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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter SS-167

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U.S. Department of Transportation
Federal Highway Administration

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

March 3, 2011

In Reply Refer To: HSST/SS-167

Mr. K. Brent Pooles, B.A., C.I.M.
Safety Base Ltd.
1036 Waverley Street
Winnipeg, MB, Canada R3T 0P3

Dear Mr. Pooles:

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

Name of system: Safety Base C10S Couplings
Type of system: Breakaway Sign Support
Test Level: NCHRP Report 350 Test Level 3 (TL-3)
Testing conducted by: N/A
Date of request: December 4, 2010
Date of completed package: December 14, 2010
Request initially acknowledged: December 10, 2010

You requested that we find this system 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 devices should meet the guidelines contained in the NCHRP Report 350 or the American Association of State Highway and Transportation Officials’ (AASHTO) Manual for Assessing Safety Hardware (MASH). Requirements for breakaway supports are those in the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals.

Decision:
The following device is found acceptable:

Description
The C10S coupler is based on the currently tested and accepted C10 version but is of a smaller size that has been developed for round, S Section and square post signs that use more compact supports and typically do not have as large base reactions.

A drawing is enclosed for reference. Similar to the larger C10 unit, it utilizes a gray cast iron coupling but with a nominal tensile capacity of 15,000 lbs. Upon impact, fracture occurs at the groove machined into the coupler and is designed and tested to fail in bending. Coupler compressive and shear ultimate strengths are in excess of the nominal tensile strength. The coupler is designed for 5/8-inch anchor bolts but may be used with female thread anchors with the provision of a short threaded stud. The coupler has a wide base that helps to protect the anchor bolts and the foundation.

As noted in the draft report you provided, the lateral load for failure is less than half of that required for the larger coupling. If one assumes that the energy required for failure of two sets of couplers is less than or approximates that of one set of the larger couplers, then the balance of energy absorbed is the translation and rotation of the supported post(s). A single post at 21 pounds per foot has already been tested with the large couplers with a change in velocity of 5.23 ft/s low speed and an extrapolated high speed velocity change of 9.74 ft/s which is well under the maximum of 16.4 ft/s. The heaviest post in this request is the 4-inch sq post at 12 lb/ft. On a 2 post installation this equates to 24 lb/ft which although is slightly heavier than the W8x21 post tested, it is close in weight, the posts are generally shorter, the attachment brackets and hardware is lighter and the W8×21 post tests were well under the very conservative extrapolated high speed calculated change in velocity.

The intent is to use the round posts on single post signs and the square posts on both single and double post signs where the posts may or may not be 7 ft apart. For these multipost signs, breakaway knee plates, similar but smaller than the units used on the W8×21 post are used.

Crash Testing
No new crash testing was conducted in conjunction with this request. However, variations of this system have been previously crash tested (acceptance letter LS-33 dated October 12, 1993, for luminaire supports and SS-97 on July 20, 2001, for sign post couplings) and FHWA has found the test results acceptable according to NCHRP 350 TL-3. The findings of this letter recognize and utilize the results of previous testing and associated acceptance letters issued for earlier variations of this system.

Findings
You have requested we accept the use of the C10S couplings for use on sign supports on the NHS. This request is accepted. The basis for this acceptance is that FHWA previously has found the system acceptable with larger varieties of the coupling. Your calculations show that the smaller C10S coupling will break at much lesser loadings. Therefore, the couplings described in the requests above and detailed in the enclosed drawings acceptable for use on the NHS, on one or two posts within a seven-foot span as noted above, under the range of conditions the original couplings were tested, when such use is acceptable to a highway agency.

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

 

Sincerely yours,

/* Signature of Michael S. Griffith */

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

Enclosures

Page last modified on December 17, 2013.
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