Guidelines for Evaluating Crashworthiness of Sign Supports and Breakaway Luminaire Poles (2024)
Chapter: Summary
SUMMARY
Guidelines for Evaluating Crashworthiness of Sign Supports and Breakaway Luminaire Poles
The objective of NCHRP Project 22-43 was to evaluate selected systems and propose modifications to the AASHTO Manual for Assessing Safety Hardware (MASH) to provide additional guidelines on selecting a critical test matrix for testing families of systems. Given the time available and budget, the two most common categories were selected for evaluation: (1) a breakaway steel luminaire pole with aluminum TB1-17 frangible transformer base, and (2) a single 2¼-in., 12-gauge perforated steel square tube (PSST) sign support. LS-DYNA models for the pole and PSST systems were developed and partially validated using available crash tests. Nine and seven full-scale crash tests were recommended for the pole and PSST families of devices. Because additional funds would have been needed to run all the desired crash tests, it was decided to provide guidelines regarding the crashworthiness of both families as best as possible.
Note that this project, NCHRP Project 22-43, proceeded with distinct objectives yet was aligned with another project, NCHRP Project 17-105, with the two projects ultimately combined. The objectives of the research were to determine the maximum height and weight of breakaway poles and hardware that will meet MASH requirements. The research efforts included (a) physical tests to correlate roof crush with pole and hardware height and weight and aid in determining critical pole configurations, and (b) dropping poles with associated hardware of varying heights and weights onto vehicles as a precursor to full-scale crash testing. The findings will be used to identify improvements to vehicle computer models to better simulate crash testing roof crush. Additionally, this research investigated whether the criterion of a 4-in.-tall object on a 5-ft chord is still applicable to the current vehicle fleet. This objective includes (a) reviewing procedures used to develop the 4-in.-tall on 5-ft-chord criteria, (b) determining whether modifications or improvements to the procedure can be implemented, and (c) conducting an analysis of procedures with the current vehicle fleet. Note that the primary emphasis will be on meeting the objectives of NCHRP Project 17-105 and considering how the new findings can contribute to the goals of this project, NCHRP Project 22-43 (i.e., identifying a critical reduced matrix for MASH evaluation of luminaire poles with TB1-17 transformer base).
Breakaway Steel Luminaire Pole with Aluminum TB1-17 Frangible Transformer Base
Two full-scale crash tests, Test Nos. TBLP-1 and TBLP-2, were conducted on luminaire poles with a TB1-17 transformer base under MASH 3-60 impact conditions at an angle of 0 degrees with impact at the vehicle’s center point. Simulation indicated that center impacts were likely more critical than impacts at the right- or left-quarter points. Additionally, an impact angle of 0 degrees would increase the likelihood of the pole landing on the center
of the roof, which has less crush stiffness. In Test No. TBLP-1, the 50-ft-tall pole with dual 15-ft-long mast arm and a weight of 1,015 lb was impacted by a 1100C small car, and the base was activated, but the test failed MASH criteria due to excessive occupant impact velocity (OIV). In Test No. TBLP-2, the 30-ft-tall pole with a single 30-ft-long mast arm and a weight of 824 lb was impacted by a 1100C small car, but the desired activation of the TB1-17 transformer base did not occur and the test failed MASH criteria. The LS-DYNA models were then updated and validated to match the test results.
Next, simulations were conducted on various pole configurations with a wide range of design parameters, including pole height and mast arm length with single and dual mast arms, under MASH 3-60, 3-61, and 3-62, with varying vehicle impact points. The validated simulations showed the potential for evaluating the crashworthiness of poles in terms of OIV. However, the inconsistent behavior of poles after base fracture observed in various full-scale crash tests presents a challenge in predicting roof crush through simulation. Additionally, inaccuracies in the vehicle roof model introduce additional complexity to the problem. These challenges will be further investigated in the continuation of the current project.
