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 these guidelines. As we approach MASH implementation agreement sunset dates for National Cooperative Highway Research Program (NCHRP) 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 need to mount chain link fences on top of these barriers to serve different purposes. In other cases, due to space restrictions, signs or even light poles are placed on top of such barriers. When DOTs mount these objects on top of barriers, the crashworthiness of the system is not necessarily guaranteed.
The purpose of this research is to investigate the crashworthiness of a 36-inch tall concrete single slope median barrier with chain link fence mounted on top under MASH 2016 evaluation criteria. The structural capacity and the occupant risk factors of such proposed barrier system will be evaluated with respect to MASH Test Level (TL) 4 criteria through full-scale crash testing.
The information compiled from this research will provide the FHWA and State DOTs with an acceptable concrete median barrier system with fence mounted on top, evaluated under MASH 2016 TL-4 conditions. 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 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. 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 NHS are required to meet 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 4409 lb to 5000 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 results 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 a result of the test impact. Although these evaluation criteria are applicable to all roadside safety devices testing, they are most relevant for sign support design and testing. In addition, little evaluation of sign supports has been performed with larger vehicles such as the pickup. Systems that have been demonstrated to be crashworthy for passenger cars may not be geometrically compatible with pickup trucks.
In 1995, Buth and Menges conducted a research study which included the evaluation through full-scale crash testing of a 31-inch tall New Jersey safety shape concrete barrier with a vandal protection fence mounted on top. Testing was conducted following the AASHTO performance level 2 impact conditions, which included a 5562-lb pickup truck impacting the test article at a nominal speed and angle of 62 mph and 20 degrees, respectively. The purpose of the full-scale crash testing was to evaluate the strength of the section in containing and redirecting the pickup and the interaction of the vehicle with the fence.
The New Jersey safety shape concrete barrier with vandal protection fence mounted on top consisted of concrete barrier segments that were 10-ft in length and 31 inches in height. The barrier was 6 inches wide at the top and 15 inches wide at the base. The vandal protection fence was mounted on 7.25-ft long × 2.875-inch OD (schedule 40 pipe) straight posts mounted to the back of the barrier. Attached to these posts were three 1.66 inches OD (schedule 40 pipe) horizontal line rails spaced 3 ft, with 1 inch × 1 inch wire fabric. Height to the top of the fence is 6 ft above the safety shape, for a total height of the installation of 8.7 ft above the road surface.
After the vehicle contacted the fence, the middle horizontal line rail pulled out of the connection at upstream side of post. The installation received minimal damage. As a result of the interaction with the vehicle during the impact event, the lower edge of the wire fabric was pushed behind the lower horizontal line rail. The middle horizontal line rail was disconnected on the upstream side and the post anchor was pushed back 0.5 inches. Maximum deflection of the fence resulted being 5.6 inches and maximum residual deformation was 3 inches. The vehicle remained upright during and after the impact event and occupant risk factors were within acceptable value. The vehicle sustained moderate damage. The floorpan, frame, and front axle were deformed and the windshield was cracked. There was a small folding running diagonally in the floorpan of the occupant compartment and there was 2.8 inches deformation into the occupant compartment of the firewall on the passenger side of the vehicle. The impact performance of the vandal protection fence on New Jersey safety shape bridge railing was considered satisfactory according to the guidelines set forth in AASHTO.
In 1972, Post et al. conducted a research study which included the evaluation through full-scale crash testing of a rigidly fixed 32-inch tall Texas CMB barrier with chain link fabric fence and a luminaire mounted on top. The Texas CMB is similar to the New Jersey Median Barrier in dimensions and shape. A total of four tests were conducted with the purpose to
The Texas CMB concrete barrier with chain link fabric fence and luminaire hardware on top consisted of concrete barrier segments that were 50-ft in length and 32 inches in height. The barrier was 8 inches wide at the top and 27 inches wide at the base. The rigid 45-ft luminaire pole was mounted on top of the Texas CMB barrier and anchored to it with use of four 1-1/4 inches diameter and 30 inches long AISI 1040 bolts. A 3-ft tall #9 gauge chain link fabric fence of 1 inch mesh was also mounted on top of the Texas CMB barrier.
The first test on the rigid concrete median barrier was conducted with the 4000-lb large sedan impacting the test article at impact conditions of 62.4 mph and 25 degrees. The centerline of the vehicle was directed as the centerline of the luminaire support. The vehicle was contained and redirected, however the severely damaged impacting front quarter and wheel of the vehicle caused it to swerve back toward the barrier. The door on the driver’s side was also sprung open and the windshield was cracked.
The second test on the rigid concrete median barrier was conducted with the 4000-lb large sedan impacting the test article at impact conditions of 55.7 mph and 25 degrees. The vehicle was contained and redirected, and it was slightly less damaged than in the first test (door was not sprung open).
The third test on the rigid concrete median barrier was conducted with the 4000-lb large sedan impacting the test article at impact conditions of 60.9 mph and 7 degrees. The vehicle was contained and redirected, with a maximum climb of approximately 18 inches. The relative minor damage consisted of bumper and sheet metal crushing.
The fourth test on the rigid concrete median barrier was conducted with the 4000-lb large sedan impacting the test article at impact conditions of 60.7 mph and 15 degrees. The vehicle was contained and redirected. Sheet metal contact caused relative minor damage to the fence. The damage in the vehicle in this test was somewhat less than the damaged vehicles in the previous CMB-1 and CMB-2 tests that were run at larger impact angles.
The research objective is to investigate the crashworthiness of a 36-inch tall concrete single slope median barrier with chain link fence mounted on top under MASH 2016 evaluation criteria. Engineering analysis will aid in the selection of system details to be considered for testing. The structural capacity and the occupant risk factors of such proposed barrier system will be evaluated with respect to MASH TL-4 criteria through full-scale crash testing.
The information compiled from this research will provide the FHWA and State DOTs with a MASH 2016 TL-4 acceptable concrete median barrier system with fence mounted on top. 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 median barrier system and of each of its components in the format required for inclusion in hardware standards documents of the Task Force 13.
The researchers will perform a literature review of concrete barrier systems that have been investigated and/or tested with attachments on top, giving priority to fence systems mounted to concrete barrier systems. In addition, the researchers will complete an investigation of the most practical and utilized installation conditions for fences on top of concrete barriers. In order to complete this investigation, it is anticipated that the researchers will poll the DOT Members. Results of the conducted survey will help to identify common design details of the roadside device to be considered for further investigation, as well as common practices for installation and implementation of the system to be considered for evaluation in future tasks.
Based on the results from Task 1, the researchers will determine design details of the fence mounted on the concrete barrier system to be advanced for testing.
Based on engineering evaluation and Member poll results, the researchers will determine the design details and installation characteristics for the combination fence-concrete median 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 DOT members to determine details of the installation characteristics of the system to be constructed.
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.
Two full-scale crash tests will be performed according to MASH 2016 TL-4. One full-scale test will involve a 5000-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 occupant risk of the combination fence-concrete median barrier system, as well as the passenger vehicle stability during the impact event.
A second 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 combination fence-concrete median 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 the Task Force 13.
|TTI Research Supervisor:
Chiara Silvestri Dobrovolny, Ph.D.
Associate Research Scientist
Texas A&M Transportation Institute
College Station, Texas 77843-3135
|Pooled Fund Technical Representative:
Derwood C. Sheppard, Jr., P.E., M.Eng.
Standard Plans Publication Engineer
FDOT Road Design Office
Florida Department of Transportation
605 Suwannee Street
Tallahassee, FL 32399-0450