**3. Structural health monitoring system (SHM)**

and sectional properties. The details of the structural components of the bridge are given in **Figure 3**. For elaborate sectional properties, all points of the components are precisely determined depending on the project drawings, and thus much more realistic dimensions of them are adopted. Based on these project specifications and general properties of the Bosphorus Bridge, FE model of the bridge is developed utilizing the spine-beam modeling approach as

Since the bridge was made of structural steel, modal damping ratio of ξ = 0.02 is also considered to calculate the proportional structural damping for the bridge. The first 50 natural frequencies and associated mode shapes are obtained and the first five modes and associated frequencies are shown in **Figure 5**. From the analysis, the main deck of the bridge is obtained to be effective for lateral and vertical response of the bridge to a dynamic input. Particularly, modal participating total mass ratio for transverse direction of the main deck is determined as 60% at the end of the first five modes directly pertinent to the main deck mode shapes. Compared to modal participating total mass ratio of 96% at the end of the fifty modes, this value indicated the efficiency of the main deck mode shapes on the dynamic response of the bridge. Similar single mode shapes are also determined for the tower after the main deck mode shapes. All these single mode shapes of the main deck and the tower are seen in the first ten mode shapes. The other mode shapes are obtained as the combination of these single mode shapes. Based on these consequences, the main deck and the tower dynamic response are estimated to dominate the dynamic behavior of the Bosphorus Bridge. The obtained analytical modal frequencies are also compared with those from the experimental results so as to verify/calibrate FEM of the

shown in **Figure 4**.

48 Bridge Engineering

bridge.

**Figure 4.** 3-D full-scale FE model of the Bosphorus Bridge.

In general, the components of a SHM system are; (i) Sensory systems (ii) Data acquisition and transmission systems (iii) Data processing and control system (iv) Data management systems (v) Structural evaluation system [18]. Design of SHM systems is based on clearly describing the monitoring objectives. Therefore, working SHM designer together with bridge designer is inevitable to identify the objectives very well. Considering the objectives, monitoring requirements should be properly identified [19]. The requirements can be considered as below;


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].

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

**Figure 6.** General layout for the sensors of the SHM system of the bridge.

**4.1. Description of extreme wind event**

**4.2. System identification of the bridge**

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

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Before the identification, data processing on the acceleration data is implemented. Firstly, base-line correction is performed to get rid of offset value. The next step is to remove linear trends from the data using de-trend technique. Besides, the mean value (DC) of the data

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


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

Structural Identification (St-Id) Concept for Performance Prediction of Long-Span Bridges http://dx.doi.org/10.5772/intechopen.71558 51

**Figure 6.** General layout for the sensors of the SHM system of the bridge.