The full-scale crash tests and simulations revealed a series of trends in crash performance of poles with various configurations. Some highlights of these trends are:
- None of the simulations or full-scale crash tests exceeded MASH maximum limits for occupant ride-down acceleration (ORA) values, lateral and longitudinal values, or roll and pitch values.
- Critical measures for evaluating luminaire poles include ensuring that the pole base breaks away on impact, limiting longitudinal OIV to below 16 ft/s, limiting maximum intrusion into the occupant compartment (i.e., roof crush) to less than 4 in., and preventing any part of the luminaire from penetrating the vehicle.
- MASH 3-60 impacts appeared to be more critical in terms of roof crush than MASH 3-61 and 3-62 impacts. Center impacts were generally found to be more critical than right- or left-quarter impacts for roof crush, primarily due to the longitudinal fall of the pole on the vehicle’s roof.
- Roof crush concerns were noted for poles weighing over 500 lb in MASH 3-60 impacts. As a result, it is recommended to conduct one full-scale crash test on any pole configuration to evaluate its performance under this critical impact condition.
- In some cases that involve left- or right-quarter impacts at a 25-degree angle, the base did not break away, resulting in high OIV values. This behavior could be attributed to modifications that strengthened the base’s corners. Validation through full-scale crash testing under these conditions is necessary to assess OIV criteria.
- MASH 3-61 impacts did not result in vehicle contact with the pole or intrusion of the pole into the occupant compartment.
- OIV values increased as the total system weight increased in MASH 3-61 impacts, particularly for heavy poles weighing over 800 lb. Full-scale crash testing is required to determine specific critical pole characteristics.
- Center impacts at a 0-degree impact angle in MASH 3-61 appeared to be suitable for assessing OIV.
- MASH 3-62 impacts generally resulted in lower OIV values than MASH 3-61 impacts, suggesting that luminaire poles should be evaluated only under MASH 3-61 to meet OIV criteria.
- None of the MASH 3-62 impacts led to vehicle contact with the pole or pole intrusion into the occupant compartment. Full-scale crash tests may not be necessary for these conditions.
Table 1. Draft guidelines suggested for MASH evaluation of luminaire poles with a TB1-17 transformer base (not applicable before validation).
| Pole Configuration | MASH 3-60 | MASH 3-61 | MASH 3-62 |
|---|---|---|---|
| Short Poles (H ≤ 20 ft and W ≤ 450 lb) | ▶ One test needed for roof crush evaluation: 3-60-CE-0 ▶ One test needed for OIV evaluation: 3-60-RQ/LQ-25 |
No test needed | No test needed |
| Medium Poles (20 ft < H < 40 ft and W ≤ 800 lb) | ▶ One test needed for roof crush evaluation: 3-60-CE-0 ▶ One test needed for OIV evaluation: 3-60-RQ/LQ-25 |
No test needed | No test needed |
| Tall Poles (H ≥ 40 ft and W > 800 lb) | ▶ One test needed for roof crush evaluation: 3-60-CE-0 ▶ One test needed for OIV evaluation: 3-60-RQ/LQ-25 |
One test needed for OIV evaluation: 3-61-CE-0 | No test needed |
Table 2. Full-scale crash tests required to validate suggestions – luminaire poles with a TB1-17 transformer base.
| Pole Configuration | MASH 3-60 | MASH 3-61 | MASH 3-62 |
|---|---|---|---|
| Short Poles (H ≤ 20 ft and W ≤ 450 lb) | One test needed: 3-60-CE-0 (Roof Crush) |
No test needed | No test needed |
| Medium Poles (20 ft < H < 40 ft and W ≤ 800 lb) | Two tests needed: 3-60-CE-0 (Roof crush) 3-60-LQ/RQ-25 (OIV) |
One test needed: 3-61-CE-0 (OIV) |
No test needed |
| Tall Poles (H ≥ 40 ft and W > 800 lb) | One test needed: 3-60-LQ/RQ-25 (OIV) |
One test needed: 3-61-CE-0 (OIV) |
One test needed: 3-62-CE-0 (OIV) |
- Low-speed pickup truck tests are not included in existing MASH standards. Further research is needed to assess their necessity and compare their criticality with MASH 3-60 and MASH 3-61 tests.
