**5. Utilization, decision-making and performance prediction**

After the destructive earthquakes in last 2 decades in Turkey, Izmit (1999) and Duzce (1999) earthquakes, the public awareness of structural earthquake safety and performance of the existing structures in Turkey has increased progressively. General Directorate of Turkish State Highways (KGM) conducted a number of rehabilitation projects (JBSI) [23] for the most critical long-span bridges in Turkey, such as the Bosphorus Bridge. The related studies for the Bosphorus in literature were basically focused on the uniform support earthquake analysis (U-sup) of the bridge. Therefore, the multi-point earthquake analysis (Mp-sup) is required to better understand the seismic behavior of the Bosphorus Bridge. Considering these recommendations, this chapter aims at determining the effects of spatially varying earthquake motion on the Bosphorus Bridge using the calibrated FEM of the bridge in the previous section.

In order to simulate site-specific ground motions, the geographic coordinates of the bridge's support points have firstly to be determined. As indicated in **Figure 11**, the support coordinates of the bridge are obtained depending on the general coordinates of the bridge. **Figure 11** also presents the general considerations of the multi-point earthquake analysis of the bridge. Taking the scenario earthquake of Mw = 7.4 predicted to occur with the probability of 70% in next 30 years in Istanbul and these coordinates of the bridge into consideration, the stochastic modeling technique proposed in [24] is used to generate spatially varying site-specific earthquake ground motions.

The simulation process is performed and the acceleration ground motion time-histories (ATH) are generated for the Bosphorus Bridge. Although the process yields to the ATHs, the displacement ground motion time histories (DTH) need to be obtained for the multi-point earthquake analysis. Therefore, the DTHs are presented in **Figure 12** instead of the ATHs. As shown in **Figure 12**, the triple-direction (two horizontals and one vertical) ground motions are generated for the each considered multi-point, A, B, C and D. Total number of twelve ground motions are defined for the analysis.

**Figure 13**. All these results demonstrate the importance of the behavior of the deck. As to the main cable, the axial force of the main cable at the tower top-saddle also increases relatively. This increase is mostly related to the displacement of the deck and high increase in the tensile strength of the main and the back-stay cables. The results exhibit again the

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Another important point of the bridge is the base-section of the tower columns, which is first considered for the retrofit investigation for long-span bridges. The maximum value of the sectional forces including the axial force, the shear force and the bending moment is given in **Figure 13**. Due to noticeably high increase in the tensile axial force of the main and the

**Figure 13.** Comparison sectional force of the critical elements/points of the bridge.

efficiency of the deck.

**Figure 11.** Geographic coordinates and multi-support earthquake analysis considerations.

**Figure 12.** Simulated spatially varying site-specific multi-point earthquake records.

In **Figure 13**, the variation of the tensile strength of the main and the back stay cables is presented. The tensile strength value of the main cable increases as 74% and 78% under the Mp-sup compared to the U-sup and JBSI retrofit project, respectively. The different percentage change reveals that JBSI retrofit project is highly conservative in terms of sectional forces. Similar percentage change is obtained for the back-stay cable as shown in **Figure 13**. All these results demonstrate the importance of the behavior of the deck. As to the main cable, the axial force of the main cable at the tower top-saddle also increases relatively. This increase is mostly related to the displacement of the deck and high increase in the tensile strength of the main and the back-stay cables. The results exhibit again the efficiency of the deck.

Another important point of the bridge is the base-section of the tower columns, which is first considered for the retrofit investigation for long-span bridges. The maximum value of the sectional forces including the axial force, the shear force and the bending moment is given in **Figure 13**. Due to noticeably high increase in the tensile axial force of the main and the

**Figure 13.** Comparison sectional force of the critical elements/points of the bridge.

In **Figure 13**, the variation of the tensile strength of the main and the back stay cables is presented. The tensile strength value of the main cable increases as 74% and 78% under the Mp-sup compared to the U-sup and JBSI retrofit project, respectively. The different percentage change reveals that JBSI retrofit project is highly conservative in terms of sectional forces. Similar percentage change is obtained for the back-stay cable as shown in

**Figure 11.** Geographic coordinates and multi-support earthquake analysis considerations.

56 Bridge Engineering

**Figure 12.** Simulated spatially varying site-specific multi-point earthquake records.

back-stay cables, the axial force of the tower directly increased as 56% and 60% according to the U-sup and JBSI retrofit project, respectively. Although the shear force of the main cable at the tower top-saddle decreases, the shear force of the tower at the base considerably increases since the deck forces the tower at the level of the expansion joints (tower-deck connections) leading to high additional shear force. Therefore, the bending moment of the tower at the base increases highly as similar percentage increase to that of the shear force as shown in **Figure 13**.

**Author details**

Address all correspondence to: selcukbas@itu.edu.tr

Mechanics Division. 1977;**103**(6):1089-1104

1978;**104**(12):1845-1858

2008;**133**(8):1051-1066

061/(ASCE)BE.1943-5592.0000705

Structural Engineering. 2013;**139**(10):1703-1715

Vibration. 2011;**330**(6):1196-1210. DOI: 10.1016/j.jsv.2010.09.024

1/1467-8667.t01-1-00312

Department of Civil Engineering, Faculty of Engineering, Bartin University, Central Bartin,

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

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59

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St-Id. USA: American Society of Civil Engineers (ASCE); 2013

2017;**22**(6):1-15. DOI: 10.1061/(ASCE)BE.1943-5592.0001086

Selcuk Bas

Turkey

**References**
