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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter B-209

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U.S. Department of Transportation
Federal Highway Administration

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

November 10, 2010

In Reply Refer To:
HSSD/B-209

Mr. Paul Fossier, P.E.
Assistant Bridge Design Administrator
Bridge and Structural Design Section, Rm. 608J
Louisiana Department of Transportation and Development
Baton Rouge, LA 70804-9245

Dear Mr. Fossier:

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

Name of system: Test Level 2 (TL-2) T-Intersection (Short Radius) Guardrail System
Type of system: W-Beam Guardrail
Test Level: NCHRP Report 350 (Report 350) TL-2
Testing conducted by: Texas Transportation Institute (TTI)
Date of request: June 13, 2008
Date of completed package: August 24, 2010
Drawing Designator: SGR40

You requested that we find this system acceptable for use on the NHS under the provisions of Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”

Requirements
Roadside safety systems should meet the guidelines contained in 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 and evaluation of T-Intersection system is the same design as originally tested in January 1989 by the Southwest Research Institute for the Yuma County Highway Department, Arizona. The test conditions were based on Performance Level 1 (PL-1) of the 1989 AASHTO Guide Specifications for Bridge Railings. The description of the crash tested T-Intersection (Short Radius) Guardrail System is as follows. The tested system consisted of a 8.0ft. radius curved section connected to a 25.0ft. long flared guardrail on the primary road and a 12.5ft. long tangent section on the secondary road. Three Controlled Releasing Terminal (CRT) posts were installed in the curved section (posts 3, 4, and 5) at 6.25ft. center-to-center spacing. Two free-standing CRT posts were installed on a 6.67ft. radius behind the curved section to decrease the vehicle stopping distance. The downstream end of the rail on the primary road transitioned into a bridge rail. The guardrail on the secondary road terminated into a standard Breakaway Cable Terminal (BCT). The bridge rail consists of 1.0ft. high concrete curb and 16 in. high W6 x 15.5posts spaced at 6ft. 3in. center to center. The bridge rail consisted of standard 12 gauge W-beam guardrail, supported by an MC 8 x 22.8 structural steel channel extending out beyond the end of the bridge deck 6ft. 3in. Layout of the test system is shown in the attached details.

Test
Level
Vehicle
(Test No.)
Vehicle
Weight
Nominal speed
(mph)
Nominal angle
(degree)
PL-1 small automobile (YC5&6) 1800 lb(817 kg) 50 mph(81 km/h) 20
pickup truck(YC4&7) 5400 lb(2450 kg) 45 mph(72 km/h) 20

Crash Testing
The original test conditions were based on Performance Level 1 (PL-1) of the 1989 AASHTO Guide Specifications for Bridge Railings, summarized below. The test matrix used consisted of a 50 mph impact with a 1800 lbs. small car and a 45 mph impact with a 5400 lbs. pickup truck with various impact angles and locations.

TTI developed a comparison of Report 350 TL-2 impact conditions for terminals and crash cushions to the Yuma County test conditions (YC).

  1. Tests YC-5, YC-4, YC-6, and YC-7 are compared to Report 350 test designation 2-32, 2-33, 2-36, and 2-37, respectively. In addition to having similar impact locations when compared to the as recommended, it was also determined tests YC-5, YC-4, YC-6, and YC-7 have more severe impact conditions (due to increased vehicle mass and/or velocity) than required in Report 350 test conditions. Additional crash test comparison and recommendations as follows:
    1. Under Report 350 Test 2-30, the 820C test vehicle impacts the curved section (terminal) head-on with one-quarter point offset at a speed of 43.5 mph. Under Report 350 Test 2-31, the 2000P test vehicle impacts the curved section (terminal) head-on at a speed of 43.5 mph. These two tests are considered less severe than Report 350 Tests 2-32 and 2-33 which impact the curved section at an angle of 15 degrees relative to the tangent section of rail along the primary roadway.
    2. Report 350 Test 2-30 falls within the impact envelope of YC-5. Similarly, Report 350 Test 2-31 falls within the impact envelope of YC-4. Therefore, TTI concludes that Report 350 Tests 2-30 and 2-31 conditions are satisfied using the aforementioned YC tests.
    3. Report 350 Test 2-38 specifies a 43.5 mph impact with a 2000P vehicle at an angle of 20 degrees at the Critical Impact Point (CIP). While Test 2-37 is intended primarily to evaluate structural adequacy and vehicle trajectory criteria, Test 2-38 differs in purpose from Test 2-37 in that it is intended to evaluate the potential for pocketing or snagging at the bridge rail end. Since Report 350 Test 2-38 falls within the impact envelope of YC-4 and YC-7, TTI concludes that Report 350 Test 2-38 conditions are satisfied.
    4. Report 350 Test 2-39 specifies a 43.5 mph reverse direction impact with a 2000P vehicle at an angle of 20 degrees at the midpoint of the tangent section of rail along the primary roadway. Reverse direction evaluates potential for snagging on a terminal anchor assembly or crash cushion. The short radius guardrail does not have an anchorage assembly along the primary roadway. This condition is no different than impacting a standard guardrail in the opposite direction. Therefore, Test 2-39 is considered unnecessary based on engineering review.
  2. Yuma County short radius guardrail original design also incorporated two free standing CRT posts behind the curved rail to dissipate energy if the impacting vehicle and reduce the stopping distance. In 1995 and 2001, TTI conducted two separate dynamic pendulum impact tests on CRT posts to evaluate their performance. Based on these impact studies, the average energy absorbed by a single CRT post energy impacted about its strong axis is 9.45 kip-ft. This is 8.3 percent and 3.4 percent of the initial kinetic energy of the 820C and 2000P vehicles, respectively. Since impact conditions do not guarantee both posts breaking about their strong axes, these percentages represent an upper bound on the effectiveness of the free-standing CRT posts. In addition and under many impact scenarios, one or both posts may be missed. In addition, the maximum deflection of the barrier is controlled by the 2000P vehicle. If dynamic deflection is assumed to be proportional to the kinetic energy of the impacting vehicle, removal of the two CRT posts would result in an increase in deflection from 20 ft. to 21.4 ft. Therefore, TTI recommends these two free standing CRT posts can be removed with no significant change in the performance of this system.

Findings
We concur with your findings and recommendations as stated herein. In addition, FHWA concurs with as requested waiver of NCHRP Report Test 2-39 for this installation in that the critical impact point (CIP) is located where the system transitions from 'stiff' portion (i.e., the transition or the Length of Need) to a 'less-stiff' portion (i.e., radius with CRT posts.).

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

 

Sincerely yours,

/* Signature of George E. Rice */

for

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

Enclosures

Page last modified on June 24, 2011.
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