(The
shortest, also called the Bradfield Highway,
is found on the Story Bridge in Brisbane).
At 48.8 m (151.3 feet) wide, it is the
widest bridge in the world (Guinness World
Records, 2004). The bridge deck portion
of the highway is 1.15 km (0.71 miles)
long. It is concrete and lies on trimmers
(beams that run along the length of the
bridge). The trimmers themselves rest
on steel beams that run along the width
of the bridge. The trimmers and beams
are visible to boats that pass underneath
the bridge. The arch is composed of two
28-panel arch trusses. Their heights vary
from 18 m (55.8 feet) at the center of
the arch to 57 m (176.7 feet) (beside
the pylons). The arch span is 503 m (1,559
feet) and the weight of the steel arch
is 39,000 tons. The arch's summit is 134
m (415.4 feet) above mean sea level, though
it can increase by as much as 180 mm (7
in) on hot days as the result of steel
expanding in heat.
Two
large metal hinges at the base of the
bridge accommodate these expansions and
contractions and thereby prevent the arch
from being damaged. The two pairs of pylons
at each end are about 89 m (276 feet)
high and are made of concrete and granite.
Abutments, which support the ends of the
bridge, are contained at the base of the
pylons. They prevent the bridge from stretching
or compressing due to temperature variations.
Otherwise, the pylons serve no structural
purpose and are primarily to visually
balance the bridge itself. They were originally
not part of the design but were added
later to allay concerns about structural
integrity — ironically, as the pylons
do not actually touch the bridge (except
at road level). Although inserted into
the designs for their aesthetic value,
all four pylons have now been put to use:
a museum and tourist centre with a lookout
of the harbour is contained in the southeastern
pylon.
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