The underlying philosophy of the Manual for Assessing Safety Hardware (MASH) in the development of the testing guidelines is that of “worst practical conditions.” Crash testing is conducted under idealized conditions so that impact performance can be evaluated and compared under controlled situations. While MASH requires to test roadside safety hardware on flat level ground conditions, variations in field installation characteristics can materially affect the performance of some roadside safety features. This can be of particular relevance for breakaway structures (i.e., signs) installed on slopes.
A breakaway support is designed to lessen the impact to a vehicle if struck and thereby minimize injury to occupants and damage to vehicles. It is unknown how a breakaway mechanism might perform under impacts that happen on sloped terrain rather than on flat level ground. Some of the questions that are left unknown are whether the release mechanism of the breakaway structure might be affected by the different installation geometry and by the resulting impact vehicle dynamics. Also, it is unknown whether the post-impact behavior and trajectory of the test article due to the installation on a sloped terrain might result in a potential increase for occupant risk and unacceptable vehicle damage.
This research project should:
1) assess crashworthiness of a large breakaway sign support per MASH Test Level 3 (TL-3) conditions on flat level ground,
2) investigate the most practical and utilized installation conditions for large breakaway sign support on sloped terrain,
3) determine the most critical characteristics within this envelope of conditions, and 4) assess crashworthiness of the large breakaway sign support in combination with sloped installation condition.
It is anticipated that a negative slope should be considered within this study.
The information compiled from this research will provide the Federal Highway Administration (FHWA) and State Departments of Transportation (DOTs) with a first understanding on how a large sign slipbase breakaway support structure reacts to impact events when installed on a given slope under MASH 2016 Test Level 3 impact conditions. Outcomes from this research and testing study will support implementation guidelines review for large slipbase breakaway support structures on roadside slopes.
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 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 TL-3 requirements.
The structural adequacy MASH 2016 test for TL-3 conditions consists of a 5000-lb pickup truck (denoted 2270P) impacting a barrier at 62 mph and 25 degrees with respect to the roadway. The severity MASH 2016 test consists of a 2420-lb passenger car (denoted 1100C) 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 3-11 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.
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 supports 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.
The underlying philosophy of MASH in the development of the testing guidelines is that of “worst practical conditions.” Crash testing is conducted under idealized conditions so that impact performance can be evaluated and compared under controlled situations. While MASH requires to test roadside safety hardware on flat level ground conditions, variations in field installation characteristics can materially affect the performance of some roadside safety features. This can be of particular relevance for breakaway structures (i.e., signs) installed on slopes.
A breakaway support is designed to lessen the impact to a vehicle if struck and thereby minimize injury to occupants and damage to vehicles. It is unknown how a breakaway mechanism might perform under impacts that happen on sloped terrain rather than on flat level ground. Some of the questions that are left unknown are whether the release mechanism of the breakaway structure might be affected by the different installation geometry and by the resulting impact vehicle dynamics. Also, it is unknown whether the post-impact behavior and trajectory of the test article due to the installation on a sloped terrain might result in a potential increase for occupant risk and unacceptable vehicle damage.
Once a sign support system is installed on a slope, the local mounting height of the sign (calculated from ground level at the location of installation) will be greater than that for the same system installed on flat, level ground. For a general installation of a sign support system on a slope at an offset distance “x” from the slope break point, the depth “y” of the slope at the particular installation location contributes to an increase in the length of the support post and local mounting height of the sign (Figure 1).
Figure 1. Effective Pole Height Variation for Sign Support Installation on Slope.
An additional consideration related to sign installations on slopes is related to the actual vehicle bumper impact (BI) location on the sign support. When an errant vehicle enters a roadside slope, certain factors influence its trajectory. These factors include the geometry of the slope, the encroachment speed, and encroachment angle at which the vehicle enters the slope.
The bumper trajectory and offset distance of the support installation from the slope break point determine the height of bumper contact above the local ground elevation. Consequently, the effective height of the post above the bumper can vary.
