Chuck Plaxico

FINITE-ELEMENT MODELING OF GUARDRAIL TIMBER POSTS AND THE POST-SOIL INTERACTION

The performance of many guardrail terminal systems is dependent on the strength of timber guardrail posts and soil conditions. Accurately simulating the breakaway characteristits of guardrail posts mounted in soils is an important issue concerning researchers in the roadside safety community. Finite-element analysis is one method that can be used to evaluate roadside hardware designs, but good simulations are contingent on developing accurate models of the components. A description is provided of the development of a model of a breakaway timber post and soil system used in the breakaway cable terminal (BCT) and the modified eccentric loader terminal (MELT). The model is described and simulation results are compared with data from physical tests of BCT/MELT posts.

Improvements to the weak-post W-beam guardrail

The weak-post W-beam guardrail has been widely used in the Northeast for many decades. Weak-post guardrails are characterized by larger dynamic deflections in a collision and are considered more forgiving than other, stiffer barriers. Most states have experienced good performance with these barriers over the last several decades if adequate clear space is provided behind the barrier. Unfortunately, recent crash tests of the standard weak-post W-beam guardrail involving a 2000-kg pickup truck resulted in a series of unacceptable test results, including overriding and penetrating the guardrail. Design modifications to the weak-post W-beam guardrail were explored by using finite element simulations and full-scale crash tests. An improved version of the weak-post W-beam guardrail system was developed and tested, and this was found to satisfy the requirements of NCHRP Report 350 for Test Level 3.

PERFORMANCE OF W-BEAM SPLICES

Structural failure of post-and-beam W-beam guardrails during impact sometimes is due to the rupture of the W-beam rail where two sections are spliced together with bolts. Summarized is a study of the mechanics of failure of the splice connection. The causes of rupture are identified, and a design alternative is formulated that will reduce the likelihood of rupture of the splice connection. The tensile forces in the W-beam rail and the mode of deformation of the splice connection during impact were critical factors considered in the study. The results of full-scale crash tests, laboratory tests, and finite element analysis indicate that relocating splices to midspan locations would greatly reduce the chance of observing a rupture of the guardrail in full-scale crash tests and in real-world collisions.

Impact Performance of the G4(1W) and G4(2W) Guardrail Systems: Comparison Under NCHRP Report 350 Test 3-11 Conditions

Several types of strong-post W-beam guardrails are used in the United States. Usually the only difference between one type of strong-post W-beam guardrail and another is the choice of post and block-out types. The impact performance of two very similar strong-post W-beam guardrails are compared—the G4(2W), which uses a 150×200 mm wood post and the G4(1W), which uses a 200×200 mm wood post. Although G4(2W) is used in numerous states, G4(1W) is now common only in the state of Iowa. The performance of the two guardrails has been presumed equal, but only one full-scale crash test has been performed on G4(1W) and that was over 30 years ago, using a now-obsolete test vehicle. The nonlinear finite element analysis program LS-DYNA was used to evaluate the crashworthiness of the two guardrails. The G4(2W) guardrail model was validated with the results of a full-scale crash test. A model of the G4(1W) guardrail system was developed, and the deflection, vehicle redirection, and occupant risk factors of the two guardrails were compared. The impact performance of the two guardrails was quantitatively compared using standard techniques. The analysis results indicate similar collision performance for G4(1W) and G4(2W) and show that both satisfy NCHRP Report 350 Test 3-11 safety performance requirements.

Improved Truck Model for Roadside Safety Simulations: Part II—Suspension Modeling

The 2000-kg pickup truck is a very important vehicle in roadside safety research because it is specified in many of the tests in NCHRP Report 350. The characteristics of the pickup truck make it a very demanding crash test vehicle. Because the 2000-kg pickup truck is an important crash test vehicle, it was the very first vehicle chosen for development of a finite element model. The nonlinear finite element program LS-DYNA has become an important feature of roadside hardware design and analysis in recent years, and much of the success of these modeling efforts is partly caused by the availability of a good 2000-kg pickup truck model. Like all models, the model has evolved over the past decade. New features and improvements have been added continuously to the model by many different teams to solve specific analysis problems. One particular area where there has been a great deal of activity is in the area of modeling the suspension properties of the vehicle. Suspension response is particularly important for 2000-kg pickup truck impacts because the vehicle often experiences stability problems in impacts with roadside hardware. A number of improvements and modifications to Version 9 of the NCAC 2000-kg pickup truck model are summarized. These improvements involved changing the finite element model, changing element properties, and obtaining suspension response properties from physical tests. The 2000-kg truck model was then validated against a series of low-speed, live-drive tests with an instrumented pickup truck. The improved model provides more realistic vehicle suspension response than earlier models and should prove to be a valuable addition to future finite element modeling activities.

