1200 New Jersey Ave., SE

Washington, D.C. 20590

May 6, 2011

FHWA: HSST: WLongstreet: ms: x60087:5/5/11

File: h://directory folder/HSST/ CC-116 Steel Backed Timbered Guardrail 

Tangent End Terminal.docx Unit.docx

 

In Reply Refer To:

HSST/CC-116

Mr. Scott Whittemore

Highway Safety Engineer

Federal Highway Administration

Eastern Federal Lands Highway Division

21400 Ridgetop Circle 

Sterling, VA 20166

Dear Mr. Whittemore:

This letter is in response to your request for the Federal Highway Administration’s (FHWA) 

acceptance of a roadside safety device for use on the National Highway System (NHS).

Name of device: Steel Backed Timber Guardrail Tangent End Terminal

Type of device: Timber Guardrail Gating End Terminal

Test Level: NCHRP 350 TL-2

Testing conducted by: National Crash Analysis Center 

Date of request: November 22, 2010

Task Force 13 Designator: SED01

You requested that we find this device acceptable for use on the NHS under the provisions of the 

National Cooperative Highway Research Program (NCHRP) Report 350 testing guidelines. 

Requirements

Roadside safety devices should meet the guidelines contained in the NCHRP Report 350. The 

FHWA memorandum “ACTION: Identifying Acceptable Highway Safety Features” of 

July 24, 1997, provides further guidance on crash testing requirements of longitudinal barriers.

Description

The Steel Backed Timber (SBT) Guardrail Tangent End Terminal overall length is 12.42 meters

(40 feet 9 inches). It consists of nine (9) wood rail segments and nine (9) wood posts. The rail 

segments’ cross-sectional area is 150 millimeters x 250 millimeters (6 inches x 10 inches), 

similar to the cross-section of the steel-backed timber SBT barrier rail segments. The length of

the first rail segment is 1.6 meters (5 feet 3 inches). The last rail segment (number 9) is 

1.75 meters (5 feet 9 inches) in length. The remaining rails (numbers 2-8) are 1.676 meters 

(5 feet 6 inches) in length. These rail segment lengths are about half the rail segment lengths of 

the SBT barrier and were selected such that the spacing between the posts is 1.22 meters (4 feet). 
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Both ends of the rail segments are cut at a 1:2 (26 degree) angle, with the exception of first rail 

segment, which has one end at 90 degree, and the last rail segment, which has one end cut at a 

1:1 (45 degree) angle. The first rail of the SBT guardrail is also cut at a 1:1 (45 degree) angle to 

fit the end of the terminal.

The posts are 150 millimeters x 250 millimeters (6 inches x 10 inches) in cross-section and 

2.1 meters (6 feet 11 inches) in length. The post embedment in the ground is 1.43 meters

(4 feet 8 inches), similar to the SBT barrier post embedment. Holes are drilled in the posts at 

ground level. The hole diameter for the first three posts is 115 millimeters (4.5 inches) and the 

hole diameter for the remaining six posts is 75 millimeters (3 inches). Steel tubes encase the 

embedded portion of the wood posts to ensure consistent failure of the posts at ground level. The 

tubes are 150 millimeters x 250 millimeters x 4.75 millimeters (6 inches x 10 inches x 3/16 inch) 

in cross-section and 1.422 millimeters (4 feet 8 inches) length.

The rail segments of the end-terminal are connected to the posts through steel splices and wood 

blockouts. This setup is based on the original SBT barrier connection but was modified to allow 

for release of the rail segments from the posts in head on impacts and to absorb some the vehicle 

kinetic energy. Details A, B, and C in Figure 8 show the end-terminal rail-to-post connections. 

At each post, the ends of two adjacent rail segments are connected to the steel splice (splice 

plate?) through lag screws. The lag screws are 16 millimeters (5/8 inch) in diameter and 

100 millimeters (4 inches) in length. Three lag screws are used to connect the splice to the rail 

segment that is closer to the start of the terminal while four lag screws are used for the adjacent 

rail segment. With this configuration, the rail segment closer to the start of end-terminal will 

separate from post before the adjacent rail segment and allow for a more controlled deceleration 

of the vehicle during the head on impact. The three lag screws are placed in 22 millimeter x 

178 millimeter (7/8 inch x 7 inches) slotted holes and placed such that the screws pull out from 

the rail segments one at a time. This design reduces the impact forces while maximizing the 

energy absorbed during the head on impact. This setup was also configured such that the rail 

segments will not separate from the post during the redirection impacts.

The steel splices and wood blockouts are connected to the posts using 16 millimeter (5/8 inch) 

diameter bolts. The bolts are 380 millimeters (15 inches) in length. The steel splices are 

150 millimeters x 750 millimeters x 10 millimeters (6 inches x 30 inches x 3/8 inch). The wood 

blockouts are 300 millimeters x 225 millimeters x 100 millimeters (12 inches x 9 inches x 

4 inches). Both are of similar size to the original SBT barrier.

