**4. Structural identification (St-Id)**

• The parameters to be monitored (wind, displacement, temperature)

Following identification of the monitoring parameters, the next step is to determine the number of sensors based on the monitoring objectives. In order to compensate for the requirements of the monitoring, it is important to select which types of sensor are used. For this purpose, technical specifications of the sensors including measurement range, sampling rate, sensitivity, resolution, linearity, stability, accuracy, repeatability, frequency responses should be identified clearly. With the daily traffic capacity of 195 thousands vehicles, the Bosphorus Bridge serves as its significant function without no traffic interruption in the transportation network of Istanbul. Due to the seismicity and location of Istanbul, the bridge has been subjected to many critical loading events from earthquake, wind, heavy traffic to marathon. Therefore, it is a crucial issue to track the structural response of the Bosphorus Bridge under these type of excitations. The bridge was decided to be donated with SHM system due to the unpredicted failure of hanger rope in 2004. With the total number of 258 channels and 168 sensors varying from accelerometers, tilt meters, force transducers, strain gauges, laser displacement, GPS, thermocouples to weather stations, a permanent structural health monitoring system (SHM) is developed considering the bridge's own characteristics and critical points to be monitored [20].

The quantity and location of the sensors are determined after certain temporal installation tests on its SHM system. In **Table 1**, the number and type of sensors installed on the bridge are listed with their preferences. Moreover, general sensor arrangement of the SHM system of

**Type Preferences Quantity** Accelerometers Measuring range of +/− 2 g 19 Tiltmeters Measuring range (°): ±14.50 15 Force transducer Measuring range (mm): ±1.50 mm 12 Strain gauges Resistance tolerance (%): ±0.30 70 Laser displacements Measuring range (mm): 200–2000 8 GPS Precision (mm): 0.2 5 Thermocouples Accuracy (%): ±0.10 33 Weather station Wind speed range (mph): 0–130 6 Total 168

• The design value and measurement range of the parameters

• The spatial and temporal properties of the parameters

• The environmental condition of the monitoring

• The accuracy requirements

50 Bridge Engineering

• The duration of the monitoring

the bridge is presented in **Figure 6**.

**Table 1.** Sensor types and quantity of the SHM system of the bridge [20].

#### **4.1. Description of extreme wind event**

Strong winds are not very frequent in Istanbul due to its location. However, during the daytime on April 18 2012, a strong storm occurred in Istanbul. It was the first time that the bridge experienced such a high wind. According to measurement of Turkish Meteorology Service, the peak wind-speed reached to 122 km/h. Although ultimate design wind speed of the bridge is 162 km/h, the bridge was closed to the traffic for a period of time as a precautionary measure. The change of wind speed with time is shown in **Figure 7**. This variation is also obtained by weather stations installed on the bridge. As seen from **Figure 7**, the mean wind speed before the storm was around 20 km/h. However, it suddenly increased to 100–120 km/h in 10 minutes [21]. This variation is also verified with meteorology data as shown in **Figure 7**. In addition, the excitation of the strong wind load is also corrected with the other weather station data recorded from different critical points of the bridge. As seen from **Figure 8**, the lateral wind direction is verified with the polar charts through SHM data at the deck midspan, and the bridge was determined to be greatly induced in N-S direction during the critical wind event.
