**4.** *In situ* **measurements of the dynamic characteristics of stone bridges**

## **4.1. Four studied stone bridges**

**Figure 11.** Connection with wooden beams and iron inserts between the primary and secondary arches of the Plaka

**Figure 12.** Iron ties used to connect the two opposite faces of the primary arch in Plaka Bridge.

**Figure 13.** Iron ties used to connect the two opposite faces of the primary arch in Tsipianis Bridge.

Bridge.

84 Structural Bridge Engineering

In measuring the dynamic response of four stone bridges, two types of excitation were mobilized. The first, namely ambient excitation, mobilized the wind, despite the variation of the wind velocity in amplitude and orientation during the various tests. Due to the topography of the areas where these stone bridges are located, usually a relatively narrow gorge, the orientation of the wind resulted in a considerable component perpendicular to the longitudinal bridge axis (**Figures 15a**, **17a**, **18a** and **19a**). This fact combined with the resistance offered to this wind component by the façade of each bridge produced sufficient excitation source resulting in small amplitude vibrations that could be recorded by the employed instrumentation. For this purpose, the employed SysCom triaxial velocity sensors had a sensitivity of 0.001 mm/s and a SysCom data acquisition system with a sampling frequency of 400 Hz. All the obtained data were subsequently studied in the frequency domain through available fast Fourier transform (FFT) software [4, 5]. This wind orientation relative to the geometry of each bridge structure coupled with the bridge stiffness properties could excite mainly the first symmetric out-of-plane eigen-mode, as can be seen in **Figure 16c** for the Konitsa Bridge. The variability of the wind orientation could also excite, although to a lesser extent, some of the other in-plane and out-of-plane eigen-modes (see **Figure 16c** for the Konitsa Bridge).

The second type of excitation that was employed, namely vertical in-plane excitation, was produced from a sudden drop of a weight on the deck of each stone-masonry bridge [6, 7]. This weight was of the order of approximately 2.0 kN that was dropped from a relatively small height of 100 mm, so as to avoid even the slightest damage to the stone surface of the deck of each bridge. Again, the level of this second type of excitation was capable of producing mainly vertical vibrations and exciting the in-plane eigen-modes of each structure that could be captured by the employed SysCom triaxial velocity sensors with a sensitivity of 0.001 mm/s and a SysCom data acquisition system with a sampling frequency of 400 Hz. All the obtained data were subsequently studied in the frequency domain through available FFT software. In **Figure 15c**, the velocity measurements are depicted along the three axes (*x*-*x* horizontal outof-plane, *y*-*y* horizontal in-plane and *z*-*z* vertical) as they were recorded during a typical sampling with the wind excitation. In **Figure 15d**, the velocity measurements are again depicted along the three axes (*x*-*x* horizontal out-of-plane, *y*-*y* horizontal in-plane and *z*-*z* vertical) as they were recorded during a typical sampling with the drop weight excitation. As can be seen, the drop weight excitation could produce at the dominant frequencies vibrations at least one order of magnitude larger than the wind excitation. From these measurements, an attempt was also made to obtain an estimate of the damping ratio for the dominant in-plane and out-of-plane frequencies. As is depicted in **Figure 16a** for the wind excitation, the main symmetric out-of-plane vibration that is excited by the wind has a dominant period of 2.539 Hz and a corresponding damping ratio approximately 1.7%. Similarly, as is depicted in **Figure**

**Figure 15.** (a) Konitsa Bridge: wind excitation; (b) drop weight excitation; (c) vibration measurements from wind excitation recorded by the triaxial velocity sensor located at the crown of the Konitsa Bridge; and (d) vibration measurements from drop weight excitation at the crown of the bridge recorded by the triaxial velocity sensor located at the middle of the Konitsa Bridge.

**16b** for the drop weight excitation, the main symmetric in-plane vibration that is excited by the drop weight has a dominant period of 7.715 Hz and a corresponding damping ratio approximately 2.7%. This increase in the damping ratio value for this latter dominant frequency must be attributed to the relatively larger amplitudes of vibration that are produced from the drop weight excitation than from the wind excitation, as already underlined. All vibration measurements of the dynamic response of the Konitsa Bridge for either type of excitation were utilized to extract the eigen-frequencies depicted in **Figure 16c** together with the approximate shape of the corresponding eigen-modes.

vertical vibrations and exciting the in-plane eigen-modes of each structure that could be captured by the employed SysCom triaxial velocity sensors with a sensitivity of 0.001 mm/s and a SysCom data acquisition system with a sampling frequency of 400 Hz. All the obtained data were subsequently studied in the frequency domain through available FFT software. In **Figure 15c**, the velocity measurements are depicted along the three axes (*x*-*x* horizontal outof-plane, *y*-*y* horizontal in-plane and *z*-*z* vertical) as they were recorded during a typical sampling with the wind excitation. In **Figure 15d**, the velocity measurements are again depicted along the three axes (*x*-*x* horizontal out-of-plane, *y*-*y* horizontal in-plane and *z*-*z* vertical) as they were recorded during a typical sampling with the drop weight excitation. As can be seen, the drop weight excitation could produce at the dominant frequencies vibrations at least one order of magnitude larger than the wind excitation. From these measurements, an attempt was also made to obtain an estimate of the damping ratio for the dominant in-plane and out-of-plane frequencies. As is depicted in **Figure 16a** for the wind excitation, the main symmetric out-of-plane vibration that is excited by the wind has a dominant period of 2.539 Hz and a corresponding damping ratio approximately 1.7%. Similarly, as is depicted in **Figure**

**Figure 15.** (a) Konitsa Bridge: wind excitation; (b) drop weight excitation; (c) vibration measurements from wind excitation recorded by the triaxial velocity sensor located at the crown of the Konitsa Bridge; and (d) vibration measurements from drop weight excitation at the crown of the bridge recorded by the triaxial velocity sensor located at the

middle of the Konitsa Bridge.

