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U.S. Department of Transportation

Federal Highway Administration

400 Seventh St., S.W.
Washington, D.C. 20590

August 30, 2007

In Reply Refer To: HSSD/CC-100

Dr. Hayes Ross
Texas Transportation Institute
3135 TAMU
College Station, TX 778433-3135

Dear Dr. Ross:

In your letter of May 16, 2007, you requested the Federal Highway Administration’s (FHWA) acceptance of a 31-inch (787 mm) high Slotted Rail Terminal (SRT-31) as a National Cooperative Highway Research Program (NCHRP) Report 350 Test Level 3 (TL-3) system. To support this request, you provided the Texas Transportation Institute (TTI) reports dated May 2007, entitled “NCHRP Report 350 Test 3-30 of the SRT-31” and “NCHRP Report 350 Test 3-35 of the SRT-31”. You also provided earlier completed TTI reports on crash testing of a new terminal for cable/wire rope guardrail and on testing of T-31 W-beam guardrail, test videos, and electronic copies of the drawings.

Barrier end treatments should meet the guidelines contained in the NCHRP Report 350, "Recommended Procedures for the Safety Performance Evaluation of Highway Features". FHWA Memorandum “ACTION: Identifying Acceptable Highway Safety Features” of July 25, 1997 provides further guidance on crash testing requirements of longitudinal barriers.

Product description
The SRT-31 was developed to be used for terminating 31-inch (787 mm) high strong post W-Beam guardrail systems including 31-inch (787 mm) high Midwest Guardrail System (MGS) with blockouts and 31-inch (787 mm) high T-31 W-beam Guardrail System without blockouts. The design of SRT-31 is based on the previously accepted Slotted Rail Terminals (FHWA acceptance letter CC-72 of December 18, 2000) with the following changes:

  1. It is an all-steel terminal.
  2. The guardrail is raised to 31 inches (787 mm) throughout the terminal length.
  3. No blockouts are used between rail and post at post locations 1 through 6.
  4. Upper part of Cable Release Post (CRP) is modified to accommodate 31-inch (787 mm) high guardrail at post 1.
  5. Two 12.5 ft (3.8 m) slotted W-beam panels are used between posts 1 and 5, as is currently used in the 27-inch (686 mm) high terminal.
  6. Special 9.375 ft (2.86 m) long W-beam panel is used beginning at post 5. This creates mid-span splices between posts 6 and 7, and beyond. If SRT-31 is used with 31-inch (787 mm) systems where W-beam splices are located at posts, a standard 150-inch (3810 mm) long panel is placed between posts 5 and 7.
  7. Steel yielding terminal posts (SYTP) are used at post positions 2 through 6.
  8. W-beam is disconnected at posts 2 through 5.
  9. W-beam backup plates are used at post positions 2 and 4.
  10. Shelf angle is used at post position 2 to provide rail support for vertical forces that occur during redirection tests.
  11. Standard W6 x 8.5 (W150 x 12.6) or W6 x 9 (W150 x 14) line post is used at post position 7.

Design details of the SRT-31 are given in Enclosure 1. It should be noted that in the process of development of the described design you initially designed a system which used 12-inch (305 mm) blockouts and no shelf angle at post location 2. With the acceptance of 31-inch (787 mm) high T-31 W-beam Guardrail System without blockouts (FHWA acceptance letter B-140 of November 3, 2005) you decided to develop the described above 31-inch (787 mm) SRT system with no blocks as a more cost effective design.

Test article installations
Details of the SRT-31 installation as used in test 3-35 are provided in Enclosure 2. Essentially the same installation was used in test 3-30, except for the changes in the SRT-31 itself, as described above.

The NCHRP Report 350 requires that in order for barrier end treatments to meet test level 3 (TL-3) criteria they must successfully pass tests 3-30, 3-31, 3-32, 3-33, 3-34, 3-35 and 3-39. You conducted only test 3-30 on the SRT-31 system with blockouts and test 3-35 on the final design of the SRT-31 system (with no blocks) and provided explanations on why in your opinion the rest of the tests can be waived. Upon review of your explanations and crash test performance demonstrated in conducted tests we are willing to agree with your assumptions. Specifically, we agree that:

Your justification of the redundancy of test 3-34 (820C, redirection, 15 degrees at CIP of gating part of the device) was based on the comparison of it to Test 3-10 (820C, redirection, 20 degrees) conducted on the T-31 W-beam Guardrail System. Upon our review we are willing to agree that test 3-34 at a 15 degree impact with post 2 of the SRT-31 system would be very similar in impact severity to test 3-10 at a 20 degree impact angle with the T-31 system. Also, the detachment of rail in SRT-31 system at post positions 2 through 5 resulting in lower impact resistance of the SYLP posts will further improve vehicular performance in test 3-34 of the SRT-31 system in comparison to test 3-10 of the T-31 system. Further, because test 3-34 was successfully conducted on the original SRT designs which had a more critical parabolic flare, we are willing to agree that SRT-31 would perform better in test 3-34 than the original design.

According to the information you provided, SRT-31 performed successfully in both tests 3-30 and 3-35. The summaries of tests results are presented in Enclosure 3.

In test 3-30 SRT-31 slowed the 820C vehicle as the vehicle gated behind the terminal in a controlled manner subsequently coming to rest on the back side of the terminal. The vehicle remained upright during and after the collision event. No penetration of the occupant compartment occurred and maximum occupant compartment deformation was 0.7 inch (18 mm) in the driver’s side door panel area near hip height. Occupant risk factors were within the preferred limits.

In test 3-35 SRT-31 contained and redirected the pickup truck. The rail element separated from all posts of the system, but the anchor cable maintained attachment keeping the rail adjacent to the posts. No occupant compartment deformation occurred. The pickup truck remained upright during and after the collision event. No occupant compartment deformation occurred, and occupant risk factors were within the preferred limits. Based on the results of test 3-35, the beginning of length-of-need of the SRT-31 is at post 3, approximately 12.5 ft (3.8 m) from the end.

In summary we agree that 31-inch (787 mm) high Slotted Rail Terminal (SRT-31) as described above meets the appropriate evaluation criteria for the NCHRP 350 TL-3 devices and may be used with 31-inch high strong post W-Beam guardrail systems including 31-inch (787 mm) high Midwest Guardrail System (MGS) with blockouts, the 31-inch (787 mm) high T-31 W-beam Guardrail system without blockouts, and the 31-inch (787 mm) high Gregory Mini Spacer (GMS) Guardrail System without blockouts at all appropriate locations on the National Highway System (NHS) when selected by the contracting authority, subject to the provisions of Title 23, Code of Federal Regulations, Section 635.411, as they pertain to proprietary products. Please note also that this acceptance is based on the reported crash performance of your posts and is not meant to address their installation, maintenance or repair characteristics.

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

This acceptance letter shall not be construed as authorization or consent by the FHWA to use, manufacture, or sell any patented device for which the applicant is not the patent holder. The acceptance letter is limited to the crashworthiness characteristics of the candidate device, and the FHWA is neither prepared nor required to become involved in issues concerning patent law. Patent issues, if any, are to be resolved by the applicant.

Sincerely yours,

Signature of George E. Rice, Jr.

George E. Rice, Jr.
Acting Director, Office of Safety Design
Office of Safety



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