing the major findings of this verification effort was completed in July 2000 and is available as Report DOT/FAA/AR-00/33.
The program FEAFAA implements a version of the FAA 3D-FEM model for rigid pavements involving 9 slabs. The current version of FEAFAA can be downloaded here. FEAFAA is made available as a research and analysis tool, but is not an FAA standard and is not intended as a design procedure.
The rigid pavement structural model continues to undergo development. Shown below are some sample results of 3D finite element analysis of airport pavement structures using the FEAFAA program. To see a full-size view, click on a thumbnail image.
B-777 Stress
View showing the distribution of bending stress in a rigid pavement slab. Note the six well-defined stress peaks corresponding to the six tires of the B-777 main gear assembly. The “unloaded” slab opposite the gear is not shown to expose the edge of the loaded slab. Stresses are compressive (negative) on the top and tensile (positive) on the bottom of the loaded slab.
B-757 Deflection
Section of the deflection basin computed for a B-757 main gear. The gear is loading the edge of the concrete slab. The vertical scale has been exaggerated to show the deformation of the subbase layers. The structure appears to be shallow; however, the bottom layer of elements has an “infinite” formulation, simulating the response of an infinitely deep elastic subgrade.
Advanced Material Models
A major advantage of the 3D finite element method of structural analysis is that it can incorporate specialized models that accurately reflect material behavior. For example, unbound layers in flexible pavements may exhibit strain-dependent behavior (strain-hardening or strain softening) that is not captured by layered elastic analysis. Research conducted under FAA sponsorship at the FAA Center of Excellence for Airport Technology (CEAT) may lead to new models for advanced design procedures based on 3D finite element analysis.
Performance And Failure Models
Accurate analysis of the pavement response to a given aircraft load is necessary but not sufficient for design. In addition, it is essential to have reliable predictions of the failure life of a pavement. In the advanced FAA design procedures, failure models are in the form of regression functions relating levels of stress of strain produced by a passing aircraft gear to the number of coverages to failure. The response is based on a mechanistic analysis such as the three-dimensional finite element method or layered elastic analysis, while the failure models are based on traffic tests of full-scale pavement structures. Hence, this methodology belongs to the family of mechanistic-empirical design methods.
The National Airport Pavement Test Facility (NAPTF) was built to produce reliable pavement performance and failure data for a variety of pavement structures, subgrade strengths and landing gear configurations. The testing vehicle can simulate repeated loading by aircraft weighing up to 1 million pounds. Data from the NAPTF is now being used to develop advanced failure models that are applicable to the new generation of aircraft landing gears, including the 6-wheel B777, A380, and future models.
Technical Reports
Several informative Technical Reports have been published and may be downloaded free of charge from the FAA William J. Hughes Technical Center Library. Reports are in pdf format.
The above reports may also be ordered from National Technical Information Service (NTIS). The following additional titles are available from NTIS:
- Advanced Pavement Design: Finite Element Modeling for Rigid Pavement Joints. Report I - Background Investigation, DOT/FAA/AR-95/85 [NTIS Order no. AD-A325 108/9INZ
- Field Verification of a 3D Finite Element Rigid Airport Pavement Model, DOT/FAA/AR-00/33 [NTIS Order no. PB2001-101160INZ]
Other technical reports are currently being drafted. Titles of these reports will be posted here as they are available.
Contact Project Lead: Dr. David R. Brill, ANG-E262
Last Update: 04/03/2012