Wadi Leban : a 100% optical fiber instrumentationelixandre
Structural health monitoring of the Wadi Leban Bridge in Saudi Arabia: A 100% optical fiber instrumentation
A jewel from Saudi civil engineering, the Wadi Leban cable-stayed bridge in Riyadh (Saudi Arabia) is on a strategic road to the capital city. The Saudi Ministry of Transport has decided to highly monitor its structural health through a 100% fiber optic instrumentation proposed and installed by Cementys.
The aim of the project is to monitor the ageing of the concrete structure, through a continuous and long-term follow-up of structural data. Wadi Leban is located in a particularly harsh environment since it is exposed to extreme weather conditions and traffic: temperature gradient above 60°C (140 °F) from summer to winter, violent storms and sandstorms, sunshine, heavy traffic…
Fiber Bragg Grating technology selected by Cementys has quickly established itself as the best device to instrument Wadi Leban. Not only traditional electrical measurement technologies reach their limits dealing with the large dimensions of the bridge (763 m length between supports, 176m high), requiring a major wiring operation, but also the optical fiber instrumentation offers many advantages such as electromagnetic immunity, multiplexing or also to place remotely the datalogger.
Many factors can cause deterioration pathologies of a cable-stayed bridge: load exceeding the design limits (overload, wind, thermal expansion…), concrete creep, rod and pre-stressing steel relaxation, corrosion or differential settlement. Monitoring such a work requires a precise knowledge of its structural behavior, combining the measurement of global parameters (global behavior of the structure, potentially a consequence of local reactions) and local parameters (local phenomena only). The purpose of a high-quality instrumentation is to reduce the number of sensors and install them wisely in order to ensure an optimal and easily interpretable monitoring.
Regarding Wadi Leban Bridge, Cementys has advised and installed a monitoring system composed of 43 optical fiber sensors. These Fiber Bragg Grating (FBG) sensors measure local parameters, crack openings (CrackLux®) among others, and global parameters: pylons’ tilt (ClinoLux®) main span vertical displacement (TassoLux®), seals’ opening at the ends (DeltaLux®) and rod tension changes (DynaLux®). The tilt angle of pylons reflects a balancing phenomenon of stay cables or a concrete creep in the pylon. Measuring vertical displacement at mid-span is an indicator for concrete creep of the deck, tension loss of pre-stressing tendons or stay cables. Monitoring the openings of seals enables to observe thermal expansion phenomena that may affect the work in case of internal stresses. Finally, the analysis of differential loads in a few rods (8 rods are instrumented out of 240) enables to monitor the global evolution and tension of all rods. Each sensor incorporates two Fiber Bragg Gratings (FBG), to measure and then offset the thermal effects. Environmental parameters are also monitored: outdoor temperature, relative air humidity, wind speed and direction, traffic on the bridge (camera).
The wiring architecture benefits from the multiplexing opportunity offered by fiber optic sensors: all sensors are gathered and connected to only seven measurement channels (7 single-mode optical fibers). Using a cable containing several optical fibers over the whole length and height of the bridge, enables changes of the monitoring system (addition or modification of sensors).
A single BraggLogger® datalogger, located at the base of a pylon, collects all the measurements data every 30 min and transfers it to a remote server. This server allows long-term storage and data transmission on THMInsight®, the secure Web server of Cementys.
Currently, the monitoring system is in continuous operation state for several months. After the learning phase, alert thresholds will be defined and configured in the control panel to generate automatically email alerts. Thus, such as a doctor, the client can monitor the ageing signs of Wadi Leban Bridge and optimize the maintenance management. The real-time health monitoring of the work enables a much more reliable diagnosis than simple spot inspections or non-destructive measurements.