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| Structural Cable
Force Monitoring |
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More than other type of structure, suspension and cable-stayed bridges have been the object of dedicated monitoring systems, enabling the characterization of the response and the identification of the cable tension force is the main objective in these structural health monitoring systems.
A normal cable force state of a suspension bridge is extremely vital to the bridge safety. For cable force monitoring of suspension bridges, using an indirect measurement with
FastTracer vibration sensor can meet the engineering requirements.
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FastTracer is a
tri-axial MEMS accelerometer, which can be connected to a PC by USB port so that
vibration measurement can be made easily. WIFI module is available in case
wireless communication with PC is needed.
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A simple supported cable with uniformly distributed mass per unit length π and length π, bending stiffness πΈπΌ, tensioned with a force π, vibrates with a natural frequency of ππ‘β order ππ π defined by
the formula on the left.
Therefore, if the mechanical characteristics of
the cable are know the measurement of one
cable frequency can provide one estimate of the
cable force.
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In an attempt to obtain sufficient accuracy in the
identification phase, free vibration tests are normally
required during which cables are manually or
mechanically excited. For long span cables excitation
is more effective at a sufficient distance from the
cable anchorage. An example of bridge cable test is
depicted in the Figure (left), in which a crane was adopted to
manually excite the cables at a height of about 10 m.
Therefore, to calculate the final tension force π, the Fast Fourier Transform (FFT) is derived from
FTAnalyzer software which is associated at the
FastTracer system.
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Next, the obtained results are depicted in a graph, that
shows the variation of frequency respect to the mode
number. According to the Equation (1) and the regression curve of the vibrational data measured, the final value of the cable force is computed.
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