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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter LS-72

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U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Ave. S.E.
Washington, D.C. 20590

March 30, 2011

In Reply Refer To: HSST/LS-72

Mr. Zach Thiemann, E.I.T.
Associate Product Development Engineer
Valmont Industries, Inc.
7002 North 288th Street
P.O. Box 358
Valley, NE 68064-0358

Dear Mr. Thiemann:

This letter is in response to your request for the Federal Highway Administration (FHWA) acceptance of roadside safety systems for use on the National Highway System (NHS).

Name of system: Valmont Decorative Clamshell Base Covers
Type of system: Breakaway Luminaire Support Structure
Test Level: NCHRP Report 350 Test Level 3
Testing conducted by: Valmont-MwRSF/UNL Pendulum Facility
Date of request: September 24, 2010
Date of completed package: September 24, 2010
Request initially acknowledged: September 30, 2010

You requested that we find these systems acceptable for use on the NHS under the provisions of the National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”

Requirements
Roadside safety devices should meet the guidelines contained in the National Cooperative Highway Research Program (NCHRP) Report 350 or the American Association of State Highway and Transportation Officials’ (AASHTO) Manual for Assessing Safety Hardware (MASH). Requirements for breakaway supports are those in the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals.

Decision:
The following devices were found acceptable:

Description
Clamshell base covers are installed on luminaire poles for aesthetic purposes. The Valmont Industries system described in this submission includes (1) a breakaway system; (2) a decorative clamshell base; (3) a pole; and (4) a luminaire arm.

For the breakaway system, four 1 inch (25 mm) diameter TRANSPO Industries double neck, pole safe, couplings were used to fasten the pole to the foundation as shown in Enclosure 1. These couplers were previously accepted in FHWA acceptance letter LS-45B dated on January 16, 1997.

Valmont Industries evaluated five different styles of decorative clamshell bases in this request namely, (1) Huntington; (2) Washington; (3) Harrisburg; (4) Memphis; and (5) Osceola. Each may be fabricated in multiple sizes to accommodate various luminaire poles. The decorative clamshell bases are cast in two halves from either a 319F or 356F aluminum alloy. The two halves are bolted together using four 3/8 inch (10 mm) diameter bolts, two near the top and two near the bottom. The clamshell does not attach directly to the pole but only wraps around the base of the pole and rests on the ground. Design details for each of the models are shown in Enclosure 4 through 24.

Valmont Industries fabricates various poles and luminaire arms. In order to test the system, one of the Valmont Industries taller, thicker, and heavier poles was selected. The round steel pole was fabricated with a 7-gauge (4.55 mm) wall thickness and had a height of 38 feet, 6 inches (11.7 m). The nominal height to the luminaire was 45 feet (13.7 m). The pole had a top outside diameter of 4-1/2 inches (114 mm) and a bottom outside diameter of 10-3/8 inches (263 mm). A handhole was located on the pole centered at a height of 1 foot, 6 inches (0.5 m) from the base of the pole. Two mounting points for luminaire arms were located 6 inches (152 mm) and 33 inches (838 mm) from the top of the pole. The pole is shown in Enclosure 2. The pole base plate, 1 inch (25 mm) thick by 14 inches (356 mm) square, was welded to the bottom of the pole. The bolt circle was 13-1/2 inches (343 mm) in diameter.

The luminaire arm attached to the pole had a length of 15 feet (4.6 m) as shown in Enclosure 3. The arm was braced with two struts at locations of 5 feet and 10 feet (1.5 m and 3m) laterally from the pole.

Crash Testing
Pendulum testing was conducted on the test articles described above by Valmont-MwRSF/UNL Pendulum Facility. The testing was conducted according to NCHRP 350 test designation 3-60. The following table shows test IDs, clamshell styles tested, and weights of pieces of each test article.

Test ID Clamshell Style Clamshell Size (inch)* Clamshell Weight (lb) Pole Weight (lb) Luminaire Arm Weight (lb)
LST-353-04 Huntington 24 47 548 175
LST-353-05 Washington 30 113 548 175
LST-381-01 Memphis 18 57 571 165
LST-381-02 Harrisburg 22 56 571 165
LST-381-03 Osceola 24 89 571 165

* Base Diameter
Summaries of test results for tests LST-353-04 and LST-353-05 are shown in Enclosures 25 and 26 respectively. Enclosures 27 through 29 summarize test results for tests LST-381-01 through LST-381-03 respectively.

