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

The Ashton Bridge contains 48 inclined fully locked coil hanger cables that are connected to its arch ribs and tie beams by steel fork sockets. Ambient Vibration Testing was used to estimate the cable forces after construction and compare them to predicted forces. Local finite element models of each cable were created. The fork socket drawings supplied by the manufacturer were used to model the sockets in order to simulate the stiffness properties. A 3rd order theory nonlinear analysis was performed on the local models followed by form finding. Thereafter the natural frequencies were computed using the Lanczos method. The frequencies in the local models were iterated until they matched the ones from the site measurements. When the two frequencies matched, the tension in the cable was evaluated. Due to the large variations in the forces of cables 1 and 24 on both arches compared to other cable groups in the bridge, they were considered as measurement errors. However the primary method for engineer’s approval relied on lift-off tests that ensured that the design force was achieved in the cables. The total variation between the forces estimated from the measured data and the forces predicted by the bridge’s finite element model was –3.18%. This was deemed as acceptable.