U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000


Skip to content
FacebookYouTubeTwitterFlickrLinkedIn

Safety

eSubscribe
eSubscribe Envelope

FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter LS-74

Memorandum


DOT logo
U.S. Department of Transportation
Federal Highway Administration

Download Version
PDF [1.05 MB]

SENT VIA ELECTRONIC MAIL


Subject: ACTION: Minnesota DOT Braced Leg
Sign Support Eligibility Letter SS-170
Date: November 30, 2012
From: Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety
In Reply Refer To:
HSST
To: William Stein, Safety Engineer
Minnesota Division

This memorandum is in response to your request for the Federal Highway Administration (FHWA) to review a roadside safety system for eligibility for reimbursement under the Federal-aid highway program.

Name of system: Minnesota Type "C" and "D" Braced Leg Sign Supports
Type of system: U-Channel Sign Support System
Test Level: NCHRP Report 350 Test Level 3
Testing conducted by: Texas Transportation Institute
Date of request: November 23, 2010
Date of completed package: September 26, 2012
Initially acknowledged: December 13, 2010

Decision
The following device is eligible, with details provided below and in the drawings attached as an integral part of this memorandum:

Based on a review of crash test results submitted certifying the device described herein meets the crash test and evaluation criteria of the National Cooperative Highway Research Program (NCHRP) Report 350, the device is eligible for reimbursement under the Federal-aid highway program. Eligibility for reimbursement under the Federal-aid highway program does not establish approval or endorsement by the FHWA for any particular purpose or use.

The FHWA, the Department of Transportation, and the United States Government do not endorse products or services and the issuance of a reimbursement eligibility letter is not an endorsement of any product or service.

Test Descriptions
There were no tests performed pursuant to this request for product acceptance.

The FHWA, the Department of Transportation, and the United States Government do not endorse products or services and the issuance of a reimbursement eligibility letter is not an endorsement of any product or service.

Requirements
To be found eligible for Federal-aid funding, roadside safety devices should meet the crash test and evaluation criteria contained in the NCHRP Report 350 or the American Association of State Highway and Transportation Officials’ Manual for Assessing Safety Hardware (MASH).

Description
You requested that we review this system using two posts within a seven-foot span under the provisions of the NCHRP Report 350.

The dual-post test installation consisted of an 8-foot wide by 7-foot tall plywood sign panel mounted on two steel U-channel supports spaced 39 inches apart. The supports were joined in a nested splice with two stubs driven into "strong soil." The stubs were 84 inches long with an embedment depth of 42 inches. The length of the structural splice between the stub and the sign riser post was 30 inches, with the bottom of the riser post 12 inches above ground level. Structural splices for u-channel posts should be a minimum of 18-inches long with two fasteners at each end of the splice piece.

The sign supports were braced with two additional supports posts mounted diagonally at a slope of ½12:1 to the rear of the installation. The braces were attached to the front supports with A-frame brackets at the sign panel and anchored to two 48-inch stubs, located 5 feet, 3 inches behind the front support stubs at 27-inch on center spacing. The rear stubs were also driven into the crushed limestone to a depth of 44 inches leaving 4 inches projecting above ground level.

All posts and stubs were 2.5 pound-per-foot rerolled rail steel u-channel posts. Stub-post connections and posts splices were joined with 5/16-inch by 1 ½-inch alloy bolts, nuts, and washers. Both posts were struck during the crash tests. A drawing of the test installation is enclosed for reference.

Later crash testing of a three-post support used a similar design with 3-pound-per-foot posts, but only two of the supports were impacted. Additional posts may be used to support wider signs, but no two posts may be closer than 7 feet.

Crash Testing
Four tests were conducted under NCHRP Report 230 criteria in 1988 and 1989. NCHRP Report 230 Tests 62 and 63 were conducted on both the Type "C" dual-post installation and the Type "D" triple-post installation. The dual-post 2.5-pound-per-foot installation yielded to the vehicle in both the high- and low-speed tests and allowed it to proceed downrange. In the low-speed test of the triple-post installation with two posts impacted, the two 3-pound-per-foot diagonal braces that were impacted by the test vehicle yielded by breaking from their stubs, but the two signposts subsequently brought the vehicle to a controlled stop. In the high-speed test both of the sign posts and diagonal braces fractured, allowing the test vehicle to proceed through. The third post that was not impacted held the sign in place. Copies of the Test Data Summary Sheets are enclosed for reference. Type "C" and "D" refer to the number of u-channel rails used as lateral braces on the back of "A Frame" signs. Type "C" is a single brace while Type "D" is a dual brace as noted on page 2 of 3 of the attached drawings.

Since NCHRP Report 230 tests 62 and 63 use the same 820-kg test vehicle as specified in NCHRP Report 350 and the evaluation criteria are comparable, these tests are considered compliant with NCHRP Report 350. The bottom of the signs that span two or three posts should be a minimum of 7 feet above the ground. Supplemental signs placed on just one of the posts may be mounted at 5 feet above the ground.

The results of this testing was known to the researchers who conducted a series of sign support tests at the FHWA’s Federal Outdoor Impact Laboratory in McLean, Virginia, from 1989 to 1991. When designing a braced-leg system of u-channel posts they made two significant changes to the Minnesota design. The first was to use a breakaway splice for the upright sign posts where the signpost overlapped the stub by six inches, with the stub projecting no more than 4 inches above the ground. The second innovation was to use aluminum fasteners to ease the separation of both the upright and diagonal posts from their respective stubs. These changes have the potential for improving the Minnesota braced leg design.

Summary and Standard Provisions
Therefore, the system described and detailed in the attached form is eligible for reimbursement and may be installed under the range of conditions tested.

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

Attachment

Page last modified on September 27, 2013.
Safe Roads for a Safer Future - Investment in roadway safety saves lives
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000