RESISTING EXTREME EVENTS - Bridges - page 69

Of directions of shaking, the longitudinal direction tends to be the
most dangerous. This stands to reason. Under static conditions, the structure
is primarily meant to resist downward forces of the superstructure, and since
the bridge is longer than wide, to resist tipping toward one side or another.
But longitudinal shaking imposes forces that the bridge may be least prepared
to resist, as seen in figure 6.1.
What can the earthquake engineer do? One thing she cannot do is
reduce the intensity of the quake force. No one can stop these enormous
geological forces. One option is to strengthen resistance by making the
structure stronger so it can resist earthquake force; another is to make the
foundation and footings stronger. These are expensive options that may
nonetheless prove inadequate in the face of the tectonic energy nature has
in store.
Earthquake engineers have developed additional countermeasures,
among which energy dissipation devices are especially sophisticated. To see
how they work, let us imagine a north-south girder bridge subjected to
longitudinal shaking, in which one phase of the shaking moves the girders
in a northward direction (the opposite phase moves it back southward).
Anticipating such an event, engineers attach a damper connecting
the girder to the bridge's north abutment. The damper is a large cylinder
containing a piston filled with viscous fluid. (A viscous fluid, like honey as
compared to water, is thick and sticky and more resistant to stresses that
would make it flow.) The piston rod is attached to the abutment and the
cylinder to the underside of the girder (figure 6.2). When the northward
movement occurs, the girder suddenly shifts toward the abutment, squeez-
ing the piston into the cylinder. The viscous, incompressible substance in
the cylinder cushions the movement of the piston. As a result, the girder
accelerates less than the ground does—the damper dissipates the earthquake
energy, mitigating harm to the structure.
Alternatively, engineers can install a bearing between the girder and
the column or abutment on which it rests. Before we describe bearings,
Bridge Span
Column
Figure 6.1. Bridge span unseated by longitudinal shaking.