The American Association of State Highway and Transportation Officials (AASHTO) recently published an updated 2016 edition of the Manual for Assessing Safety Hardware (MASH) document. Along with this, the Federal Highway Administration (FHWA) and AASHTO developed a revised joint implementation agreement which establishes dates for discontinuing the use of safety hardware that has met earlier crash testing criteria for new installations and full replacements on the National Highway System (NHS). Although some barrier testing was performed during the development of the updated criteria, many barrier systems and other roadside safety features have yet to be evaluated under the proposed guidelines. As we approach MASH implementation agreement sunset dates for NCHRP Report 350 devices, evaluation of the remaining widely used roadside safety features using the safety-performance evaluation guidelines included in MASH 2016 is needed.
Concrete median barriers are used by Departments of Transportation (DOTs) as permanent and temporary barriers for providing separation of traffic. Typically, the crashworthiness of these barriers is tested and evaluated through full-scale crash testing conducted per current roadside safety device standards. Occasionally, DOTs have the need to flare the concrete barrier length of need (LON) around fixed objects, such as bridge piers. No current recommendations are available to guide barrier flare rate around such fixed objects, while still maintaining barrier crashworthiness. Although the current practice is to flare the cast-in-place concrete barrier at a maximum 20:1 rate, no full-scale crash testing has been conducted to determine the crashworthiness of the system at this condition or at a flare rate that might be considered more critical. Flaring a concrete barrier directly affects the impact angle of run-off-road errant vehicles, increasing the impact severity of such vehicle, and creating opportunity for pocketing, vehicle instability, and/or occupant interaction with the shielded fixed object.
The purpose of this research is to investigate the critical flare rate and corresponding crashworthiness of a concrete median barrier flaring around a fixed object. The structural capacity and the occupant risk factors of such proposed guardrail system will be evaluated with respect to MASH Test Level (TL) 4 criteria through computer simulations and full-scale crash testing.
The information compiled from this research will provide the FHWA and State DOTs with an acceptable critical flare for cast-in-place concrete barrier system flaring around a fixed object under MASH 2016 TL 4 conditions. A successfully crash-tested flared concrete barrier system can be applied in situations where flaring a concrete barrier is a need to shield errant vehicles from fixed objects. A concrete barrier crashworthy higher flare rate would result in reduction in flaring length for location where flaring space is an issue. A successfully crash-tested system would reduce the risks of injury or fatality for impacting errant vehicles.
The 2016 MASH edition is the latest in a series of documents that provided guidance on testing and evaluation of roadside safety features (1). The original MASH document was published in 2009 and represents a comprehensive update to crash test and evaluation procedures to reflect changes in the vehicle fleet, operating conditions, and roadside safety knowledge and technology (2). The MASH documents supersede the National Cooperative Highway Research Program (NCHRP) Report 350, ‟Recommended Procedures for the Safety Performance Evaluation of Highway Features” standards (3).
The FHWA issued a January 7, 2016 memo mandating the AASHTO/FHWA Joint Implementation Agreement for MASH with compliance dates for installing MASH hardware that differ by hardware category. After December 31st 2019, all roadside safety devices must have been successfully tested and evaluated according to the 2016 MASH standard edition. The Federal Highway Administration (FHWA) will no longer issue eligibility letters for highway safety hardware that has not been successfully crash tested according to the 2016 MASH edition evaluation criteria. At a minimum, all barriers on high-speed roadways on the National Highway System (NHS) are required to meet Test Level3 (TL-3) requirements.
The structural adequacy MASH 2016 test for TL-4 conditions consists of a 22,000-lb single unit truck (SUT) (denoted 10000S) impacting the barrier at 56 mph and 15 degrees with respect to the roadway (Test 4-12). The severity MASH 2016 test consists of a 5000-lb pickup truck (denoted 2270P) (Test 4-11) and a 2420-lb passenger car (denoted 1100C) (Test 4-10) impacting the barrier at 62 mph and 25 degrees with respect to the roadway.
MASH was developed to incorporate significant changes and additions to procedures for safety-performance evaluation, and updates reflecting the changing character of the highway network and the vehicles using it. For example, MASH increased the weight of the pickup truck design test vehicle from 4,409 lb to 5,000 lb, changed the body style from a ¾-ton, standard cab to a ½-ton, 4-door, and imposed a minimum height for the vertical center of gravity (CG) of 28 inches. The increase in vehicle mass represents an increase in impact severity of approximately 13 percent for Test 4-11 with the pickup truck design test vehicle with respect to the impact conditions of NCHRP Report 350. The increased impact severity may, therefore, result in increased impact forces and larger lateral barrier deflections compared to NCHRP Report 350.
The impact conditions for the small car test have also changed. The weight of the small passenger design test vehicle increased from 1800 lb to 2420 lb, and impact angle increased from 20 degrees to 25 degrees with respect to the roadway. These changes represent an increase in impact severity of 188 percent for Test 4-10 with the small car design test vehicle with respect to the impact conditions of NCHRP Report 350. This increase in impact severity might result in increased vehicle deformation and could possibly aggravate vehicle stability.