Note that the LS-DYNA simulations were significantly improved (compared to pretest simulations) in predicting the impact behavior of luminaire poles. However, the validation process is incomplete without running the full set of tests. For instance, simulations for poles representing MASH 3-61 and 3-62 impact conditions lack full-scale crash test data for validation, which makes it challenging to develop guidelines and suggest modifications to MASH for these systems. Thus, the concept of identifying a family of devices and using a reduced test matrix to determine crashworthiness could not be fully demonstrated. Using the available full-scale crash tests and updated simulations, preliminary guidelines for MASH evaluation of poles with a TB1-17 base were drafted, as shown in Table 1. To validate these suggestions and to further refine the pole categories, a series of full-scale crash tests are necessary, as shown in Table 2, and would also provide data for further validation of the simulations.
Single 2¼-in., 12-Gauge PSST Sign Support
The second type of device selected for analysis and evaluation under this project was the PSST sign support system. This system was found to be widely used across the nation, and based on a survey from NCHRP Project 03-119 (Rasmussen et al. 2018), had a high ranking and percentage of use by state departments of transportation (DOTs).
The analyses began with model validations of the PSST sign support system using available full-scale crash tests. On completing the validations, a matrix of simulations was set up and conducted to investigate the effects of different parameters on MASH performance within a family of PSST sign support systems. Table 3 highlights the various parameters that were investigated in the matrix of simulations. PSST systems with various panel sizes were analyzed, ranging from 1 ft × 1½ ft to 4 ft × 5 ft. Gauge thicknesses of 0.08 in., 0.1 in., and 0.12 in. were used for the sign panels, with aluminum assigned as the material for all models. The post size for this family of devices was selected to be a 2¼-in. × 2¼-in. cross-section with a gauge thickness of 12. A 7-ft mounting height to the bottom of the sign panel, the most common configuration, was used for all models. The anchor size was selected to accommodate the post sizes (2¼ in. × 2¼ in. by 12-gauge thickness), using ASTM A1011 Grade 50 steel for both the post and anchor. The anchor embedment was set at 36 in. into the ground in all models. These parameters were chosen based on what was commonly used by state DOTs determined under NCHRP Project 03-119. MASH standard soil was used for the ground surrounding the anchor. Models at three MASH impact configurations (3-60, 3-61, and 3-62) were employed in the matrix of simulations. Additionally, different impact locations (center, driver-side offset, and passenger-side offset) and different impact angles (0, +25, and –25 degrees) were incorporated into the matrix of simulations.
Close to 400 simulations were performed and analyzed, generating a considerable amount of information. The critical MASH metrics for the PSST system were the OIV, windshield intrusion, and roof deformation. Excessive windshield penetration was by far the most common cause of failure for the PSST systems.
The results from the simulations were used to select full-scale crash tests to validate the analyses and establish guidelines for potential incorporation into MASH for testing a family of sign support devices. Five full-scale crash tests were conducted on PSST systems. All five tests used the same post: a 2¼-in., 12-gauge single PSST post, which is a criterion for belonging to the same family of devices. The five tests were diversified in terms of impact configuration (MASH Test Nos. 3-61 and 3-62), panel size (1 ft × 1.5 ft, 4 ft × 5 ft, and 3 ft × 3 ft), impact location (center and offset), and impact angle (0 and 25 degrees).
