# Dynamic Identification of Tensile Force in Tie-Rods by Interferometric Radar Measurements
## 4 Experimental Setup  
The capability of the interferometric radar technique for application to the estimation of tensile force in tie-rods was tested in an experimental campaign carried out at Laboratory \"[PERSON]\" of the Polytechnic University of Bari. To this aim, a suitable experimental device for applying known tensile forces in rods and cable was built (Figure 3) to reproduce the working conditions of a tie-rod like that employed in masonry constructions. In particular, this device consists of a supporting structure composed of 2 horizontal beams HEA 140 connected by two vertical high stiffness steel plates; a hydraulic jack and two clamping devices for tensioning the tie-rod; a load cell for directly measuring the real value of the tensile force applied. It is worth noticing that the experimental setup was assembled in such a way as to guarantee symmetric constraint conditions at the two ends of the tie-rod. As the tie-rod, a steel strand with geometric, inertial and mechanical parameters (equivalent circular cross-section) reported in Table 2 was employed for the experiments. In particular, three values of the tensile force were applied to the tie-rod: 50, 100 and 150 kN. For each tensile force level, the vibrations of the steel tie-rod were measured simultaneously through both a radar interferometer and some accelerometers. Specifically, the employed interferometric radar system is the customized version of the IBIS-FS manufactured by IDS GeoRadar (Pisa, Italy) described in Section 3. The sampling frequency employed for the tests was 297.62 Hz, the maximum distance between \begin{table}
\begin{tabular}{c c} \hline \hline \(L\) (m) & 2.59 \\ \(A\) (cm\({}^{2}\)) & 2.27 \\ \(\rho\) (kg/m) & 1.75 \\ \(J\) (cm\({}^{4}\)) & 0.374 \\ \(E\) (MPa) & 210,000 \\ \(G\) (MPa) & 80,770 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Geometric inertial and mechanical parameters of the tie-rod. Figure 3: The experimental device for applying known tensile forces to tie-rods. the instrument and the tie-rod was 10 m, and the IBIS-ANT5-H12V39 antennas were employed, having a power gain of 18 dBi, and a beamwidth at \(-3\) dB of \(12^{\circ}\) in the azimuthal direction and of \(39^{\circ}\) in the elevation direction. This kind of antennas is one of the most directional available, capable of concentrating the beam at a restricted solid angle. Indeed, to obtain good quality data, the length of the portion of the tie-rod falling into a single range bin should be minimized. On the other hand, there is the need to make as much as possible parallel the radial direction defined by the axis of radar antennas and the direction of transverse displacements of the tie-rod. Therefore, the relative position between the radar and the tie-rod was accurately studied before performing the tests and set in such a way as to be a right balance between the above two requirements, as described by Figure 4a. Moreover, three uniaxial accelerometers were installed at 1/4, 1/2 and 3/4 of the tie-rod length (see Figure 3) oriented such that only transverse vibrations in the horizontal plane were measured. The featuresof the accelerometers are reported in Table 3. The acquisition and preprocessing of accelerometric data have been performed by a data acquisition system HBM MGCplus. Vibrations of the tie-rod were excited by lightly striking it with an impact hammer. Notice that the intensity of the impulse was maintained very low to avoid damaging the accelerometers, but if only interferometric radar tests are made, it is possible to hit the tie-rod with much more energy, thus enhancing the amplitude of the vibrations and, therefore, the accuracy of the measurements. A preliminary check clearly showed that interferometric radar data were inconsistent with the oscillating behavior of the cable put in vibration acquired by the accelerometers.