Development of a Low-cost Automated Tension Estimation System for Cable-stayed Bridges

Cable tension force is one of the most important structural parameters to monitor in cable-stayed bridges. For example, cable tension needs to be monitored during construction and maintenance to ensure the bridge is not overloaded. To economically monitor tension forces, this study proposes the use of an automated wireless tension force estimation system (WFTES) developed solely for cable force estimation. The design of the WFTES system can be divided into two parts: low-cost hardware and automated software. The low-cost hardware consists of an integrated platform containing a wireless sensing unit constructed from commercial off-the-shelf components, a low-cost commercial MEMS accelerometer, and a signal conditioning board for signal amplification and filtering. With respect to the automated software, a vibrationbased algorithm using estimated modal parameters and information on the cable sag and bending stiffness is embedded into the wireless sensing unit. Since modal parameters are inputs to the algorithm, additional algorithms are necessary to extract modal features from measured cable accelerations. To validate the proposed WFTES, a scaled-down cable model was constructed in the laboratory using steel rope wire. The wire was exposed to broad-band excitations while the WFTES recorded the cable response and embedded algorithms interrogated the measured acceleration to estimate tension force. The results reveal the embedded algorithms properly identify the lower natural frequencies of the cable and make accurate estimates of cable tension. This paper concludes with a summary of the salient research findings and suggestions for future work. INTRODUCTION In recent tears, there has been rapid growth in the number of long-span bridges constructed worldwide. Suspension and cable-stayed designs remain popular choices for long-span bridges. Within these bridges, cables are important load carrying elements; given their importance, cable force is an important parameter that should be routinely measured. In particular, there is a need to continuously or semi-continuously monitor cable forces during construction and maintenance. Cable tension measurement methods can be classified into two categories: static (or, direct) and vibration (or, indirect) methods. Static methods directly measure tension force using a load cell or a hydraulic pressure-meter installed at the end of the cable. In contrast, vibration methods estimate cable tension using natural frequencies, material properties and geometric features. Given that static methods require complex and expensive instrumentation, vibration methods are more widely used because they require less invasive instrumentation and are easier to implement in a cost-effective manner. Vibration-based techniques employ accelerometers mounted to the surface of the cable to record the ambient vibration response; cable natural frequencies may be easily obtained from these responses. Vibration methods find their origin in flat taut string theory [1]. Direct application of flat taut string theory can be difficult