Figure 14. BCT speared on a 817.2-kg (1800-lb) car.
Figure 13. Breakaway cable terminal (BCT).
problem without creating a launching problem
material properties of candidate composite mate-
and became widely used. However, full-scale tests
rials was conducted. We also decided that the pro-
have shown that the BCT can spear or overturn
posed test prototype W-beam should have tension
825-kg (1,800-lb) cars in end-on impacts, as shown
and flexure strength properties comparable to
in Figure 14, because the steel W-beam rail is too
those of the existing Class A steel W-beams. Lami-
stiff (Kimball et al. 1982). A substitute for the steel
nates would be designed and fabricated with
W-beam rail, made of a composite material that
strength, modulus, and other mechanical and dur-
is not as stiff as steel, was therefore also consid-
ability characteristics similar to FRP materials cur-
ered for developing a retrofit design for the BCT.
rently available in the market.
The 12.7-mm- (0.5-in.-) thick composite W-
We aimed at making the flexural stiffness of
beam design was intended to be a one-for-one
one set of specimens match the AASHTO Class A
substitute for a steel W-beam. This design is
steel W-beam, and some additional beams at a
shown in Figure 15. It was to have the same ten-
lower stiffness. The turned-down guardrail termi-
sile capacity in the longitudinal direction and the
nal, also known as Texas Twist, was developed in
same lateral stiffness as a steel W-beam. Produc-
the late 1960s because the ends of W-beam rails
ing these W-beam sections by the pultrusion pro-
were observed to spear into cars in end-on
cess could result in significantly lower production
impacts. Currently, about 250,000 turned-down
costs, but would require heavy capital outlay for
Texas Twist terminals are used in this country
tooling. It was therefore decided to manufacture
(McDevitt and Dutta 1993). The twisted versions
the prototypes by another available low-cost
differ in detail, but all have a W-beam rail that is
manufacturing technique.
twisted through 90, turned down to the ground,
and bolted to a recessed concrete block, as shown
Design analysis
in Figure 13. Turned-down terminals have been
An engineering analysis was conducted based
very popular because they have a lower initial
on the performance of the beam. The key design
cost than any other terminal. However, crash tests
equation for limiting beam deflection takes the
have shown that the turned-down W-beam forms
form
a ramp that can launch cars (Hinch et al. 1984).
The turned-down steel W-beam is too stiff for
δ = f(P)/(EI)
(1)
825-kg (1,800-lb) cars to push it to the ground and
which shows that the deflection δ is dependent on
pass over it. Consequently, the Federal Highway
the product EI, the flexural stiffness. The numer-
Administration has banned the use of turned-
ator f(P) is a function of the applied load, depend-
down terminals on strong-post guardrails on
ing on the beam geometry and supporting condi-
Federally-funded, high-speed, high-volume roads
tions. Among the common engineering mate-
(Willett and Bennett 1990). Turned-down termi-
rials, E is 206.7 GPa (30 million psi) for steel and
nals are still used on low-speed, low-volume facil-
68.9 GPa (10 million psi) for aluminum. The val-
ities, and on weak-post guardrail systems.
ues for polymeric materials range from 172.25
The breakaway cable terminal (BCT) was
MPa (25,000 psi) (polyurethane) to about 20.67
developed in the 1970s. It solved the spearing
13
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