Innovative experimental assessment of direct and drive‑by monitoring on two truss bridges

• Field validation of drive-by monitoring is conducted on two operational bridges. • Drive-by monitoring is compared with conventional direct measurements. • The influence of vehicle speed and deck surface condition is evaluated. • The results show that dominant lower bridge modes can be identified. • The method can be applied as a rapid and low-cost screening tool for bridge monitoring. Drive-by monitoring has gained increasing attention as a cost-effective alternative to conventional direct bridge vibration measurements. This study presents a field-based comparison between direct and drive-by vibration monitoring conducted on two large steel truss bridges in Vietnam, namely Thang Long Bridge and Chuong Duong Bridge. Vertical acceleration responses were collected using fixed sensors installed on the bridge deck and accelerometers mounted on a test vehicle crossing the bridges at controlled speeds from 20 to 50 km/h. For Thang Long Bridge, direct measurements identified nine vertical bending modes below 5 Hz, while the drive-by approach consistently captured the first two bending modes. The minimum frequency estimation error was as low as 0.2% at vehicle speeds of 30–35 km/h, whereas errors increased at other speeds, reaching up to approximately 15% at 50 km/h. For the Chuong Duong Bridge, two dominant modes were successfully identified, with frequency errors ranging from 1.5% to 9.9%. The best performance was obtained at speeds near 30 km/h. The results indicate that drive-by monitoring can provide reliable estimates of low-order vertical bending frequencies under suitable operating conditions, while higher-order and torsional modes remain challenging to identify.