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Structural Cable Force Monitoring
Bridge Cable Force Monitoring

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.

Multi FastTracer connected to PCB by USB cable
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.
cable force calculation by mode frequencies  
A simple supported cable with uniformly distributed mass per unit length π‘š and length 𝑙, bending stiffness 𝐸𝐼, tensioned with a force 𝑇, vibrates with a natural frequency of π‘–π‘‘β„Ž order 𝑓𝑠𝑖 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.

Cable excited and vibration measured by FastTracer

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 𝑇, the Fast Fourier Transform (FFT) is derived from FTAnalyzer software which is associated at the FastTracer system.

vibration spectrum of bump test of cable

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.

frequency vs mode number of cable
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Case story of cable force monitoring
FastTracer datasheet
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