Ernest Heymsfield

Application of the 2-D constant strain assumption to FEM elements consisting of an arbitrary number of nodes

Abstract The formulation for the constant strain element is revisited to develop multi-noded elements that can be used as transition elements in a finite element mesh. Although the constant strain approach is computationally attractive, spurious force-free displacement modes arise for elements consisting of more than three nodes. These unstable mode shapes are typified by “hourglassing” which develops in quadrilateral elements when constant strain is assumed within the element. The means for stabilizing spurious mode shapes for quadrilateral elements is well documented in the literature, however, in this paper, a general formulation for stabilizing forces is presented for elements having an arbitrary number of nodes and therefore is not restricted to quadrilateral elements. This paper examines the use of meshes consisting of constant strain elements created from polygons having differing numbers of element nodes. The effectiveness of the stabilization procedure is illustrated along with “patch test” examples to assess the consistency of the approximation. The elements are shown to be surprisingly robust, yielding reasonable results even when poorly designed mesh transitions are used.

Sensitivity Analysis of Engineered Material Arrestor Systems to Aircraft and Arrestor Material Characteristics

In most aircraft overruns, an aircraft comes to a stop within 1,000 ft (305 m) of the runway threshold. In response, U.S. airfields are required by FAA to have a 1,000-ft runway safety area in addition to the design runway length in case of aircraft overruns. At airfields where space is limited, airport operators need to consider alternative solutions. One solution is for an airport operator to reduce runway length and therefore limit aircraft types landing at their airport. Conversely a more attractive solution is for an airport to implement an engineered material arrestor system (EMAS). An EMAS is a passive system comprising a cementitious type material. Aircraft deceleration occurs because of drag forces that are incurred by the aircraft landing gear as the aircraft passes through and crushes the arrestor material. Parameters significantly influencing a B727 or B747 aircrafts stopping distance as it traverses an EMAS are presented. The computer code, ARRESTOR, developed for FAA, was used to compute the aircraft stopping distances. Five aircraft parameters (mass moment of inertia, weight, wheel friction, reverse thrust, and center of gravity location) for each aircraft type are examined, along with two arrestor material types (low-density concrete and phenolic foam) and three arrestor bed geometries. Each parameter is individually varied to investigate stopping distance sensitivity. Twenty-one overrun simulations were used in the aircraft parameter study, and 100 overrun simulations were used to determine an optimal material compressive strength. Arrestor bed geometry was investigated by using 32 overrun simulations. Study results are summarized in plots and tables.

Predicting Aircraft Stopping Distances within an EMAS

An overrun is an accident/incident in which an aircraft is unable to stop within the design runway length. To minimize the adverse consequences of an overrun, the Federal Aviation Administration (FAA) requires airports to have a runway end safety-area 305 m (1,000-ft) long beyond the runway design length. However, many U.S. airports are unable to satisfy this requirement without limiting the aircraft mix using the airport. In response, the FAA permits installing an engineered materials arrestor system (EMAS) as an alternative solution. Four aircraft types are investigated in this paper for stopping-distance behavior within an EMAS, as follows: (1) B737-900ER, (2) B757-300, (3) B767-400ER, and (4) B747-400ER. Stopping distances are evaluated using a base arrestor bed configuration and base arrestor material. Aircraft strut behavior, aircraft pitch moment of inertia, and bogie weights are proprietary to aircraft manufacturing companies; therefore, approximate values for load-stroke behavior, damping, pitch moment of inertia, and bogie weights are developed in this paper. Besides the base arrestor material, a suite of five low-density concrete mixes with varying stress-strain behavior are investigated for their impact on aircraft stopping-distance. In addition, aircraft stopping-distance as a function of arrestor bed configuration is investigated.

