Tower Bridge - Pushing the Limits: New Adventures in Engineering - page 46

Tower Bridge
Tower Bridge is to London what the Golden Gate Bridge is to San Francisco. Tourists flock to these
instantly recognizable landmarks, and their images are ubiquitous in the souvenir shops of their re-
spective cities, appearing on everything from T-shirts to tea-spoons. Although many West Coast
tourists can identify the Golden Gate as a suspension bridge—indeed, as one of the largest and
grandest examples of the form—few visitors to London can properly categorize Tower Bridge. In
fact, many nonspecialist engineers have a difficult time classifying the unique Victorian structure,
for it is one of the most unusual bridges in the world. Like many unusual artifacts, its form arose
from the unique conditions under which it was designed and built.
During the mid-nineteenth century, London Bridge, just upstream from where Tower Bridge now
stands, was the first barrier that tall-masted oceangoing ships encountered in their journey up the
Thames. Some ships anchored immediately downstream of the old bridge, in the deeper midstream
water known as the pool. In that location, where the banks of the river were low, ships were loaded
and unloaded by smaller, shallow-draft vessels known as lighters. The Tower of London stood then,
as it does now, beside that part of the river, on naturally rising ground, and immediately upstream
from St. Katharine's Dock.
A need for additional river crossings developed with the growth of Greater London during the
latter half of the nineteenth century. London Bridge, always a bottleneck, had become unbearably
crowded, and so a new span was proposed to be constructed a little way downriver. Financing for
the project was assured through the well-endowed Bridge House Estates Trust, which had its origins
in the thirteenth century, when tolls were collected on London Bridge and set aside for its main-
tenance. The ample fund came to be employed for transportation improvements beyond London
Bridge proper.
Designing a new bridge across the Thames in the latter nineteenth century was a formidable task,
not so much for structural reasons but because of the historical, topographical, and commercial con-
straints imposed by the location. The north-side approaches to the new bridge were placed most
naturally between the Tower of London, the bridge's namesake, and St. Katharine's Dock, where
the banks of the river are low. The bridge either had to be a high-level crossing, so as not to ob-
struct shipping, or have a movable span. A bridge with high clearance would have necessitated long
approaches, which would have added to the expense, not to mention presented an unwelcome struc-
ture beside the Tower of London. By incorporating a movable span in the center of the bridge, the
side spans could be built close to the water and hence minimize the approach viaducts.
Constraints clearly affect the nature of bridge design as they do all of engineering. Since they
must be taken into account, usually through compromise, few bridges can be first sketched on paper
the way they are finally built, and Tower Bridge was not one of the exceptions. Among the early
designs was a low-level crossing of the kind once proposed across the Delaware River at Philad-
elphia: one with a divided roadway to eliminate the backup of traffic that accompanies the operation