Tower Bridge - Pushing the Limits: New Adventures in Engineering - page 50

not do across the two-hundred-foot shipping channel mandated by Parliament. A bascule bridge, by
contrast, incorporates a movable deck that is counterweighted, requiring relatively little power to
raise and lower it.
The bascule design presented an alternative in crowded cities to the tall and generally unattractive
lift-bridge concept favored by Waddell. Indeed, a bascule design could so conceal the massive coun-
terweights and machinery required for its operation that the movable nature of the bridge might
hardly be noticed when it was not open. For example, the Arlington Memorial Bridge, completed
in 1932 between Arlington Cemetery and the Lincoln Memorial and considered to be one of the
most beautiful bridges in Washington, D.C., has a well-concealed steel bascule span between its
graceful stone-faced concrete arches. In late-Victorian London, however, a bascule on the scale of
Tower Bridge was a new concept and one that held much interest for engineers, almost more so for
its machinery than for its structure.
Arlington Memorial Bridge bascule span
The need to obviate any uneven settlement, so that the hundred-foot-long and fifty-foot-wide
roadway of the thousand-ton bascule leaves would remain aligned for smooth operation, was among
the prime reasons that substantial foundations were required for Tower Bridge. The power for mov-
ing the leaves was initially provided by two 360-horsepower steam-pumping engines, which pres-
surized water in accumulators to 850 pounds per square inch. That water pressure then drove rotary
hydraulic engines, which in turn activated gearing that raised and lowered the bascule leaves. All
machinery was installed in duplicate so that the breakdown of any one part would not jeopardize
the efficient operation of the bridge. It is said that the only time the system did not work perfectly
was the first time the machinery was tried, although reportedly on at least one hot summer day, in
1968, the bridge jammed because of the heat.
The principal power source was in the Surreyside (south) abutment of the bridge, and the water
pressure was conveyed to the Middlesex (north) pier via piping that was located in the elevated
walkways. It is unlikely that this pressure conduit was the primary reason for including the high-
level girders between the bridge towers, for duplicate machinery could have been located on either
side of the bridge. Rather, the elevated walkways, which were erected cantilever fashion out from
the steelwork of the towers because Parliament did not allow scaffolding to obstruct the river during