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June 20, 1997
Refer to: HNG-14/SS-60B
Mr. Clifford M. Dent
President
Dent Breakaway Ind., Inc.
10 A Town Plaza
Suite 130
Durango, Colorado 81301
Dear Mr. Dent:
This is in response to your December 17. 1996, letter to Mr. Nicholas Artimovich requesting that the Federal Highway Administration (FHWA) accept your company's breakaway couplings for use in various support systems on the National Highway System (NHS). The couplings are ASTM A325 fasteners with special dimensional requirements (A325 breakaway couplings). Transmitted with your letter were three December 1996 report of pendulum testing conducted by the Southwest Research Institute on single-post supports, a video of the tests, drawings of the hardware, and other documentation. Additional information and drawings were sent on March 19 in response to our request.
Requirements for breakaway supports are those in the American Association of State Highway and Transportation Officials' (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. We also recognize the testing and evaluation guidelines in the National Cooperative Highway Research Program (NCHRP) Report 350 Recommended Procedures for the Safety Performance Evaluation of Highway Features.
The three pendulum tests are summarized in the table below. A brief description of the test articles follows the table.
| Test number |
LP-1 |
SP-1 |
CR-1 |
| Support Type |
Steel Luminaire Support |
Perf. Square Sign Post |
Motorist Aid Callbox |
| Support Size |
305 mm Diameter |
64 mm Square |
126 mm Diameter |
| Wall Thickness |
7 Gage |
12 Gage |
Schedule 40 |
| Coupling Designation |
A-LP |
A Modified * |
A |
| Thread Diameter |
25.4 mm |
12.7 mm |
12.7 mm |
| Diameter @ Neck Between Cones |
7.1 mm |
7.1 mm |
7.1 mm |
| Radius of Groove Between Cones |
1.6 mm |
1.6 mm |
1.6 mm |
| Radius of Cone Bases |
50.8 mm |
25.4 mm |
25.4 mm |
| Height of Flats @ Cone Bases |
1.6 mm |
1.6 mm |
1.6 mm |
| Distance Between Cone Bases |
38.1 mm |
9.5 mm |
12.7 mm |
| Bolt Circle Diameter |
381 mm |
(single bolt) |
220 mm |
| Test Article Mass |
461 kg |
20 kg |
88.5 kg |
| Soil Type ** |
n/a |
n/a |
n/a |
| Height |
19.81 m |
3.7 m |
3.28 m ** |
| Impact Speed |
9.8 m/s |
9.8 m/s |
9.8 m/s |
| Pendulum Mass |
816 kg |
816 kg |
816 kg |
| Velocity Change |
3.1 m/s |
0.5 m/s |
2.0 m/s |
| Est. Velocity Change @ 100 km/h **** |
4.8 m/s |
0.6 m/s |
2.8 m/s |
| Stub Height |
19 mm |
89 mm |
12 mm |
* The Type A coupling (nominally 114.3 mm) was modified for this application. Length of the coupling was 51 mm.
** All supports were mounted to the rigid test frame, simulating concrete foundations in strong soil.
*** Height of the callbox was 1 meter above grade.
**** High Speed extrapolation based on the procedure in FHWA Notice N 5040.20.
Test Article Descriptions:
LP-1: The Type A-LP A325 breakaway coupling was a I-inch (25.4-mm) diameter galvanized device (see Enclosure 1) used to attach the luminaire pole base plate to a foundation plate in the test texture (see Enclosure 2) The pole base plate was a conventional non-breakaway four-bolt design.
SP- 1: The test article consisted of 3700-mm long by 64-mm perforated square steel tubing (12-gage wall, grade 50 steel.) The tube was bolted to a ground to pole adapter bracket (see Enclosures 3A and 3B) using two 3/8-inch (9.5-mm) diameter carriage bolts. The bracket was bolted (using 3/8-inch carriage bolts) to a section of 76.2-mm square steel tube that was welded to the test fixture. One ½-inch (12.7-mm) diameter galvanized A325 breakaway coupling (see Enclosure 4) was used in the ground to pole adapter bracket.