Findings
According to the NCHRP 350, test designation 3-60 and test designation 3-61 are to be conducted for support structures for Test Level 3 approval. In both tests full scale automobile testing with an 820C small car is required. As described before, five pendulum tests were conducted as a surrogate for a low speed test with an 820C vehicle. In these tests occupant impact velocity, occupant ridedown acceleration, and maximum vehicle speed change (where measured) were found to be within the acceptable limits of NCHRP 350. The couplings and luminaire poles broke away in a consistent and predictable manner. The base of the pole rotated away from impact to allow a vehicle to travel underneath it. Therefore, the test article passed the low speed test.

The breakaway system used includes TRANSPO Industries double neck bolts. In tests LST-381-01 through LST-381-03, two bolts broke at the upper neck. In test LST-353-04, one bolt broke at the upper neck and in test LST-353-05 all bolts broke at the lower necks. For those bolts that broke at the upper neck, remaining stubs were 6 inches above the ground whereas those broke at the lower neck had stubs remaining that were 3 inches above the ground. The AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries, and Traffic Signals require that any “substantial remains” of the breakaway support not project greater than 4 inches above ground level to avoid vehicle undercarriage snagging. It was judged that the stubs in these crash tests that measured more than 4 inches would not present a significant hazard. This was due to the fact that the breakaway elements of the stubs, the lower neck, was present and thus the remaining stubs are themselves breakaway systems which would give way on subsequent impact, reducing potential snagging.

FHWA allows the results of the high speed tests to be estimated using data from the low-speed test in combination with an analytical extrapolation method described in the FHWA memorandum “Identifying Acceptable Highway Safety Features” dated on July 25, 1997. For all five test articles, the high speed test extrapolation analysis resulted in change in velocity values for the high speed tests above the NCHRP 350 limit of 5.0 m/s and all five test articles failed the test.

The equation for extrapolation to obtain results for the high speed test includes mechanical properties of the test article, the speed at which low speed and high speed tests are conducted, and the speed change of the test vehicle during the low speed test. It is intuitive that in both low speed and high speed tests, a heavier pole with greater moment of inertia/radius of gyration and longer luminaire would cause more speed change for the impacting vehicle. As described in this letter, Valmont Industries selected critical poles and luminaire arm combinations (i.e., tall, heavy, thick poles with long luminaire arms) for the low speed tests. Experience has shown that any less critical (lighter) poles will cause smaller speed change for the impacting vehicles.

In your request, you have estimated the results of the high speed tests for various combinations of poles, luminaire arms, and clamshell bases that Valmont Industry fabricates using the analytical equation. In these estimations you used the speed changes obtained from the low speed tests which are more conservative speed changes for these less critical combinations. Therefore, if the result of the extrapolation for each combination satisfies the requirements of NCHRP Report 350, I agree that the combination passes test 3-61.

The following tables illustrate all combinations which passed test 3-60 and 3-61. These poles can be used with single or twin arms shorter or equal to 15 ft or top mount luminaries weigh less than 150 pounds where applicable. In these tables, the cells containing “X” represent the pole and aesthetic clamshell base covers that are compatible with each other and determined to pass both tests. It should be noted that a pole that falls between any two poles shown in these two tables is also acceptable and can be used on the NHS provided that all other conditions of this letter are satisfied.

It should be noted that the pole, luminaire arm, and clamshell base covers must meet the conditions and limitations of TRANSPO Industries breakaway couplings identified in FHWA acceptance letter LS-45B particularly the followings:

Therefore, the system described in the requests above and detailed in the enclosed drawings is acceptable for use on the NHS under the range of conditions tested, when such use is acceptable to a highway agency.