The objectives of this research project are:
The information compiled from this research will provide the FHWA and State DOTs with a first understanding on how a large sign slipbase breakaway support structure reacts to impact events when installed on a given slope under MASH 2016 TL-3 impact conditions. Outcomes from this research and testing study will support implementation guidelines review for large slipbase breakaway support structures on roadside slopes to help with a better roadside device design and implementation to reduce the risk of injury and injury severity of related crashes.
TTI will provide composite video and photographic documentation of the crash tests and a final report documenting the research and testing performed. If applicable, documentation needed to request FHWA’s acceptance of the large sign slipbase support system for use on the National Highway System will be provided.
TTI will further provide drawings of the roadside device and of each of the components of the system in the format required for inclusion in hardware standards documents of the Task Force 13.
The proposed work plan consists of six tasks.
The researchers will perform a literature review of large sign breakaways support systems that have been investigate and/or tested per MASH TL-3 conditions in the past. In addition, the researchers will complete an investigation of the most practical and utilized installation conditions for large breakaway sign support on sloped terrain. In order to complete this investigation, it is anticipated that the researchers will poll the DOT Members. Results of the conducted survey will allow 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 these large sign breakaway devices to be considered for evaluation in future tasks.
Based on the results from Task 1, the researchers will determine design details of the breakaway support system to be advanced for further investigation. The researchers will then conduct engineering analysis to investigation the crashworthiness performance of the proposed system, and will determine whether full-scale crash testing is needed to confirm crashworthiness determination.
A set of computer simulations will then serve to investigate the system behavior when impacted by errant passenger vehicles while installed on a sloped terrain. It is anticipated that a negative slope will be considered. Slope characteristics, however, as well as other installation characteristics will be determined based on the results obtained from Task 1. It is anticipated that computer impact simulations will be performed with both passenger car and pickup truck vehicles, as described in Tasks 4 and 5. The researchers anticipate investigating impact details, such as impact angles and critical impact locations based on additional vehicle dynamic analysis. Such analysis will characterize the behavior of the vehicle once entering and traversing the slope. Based on the computer simulations results, the researchers might suggest additional design modifications to improve the system performance.
Based on engineering evaluation and finite element computer simulations results, the researchers will determine the design details and installation characteristics for the breakaway support 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.
Appropriate drawings will be developed to aid in the construction of the approved systems and to guide installation procedures for the testing plan. The approved system will be constructed following MASH 2016 requirements.
Within this Task, two full-scale crash tests will be performed according to MASH 2016 TL-3 impact conditions:
MASH Test Designation 3-60: A 2420-lb vehicle impacting the support structure at a nominal impact speed of 19 mi/h and the critical impact angle between 0 and 25 degrees. This test investigates the activation of the breakaway mechanism of the support structure when impacted at low speed by a small passenger vehicle.
MASH Test Designation 3-61: A 2420-lb vehicle impacting the support structure at a nominal impact speed of 62 mi/h and the critical impact angle between 0 and 25 degrees. This test investigates vehicle stability and the potential for test article intrusion into the windshield or roof of a small passenger vehicle.
Results from Tasks 1 and 2 will dictate the critical impact angle for consideration in these planned tests.
Within this Task, one full-scale crash test will be performed according to MASH 2016 TL-3 impact conditions:
MASH Test Designation 3-62: A 5000-lb pickup truck impacting the support structure at a nominal impact speed of 62 mi/h and the critical impact angle between 0 and 25 degrees. This test investigates the potential for test article intrusion into the windshield or roof of a pickup truck vehicle.
Results from Tasks 1 and 2 will dictate the critical impact angle for consideration in this planned test.
The researchers will generate a final report of the findings from the research and testing study. Results will be reported in terms of system 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 sign support 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. Research Scientist Texas A&M Transportation Institute TAMU 3135 College Station, Texas 77843-3135 (979) 317-2687 [email protected] |
Pooled Fund Technical Representative: Derwood C. Sheppard, Jr., P.E., M.Eng. Standard Plans Publication Engineer FDOT Roadway Design Office Florida Department of Highways 605 Suwannee Street Tallahassee, FL 32399-0450 (850) 414-4334 [email protected] |
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2019-06-19