Retrofitting Precast Bridge Beams with Carbon Fiber-Reinforced Polymer Strips for Shear Capacity

Advancements in fiber-reinforced polymers (FRPs) have made this an attractive material for rehabilitation and strengthening of bridge superstructures. FRP has primarily been used with the intention of increasing the bending strength of bridge members. However, this paper investigates the use of externally placed FRP strips to increase shear capacity of short-span, 5.7 m (19 ft), precast concrete channel beam bridges. A statewide survey revealed that as many as 389 bridges in the state of Arkansas are comprised of these members. Notably, beams within these bridges were designed under provisions that did not require shear reinforcement. In this research, four sections were retrofitted using carbon fiber-reinforced polymer (CFRP) strips and load tested to failure to measure the repair effectiveness. The performance of the retrofitted sections far exceeded that of unretrofitted sections. It was concluded that the addition of the CFRP repair increased the deflection ductility at least 123%. In addition, beams retrofitted with the CFRP strips experienced at least 26% more deflection after the initiation of a shear crack; therefore reducing the risk of a catastrophic failure.

Development of Damage Model to Analyze Stabilized Soil Layers Subjected to Repetitive Aircraft Loadings

The U.S. Army Engineering Research and Development Center is developing methods to construct a contingency airfield within a short time that can support C-17 aircraft loadings. The proposed approach is to construct the runway site surface by chemically stabilizing the in-place top soil layer. To develop an optimal stabilized soil mixture, a suite of soil–chemical mixture combinations was examined. Because of the complexity and expense of full-scale testing, a numerical approach is being developed to predict stabilized soil response at contingency airfields. Results from the numerical approach will then be used to supplement data obtained from full-scale tests. Three stabilized soil mixtures were investigated by using stabilizing agents of 6% cement, 4% cement, and 4% cement plus 1.5-in. (38-mm) polypropylene fibers. These three mixtures were evaluated for strength and durability from unconfined compression and repeated load tests. A total of 33 unconfined compression load tests and 48 unconfined repeated load tests were conducted. A numerical model using damage is presented to predict stabilized soil behavior in response to repetitive loading. The damage model is calibrated by using only two material properties and two damage parameters. Results of this study indicate that stabilized soil exhibits plastic behavior and therefore a numerical model for stabilized soil must consider both damage and plastic behavior.

FIBER-REINFORCED POLYMER SHEAR STRENGTHENING OF SHORT-SPAN, PRECAST CHANNEL BEAMS IN BRIDGE SUPERSTRUCTURES

A national study concluded that at least 14 states use precast channel beams for their bridge superstructures. Before the mid-1970s short-span (19-ft) precast channel beam bridges were designed by the Arkansas State Highway and Transportation Department for H15 loading without any provision for shear reinforcement. A recent statewide survey has identified 389 of these bridges that remain in service. About 3% are load posted. Approximately one-third of the 389 bridges are exhibiting deterioration that may be serious. During a recent investigation by the University of Arkansas, 33 precast channel beams were evaluated for structural strength. It was determined that many of these units have inadequate shear capacity. A shear crack frequency distribution curve was developed through examination of the 33 tested beams. A simple shear strengthening technique is presented consisting of fiber-reinforced polymer (FRP) strips as external stirrups. The technique has been developed so that it can be easily implemented in the field. The FRP strips are spaced to prevent the full formation of a diagonal crack between two adjacent strips and designed to ensure a flexural failure rather than a sudden shear failure. Four beams retrofitted with the proposed retrofit technique and two control beams were load tested to failure to evaluate the effectiveness of the technique. Results from the experiment show that the retrofitted beams exhibited at least a 290% increase in deflection ductility and a 418% increase in energy ductility.

