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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-184A

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U.S. Department of Transportation
Federal Highway Administration

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

September 23, 2009

In Reply Refer To: HSSD/B-184A

Mr. Rick Mauer
Outside National Sales Representative
Nucor Steel Marion, Incorporated
P.O. Box 837
Greenland, NH 03840

Dear Mr. Mauer:

This letter is in response to your request for the Federal Highway Administration (FHWA) concurrence in the deflection distance for a roadside safety system for use on the National Highway System (NHS).

Name of system:Nu-Cable with 20 foot post spacing
Type of system:Cable Barrier
Test Level:NCHRP Report 350 TL-4
Testing conducted by:Holmes Solutions
Date of request:April 9, 2009
Date of completed package:July 10, 2009

You requested that we concur in the design deflection distance for this system which is 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 systems should meet the guidelines contained in the NCHRP Report 350. The FHWA Memorandum: ACTION: “Identifying Acceptable Highway Safety Features” of July 25, 1997, provides further guidance on crash testing requirements of longitudinal barriers.

Description
The Test Level 4 (TL-4) Nu-Cable barrier system consists of four steel cables mounted on 4 lb/ft Nucor Steel Marion Rib-Bak U-channel posts. This system was found acceptable in the following FHWA acceptance letters:

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Crash Testing
The Nucor Nu-Cable System consists of four 19-mm (3/4-in) wire rope cables attached at heights of 380 mm, 640 mm, 790 mm, and 890 mm (15 in, 25 in, 31 in, and 35 in) above ground level. The lower two cables are attached using locking hook bolts which connect directly to the U-posts. The top two cables are supported by a top clip which engages with the top of the U-post section.

All posts in the Nucor Wire Rope Barrier System were installed directly into plastic sockets that have been cast into 300-mm diameter x 750-mm (12-in x 30-in) deep concrete foundations. The posts were spaced at 6.1-m (20-ft) centers along the length of the barrier. The overall barrier length was 103.7 m (340 feet) plus an additional 8.0 m (26.3 feet) of terminal end at either end of the barrier.

The tested system was identical to that found acceptable in FHWA acceptance letter B-184 with the addition of a stainless steel retaining band holding the top of the hanger clip against the post. This band prevents the cables from prematurely disengaging from the U-channel posts.

The intent of the crash test was to establish the “design deflection distance” for the TL-4 Nu-Cable system using the 2000P test vehicle. The test designations for this impact is NCHRP Report 350 Test 3-11 or 4-11 as all vehicle and impact parameters are the same regardless of the test level rating of the barrier.

The 2000P vehicle impacted with a velocity of 99.5 km/h at an angle of 25 degrees as seen in the enclosed test data summary sheet. The vehicle remained in contact with the barrier and had a maximum dynamic deflection of 2.55 m (8.37 feet). Occupant risk values and vehicle trajectory for this test were acceptable.

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

Although the barrier performed well under ideal test impact conditions with the two test vehicles, the likelihood of passenger car underrides of any cable system may increase as the post spacing increases, particularly when the barrier is installed on non-level or slightly irregular terrain and the cables are not restrained from lifting at each post. Consequently, some transportation agencies have limited post spacing to approximately 6m (20 feet) for cable barriers. The dynamic deflection of the barrier is likely to increase when it is installed along the convex sides of horizontal curves, and when distances between anchorages exceed the 100m (330-foot) test length.

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

 

Sincerely yours,

Signature of George E. Rice, Jr.

for David A. Nicol, P.E.
Director, Office of Safety Design
Office of Safety

Enclosures

Page last modified on June 24, 2011.
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