Pole Aesthetic Clamshell Base Covers
Family - Type Shaft Length (ft) Shaft Length (ft) Thickness (ga.) Base O.D. (in.) Huntington Washington Memphis Harris- burg Osceola
24 17 12 30 24 18 18 15 22 24 17
DS30 20 21 11 5.90   X       X   X     X
DS30 25 26 11 5.90   X       X   X     X
DS30 30 31 11 6.60 X       X     X X   X
DS30 35 36 11 7.30 X       X   X X X   X
DS30 35 36 11 8.50 X       X   X X X    
DS30 39 40 11 9.00 X       X   X   X    
DS50 17 20 11 5.90   X       X   X     X
DS50 17 20 11 6.50 X       X     X X   X
DS50 22 25 11 5.90   X       X   X     X
DS50 22 25 11 7.00 X       X     X X   X
DS50 27 30 11 6.60 X       X     X X   X
DS50 27 30 11 7.50 X       X   X X X   X
DS50 30 32 11 6.60 X       X     X X   X
DS50 30 32 11 8.00 X       X   X X X X  
DS50 32 35 11 7.30 X       X   X X X   X
DS50 32 35 11 8.00 X       X   X X X X  
DS50 37 40 11 9.00 X       X   X   X    
DS50 42 45 11 9.50 X           X   X    
DS60 20 25 11 6.50 X       X     X X   X
DS60 20 25 11 7.00 X       X     X X   X
DS60 25 30 11 7.00 X       X     X X   X
DS60 25 30 11 7.50 X       X   X X X   X
DS60 25 30 11 8.00 X       X   X X X X  
DS60 30 35 11 8.00 X       X   X X X X  
DS60 30 35 11 8.50 X       X   X X X    
DS60 35 40 11 9.00 X       X   X   X    
DS70 22 25 11 7.00 X       X     X X   X
DS70 27 30 11 7.50 X       X   X X X   X
DS70 27 30 11 8.00 X       X   X X X X  
DS70 32 35 11 8.00 X       X   X X X    
DS70 32 35 11 8.50 X       X   X X X    
DS70 32 35 11 9.00 X       X   X   X    
DS70 37 40 11 9.00 X       X   X   X    
DS70 37 40 11 9.50 X           X   X    
DS32 28.5 30 11 7.50 X       X   X X X   X
DS32 28.5 30 11 8.00 X       X   X X X X  
DS32 33.5 35 11 8.00 X       X   X X X X  
DS32 33.5 35 11 8.50 X       X   X X X    
DS32 33.5 35 11 9.00 X       X   X   X    
DS32 38.5 40 11 9.00 X       X   X   X    
DS32 38.5 40 11 9.50 X           X   X    
DS32 38.5 40 11 10.00 X           X   X    
DS32 43.5 45 11 9.50 X           X   X    
DS32 43.5 45 11 10.00 X           X   X    
X = Passes Requirements for all DS70 Luminaire Arm or Top Mount Possibilities
  = Aesthetic Clamshell Base Not Compatible With Pole
Pole Aesthetic Clamshell Base Covers
Family - Type Shaft Length (ft) Shaft Length (ft) Thickness (ga.) Base O.D. (in.) Huntington Washington Memphis Harris- burg Osceola
24 17 12 30 24 18 18 15 22 24 17
DS210 20 20 11 5.90   X       X   X     X
DS210 20 20 11 6.50 X       X     X X   X
DS210 25 25 11 5.90   X       X   X     X
DS210 25 25 11 7.00 X       X     X X   X
DS210 25 25 7 7.00 X       X     X X   X
DS210 30 30 11 6.60 X       X     X X   X
DS210 30 30 11 8.00 X       X   X X X X  
DS210 35 35 11 7.30 X       X   X X X   X
DS210 35 35 11 8.50 X       X   X X X    
DS210 35 35 11 9.50 X           X   X    
DS210 39 39 11 7.82 X       X   X X X   X
DS210 39 39 11 9.00 X       X   X   X    
DS210 45 45 11 10.00 X           X   X    
DS210 50 50 11 10.00 X           X   X    
FL210 20 20 11 6.72 X       X     X X   X
FL210 25 25 11 7.30 X       X   X X X   X
FL210 25 25 7 7.75 X       X   X X X   X
FL210 30 30 11 8.00 X       X   X X X X  
FL210 30 30 7 8.00 X       X   X X X    
FL210 35 35 11 8.45 X       X   X X X    
FL210 35 35 7 9.00 X           X   X    
FL210 39 39 11 9.00 X       X   X   X    
FL210 39 39 7 10.00                      
Custom 20 23 7 6.00   X       X   X     X
Custom 20 23 7 7.00 X       X     X X   X
Custom 25 28 7 6.00   X       X   X     X
Custom 25 28 7 7.00 X       X     X X   X
Custom 25 28 7 7.75 X       X   X X X   X
Custom 25 28 7 8.00 X       X   X X X    
Custom 30 33 7 7.00 X       X     X X   X
Custom 30 33 7 7.75 X       X   X X X   X
Custom 30 33 7 8.00 X       X   X X X    
X = Passes Requirements for all DS70 Luminaire Arm or Top Mount Possibilities
  = Aesthetic Clamshell Base Not Compatible With Pole

Please note the following standard provisions that apply to FHWA letters of acceptance:

 

Sincerely

/* Signature of Michael S. Griffith */

Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety

29 Enclosures

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