Malcolm H. Ray

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.

Repeatability of Full-Scale Crash Tests and Criteria for Validating Simulation Results

A method of comparing two acceleration time histories to determine whether they describe similar physical events is described. The method can be used to assess the repeatability of full-scale crash tests and it can also be used as a criterion for assessing how well a finite-element analysis of a collision event simulates a corresponding full-scale crash test. The method is used to compare a series of six identical crash tests and then is used to compare several finite-element analyses with full-scale crash test results.

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.

CHARACTERIZING GUARDRAIL STEEL FOR LS-DYNA3D SIMULATIONS

Finite-element models have three parts: geometry, connections, and material properties. As the visible parts of a model, geometry and connections are generally carefully considered. Material properties often are not chosen with the same degree of care although they are equally important to obtaining good results. Accurate simulations of vehicles striking roadside hardware require an understanding of both the material behavior and the mathematical material models in LS-DYNA3D. A method for comparing LS-DYNA3D simulations with typical ASTM materials tests is described. The behavior and modeling parameters of guardrail steel (AASHTO M-180 Class A Type II) are examined in this study. Experimental and simulation results of quasistatic coupon tests are compared for AASHTO M-180 Class A Type II guardrail steel, and parameters for guardrail steel are recommended.

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.

Finite-Element Model of Modified Eccentric Loader Terminal (MELT)

Improving the performance of guardrail terminals and end treatments in impacts with passenger vehicles has been an active area of research over the past decade. One particular W-beam guardrail terminal that has been the focus of recent full-scale crash testing is the Modified Eccentric Loader Terminal (MELT). The development of a nonlinear, finite-element model of a recent modification of the MELT is being used to learn more about the performance of this type of guardrail terminal. A finite-element model of the MELT was developed using the TrueGrid preprocessor and the LS-DYNA3D finite-element software was used to perform the analysis. Results of the analysis are compared with data from a full-scale crash test involving a small passenger car.

VIDEOLOG ASSESSMENT OF VEHICLE COLLISION FREQUENCY WITH CONCRETE MEDIAN BARRIERS ON AN URBAN HIGHWAY IN CONNECTICUT

In-service performance evaluation is the process of assessing the performance of roadside safety hardware under real-world service conditions. The purpose of in-service evaluation is to determine and document the manner in which a safety feature performs during a broad range of collision, environmental, operational, and maintenance situations for typical real-world site and traffic conditions. An in-service performance evaluation of the concrete median barrier (CMB) in Connecticut was undertaken to determine how often CMBs are struck and how often such collisions are reported to the police. The method used to perform the inservice performance evaluation of the CMB is described, and the results are presented. Data were collected for a section of CMB located on I-84 in Hartford, Connecticut. Accurate information about impacts with CMBs is difficult to obtain because so many collisions are unreported. Even for severe collisions, damage to the barrier is rare, and little maintenance is required. In many cases, police reports are the only means of monitoring when, where, and how severe the CMB collision was. Videologging was used to accomplish the in-service performance evaluations of CMBs to document the extent of unreported collisions.

Unreported collisions with post-and-beam guardrails in Connecticut, Iowa, and North Carolina

Presented are the results of an in-service performance evaluation of four guardrail systems: the G1 cable guardrail, the G2 weak-post W-beam guardrail, and the G4(1S) and G4(1W) strong-post W-beam guardrails. The study was focused particularly on estimating the number of unreported collisions and the true distribution of vehicle occupant injuries. The data were collected in portions of Connecticut, Iowa, and North Carolina during a 24-month data collection effort from 1997 to 1999. The collision performance was measured in terms of collision characteristics, occupant injury, and barrier damage. Unreported collisions were counted in all three areas by periodically inspecting guardrails in a specific control section. Because this type of data collection is both time-consuming and sensitive to methodology and human error, its use is not recommended for future in-service evaluation studies. Instead, the use of rates of injury-producing collisions per million vehicle kilometers traveled past the guardrail or other hardware, which can be determined from data on reported injury collisions, hardware inventory, and traffic volumes, is recommended.