Upon analyzing the test results and comparing them with the simulation predictions, differences were observed that did not significantly affect the overall conclusions drawn
Table 3. PSST sign support configurations analyzed.
| Device Family | 2¼-in., 12-Gauge Single PSST Sign Support | |
| Identical, Critical Structural Feature | Sign support section and size | 2¼-in., 12-gauge PSST |
| Potential Parameters That Provide Similar Safety Performance Under MASH Impact Conditions | Sign panel size (If applicable, a lower smaller advisory panel may potentially be considered.) |
1 ft × 1½ ft; 2 ft × 2 ft; 2½ ft × 2½ ft; 3 ft × 3 ft; 3 ft × 4 ft; 4 ft × 5 ft (recommended from NCHRP Project 03-119) |
| Sign panel mounting height | 7 ft (recommended from NCHRP Project 03-119) | |
| Sign panel material | Aluminum (recommended from NCHRP Project 03-119) | |
| Sign panel thickness | 0.08 in., 0.10 in., 0.12 in. | |
| Support embedment/foundation | 36 in. | |
| Presence of wind beams | With and without wind beams | |
from the simulation analyses. The variances were attributed to soil strength, which affected the separation timing of the post from the base sleeve. After updating the models accordingly, the simulations aligned with the test outcomes. The simulations were rerun, and the summary results were updated. Table 4 shows a summary of all cases, incorporating the results from the updated models. Based on these results, the following observations were made:
- MASH Test No. 3-60 was the least critical of the three impacts for the PSST sign support system family being considered. This test could be omitted or reduced to only one test with the largest sign panel.
- MASH Test No. 3-61 was the most critical of the three tests. It was noted that the size of the panel affects the MASH performance outcome. In some cases, medium-sized panels were more critical than smaller and larger sizes. To verify that a system with all panel sizes meets MASH, tests with the smallest and largest sign would need to be performed, and based on the outcome of these tests, an additional test may need to be performed (e.g., if the smallest panel hit the hood and the larger panel hit the roof, a test with a size in between would be needed).
- Similar effects were noted for MASH Test No. 3-62. The panel size affects the MASH performance, and the middle panel size could be more critical than the smallest and largest sizes. Testing should start with the two extreme cases (smallest and largest sizes), and based on the outcome, additional testing with in-between sizes may be needed.
- It was noted that impact locations and impact angle can have an important effect on the outcome of the test. In some cases, offset impacts and impacts at an angle were more critical than the no-offset impact. In other cases, the no-offset impact was more critical. Tests with and without offset are needed.
Considering all impacts, none of the analyzed configurations were found to meet MASH criteria for all three tests (3-60, 3-61, and 3-62). This highlights the difficulty of developing a system that meets current MASH criteria.
A matrix of tests is currently defined in MASH for sign support systems (see Table 5). Full-scale testing of all configurations in this matrix for a family of PSST systems would be cost-prohibitive. Using the results from the simulations, along with the full-scale crash tests and insights gained from existing literature, the research team developed a preliminary matrix for MASH testing of a family of small-sign support systems. Instead of running the three impacts (3-60, 3-61, and 3-62) for each configuration in the family of devices, the new matrix incorporates fewer key tests on select critical configurations. The new matrix reduces the number of required tests without compromising the evaluation and safety of the different configurations within the family of small-sign support systems. Table 6 depicts the preliminary test matrix for the analyzed family of PSST systems. Only Test Level 3 (TL-3) is included in the table, but similar tests can be adopted for the other test levels. The same table can be adopted for other small-sign support systems after further analysis and testing.
In the revised matrix, the impact speeds, test vehicles, and evaluation criteria remained unchanged from the original MASH recommendations. The impact angles and locations were updated to reflect worst-case scenarios. According to the revised preliminary test matrix, a minimum of five tests are needed for each family of devices, with the possibility of requiring two additional tests depending on the outcomes of the initial five tests. Detailed justifications for the selection of tests and the updated impact conditions are included in Section 7.2 of this report.
