Refer to: HSA-10/SS-106

Mr. Charles R. Norton

Trinity Industries

950 West 400 South

Centerville, UT 84104

Dear Mr. Norton:

Thank you for your letter of January 30, 2002, requesting Federal
Highway Administration (FHWA) acceptance of your company's Dent Type A
Breakaway Bolts as breakaway couplings for use on the National Highway
System (NHS). You requested that we find this system acceptable when two
posts are needed to support a sign on the NHS under the provisions of
National Cooperative Highway Research Program (NCHRP) Report 350
"Recommended Procedures for the Safety Performance Evaluation of Highway
Features." The Dent system has been previously found acceptable for use
in various single-post configurations, including signs, motorist-aid
callboxes, and luminaire supports in FHWA Acceptance Letters beginning
in 1995. These letters also covered couplings with various shear-neck
diameters.

**Introduction**

Prior testing of the supports was in compliance with the guidelines
contained in the NCHRP Report 350, Recommended Procedures for the Safety
Performance Evaluation of Highway Features. Requirements for breakaway
supports are those in the American Association of State Highway and
Transportation Official's Standard Specifications for Structural
Supports for Highway Signs, Luminaires and Traffic Signals.

**Testing**

Pendulum testing was conducted on the Dent Type A Bolts, having a
necked-down cross section

area of 7.11 mm or 0.28 inch. A recap of those tests is presented below.

  ------ ----------- ---------- ---------- ----------- ----------- --------
  Test   FHWA SS \#  Speed      Bolt       Post mass   Occup.      Delta V
  \#                            Circle                 Speed       

  CD 4   SS-60       35.28 km/h 220 mm     70.8 kg     None        1.5 m/s

  LP 1   SS-60B rev  35.28 km/h 381 mm     461 kg      n/r         3.1 m/s

  SP 1   SS-60B rev  35.28 km/h (1 bolt)   20 kg       n/r         0.5 m/s

  CB 1   SS-60B rev  35.28 km/h 220 mm     88.5 kg     n/r         2.0 m/s
  ------ ----------- ---------- ---------- ----------- ----------- --------

Occup. Speed: Occupant Impact Speed: Speed at which a theoretical front
seat occupant will contact the windshield. In meters per second

Delta V: Speed change of the test vehicle. In meters per second.

**Findings**

Your request is to use the Dent Type A system on two structural posts,
placed within a span of

7 feet, to support a traffic sign. From the testing that was conducted
on single post supports using the Type A Dent Bolt it is clear that the
mass of the support plays an important role. Of course, two luminaire
supports would never be placed such that two would be struck at the same
time, but the 3.1 m/s velocity change in test LP-1 of a single luminaire
support is a strong indicator that a support system using two posts of
that mass would not perform in an acceptable manner. On the other hand,
the low velocity changes seen in CD-4 and CB-1 indicate that the system
may work if the mass is limited.

There is no objective method of relating the results of a crash test of
a single post support to the performance of a dual post support.
However, in the one instance where FHWA has extrapolated a single post
test using a breakaway coupling system to dual posts, both the mass of
the supports and the velocity changes were comparable to those in CD-4
and CB-1. In addition, recent testing of dual post supports using
breakaway couplings confirmed FHWA's earlier extrapolations.

Therefore, breakaway systems on one or two posts using the Type A Dent
Breakaway Bolt are acceptable for use as Test Level 3 devices on the NHS
under the range of conditions tested, when proposed by a State, subject
to the following limitations:

1\. Supports shall be mounted to a structural concrete foundation that
will not move in the soil if the support is struck by a vehicle.

2\. This extrapolation is limited to the Type A Dent bolt having a
necked-down cross section area of 7.11 mm (0.28 inches) in a bolt circle
of approximately 220 mm (8.6 inches).

3\. The couplings may be used in three- or four-bolt slip bases, but may
not be used on inclined slip bases.

4\. The maximum mass per foot of sign post shall not exceed 27 kg/m (18
pounds per foot.) This is in agreement with the maximum post size that
was found acceptable for dual slip base support installations discussed
in FHWA acceptance letter SS-25 dated June 4, 1991.

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

-   Our 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, it reserves the right to modify or
    revoke its 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 they will
    meet the crashworthiness requirements of FHWA and NCHRP Report 350.

-   To prevent misunderstanding by others, this letter of acceptance,
    designated as number SS-106 shall not be reproduced except in full.
    As this letter and the supporting documentation, which support it,
    become public information, it will be available for inspection at
    our office by interested parties.

The Dent Breakaway Bolts are patented products and considered
\"proprietary.\" The use of proprietary devices specified on Federal-aid
projects, except exempt, non-NHS projects: (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 enclosed.

> Sincerely yours,
>
> A. George Ostensen

Program Manager, Safety

Enclosure

FHWA:HSA-10:NArtimovich:tb:x61331:3/11/02

File: SS106Dent2020fin.wpd

cc: HSA-10 (Reader, HSA-1; Chron File, HSA-10;

N. Artimovich, HSA-10)