**2. Finite element modeling and modal analysis**

As in many disciplines, the application of a detailed accurate finite element model in bridge engineering field has been relatively significant to properly assess structural performance of bridges. The major leaps in computation technology enable to make advanced FE modeling and analysis. Due to the complexity of long-span cable-supported bridges, these structures are modeled based on certain assumptions: (a) simplified spine-beam model, (b) multiscale (hybrid) FE model. The spine-beam model that enables to reduce degree-of-freedom (DOF) of bridge structures presents the results from the global behavior of bridges. Using the spine-beam model, modal analysis can be readily conducted to obtain the effective mode frequencies and associated mode shapes. In addition, preliminary FE model that is developed based on as-built project drawings can also be validated using this modeling technique. Therefore, the spine-beam modeling technique helps to check the considered equivalent sectional properties of structural components. On the other hand, multi-scale modeling becomes very significant upon making analysis for local structural component. This modeling technique also provides that different element types, beam, solid, shell, and truss, are used together to establish 3D full-scale bridge FE model. Accordingly, both modeling techniques are utilized for different goals, and generally, they have been used together for structural analysis of bridge structures.

The modal analysis of structures is a powerful tool for earthquake excitation analysis of structures. Through this analysis, the response of structures to dynamic input can be estimated and certain outcomes related to dynamic inputs can be explained [17]. For large-scale bridge structures with different large size of structural component, such as main deck, tower etc., the mode shapes may show which component dominates the dynamic response of long-span bridges [14–16].

The bridge's structural components of the tower, the main deck, the portal beams, and approach span are modeled as equivalent frame element corresponding their mechanical

**Figure 3.** Sectional properties of the Bosphorus Bridge [17].

the Asian and the European. When opened to traffic in 1973, the bridge was classified as the 4th longest suspension bridge in the world according to its main span. The bridge serves as vital link on the Motorway-1 (O1) connecting the city center of Istanbul. Significant part of heavy traffic of Istanbul has been carried from the bridge along with the Fatih Sultan Mehmet Bridge named the 2nd Bosphorus Bridge located on the northern side of the Bosphorus

Taking the efficiency of St-Id into account, this approach is utilized for the Bosphorus Suspension Bridge and the results are presented in this study. Each step of the concept is handled to be conducted for the bridge. Along with establishing the numerical model of the bridge to obtain its analytical modal frequencies, the experimental study is conducted using SHM data collected during the extreme wind event. Based on the discrepancy between FEM and experimental results, model verification/calibration is performed. The calibrated model is then used for utilization of multi-point earthquake analysis of the bridge. The outcomes from this study illustrated that St-Id concept is a robust methods to properly predict the structural

As in many disciplines, the application of a detailed accurate finite element model in bridge engineering field has been relatively significant to properly assess structural performance of bridges. The major leaps in computation technology enable to make advanced FE modeling and analysis. Due to the complexity of long-span cable-supported bridges, these structures are modeled based on certain assumptions: (a) simplified spine-beam model, (b) multiscale (hybrid) FE model. The spine-beam model that enables to reduce degree-of-freedom (DOF) of bridge structures presents the results from the global behavior of bridges. Using the spine-beam model, modal analysis can be readily conducted to obtain the effective mode frequencies and associated mode shapes. In addition, preliminary FE model that is developed based on as-built project drawings can also be validated using this modeling technique. Therefore, the spine-beam modeling technique helps to check the considered

Bridge.

46 Bridge Engineering

performance of the Bosphorus Bridge.

**Figure 2.** General views of the Bosphorus Bridge.

**2. Finite element modeling and modal analysis**

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 shown in **Figure 4**.

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 bridge.

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

**Figure 5.** The first five mode frequencies and corresponding shapes of the Bosphorus Bridge.

below;

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

Structural Identification (St-Id) Concept for Performance Prediction of Long-Span Bridges

http://dx.doi.org/10.5772/intechopen.71558

49

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

Structural Identification (St-Id) Concept for Performance Prediction of Long-Span Bridges http://dx.doi.org/10.5772/intechopen.71558 49

**Figure 5.** The first five mode frequencies and corresponding shapes of the Bosphorus Bridge.
