CHAPTER 6: FRP COMPOSITE BREAKAWAY COUPLERS
amount of energy required to fracture the pole in
Background
Each year numerous injuries and fatalities
a plane near the vehicle bumper. The essential cri-
occur as a result of vehicles hitting signposts, util-
teria developed for the breakaway design were
ity poles, and light poles. Even as early as 1976,
that the breakaway pole must (1) provide an
Graf et al. (1976) noted that 1600 deaths and
acceptable momentum change (change of veloc-
100,000 injuries occurred annually as a result of
ity below 5 m/s (16 fps), (2) provide sufficient
collision with timber utility posts (Ivey and Mor-
structural integrity of the pole to withstand ice-
gan 1986). In 1985, to minimize injury to occupants
and wind-induced loads, (3) minimize post-
and damage to the impacting vehicles, the Ameri-
breakaway hazards to the vehicle and the occu-
can Association of State Highway Transport Offic-
pants, and (4) have low implementation cost. In
ers (AASHTO), in "Standard Specifications for
1978, Dinitz and Chisholm (1978) described an
Structural Supports for Highway Signs, Luminar-
innovative design of longitudinally grooved cou-
ies, and Traffic Signals," stipulated that all light
pling bolts for ground-mounted sign supports.
poles or ground-mounted signposts within 9.15 m
Later designs included notched coupling bolts for
(30 ft) of a highway shall be placed on breakaway
connecting the base plate to the anchor bolts for
supports. The AASHTO (1985) required that for a
ground-mounted supports.
standard 816.5-kg (1800-lb) vehicle striking a
breakaway support at a speed between 32.2 and
Breakaway mechanisms
96.6 km/hr (20 to 60 mi/hr), the velocity change
As stated before, a breakaway mechanism
must not exceed 4.88 m/s (16 ft/s). A higher
should reduce the amount of energy required to
change of velocity results in more severe occupant
fracture the pole. Under normal conditions, it is
injury. As a result, designs of many pre-1985
the environmental loads, like wind and ice, that
breakaway couplings were reevaluated and new
provide bending stress at the base of the pole.
design approaches developed. In 1989, Ross et al.
During vehicular impact, shearing is the primary
(1989) in NCHRP Report 318, documented the
mechanism. Therefore, the ideal breakaway con-
evaluation of the impact performance of roadside
figuration would be the one that weakens the
safety elements for 680.5 kg (1500 lb) and identi-
pole's shear strength without affecting its bend-
fied several potential modifications of existing
ing strength significantly. Theoretically, this pro-
hardware that would accommodate vehicles
vides a situation in which an anisotropic mate-
weighing as little as 567.2 kg (1250 lb). From their
rial like FRP composite, which is very strong in
the fiber direction and weak in the cross-fiber (90)
test results on the breakaway sign supports with
slip-base design, which were impacted at 32.2 km/
direction (see Chap. 2), would be a desirable mate-
hr (20 mi/hr) and 96.6 km/hr (60 mi/hr) and
rial. Besides, this material has the other advan-
which produced only a 3.05 m/s (10 ft/s) and a
tages of lighter weight and high durability. In this
3.36 m/s (11 ft/s) change in velocity, respectively,
study, the application of FRP material for design-
they concluded that, in general, slip-base supports
ing a breakaway mechanism was explored.
will pose no serious damage to vehicles in the
680.5-kg (1500-lb) weight range. However, cast
FRP design approach
aluminum transfer bases widely used as break-
The FRP breakaway coupling design was aimed
away devices for roadside luminaries supports
at providing both a high tensile load capacity and
would not satisfy either the NCHRP Report 230
a low breakaway force requirement. This could
(1981) evaluation criteria (Michie 1981), or the
be achieved by the composite material having the
AASHTO (1985) standards. The Ross et al. (1989)
fibers carry the major tensile load, while the shear
study provided the impetus to research and rede-
force acts at 90to the fibers. Commercially avail-
sign of the breakaway supports for light poles, sign
able pultruded glass-fiber-reinforced FRP bars
posts, and utility poles.
were considered as the basic raw material for the
The impact dynamics between an errant vehicle
breakaway coupler. A discussion with an indus-
trial expert* on breakaway coupler design resulted
and a pole depend on numerous variables, such
in the decision to develop a design based on the
as: the size, weight, and crush characteristics of the
vehicle; the impact velocity; and the geometry and
strength characteristics of the pole. A built-in
* T. Husain, Transpo Industries, New Rochelle, New York,
breakaway mechanism in the pole reduces the
personal communication, 1995.
47
Previous Page
