# Performance Assessment of a Renovated Precast Concrete Bridge Using Static and Dynamic Tests
## 4 Measurement System  
### Accelerometers and Strain Gauges  
A complex measurement system (able to measure accelerations, strains, and temperatures simultaneously by up to four devices) using National Instruments equipment was used. It was developed by one of the authors in collaboration with mechanical engineers [21]. It consisted of two devices for this procedure: NI cRIO (National Instruments Compact Reconfigurable Input Output) 9067 with four NI 9234 modules, and NI cRIO 9074 with two NI 9234 modules. The NI 9234 input module is an analogue-to-digital converter capable of measuring acceleration. The number of modules used allowed us to measure 24 channels. There was also one NI 9237 input module for the measurement of strain gauges. Temperature measurements formed an important part of the test. One NI 9211 input module allowed for measuring air temperature, as well as the structure temperature, using contact temperature sensors. Four temperature sensors, two strain gauges and 24 accelerometers were used simultaneously.  
Accelerations in the Z-direction (vertical) were measured with 18 sensors (labelled with a red arrow); accelerations in the Y-direction (lateral) were measured with three sensors (labelled with a blue arrow, and accelerations in the X-direction (longitudinal) were measured with one sensor (labelled with a purple arrow). Two accelerometers were mounted by a magnetic pad on the external pre-stressed cables approximately in the middle of span no. 2 (Figure 8). Two measuring polygons using two measuring devices with separately connected accelerometers were placed along the footpaths to obtain the torsional mode shapes. The first polygon with eight accelerometers was on the north side, the second one with 16 accelerometers was on the south side. The synchronization of measuring devices was carried out via an FTP (file transfer protocol) using an FTP cable placed under the bridge girder to be protected from traffic. This arrangement of the devices reduces the measurement preparation time, as the necessary cable lengths were shortened.  
Figure 4: _Cont._  
Accelerometers (PCB--PicoCoulombB--Piezotronics 393B31) in combination with the NI 9234 input modules, measured frequency higher than 0.5 Hz with an interval of amplitudes \(\pm\)4.9 m/s\({}^{2}\). The resonant frequency of the sensors is over 0.4 kHz. The high sensitivity of these sensors makes them suitable for ambient vibration measurements. Accelerometers (MMF--Metra Mess--und Frequenztechnik--KS48C) were also applied with a higher range of amplitudes (\(\pm\) 58.9 m/s\({}^{2}\)) and the resonant frequency was above 7 kHz. Strain measurements were performed using HBM (Hottinger Baldwin Messtechnik) 50/120 LY41 strain gauge sensors (in the \"Half Bridge\" set up, i.e., one strain gauge is measuring the structure and one strain gauge is a dummy), suitable for concrete structures. Sensor resistance was 120 \(\Omega\), and strain factor was 2.09. K-type thermocouple contact sensors (SENSIT type TCS190) were used for the measurement of the structure temperature. Air temperature was measured by the SENSIT type TG4 thermocouple. Both air and structure temperatures were measured during the test (Table 5).