MIDWEST STATES POOLED FUND PROGRAM
Guidelines for Placement of Breakaway Light Poles Behind Midwest Guardrail System
Sponsoring Agency Code
TPF-5(193) Supplement 87
Light poles are a commonly found along most highways due to the need to provide proper illumination to critical areas of the road. It is not uncommon for light poles to be placed in areas where guardrail is present as well. In these situations, light poles must be placed sufficiently close to the roadway while not interfering with the performance of the guardrail system. However, several concerns exist when placing light poles in close proximity to guardrail that may affect its ability to safely contain and redirect vehicles. First, interaction between a deflected guardrail system and a pole may create unwanted stiffening or hinging of the barrier system about the pole, which may cause pocketing and increased loading to the guardrail. The pole may also present a snag hazard to impacting vehicles, which may cause increased vehicle decelerations and instabilities. Third, interaction between the guardrail posts and light pole may affect barrier performance. While the use of breakaway light poles may mitigate these concerns to some degree, the interaction between a guardrail system and a closely-positioned light pole requires further investigation.
The Illinois Tollway currently has many instances where light pole placement is desired directly behind W-beam guardrail in order to provide adequate road illumination. Illinois Tollway and Illinois Department of Transportation have recently adopted the Midwest Guardrail System (MGS) as their standard W-beam guardrail design. The current Illinois Tollway standard for light poles is to place the pole no closer than the minimum barrier clearance distance of 28” (standard 6’-3” post spacing), 23” (½-post spacing), and 14” (¼-post spacing), as shown in Figure 1. In order to accommodate poles positioned closer than the minimum barrier clearance distance, an investigation should be conducted to determine if the minimum standard distance for breakaway light poles can be reduced, and if so, determine the optimal position of the light pole with respect to the guardrail system. It is anticipated that computer simulation/modeling with non-linear finite element analysis will be conducted to reduce the number of required crash tests. Crash tests should be conducted according to the AASHTO MASH Test Level 3(TL-3) impact safety criteria.
Limited research has been conducted regarding the placement of light pole structures behind guardrail. A study for the Ohio Department of Transportation by James Kennedy, Battelle, conducted a series of crash tests on light poles that were offset behind round, wood-post, W-beam guardrail with a top mounting Limited research has been conducted regarding the placement of light pole structures behind guardrail. A study for the Ohio Department of Transportation by James Kennedy, Battelle, conducted a series of crash tests on light poles that were offset behind round, wood-post, W-beam guardrail with a top mountingheight of 27 in. For this study, researchers looked at light poles with aluminum transformer bases at lateral offsets of 6 in. and 18 in. behind the back side of the guardrail. Testing was conducted according the NCHRP Report No. 350 TL-3 criteria with both the 820C and 2000P vehicles. The study varied the impact location relative to the pole to evaluate varied pole placement (i.e., near point of impact and at a location near point of maximum barrier deflection). The results from these tests did not indicate that placement of light poles behind the barrier system was a hazard. However, the testing performed in the study did not use consistent TL-3 impact speeds for all tests, occupant risk values were not calculated for all of the testing where vehicles impacted both the guardrail and the light pole, and the testing did not critically impact the pole in all tests. The testing did demonstrate significant vehicle damage and snag due to contact with the light poles.
The Illinois Tollway current uses the MGS system for their standard W-beam guardrail. The MGS system provides many safety benefits, including increased barrier capacity, improved capture and redirection of high center-of-gravity vehicles, and a wide range of special applications for slopes, long-spans, curbs, and other non-standard installations. However, the MGS also has increased deflections when compared to previous strong-post W-beam guardrail systems with 27-in. top mounting heights. Thus, interactions between the barrier system and adjacent light poles may be increased during an impact event. As noted above, the Illinois Tollway currently attempts to mitigate this interaction by offsetting the light poles outside of the expected barrier deflection. This offset methodology is reasonable and safe, but it may be difficult and costly to achieve depending on the installation. Thus, there is a desire to determine if reduced lateral offsets between pole and MGS can be safely accommodated.
Illinois Tollway specifies a standard light pole which measures 50 ft tall, utilizes a 10-ft to 15-ft long mast arm, and is supported on a breakaway transformer based manufactured by Hapco. The breakaway transformer base is 9 in. tall and fabricated from 356-T6 aluminum. The transformer base was previously evaluated to the AASHTO Standard Specifications for Structural Supports for Highway Sign, Luminares, and Traffic Signals through a series of pendulum tests conducted at Southwest Research Institute in 1990, and the base was given an FHWA acceptance letter in that same year. This pole and breakaway transformer base will be used for the analysis and testing in this research effort.
The objective of this research effort is to develop guidance for safe placement of the Illinois Tollway standard light pole design behind the MGS. Computer simulation of various pole offsets and critical impact points will be utilized to determine the minimal pole offset for the MGS system with standard post spacing and the recommended configuration for full-scale testing. Full-scale crash testing will be conducted to evaluate the minimum proposed pole offset according to the TL-3 impact safety requirements in MASH. The results of the crash testing, computer simulation and previous testing of the MGS system will be applied to develop pole placement guidance for the MGS with ½ and ¼ post spacing.
The Illinois Tollway requests that the following objectives/deliverables bet met as part of the research effort.
