Problem
The main span of current cable-stayed bridges can reach 750 meters, and
future projects are expected to exceed 1000 meters. These structures are very
flexible, because the strength of high performance materials increases faster
than their stiffness; as a result, they become more sensitive to wind and
traffic-induced vibrations.
Large bridges are also sensitive to flutter which, in most cases is
associated with the aeroelastic damping coefficient in torsion becoming negative
above a critical wind velocity. Everyone remember the Takoma Narrows suspended
bridge which broke by flutter in torsion at a wind velocity of 42 mph on
November 7, 1940.
The situation can be improved by increasing the damping in the system. The
tendon control of a cable structure consists of placing an actuator (hydraulic
from Bosch Rexroth) at one end of the cable and controlling its motion in the
appropriate manner in order to counteract the incoming perturbations.
Control
The control is based on a force sensor measuring the tension in the cable
collocated with the active tendon. A correctly selected feedback algorithm
produces a very stable and robust energy absorbing controller.
Other possible applications
Cable structures are used in many fields of engineering, including tension
trusses for large deployable antennas in space, guyed towers or roofs in large
public buildings.
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