Estimation of Cable Forces at the Ashton Bridge through Ambient Vibration Testing

Nare, Kaboyamodimo

Estimation of Cable Forces at the Ashton Bridge through Ambient Vibration Testing

Thesis / Dissertation

2023

Abstract

The Ashton Bridge is a concrete tied arch bridge which was part of a project conducted under the auspices of the Western Cape Government. AECOM SA were the consulting engineers. The construction of bridge was started by Basil Read and completed by Haw & Inglis after the former went into business rescue. The bridge was officially opened on the 15th of August 2021 after being transversely launched to its final position. Hanger cables distribute forces and become more dynamically active as span lengths increase. It is for this reason that the accurate evaluation of forces in the hanger cables is important during construction and for bridge maintenance purposes because they are the main elements that determine the integrity of the structure. Superstructure and cable vibration data was collected on site after the final stage of cable tensioning. Finite element models of each cable were created using the software, Sofistik. Thereafter, the force in the cable models was iterated until the theoretical natural frequencies matched the ones from site measurements. The superstructure vibration test results were used to calibrate the main model and isolate cable frequencies in cases where the peak frequencies were not easily distinguishable especially for the short cables. The estimated force was evaluated by adapting a COLTO tendon prestressing specification and considering the temperature on the day of the tests. In order to investigate the impact of the sockets on the natural frequencies and force of the cables, an analysis was conducted by removing the sockets from each cable FEM one investigation and adjusting the stiffness parameters associated with the sockets in another investigation. Most of the cable forces matched the predicted ones with the total variation between the two being - 3.18%. It is recommended that the frequency measurement of Cable 1 (Robertson end) on the North Arch be retaken because it differs markedly from the other 3 similar cables. However, the primary method for engineer's approval relied on lift-off tests that ensured that the design force was achieved in all the cables. The total variation between the forces estimated from the measured data and the forces predicted by the bridge's FEM is acceptable. Large variations in the forces in short cables are attributed to their high sensitivity to temperature changes. The sockets play a significant role in the frequency and ultimately the force in the cables depending on their proportion in the cable system. Hence, they should be modelled accurately to simulate the geometric and material properties. As the length of a cable decreases, the socket proportions become larger and more influential in the cable system behaviour. Consequently, the cable may start to behave more as a beam than a cable. However, the point at which the cable transitions to beam behaviour or the combination of the cable and beam behaviour is not known and requires further study.

Keywords

Civil Engineering

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