Floating Bridges - Pushing the Limits: New Adventures in Engineering - page 20

Floating Bridges
Whatever its type, a bridge is designed to carry something over some obstacle—a road, a valley, a
river, a lake. Bridging a lake can be among the most challenging problems an engineer might face.
Walden Pond is not large enough to be called a lake, but plumbing its depths beneath the ice one
winter gave Henry David Thoreau plenty of insight into the nature of things large and small. His
1846 drawing of the pond, which he reproduced in Walden, showed it to be no more than about 150
rods long by 100 rods wide. He also measured the depth of the pond along its major and minor axes
and found it to be deepest—about one hundred feet—near where they crossed, a fact from which he
generalized about the character of men. (Thoreau tended to use the surveying units of rods—each of
which equaled 16.5 feet—for horizontal distances and the more familiar feet for vertical distances.)
At the time Thoreau surveyed Walden Pond, the state of the art of bridge building in America and
the world would have allowed a single-span suspension bridge to be thrown across the narrowest
part of the pond—where it was only fifty rods wide—but not across its widest part. Such a distance
would not be bridged in a single span until the Brooklyn Bridge was completed in 1883. Bridging
the length of Walden Pond with two spans would also have pushed the limits of mid-nineteenth-
century engineering, for setting a foundation would have required working at depths greater than
those at which workers digging the foundations of the Eads and Brooklyn bridges would experience
the then-mysterious “caisson disease” now understood to be the bends.
If even a body of water as small and placid as Walden Pond could not easily have been bridged
in the mid-nineteenth century, then how did the Persian king Xerxes throw a bridge across the
Hellespont—the strait between the Gallipoli Peninsula in Europe and Turkey in Asia—in order to
invade Greece almost twenty-five centuries ago?
The solution then, as it could have been at Walden Pond, was to use a floating bridge. In its
simplest form, such a bridge is simply a boat. In many a crowded harbor or even at a busy dock,
where boats tessellate the water by being jammed from stem to stern and from port to starboard, a
common way to reach an outer one is to walk across the inner ones. This was how, a few years ago,
a party of engineers I was with boarded a riverboat at Sandouping to begin a journey up the Yangtze
River and how we disembarked at several busy cities on our way to Chongqing.
When a boat or ship is free of its moorings, it is metaphorically a bridge, carrying its passengers
from one point of land to another. Ships are structures that float, and as such they must be designed
to withstand the forces to which they will be subjected. The first test of a vessel's strength tradition-
ally came at launch, when it slid down the ways and for the first time felt the force of buoyancy.
With the stern supported in the water and the stem still on the ways, the hull was literally a bridge
between water and land, and many larger ships were not up to the task of carrying even their own
weight. The breakup of timber ships upon being launched was long known to be a danger but still