EFFECTS OF POST AND SOIL STRENGTH ON PERFORMANCE OF MODIFIED ECCENTRIC LOADER BREAKAWAY CABLE TERMINAL

The effects of wood post strength and soil strength on the dynamic performance of guardrail systems have long been a concern of the roadside safety community. Evidence from full-scale crash tests has suggested that these parameters may significantly affect guardrail system performance. Essentially identical tests have resulted in widely varying outcomes that might be the result of various post strengths and soil conditions. A finite-element model of a common guardrail terminal—the modified eccentric loader breakaway cable terminal—was developed to examine the effects of post strength and soil strength on the overall performance of the terminal system. A matrix of 12 simulations of a particular full-scale crash test scenario was conducted with the explicit nonlinear dynamic finite-element software LS-DYNA3D to establish the combinations of post and soil strengths that produce favorable results. The parametric simulations show that certain combinations of soil and post strengths increase the hazardous possibilities of wheel snagging, pocketing, or rail penetration, whereas other combinations produce more favorable results. Conditions that will maximize the safety and reliability of the guardrail terminal system are identified.

Quantitative Method for Assessing the Level of Deterioration of Round Wood Guardrail Posts

This paper presents results of a study that developed a procedure for measuring the amount of deterioration damage of round wood guardrail posts. The guardrail posts used in the study were extracted from damaged guardrail installations in Ohio. A resistograph device was used to measure the resistance of a 1 16 in. drilling needle passing through the cross section of the guardrail posts. The deterioration damage levels of the posts ranged from severe to essentially undamaged. A data processing procedure was developed to convert the resistograph data into a quantitative damage score. Physical impact tests were then performed to correlate the damage score to the dynamic strength properties of the posts.

Method for Modeling Crash Severity with Observable Crash Data

This paper presents a new method to rank the severity of an impact with a roadside hazard that is based on observable crash data. This method has been incorporated into the third update of the Roadside Safety Analysis Program. The equivalent fatal crash cost ratio (EFCCR) is a dimensionless value that represents the severity of a crash on a scale of zero to unity, where zero represents no chance of injury and unity represents absolute certainty of a fatal injury. The method uses a census of police-reported data, which ideally covers a range of speed limits. The purpose is to determine the severity distribution of crashes in cases in which no events preceded the crashes with the hazard under evaluation and which did not result in a penetration or a rollover. The number of unreported crashes is estimated and added into the severity distribution with the assumption that the number represents crashes that led to property damage only. The average expected crash cost is then calculated and normalized to a reference speed of 65 mph so that it is directly comparable to EFCCR values calculated for other types of hazards. Unlike the earlier subjective severity index method, the new EFCCR method has its basis in observed crash data and uses a systematic approach to calculate crash severities that can be used in benefit–cost and other safety analyses.

Selection Table Development for Bridge Railing

Although six test levels have been available for 20 years to develop a longitudinal barrier, guidelines on how to choose bridge railings on the basis of specific site and traffic conditions have not been developed that take advantage of the range of crash-tested bridge railing hardware. The research reported in this paper was performed to develop such guidelines. During the guidelines’ development, several important issues emerged that quantified how the site, traffic, and cost characteristics of a project site should be characterized. This paper presents the issues and discusses the applied solutions. A sample set of bridge rail selection tables is presented with use of the solutions discussed, and the sample is compared with bridge railing selection tables documented in the literature. Although the specific example discussed in this paper involves the selection of bridge railings on the basis of site and traffic conditions, the same procedure can be used to develop most other roadside safety selection and location tables.