86 Structural Bridge Engineering

**Figure 16.** (a) Vibration measurements from wind excitation obtained from the triaxial velocity sensor located at the middle of the Konitsa Bridge; (b) vibration measurements from drop weight excitation at the middle of the bridge obtained from the triaxial velocity sensor also located at the middle of the Konitsa Bridge; and (c) measured eigen-frequencies and corresponding eigen-modes for the Konitsa Bridge.

**Figure 17.** Kokorou Bridge: (a) wind excitation and (b) drop weight excitation.

**Figure 18.** Tsipianis Bridge: (a) wind excitation and (b) drop weight excitation.

**Figure 19.** Kontodimou Bridge: (a) wind excitation and (b) drop weight excitation.


**Table 1.** Measured eigen-frequencies for four stone-masonry bridges.

**Figure 17.** Kokorou Bridge: (a) wind excitation and (b) drop weight excitation.

88 Structural Bridge Engineering

**Figure 18.** Tsipianis Bridge: (a) wind excitation and (b) drop weight excitation.

**Figure 19.** Kontodimou Bridge: (a) wind excitation and (b) drop weight excitation.

The same process was followed for measuring the dynamic characteristics of another three stone-masonry bridges (Kokorou, Tsipianis and Kontodimou) using both the wind and the drop weight excitations, as shown in **Figures 17**–**19** where the position of the employed velocity sensors is indicated. Next, by utilizing all these vibration measurements of the dynamic response of each of these studied bridges for either type of excitation, it was possible to extract the relevant eigen-frequencies that are listed in **Table 1**. At least measurements of three repetitive sampling sequences for each type of excitation, either wind or drop weight, for each bridge (Konitsa, Kokorou, Tsipianis and Kontodimou) were measured. The eigen-frequency values listed in **Table 1** are values representing an average from corresponding values that were obtained by analysing the measured response from all tests.

#### **4.2. Additional field measurements for the Konitsa Bridge**

To gain more confidence in the *in situ* measurements presented in Section 4.1, the results of an independent *in situ* campaign are also presented here and briefly compared with the corresponding results presented in Section 4.1 for the same bridge. This additional *in situ* campaign was conducted during the end of October 2015 (**Figure 20**). This almost coincides with the *in situ* campaign described in Section 4.1, which was conducted during the period from mid-November 2015 till mid of December 2015 for all four bridges. Moreover, for the Konitsa Bridge the measurements presented in Section 4.1 were obtained on the dates of 8, 16 and 20 November. Based on this timing and the constant weather conditions prevailing during this period, no influence is expected to arise from environmental conditions to all these measurements. The objective of this independent field experiment was the same, that is, to assess the dynamic characteristics of the Konitsa stone arch bridge [8] using a set of Wilcoxon high-sensitivity accelerometers (1000 V/g) integrated with a data-recording/FFT analyzer RION-S78 system depicted in **Figure 20**. Further data post-processing was performed using a set of additional FFT processing software.

**Figure 20.** RION system and accelerometer system used in Konitsa Bridge field study. (a) Location of sensor at the South part of Konitsa Bridge and (b) location of the sensor at the North part of Konitsa Bridge.

Shown in **Figure 21** are the post-wind gust bridge response (vertical acceleration) and the corresponding power spectrum associated with the trace segment between 12 and 16 s of the record [4]. The power spectrum associated with the decay segment clearly delineates (a) the symmetric vertical mode (7.75 Hz).

**Figure 21.** Decay segment of acceleration trace (vertical) and the corresponding power spectrum (7.75 Hz, damping ratio estimate of 1.6%).

**Figure 22.** Horizontal (out-of-plane) acceleration time histories recorded simultaneously at two locations on the Konitsa Bridge deck. (a) Entire trace including high wind effects; (b) post-wind free vibration. Dominant frequency of 2.56 Hz; (c) recorded vertical and horizontal spectra at crown averaged over 512 records; (d) coherence measurements between *P*o and *P*1 locations aiding mode identification (arrows indicate 100% coherence characteristic of the structure modes).

**Figure 22** depicts the horizontal (out-of-plane) acceleration time histories recorded simultaneously at two locations on the Konitsa Bridge deck and their corresponding power spectrum with dominant frequency of 2.56 Hz (first out-out-of plane eigen-mode). Comparing the eigenfrequency values obtained for the *in situ* experiments, reported in Section 4.1 (depicted in **Figures 15** and **16**), with the corresponding values obtained from this independent *in situ* experiments (depicted in **Figures 21** and **22**), very good consistency can be observed. **Figure 22c** and **b** depict the FFT-averaged Fourier spectral curves that formed the base together with the coherence plot of **Figure 22d** to identify with confidence the eigen-frequency values [9].
