Civil Engineering Reference
In-Depth Information
SUSPENSION BRIDGES
The immediately recognizable feature of the suspension bridge is the pair of
cables extending from one side of a channel across two towers to the other
side. Once a cable is suspended between the towers and anchored into the
ground at both ends, it takes on its distinctive, graceful shape. Hangers
extend downward from the cable to support the deck. Certainly the cable
must be strong enough to support enormous loads. It consists of thousands
of steel wires bound together, the whole cable then securely attached to an
anchorage on each shore. The deck and its live loads exert tension through
hangers to the main cables, which transfer forces to the abutments and the
towers, which in turn transmit them to the ground (figure 4.8).
There is a debate, though not a particularly vociferous one, about the
suspension cable's actual geometric shape. As long as the suspended cable
is acted on only by its own weight, it has a
catenary
form, the form that
any suspended rope or chain acquires. Once it has been loaded with hang-
ers holding up the deck, the cable becomes distended, taking on a more or
less parabolic shape, one group says. Then again, says another, it has loads
suspended on hangers at regular intervals, creating slight changes in cable
angle (or arc) between hangers, so the cable is more accurately described
as having a
funicular
shape. The funicular shape will depend on how many
hangers are used and how close the hangers are placed.
It is a geometric oddity that the shape of the main cable resembles an
upside-down arch. Whereas the arch acts by compression thrusting outward
along its ribs to the abutments, the cable acts by tension, pulling inward
across the towers from the anchorages.
As its main span is supported only by cables, this arrangement is
relatively light, but expensive to construct. The longer the span, the more
financially viable it is. For spans of more than about 2500 feet, it is con-
sidered the only economical option. Its further advantage is that ground-
Main Cable
Towe r
Suspenders or
Hangers
Deck
Figure 4.8. Suspension bridge.
