Seismic monitoring of ITU Triga Mark II research reactor building

The process of implementing a damage detection plan is called Structural Health Monitoring (SHM). Changes in material and geometric properties, and boundary conditions of an aerospace, civil and mechanical engineering structure are considered as damage. In civil structures, such as buildings, the damage can be due to an earthquake which can also interrupt essential utilities and functionalities of buildings. Detecting these damages and predicting their possible outcomes can be crucial for the state of the building and its safety. Buildings equipped with sensors for this purpose use SHM and near real-time monitoring which gives the ability of fast decision making in cases of emergencies. As it is well known, in recent years due to their low-cost, MEMS (Microelectromechanical Systems) sensors are widely used and being tested in monitoring of civil structures. With the developments in technology, the quality of the data collected with these sensors are improved and started to take place in SHM of civil structures. In this study, for SHM we use the seismic vibration data collected by MEMS sensors in ITU TRIGA MARK II Training and Research Reactor building. The installation of the MEMS sensors and data acquisition in the building is realized under the project 'Signal Analysis of ITU TRIGA MARK II Research Reactor' supported by ITUNOVA-TTO A.Ş. and Aim Enerji A.Ş. One of the aims of this project was to use domestic products as much as possible to decrease the foreign-source dependency. To do so twelve domestic products, SAYKAL S-TEK HPVS V1 tri-axial MEMS accelerometer sensors, are installed in the building. The building located in Istanbul Technical University (ITU) Maslak Campus was constructed in 1976 as a reinforced concrete (RC) frame building, and retrofitted in 2000s by adding RC walls between some columns. The study is performed on the full-scale RC moment resisting frame-wall building. This three–story non-symmetric building is one of the prominent structures in the area which needs regular monitoring since it contains a nuclear research reactor. During the realization of the project and in this thesis, we checked the data quality and the software system, and gave feedback to the manufacturer of the sensors for improvement. Due to the ongoing studies in the research reactor and in the building, the data acquisition is interrupted many times with technical reasons. Data gap and spike-like unwanted records are available in the data as a result of the difficulties in the acquisition system and human interferences respectively. Also, problems like not accurate time stamping are observed in the data. The complications related to the sensors are forwarded to the sensor manufacturer and some of these difficulties are fixed and some of them are still in fixing process. The feedbacks provided during this thesis helped to further improve the quality of the records. In this thesis we aim to identify the basic characteristics of the building, such as translational and torsional modal frequencies, damping and the soil structure interaction of the building. Obtained basic characteristics of the building from this thesis will be used in determination of the possible damages by continuous monitoring of the building. For this purpose, a recorded earthquake (October 15 2016 Black Sea Mw= 4.8) and two-month long ambient noise data is used. The 10-minute length recorded ambient noise data are windowed to obtain 30-second length segments, total of 76.040 segments are stacked. After mean and trend removal for both earthquake and ambient noise data, Fourier amplitude spectra (FAS), cross-spectrum, coherence and cross-phase are calculated. Using the spectral information, the translational modes are determined as 5.57 Hz, 6.4 Hz, 7.0 Hz, 9.6 Hz and 12.42 Hz. Rotational modes are determined as 8.7 Hz and 11.47 Hz. To support these findings, a simple modal frequency calculation for a clamped boundary on one end is performed by considering the height and the velocity as 10 meters and 50 m/s respectively. The calculated results support the modal frequencies obtained by the FAS of the recorded data. These results are also supported by the building's structural analyses modeling results performed by Eng. MSc. Pınar İnci Koçak under the supervision of Prof. Dr. Alper İlki. Damping ratios of the building are calculated by using the half-power technique, for the East-West and North-South components of each floor recordings. The estimations indicate that the building is more stable on North-South direction, which is expected when the asymmetric structure of the building is considered. The Soil-Structure-Interaction (SSI) is investigated by using the earthquake data and comparing roof to base transfer function to the base acceleration recordings. No prominent indication of SSI is found for the excitation caused by the Black Sea Earthquake. The outcomes of this thesis are the preliminary results of the seismic monitoring of the ITU TRIGA MARK II research reactor building. After the retrofit in the building is finalized and the data acquisition system is improved, the monitoring of the building will continue for SHM. For future studies an early warning system engagement is also planned.