Cable-stayed footbridge

Knowing

Cable-stayed footbridges are light, slender structures with closely spaced vibration modes that are sensitive to people walking across them. Knowing their actual dynamic behaviour is the starting point for assessing comfort and safety and for setting up monitoring.

The footbridge is about 60 m long and 3.4 m wide: its steel deck is carried by six stay cables connected to an asymmetric pylon. The study was carried out in 2026 for Durham University and also served as a field test of the QMEMS accelerometer measuring platform.

Measuring

The ambient vibration campaign took place on 16 July 2026 and lasted about three hours, with the bridge open to normal pedestrian traffic: no closure and no interaction with the people crossing it.

The pylon could not be accessed, so the network was designed on the deck to make the main vibration modes clearly observable: nine Smart Sensor Boxes with Epson M-A370 QMEMS accelerometers and one reference station on the ground, 30 channels in total. Each box records its data locally, with no cables to a central system. In the main frequency range the deck amplifies the motion by about 40 dB compared with the ground.

Modelling

Frequencies, damping ratios and mode shapes were identified with the SSI-UPC method (Stochastic Subspace Identification). The stabilization diagram shows stable poles from low model orders up to the highest ones: a sign of good-quality measurements.

Twelve stable modes were identified between 0 and 8 Hz, grouped in two clusters of closely spaced frequencies: about 1.8–3.7 Hz and 5.2–7.8 Hz. The mode shapes can be explored in 3D at the bottom of the page.

Outcome

Twelve modes identified in three hours, without stopping pedestrian traffic.

The split into two frequency clusters is typical of decks hung from several stay cables, where each anchorage acts as an elastic support. The first and third mode shapes confirm it: the deck does not behave as a continuous suspended beam, but as a sequence of spans on elastic supports.

The campaign also confirmed that the QMEMS platform delivers reliable operational modal analysis even on light structures with very closely spaced modes.