Drawings of the SBT Guardrail Tangent End Terminal are included as enclosures to this 

correspondence.

Crash Testing

The following methods used for analysis of the Backed Timber Guardrail Tangent End Terminal.

A. Simulation

Computer simulations were performed to evaluate the end-terminal under the seven tests 

recommended for Test Level 2 (TL-2) impact condition. Results from these simulations are 

as follows.
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The simulation results showed that the final design meets all NCHRP Report 350 

recommendations for gating end terminals under TL-2 impact conditions. Based on these 

simulation results five tests were selected for testing. These include tests 2-30, 2-31, 2-32,

2-34, and 2-35. The other two tests, 2-33 & 2-39, were omitted based on the simulation 

results that showed that they meet requirements and are less critical than the remaining five. 

The test house therefore requests to waive the remaining two tests, 2-33 & 2-39.

B. Physical Crash Testing:

The NCHRP Report 350 outlines the criteria used to determine the crashworthiness of 

roadside features, including traffic barriers and their terminals. The end terminal for the SBT 

guardrail was tested according to these requirements, but requested a waiver for two of the 

seven crash tests required for TL-2 based upon simulation and previous experience with 

gating terminals. The test house has therefore conducted the following five out of seven 

crash tests:

• Test 2-30: An 820C small car impacting the end-terminal at an angle of 0 degrees and a 

speed of 70 km/hr (43.5 mi/hr). The vehicle strikes the end-terminal head on with an 

offset of 1/4 the width of the vehicle.

• Test 2-31: A 2000P pickup truck impacting the end-terminal at an angle of 0 degrees and 

a speed of 70 km/hr (43.5 mi/hr). The vehicle strikes the end-terminal head on at the 

vehicle centerline. 

• Test 2-32: An 820C small car impacting the end-terminal at an angle of 15 degrees and a 

speed of 70 km/hr (43.5 mi/hr). The vehicle strikes the end-terminal head on at the 

vehicle centerline. 

• Test 2-34: An 820C small car impacting the end-terminal at an angle of 15 degrees and a 

speed of 70 km/hr (43.5 mi/hr). The vehicle strikes the end-terminal at the critical impact 

point (selected between posts 2 and 3 of the end-terminal). 

• Test 2-35: A 2000P pickup truck impacting the end-terminal at an angle of 20 degrees 

and a speed of 70 km/hr (43.5 mi/hr). The vehicle strikes the end-terminal at the start of 

length of need point (selected at post 4 of the end-terminal). 

Crash test summaries of the above mentioned tests are provided as enclosures to this 

correspondence.

Findings

Based on the successful completion of the aforementioned simulation and NCHRP Report 350 

tests, we agree that the SBT Guardrail Tangent End Terminal meets the evaluation criteria for 

NCHRP 350 on gating end terminals at TL-2 impact conditions. In addition we also agree to the 

waiver of tests 2-33 & 2-39. Test 2-33 was a critical test specifically for turn-down terminals 

which are no longer recommended as an acceptable roadside safety installation. In addition, test 

2-39 is only specified if the terminal is positioned where being struck in the reverse direction at 

high speed is deemed to be likely. This should be conveyed to the agency within the system 

specifications.

Therefore, the system described in the request above and detailed in the enclosed drawings is 

acceptable for use on roadways in Federal Lands jurisdiction only under the range of conditions 

tested, when such use is deemed acceptable by the engineer.
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Please note the following standard provisions that apply to the FHWA letters of acceptance:

• This acceptance provides a AASHTO/ARTBA/AGC Task Force 13 designator that 

should be used for the purpose of the creation of a new and/or the update of an existing 

Task Force 13 drawing.

• This acceptance is limited to the crashworthiness characteristics of the devices and does 

not cover their structural features, nor conformity with the Manual on Uniform Traffic 

Control Devices.

• Any changes that may adversely influence the crashworthiness of the device will require 

a new acceptance letter.

• Should the FHWA discover that the qualification testing was flawed, that in-service 

performance reveals unacceptable safety problems, or that the device being marketed is 

significantly different from the version that was crash tested, we reserve the right to 

modify or revoke our acceptance.

• You will be expected to supply potential users with sufficient information on design and 

installation requirements to ensure proper performance.

• You will be expected to certify to potential users that the hardware furnished has 

essentially the same chemistry, mechanical properties, and geometry as that submitted for 

acceptance, and that it will meet the crashworthiness requirements of the FHWA and the 

Manual for Assessing Safety Hardware.

• To prevent misunderstanding by others, this letter of acceptance is designated as number 

CC-116, and shall not be reproduced except in full. This letter and the test 

documentation upon which it is based are public information. All such letters and 

documentation may be reviewed at our office upon request.

• 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,

Michael S. Griffith

Director, Office of Safety Technologies 

Office of Safety