CB-1: The test article consisted of a 3280-mm long by 127-mm diameter round
steel pole mounted to a 257-mm diameter plate with a motorist aid call box
and a 900-mm aluminum sign panel attached to the pole. No antenna was attached
to the pole. Four 1-inch (12.7-mm) diameter galvanized A325 breakaway couplings
(see Enclosure 5) were used to attach the pole to a foundation plate in the
test fixture.
Because of the results of the pendulum testing and the high-speed extrapolations met the stub height and change-in-velocity criteria recognized by the FMWA, your company's A325 breakaway couplings with 0.280-inch (7.11-mm) necked down breakaway cross sections will be acceptable for use on the NHS, in the range of conditions tested, when proposed by a State. The following limitations and conditions also apply:
For sign supports:
For luminaire supports:
For all installations:
All supports shall be mounted to a structural concrete foundation that will not move in the soil if the support is struck by a vehicle.
You requested acceptance of a larger bolt circle diameter and higher and heavier luminaire poles than we have found acceptable. We have limited our acceptance of the bolt circle diameter to the dimension that was tested because we believe increasing bolt circle diameters will increase the difficulty of actuation when breakaway couplings are used. We have limited the maximum mass and height because pendulum testing of luminaire supports does not reveal the consequences of a support falling on the roof of an impacting vehicle. Based on test observations and engineering judgment, the FHWA has set upper limits on the support masses and heights it will find acceptable even where analysis or testing appear to indicate acceptability of greater mass or height supports.
You also requested that the bracket tested for square tubes in test SP-1 be accepted whether the size of the post fits inside the upper portion of the bracket. This variation is acceptable as long as the posts are at least as stiff as the tested 12-gage 64-mm square perforated tube. The section modulus of the tested tube (10, 520 mm3) may be used when comparing other post sections. Posts with lesser section properties should not be used unless verified by crash testing.
Your drawings also included bracket designs for use with round tubes (see Enclosures 6A and 6B) and a u-channel post and we infer that you desire our action on these as well. The round tube post will be acceptable, subject to the same restriction on the stiffness of the sign post as mentioned above. However, our acceptance does not extend to u-channel posts as their open section gives them a greater tendency to bend that square or round tubes. Please note that only the bracket design with the 3/8-inch (9.5-mm) gap, for use with the Type A (modified) couplings is acceptable. The use of larger couplings with these brackets is not acceptable unless crash tested with successful results.
We should point out that we expect the couplings to be undersized for use with the structure the size of the luminaires supports used in the testing. We would suggest that your design guidance be based on static and fatigue testing which, we understand, is currently being conducted.
Our acceptance is limited to the breakaway characteristics of the tested supports when used with your company's couplings and does not cover their structural features. Presumably, you will supply potential users with sufficient information on design and installation requirements to ensure proper performance. We anticipate that the States will require certification from Dent Breakaway Industries that the hardware furnished has essentially the same mechanical properties and geometry as that used in the crash testing, and that it will meet the FHWA change in velocity requirements.
The Dent Breakaway Coupling is a proprietary product. To be used in Federal-aid projects, except exempt, non-NHS projects, proprietary products: (a) must be supplied through competitive bidding with equally suitable unpatented items; (b) the highway agency must certify that they are essential for synchronization with existing highway facilities or that no equally suitable alternative exists; or (c) they must be used for research or for a distinctive type of construction on relatively short sections of road for experimental purposes. Our regulations concerning proprietary products are contained in Title 23. Code of Federal Regulations, Section 635.411, a copy of which is Enclosure 7.
Sincerely yours,
[Original signed
by David A. Price] for
Dwight A. Horne, Chief
Federal-Aid and Design Division
7 Enclosures