Structural Evaluation of Precast Concrete Channel Beams in Bridge Superstructures

During the period from the mid-1950s through the mid-1970s a large number of bridges were constructed throughout Arkansas using a 5.79-m (19-ft) long, precast, non-prestressed, concrete channel beam that was then standard. A survey of highway departments has identified 12 states that have used a similar bridge element in the past. It has been determined that nearly 400 of these bridges remain in use in Arkansas alone. Recently, the Arkansas State Highway and Transportation Department discovered that a number of these sections are exhibiting potentially serious deterioration. The deterioration appears to have been initiated by corrosion of the flexural reinforcement in the beam stems. An additional issue is that these beams were fabricated without any shear reinforcement. Moreover, some sections are showing signs of concrete degradation. The need to determine the in-place load capacity, serviceability, and durability of these sections has reached a critical level. To date, 20 beams have been removed from existing structures and tested for their flexural load capacity and the material properties of the concrete and longitudinal reinforcement. Results have varied depending on the extent of deterioration. However, in nearly every case shear failure has controlled the load capacity of a section. Based on this research, a draft field guide, intended for use by inspection crews, is being prepared. This guide will aid inspectors in prioritizing sections for repair, rehabilitation, and removal.

Implementing Gigapixel Technology in Highway Bridge Inspections

AbstractThe current number of deficient U.S. bridges makes the evaluation of bridge condition paramount to a state highway department in developing a bridge management system. State DOTs routinely visually inspect bridges on 24-month cycles. Findings from these inspections are numerically summarized by bridge inspectors for bridge condition ratings and additionally documented with inspector notes and pictures. However, bridge inspections include a human factor, which introduces bias and subjectivity. Consequently, bridge inspection values are tainted with nonuniformity. This article presents gigapixel technology as an inexpensive quality assurance/quality control tool that can easily be implemented by a state DOT for their bridge inspections. The process includes developing gigapixel panoramas of a bridge that can later be reviewed by bridge engineers trained for this task. The proposed approach is used to supplement visual bridge inspection to improve quality control among bridge inspections. This approa...

Retrofitting Short-Span Precast Channel Beam Bridges Constructed without Shear Reinforcement

This paper investigates retrofitting precast, nonprestressed, channel beams (PCB) used in short-span bridges to improve beam shear strength and, consequently, beam ductility. Three retrofit approaches were investigated: applying carbon-fiber-reinforced polymer (CFRP) strips, applying an epoxy spray-on, and retrofitting by installing shear bars within the stems of the precast channel beam. Implanting shear bars into each precast channel beam stem was found to be the optimal retrofit based on improved beam strength, installation ease, and economics. The suitability of the shear bar retrofit was further explored by implementing the shear bar retrofit at a short-span precast channel beam bridge. Precast channel beam sections cast without shear reinforcement were used to construct Arkansas Bridge #02992 over the Flat Hollow Branch Creek. The bridge was constructed in 1955, but several of the beams used in the original construction have since been replaced with better-condition similar-style beams. The bridge i...

Development of anti-icing airfield pavement using surface-embedded heat wire

ABSTRACT Snow and ice pose a significant risk for aircraft ground operation safety. Ploughing and chemical treatment are used for snow removal, but yield long-term detrimental impacts to the airfield infrastructure and environment. A proof-of-concept airfield heated pavement system (AHPS) prototype using near-surface embedded heat wire for pavement anti-icing is presented in this paper. Surface-embedded wires were used to study heat performance and energy warranted to maintain above-freezing slab surface temperatures. This approach concentrated heat energy to the pavement surface. Testing this AHPS prototype contained the following objectives: (1) optimise surface-embedded heat wire configuration, (2) investigate surface temperature distribution and (3) summarise system heating performance in outdoor winter conditions. Preliminary laboratory testing used a 150 mm wire spacing in a serpentine configuration. At this spacing, an energy flux of 142 W/m2 produced a 6°C temperature rise in 6 h in a −14°C freezer chest. Sufficient surface temperature rise was attained in preliminary field-testing during the 2015–2016 winter season using a 213.1–697.5-W/m2 energy range. Using the presented AHPS prototype, a 25-mm layer of crushed ice melted within 1 h during an experimental ice test.