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July 15, 1992

Refer to: HNG-14/SS-29

Mr. William G. Edwards
A.B Chance Company
210 North Allen Street
Centralia, Missouri 65240-1395

Dear Mr. Edwards:

This is in response to your request for the Federal highway Administration's (FHWA) acceptance of your company's helical screw foundations (HSFs) as foundations for slip-base supports for motorist aid call boxes. Your initial letter of November 26, 1991, included the reports of crash tests Chance 1 and Chance 2 conducted by Southwest Research Institute (SWRI) along with documentation describing the foundations. You provided additional information and video of the callbox support crash testing with your March 10 letter responding to our request of January 2. Subsequent to our meeting on May 27 you submitted a copy of a crash test conducted in "weak" soil by Mobility Systems and Equipment Company (MSE) and followed that up with final details in your facsimile message of July 6.

The tests were conducted to assess the breakaway performance of steel-post motorist-aid call boxes mounted on HSFs. Requirements for breakaway supports are found in the 1985 AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. These specifications have been adopted, with minor modifications, by the FHWA. The test articles are described in the enclosed drawings.

The test results are summarized here:

Test Number







Support Type

Sch. 40 Steel

Sch. 40 Steel

Steel .266 wall

Test Article Mass, kg (lbs.) (including 23.6 kg callbox)

95 (208)

95 (208)

100 (218)

Bolt Torque, N-m (ft-lbs)

40.7 (30)

40.7 (30)

67.9 (50)

Vehicle Mass, kg (lbs)

898 (1975)

890 (1957)

825 (1815)

Soil Type (per NCHRP 230)




Impact Speed, km/hr (mph)

30.9 (19.2)

95.1 (59.1)

31.5 (19.6)

Velocity Change, m/s (fps)

0.37 (1.2)

1.28 (4.2)

1.28 (4.2)

Stub Height, mm (in)

75 (3)

75 (3)

100 (4)

All supports were steel poles, 100 mm (4 inches) in diameter with a height of 4300 mm (14 feet) to the top of the pole (base of antenna). The HSF was embedded 1500 mm (5 feet) into the soil. We believe that the low speed test in weak soil is a "worst case" scenario and that a high speed test is not warranted.

The films of the crash tests that you supplied show that the only significant damage to the test vehicles occurred when the top of the support rotated after impact and struck the rear hatch windows. We do not consider this to be significant passenger compartment intrusion. The comparison test video that you provided showed that a wooden callbox support, when tested, broke at the groundline, at bumper height, and below the call box. This left the callbox suspended in space, with the result that it severely damaged the windshield of the test vehicle and entered the passenger compartment. It is apparent that a different type support, such as a steel pipe, is required to assure that the callbox does not become a greater hazard than the support in the event of a crash.

The results of the three slip-base tests outline above meet the change in velocity and stub height requirements adopted by AASHTO and the FHWA. Therefore, your company's helical screw foundation described above is acceptable for use on Federal-aid highway projects to support slip-base callbox supports, within the range of conditions tested, if proposed by a State.

Our acceptance is limited to breakaway characteristics of the system and does not cover its structural features. Presumably, you will supply potential users with sufficient information on structural design and installation requirements to ensure proper performance. We anticipate that the States will require certification from A.B. Chance Company and/or the callbox supplier that the hardware furnished has essentially the same chemistry, mechanical properties, and geometry as that used in the tests, and that it will meet the FHWA change in velocity requirements.

Sincerely yours,

Lawrence A. Staron, Chief
Federal-Aid and Design Division


Geometric and Roadside Design Acceptance Letter SS-29

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