Aesthetic Concrete Barrier Design (2006)

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30 CHAPTER 4 AESTHETIC CONCRETE BARRIER DESIGN GUIDELINE DEVELOPMENT OBJECTIVE Demands from local communities and agencies for aesthet- ically pleasing concrete barrier alternatives have increased. Guidance regarding the application of aesthetic surface treat- ments to vertical and single-slope barriers is provided in the FHWA acceptance letter B-110. This guidance is based on a series of crash tests conducted by Caltrans.(19) However, exist- ing design procedures and guidelines do not provide sufficient information to understand the effect of aesthetic surface treat- ments on the impact performance of concrete safety shape median and roadside barriers. The objective of this research was to develop design guide- lines for aesthetic surface treatments of concrete barriers for New Jersey or F-shape profiles. These design guidelines were developed through extensive use of finite element simula- tion, in conjunction with full-scale vehicle crash testing. An overall summary of the development approach is presented below. Detailed information regarding the guideline devel- opment process is presented in subsequent chapters of this report. OVERALL SUMMARY OF THE DEVELOPMENT APPROACH To develop design guidelines for the application of aesthetic surface treatments on concrete safety shape barriers, a set of preliminary guidelines were initially developed. A parametric study was performed using finite element simulations to estab- lish these preliminary guidelines. A full-scale crash-testing phase was then conducted. The test results were subsequently used to adjust and refine the guidelines into their final form. Previous crash-testing data show that the most common failure mechanisms associated with longitudinal barrier im- pacts are excessive occupant compartment deformation (OCD) and vehicular instability (i.e., overturn). When surface asper- ities are introduced onto the face of a barrier, the primary con- cern relates to excessive OCD resulting from snagging of vehicle components (e.g., wheel) on the asperities. NCHRP Report 350 uses two basic design test vehicles: a 2,000-kg pickup truck (denoted 2000P) and an 820-kg pas- senger car (denoted 820C). The 2000P is generally considered to be the more critical of the two design vehicles in regard to assessment of OCD. For this reason, it was the primary design vehicle used in the simulation effort conducted to establish the preliminary guidelines. Simulations with the 820C were to be used as a check to determine if the preliminary relation- ships required modification based on vehicular instability or other concerns with the small passenger car. The pickup truck finite element model was validated by comparing simulation results to available crash test data for the New Jersey safety shape and single-slope concrete barri- ers. Similar comparative analyses were conducted to evaluate validation of the finite element model of the small car design vehicle. Finite element vehicle models used in roadside safety de- sign generally show good correlation with test data in regard to overall vehicle kinematics. However, little work has been done to validate the ability of these models to accurately cap- ture OCD resulting from an angled impact into a longitudinal barrier. Two primary types of OCD are of interest with respect to longitudinal barrier impacts: (1) deformation resulting from direct contact of the wheel assembly or other vehicle com- ponents with the floor board, toe pan, or fire wall and (2) de- formation induced by impact loads applied to the frame or structure of the vehicle. The direct deformation typically results from some form of wheel snagging or an increase in effective friction between the wheel and barrier that fails components of the steering system and suspension and shoves the wheel assembly rearward. This type of deformation is particularly relevant to the investigation of surface asperities. Induced deformation is caused by lateral impact loads applied to the frame rails or other structural components of the vehicle and may manifest itself in buckling of the floor board or rack- ing of the vehicle body. The mechanism by which direct OCD is generated in the finite element vehicle model may differ from the mechanism of an actual crash test vehicle due to lack of suspension fail- ure in the finite element vehicle model. For this reason, a direct measure of the vehicle’s OCD from simulation results cannot be considered deterministic for comparing with the crash test data. Thus, to evaluate vehicle OCD from simula- tion results, a surrogate measure for quantifying OCD was developed. Several available crash tests of concrete barriers were identi- fied and simulated using the finite element vehicle model. Each simulation was set up to collect several potential surrogate

31 measures of OCD. The details of these measures will be pre- sented in the next chapter. It was determined that the internal energy of the pickup truck floorboard in simulation results showed the best correlation with OCD reported in full-scale crash tests. Internal energy provides a measure of the overall deformation directly or indirectly generated in the floorboard. Truck floorboard internal energy was therefore selected as the surrogate OCD measure. By comparing the internal floorboard energies from 2,000-kg pickup truck simulations and the reported OCD values for several crash tests, thresholds for acceptable and unacceptable internal energy levels were estab- lished. Given a simulated barrier with a selected asperity con- figuration, these threshold values were used to determine the likelihood of failure due to excessive OCD. Further details are presented in Chapter 5. The New Jersey safety shape barrier was used for the dev- elopment of the preliminary and the final design guidelines. Vehicular impacts with the F-shape safety barriers are known to result in lower vehicle instabilities when compared with the New Jersey safety shape barriers. The guidelines devel- oped are, therefore, considered to be applicable to both New Jersey safety shape and F-shape concrete barriers. Generalized types of surface asperities were defined in terms of various parameters, such as the width, depth, and angle of inclination. Parametric finite element simulations were per- formed for asperity angles of 30, 45, and 90 degrees, and each simulation was assigned an outcome of “acceptable,” “marginal/unknown,” or “unacceptable” based on comparison of the internal floorboard energy with the established thresh- old values. Preliminary guidelines were then developed in terms of asperity depth, width, and angle of inclination based on the combined set of simulation outcomes. Based on these preliminary guidelines, a crash test plan was developed in which the outcome of one test determined the configuration evaluated in a subsequent test. In other words, the test matrix was adjusted as the crash tests were performed, and the results were analyzed in order to maxi- mize the information available for adjusting and finalizing the relationships for asperity depth, width, and angle. The OCD measurements from the tests enabled the adjustment of the thresholds for acceptable and unacceptable floorboard internal energy upon which the final design guidelines are based. Chapter 5 presents details of the simulation phase in the development of the preliminary guidelines, Chapter 6 pre- sents details on the testing phase of the guideline develop- ment, and Chapter 7 and the appendix present the final design guidelines.