Table 4. Summary of PSST computer simulation results.
| Impact Condition | Impact Locations and Angle | Panel Size | 1’ × 1.5’ | 2’ × 2’ | 2.5’ × 2.5’ | 3’ × 3’ | 3’ × 4’ | 4’ × 5’ | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Panel Thickness | 0.08 | 0.10 | 0.12 | 0.08 | 0.10 | 0.12 | 0.08 | 0.10 | 0.12 | 0.08 | 0.10 | 0.12 | 0.08 | 0.10 | 0.12 | 0.08 | 0.10 | 0.12 | ||
| MASH 3-60 | Center/0 Deg. | OIV (ft/s) | 5.2 | 5.6 | 5.6 | 6.6 | 7.2 | 7.2 | 7.9 | 8.2 | 8.2 | |||||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||||
| Roof Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||||
| Driver-Side Offset/0-Deg | OIV (ft/s) | 5.9 | 5.9 | 6.2 | 7.2 | 7.5 | 7.9 | 7.9 | 8.5 | 8.5 | 8.9 | 9.2 | 9.5 | 9.5 | 9.8 | 10.5 | 11.2 | 11.5 | 11.8 | |
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | ||
| Roof Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | ||
| Pass.-Side Offset/0 Deg. | OIV (ft/s) | 5.9 | 5.9 | 5.9 | 7.2 | 7.5 | 7.9 | 8.2 | 8.5 | 8.9 | ||||||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||||
| Roof Int.(in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |||||||||||
| MASH 3-61 | Center/0 Deg. | OIV (ft/s) | 3.3 | 6.2 | 6.6 | 6.2 | 6.2 | 6.9 | 6.2 | 5.9 | 6.2 | 6.2 | 5.9 | 3.9 | ||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 1.2T | 4.7T | 8.8T | 8.2T | 4.3T | 0 | ||||||||
| Roof Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 2.8 | 4.3 | 4.3 | 3.9 | 5.9 | ||||||||
| Center/–25 Deg. | OIV (ft/s) | 5.9 | 5.6 | 6.2 | 6.6 | 6.2 | 6.2 | 6.2 | 3.0 | 6.6 | 6.2 | 7.2 | 4.9 | |||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0.7T | 4.3T | 1.2T | 3.1T | 3.1T | 6.7T | 3.3T | ||||||||
| Roof Int.(in.) | 0 | 0 | 0 | 0 | 0 | 0 | 2.362 | 0 | 1.6 | 1.6 | 3.9 | 8.0 | ||||||||
| Center/+25 Deg. | OIV (ft/s) | 4.9 | 5.9 | 5.2 | 5.6 | 5.6 | 7.2 | 7.2 | 7.9 | 7.2 | 7.2 | 3.3 | 6.2 | |||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0.4T | 1.6T | 2.0T | 3.1T | 1.8T | 6.3T | 7.9T | ||||||||
| Roof Int.(in.) | 0 | 0 | 0 | 0 | 0 | 0 | 2.4 | 0 | 0 | 0 | 3.6 | 5.1 | ||||||||
| Driver-Side Offset/0 Deg. | OIV (ft/s) | 3.3 | 3.6 | 3.3 | 2.6 | 2.6 | 2.6 | 2.6 | 2.6 | 2.6 | 2.6 | 2.6 | 1.0 | |||||||
| Windshield Int. (in.) | 1.2T | 1.6T | 4.2T | 12.4T | 6.8T | 12.5T | 13.0T | 11.8T | 4.7T | 4.7T | 4.1T | 0.1 | ||||||||
| Roof Int. (in.) | 0 | 0 | 0.2 | 3.9 | 0.2 | 4.3 | 3.9 | 3.9 | 3.5 | 3.9 | 3.3 | 2.4 | ||||||||
| Pass.-Side Offset/0 Deg. | OIV (ft/s) | |||||||||||||||||||
| Windshield Int. (in.) | ||||||||||||||||||||
| Roof Int. (in.) | ||||||||||||||||||||
| MASH 3-62 | Center/0 Deg. | OIV (ft/s) | 2.0 | 2.0 | 2.3 | 2.3 | 2.6 | 2.3 | 1.3 | 2.3 | 2.3 | 2.3 | 2.3 | 2.3 | ||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 3.9 | 5.5 | 0 | 5.5 | 3.3 | 0 | 3.6 | ||||||
