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30 CHAPTER 4 AESTHETIC CONCRETE BARRIER DESIGN GUIDELINE DEVELOPMENT OBJECTIVE vehicle used in the simulation effort conducted to establish the preliminary guidelines. Simulations with the 820C were Demands from local communities and agencies for aesthet- to be used as a check to determine if the preliminary relation- ically pleasing concrete barrier alternatives have increased. ships required modification based on vehicular instability or Guidance regarding the application of aesthetic surface treat- other concerns with the small passenger car. ments to vertical and single-slope barriers is provided in the The pickup truck finite element model was validated by FHWA acceptance letter B-110. This guidance is based on a comparing simulation results to available crash test data for series of crash tests conducted by Caltrans.(19) However, exist- the New Jersey safety shape and single-slope concrete barri- ing design procedures and guidelines do not provide sufficient ers. Similar comparative analyses were conducted to evaluate information to understand the effect of aesthetic surface treat- validation of the finite element model of the small car design ments on the impact performance of concrete safety shape vehicle. median and roadside barriers. Finite element vehicle models used in roadside safety de- The objective of this research was to develop design guide- sign generally show good correlation with test data in regard lines for aesthetic surface treatments of concrete barriers for to overall vehicle kinematics. However, little work has been New Jersey or F-shape profiles. These design guidelines were done to validate the ability of these models to accurately cap- developed through extensive use of finite element simula- ture OCD resulting from an angled impact into a longitudinal tion, in conjunction with full-scale vehicle crash testing. An barrier. overall summary of the development approach is presented Two primary types of OCD are of interest with respect to below. Detailed information regarding the guideline devel- longitudinal barrier impacts: (1) deformation resulting from opment process is presented in subsequent chapters of this direct contact of the wheel assembly or other vehicle com- report. ponents with the floor board, toe pan, or fire wall and (2) de- formation induced by impact loads applied to the frame OVERALL SUMMARY OF THE or structure of the vehicle. The direct deformation typically DEVELOPMENT APPROACH results from some form of wheel snagging or an increase in effective friction between the wheel and barrier that fails To develop design guidelines for the application of aesthetic components of the steering system and suspension and shoves surface treatments on concrete safety shape barriers, a set of the wheel assembly rearward. This type of deformation is preliminary guidelines were initially developed. A parametric particularly relevant to the investigation of surface asperities. study was performed using finite element simulations to estab- Induced deformation is caused by lateral impact loads applied lish these preliminary guidelines. A full-scale crash-testing to the frame rails or other structural components of the vehicle phase was then conducted. The test results were subsequently and may manifest itself in buckling of the floor board or rack- used to adjust and refine the guidelines into their final form. ing of the vehicle body. Previous crash-testing data show that the most common The mechanism by which direct OCD is generated in the failure mechanisms associated with longitudinal barrier im- finite element vehicle model may differ from the mechanism pacts are excessive occupant compartment deformation (OCD) of an actual crash test vehicle due to lack of suspension fail- and vehicular instability (i.e., overturn). When surface asper- ure in the finite element vehicle model. For this reason, a ities are introduced onto the face of a barrier, the primary con- direct measure of the vehicle's OCD from simulation results cern relates to excessive OCD resulting from snagging of cannot be considered deterministic for comparing with the vehicle components (e.g., wheel) on the asperities. crash test data. Thus, to evaluate vehicle OCD from simula- NCHRP Report 350 uses two basic design test vehicles: a tion results, a surrogate measure for quantifying OCD was 2,000-kg pickup truck (denoted 2000P) and an 820-kg pas- developed. senger car (denoted 820C). The 2000P is generally considered Several available crash tests of concrete barriers were identi- to be the more critical of the two design vehicles in regard to fied and simulated using the finite element vehicle model. Each assessment of OCD. For this reason, it was the primary design simulation was set up to collect several potential surrogate
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31 measures of OCD. The details of these measures will be pre- inclination. Parametric finite element simulations were per- sented in the next chapter. It was determined that the internal formed for asperity angles of 30, 45, and 90 degrees, and energy of the pickup truck floorboard in simulation results each simulation was assigned an outcome of "acceptable," showed the best correlation with OCD reported in full-scale "marginal/unknown," or "unacceptable" based on comparison crash tests. Internal energy provides a measure of the overall of the internal floorboard energy with the established thresh- deformation directly or indirectly generated in the floorboard. old values. Preliminary guidelines were then developed in Truck floorboard internal energy was therefore selected as the terms of asperity depth, width, and angle of inclination based surrogate OCD measure. By comparing the internal floorboard on the combined set of simulation outcomes. energies from 2,000-kg pickup truck simulations and the Based on these preliminary guidelines, a crash test plan reported OCD values for several crash tests, thresholds for was developed in which the outcome of one test determined acceptable and unacceptable internal energy levels were estab- the configuration evaluated in a subsequent test. In other lished. Given a simulated barrier with a selected asperity con- words, the test matrix was adjusted as the crash tests were figuration, these threshold values were used to determine the performed, and the results were analyzed in order to maxi- likelihood of failure due to excessive OCD. Further details are mize the information available for adjusting and finalizing presented in Chapter 5. the relationships for asperity depth, width, and angle. The The New Jersey safety shape barrier was used for the dev- OCD measurements from the tests enabled the adjustment of elopment of the preliminary and the final design guidelines. the thresholds for acceptable and unacceptable floorboard Vehicular impacts with the F-shape safety barriers are known internal energy upon which the final design guidelines are to result in lower vehicle instabilities when compared with based. the New Jersey safety shape barriers. The guidelines devel- Chapter 5 presents details of the simulation phase in the oped are, therefore, considered to be applicable to both New development of the preliminary guidelines, Chapter 6 pre- Jersey safety shape and F-shape concrete barriers. sents details on the testing phase of the guideline develop- Generalized types of surface asperities were defined in terms ment, and Chapter 7 and the appendix present the final design of various parameters, such as the width, depth, and angle of guidelines.