1. Review previous literature and projects regarding the placement of objects behind guardrail.
2. Research the potential modification of current minimum distance in regards to the placement of breakaway light poles behind guardrail.
3. Recommend modifications to Tollway Standard Drawings and manuals based upon research.
4. Suggest guidance regarding clearance distance for similar, breakaway devices.
5. Obtain FHWA Eligibility Letter for proposed clearance distance for breakaway devices.
6. Demonstrate educational emphasis through the use of graduate-level students.
7. Provide a 1 page technical summary of report.
8. Update Tollway with quarterly briefings (electronic memo) of findings as well as student involvement.
9. Submit 2 hardcopies and 1 electronic copy (PDF) of final report.
10. Archive to a DVD the MASH TL-3 crash videos.
The research effort consists of the development of guidance for use in the safe placement of Illinois Tollway standard light poles behind the MGS, which is expected to proceed in two phases. Phase I would consist of the literature review as well as analysis and computer simulation effort to determine a minimum pole offset for the MGS system with standard post spacing. Phase II would consist of full-scale vehicle crash testing to evaluate the lateral pole offset selected in Phase I.
The both phases of the research plan are scheduled for completion within 24 months of the start of the research agreement. This would include completion of the research objectives, preparation of the final report, review of the final report by the Tollway Technical Review Panel, and final review/publishing of the report. It should be noted that the Tollway should be allowed three months prior to the end of the project to review the draft final report.
Phase I will begin with a literature search of previous W-beam guardrail (including MGS) testing with and without poles or other nearby obstacles to evaluate dynamic deflections, working widths, deflected barrier lengths, as well as vehicle pocketing and snag risks. In addition, a review will be performed on relevant breakaway light pole systems specified by the Illinois Tollway. Review of this material will provide insight on expected barrier deflection, potential hazards associated with striking obstacles within the working width, performance of breakaway light poles, and data for creation and validation of the computer simulation models.
Following the review of the literature search materials, researchers will use a combination of LS-DYNA computer simulation and engineering analysis to select a minimum lateral pole offset for the MGS system with standard post spacing and the critical impact point (CIP) for full-scale crash testing with 2270P and 1100C vehicles. The computer simulation effort will begin with creation of a model of the light pole used by the Illinois Tollway with the appropriate mass and geometry. Modeling of the breakaway mechanism of the pole will be outside the scope of this research, but the pole will be used in conjunction with models of the MGS system to evaluate interaction between the pole and the guardrail. Computer simulations will be conducted at various pole offsets and vehicle impact locations to determine the minimum safe pole offset based on rail pocketing, rail loads, vehicle capture, vehicle stability, and other factors. It is anticipated that the analysis will primarily focus on the 2270P vehicle due to its larger dynamic deflections, but simulations of the 1110C vehicle will also be conducted to ensure that the pole offset is feasible for the small car vehicle. Illinois Tollway will be included in the discussions to incorporate their feedback and comments on minimum pole offset and critical impact points.
Once the simulation and analysis of the lateral pole offset and CIP are completed, CAD details of the MGS system with nearby light pole will be developed. At the completion of the Phase I research effort, a summary report will be completed with recommendations for full-scale crash testing of the MGS with an offset light pole.
For Phase II of the research, the standard MGS barrier with standard 75-in. post spacing and an offset light pole will be constructed at the MwRSF Outdoor Proving Grounds for testing and evaluation according to the MASH guidelines for test designation no. 3-11 with the 2270P pickup truck vehicle and test designation no. 3-10 with the 1100C small car vehicle. Test no. 3-10 may potentially be waived if the selected pole offset from Phase I is large enough to alleviate concerns for interaction with the small car vehicle. However, it is assumed at this time that the small car test will be needed. The pole offset and vehicle impact points will be selected based on the simulation analysis from Phase I. The pole used will be a standard light pole specified by the Illinois Tollway. The pole dimensions will be selected to ensure a worst-case test condition. The full-scale vehicle crash tests will be conducted, documented, and evaluated by MwRSF personnel and in accordance with the MASH guidelines.
Following the successful full-scale crash testing, results will be reviewed and analyzed to evaluate the performance of the Illinois Tollway light pole when placed within a minimum offset behind the MGS. Guidance will be provided on recommended offsets for Illinois Tollway light pole placement for use with the MGS with standard 75-in. post spacing. If the full-scale crash testing proves unsuccessful, the researchers will provide guidance on potential improvements to the tested offset, and recommendations for further research and/or testing will be given.
After completion of the full-scale crash testing, a summary report of the research project will be completed detailing the as-tested barrier system with nearby pole, full-scale crash test results with evaluation, and recommendations for implementation of MGS barrier with nearby light poles. MwRSF will also prepare and submit an eligibility letter to FHWA with respect to the as-tested light pole and barrier combination. MwRSF will also prepare a one page technical summary for Illinois Tollway for their use in dissemination of the research.
The successful development and evaluation of a minimal offset for light poles placed adjacent to the MGS would allow the Illinois Tollway and the Illinois DOT to reduce light pole relocations in upcoming construction projects and avoid relocation in projects that are currently underway. Avoiding or reducing light pole relocations when minimum clearance distance is not met would reduce construction costs. In addition, the research could potentially reduce the need for supplemental lighting, planning, and analysis of lighting impacts due to necessary light pole relocation.
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