| Roof Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 4.7 | 2.4 | 0 | 3.0 | 3.2 | 0 | 3.504 | ||||||
| Center/+25 Deg. | OIV (ft/s) | 1.6 | 2.0 | 2.0 | 3.0 | 3.9 | 3.0 | 1.6 | 3.0 | 1.6 | 3.3 | 3.0 | 3.0 | |||||||
| Windshield Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 3.9 | 2.8 | 0 | 2.4 | 1.8 | 0 | 2.205 | ||||||
| Roof Int. (in.) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.8 | 0 | 1.6 | 2.8 | 0 | 3.0 | ||||||
| Center/–25 Deg. | OIV (ft/s) | |||||||||||||||||||
| Windshield Int. (in.) | ||||||||||||||||||||
| Roof Int. (in.) | ||||||||||||||||||||
Notes: 
= meets MASH criteria; 
= does not meet MASH criteria; “T”after intrusion number indicates that tearing is predicted; Int. = intrusion; blank cells, green or red, represent cases that were not run but their outcome was assessed based on results from the other simulations.
Table 5. Original MASH test matrix for support structures (AASHTO 2016).
| Feature | Test No. | Vehicle | Impact Speeda [mph (km/h)] | Impact Angleb (θ deg.) | Acceptable KE Range, [kip-ft (kJ)] | Impact Point | Evaluation Criteriac |
|---|---|---|---|---|---|---|---|
| Support Structures | 3-60 | 1100C | 19 (30) | 25 | ≤34 (41) | (c) | B, D, F, H, I, N |
| 3-61 | 1100C | 62 (100) | 25 | ≥288 (390) | (c) | B, D, F, H, I, N | |
| 3-62 | 2270P | 62 (100) | 25 | ≥594 (806) | (c) | B, D, F, H, I, N |
Notes: a See MASH Section 2.1.2 for tolerances on impact conditions; b see MASH Table 5-1; c see MASH Figure 2-5 for impact point; KE = kinetic energy.
Table 6. Preliminary test matrix for family of PSST sign support systems.
| Feature | Test No. | Vehicle | Family System Sizea | Impact Speed [mph (km/h)]b | Impact Anglec (θ deg.) | Acceptable KE Range, [kip-ft (kJ)] | Impact Pointd | Evaluation Criteriae |
|---|---|---|---|---|---|---|---|---|
| Small-Sign Support System | 3-60A | 1100C | Tallest | 19 (30) | 25 | ≤34 (41) | Offset | B, D, F, H, I, N |
| 3-61A | 1100C | Tallest | 62 (100) | 25 | ≥288 (390) | Center | B, D, F, H, I, N | |
| 3-61B | 1100C | Shortest | 62 (100) | 25 | ≥288 (390) | Offset | B, D, F, H, I, N | |
| 3-61Cf | 1100C | Midsize | 62 (100) | 25 | ≥288 (390) | Offset | B, D, F, H, I, N | |
| 3-62A | 2270P | Tallest | 62 (100) | 25 | ≥594 (806) | Center | B, D, F, H, I, N | |
| 3-62B | 2270P | Shortest | 62 (100) | 25 | ≥594 (806) | Offset | B, D, F, H, I, N | |
| 3-62Cf | 2270P | Midsize | 62 (100) | 25 | ≥594 (806) | Offset | B, D, F, H, I, N |
Notes: a See report Sections 7.2.3 and 7.2.4 for size guidance; b see MASH Section 2.1.2 for impact conditions tolerances; c see report Section 7.2.6 for impact angle; d see report Section 7.2.5 for impact location; e see MASH Table 5-1 for evaluation criteria; f may not be required (see Sections 7.2.3 and 7.2.4); KE = kinetic energy.