MASH also adopted more quantitative and stringent evaluation criteria for occupant compartment deformation than NCHRP Report 350. An increase in impact severity might result in increased vehicle deformation and could possibly result in failure of meeting the latest MASH evaluation criteria. For example, NCHRP Report 350 established a 6-inch threshold for occupant compartment deformation or intrusion. MASH, by comparison, limited the extent of roof crush to no more than 3.9 inches. In addition, MASH requires that the vehicle windshield would not sustain a deformation greater than 3 inches and would not have holes or tears in safety lining as result of the test impact.
The purpose of this research is to investigate the critical flare rate and corresponding crashworthiness of a concrete median barrier flaring around a fixed object. The structural capacity and the occupant risk factors of such proposed guardrail system will be evaluated with respect to MASH TL-4 criteria through computer simulations and full-scale crash testing.
The information compiled from this research will provide the Federal Highway Administration and State Departments of Transportation with an acceptable critical flare for cast-in-place concrete barrier system flaring around a fixed object under MASH 2016 TL 4 conditions. A successfully crash-tested flared concrete barrier system can be applied in situations where flaring a concrete barrier is a need to shield errant vehicles from fixed objects. A concrete barrier crashworthy higher flare rate would result in reduction in flaring length for location where flaring space is an issue. A successfully crash-tested system would reduce the risks of injury or fatality for impacting errant vehicles.
TTI will provide composite video and photographic documentation of the crash tests and a final report documenting the research and testing performed. TTI will further provide drawings of the flared concrete barrier system and of each of its components in the format required for inclusion in hardware standards documents of Task Force 13.
The researchers will perform a literature review of concrete barrier systems that have been considered and tested in the past with flare configurations. Based on this initial evaluation, a system design with a critical flare will be proposed for further evaluation with computer simulations. The flared system will be designed to meet MASH 2016 TL-4 criteria. Proposed initial system to be considered for evaluation is a cast-in-place single slope concrete barrier systems.
The researchers will perform finite element computer simulations to evaluate the proposed system design. A set of computer simulations will serve to investigate barrier crashworthiness behavior with respect to installation details, specifically height and critical flare rates. Researchers suggest starting by evaluating the system when flared at the critical flare rate per current practice (20:1 flare). Based on the results of the performance of the computer simulation(s), the researchers might have to modify accordingly the flare of the system and re-evaluate the newly proposed critical flare. Computer simulations will be used as design aid to determine barrier height requirement to satisfy MASH evaluation criteria. Based on the computer simulations results, the researchers might suggest additional design modifications to improve the system performance (such as lateral offset of the fixed object from face of the barrier). Also, computer simulations will be used to investigate critical impact points for the system with use of both small passenger car, pickup truck, and single unit truck.
If time and task budget permits, a similar computer modeling investigation will be performed for a cast-in-place concrete F-shape barrier, under MASH TL4 testing and evaluation criteria.
Based on engineering evaluation and FE analysis outcome, the researchers will determine design details, as well as installation and flaring characteristics for the concrete barrier system to be advanced for construction and testing.
Within this Task, the TTI researchers will work closely with the project Technical Representative and, if necessary, will coordinate with the Pooled Fund DOTs members to determine details of the installation characteristics of the system to be constructed, including representation of the fixed object to be shielded.
Appropriate drawings will be developed to aid in the construction of the approved system and to guide installation procedures for the testing plan. The approved system will be constructed following MASH 2016 requirements.
Three full-scale crash tests will be performed according to MASH 2016 TL-4. Engineering analysis and FEA outcomes will determine the order in which the three full-scale tests will be conducted, based on higher risk of failing MASH 2016 evaluation criteria. One full-scale test will involve a 2,420-lb passenger car impacting the barrier system at 62.2 mph nominal speed with a nominal orientation of 25 degrees relative to the roadway. The test will be assessed according to the evaluation criteria set in MASH 2016 standards. This test will investigate the occupant risk of the barrier system during the impact event.
A second full-scale crash test will involve a 5,000-lb pickup truck impacting the barrier system at 62.2 mph nominal speed with a nominal orientation of 25 degrees relative to the roadway. The test will be assessed according to the evaluation criteria set in MASH 2016 standards. This test will investigate the passenger vehicle stability during the impact event, as well as any potential for occupant interaction with the fixed object.
A third full-scale crash test will involve a 22,000-lb single unit truck impacting the rail at 56 mph nominal speed and at nominal orientation of 15 degrees relative to the roadway. The test will be assessed according to the evaluation criteria set in MASH 2016 standards. This test will investigate the structural capacity of the barrier system.
The researchers will generate a final report of the findings from the research and testing study. Results will be reported in terms of barrier performance, vehicle stability and occupant risk factors and will be compared with respect to MASH 2016 crashworthiness criteria. The researchers will further provide drawings of the tested system and of each of its components in the format required for inclusion in hardware standards documents of Task Force 13.
|TTI Research Supervisor:
Chiara Silvestri Dobrovolny, Ph.D.
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Texas Department of Highways
125 East 11th Street
Austin, TX 78701-2483