The Dynamic Characteristics And Reliability Assessment Of A Beam Inbrick-concrete Structure

This paper choose a beam in brick-concrete structure of the 1970 s as an object of real time monitoring on the wall surface of the building, to research the dynamic characteristics of the beam with heavy hammer hammering method and road roller vibrating fixed-point method, and to assess it’s reliability based on monitoring result, which come from FBG strain sensors. The main conclusions are as follows:(1) When using heavy hammer hammering method, the peak acceleration on heavy hammer is obviously bigger than the peak acceleration on the beam, the reason is that heavy hammer is incentive, and beam is the corresponding response. The peak acceleration on the beam follows the law of exponential decay. From the law of exponential decay, each order damping ratio can be determined. The average horizontal and vertical natural frequency when heavy hammer hammering is 4.068 Hz and 5.80 Hz. The natural frequency of the beam in horizontal direction is close to the vertical, first order and second order natural frequencies is around 76.2 Hz and 125.2 Hz. The natural frequencies in horizontal and vertical directions in beam is approximate when using the road roller vibrating fixed-point method. The first three order natural frequencies is 11.63 Hz, 39.94 Hz and 60.54 Hz in turn, and corresponding damping ratios are 0.010, 0.011 and 0.006. The calculated frequency of beam are 11.07、44.30 and 99.67 Hz. It compared with test results, the first order natural frequency is basically the same, the second order and third order calculated frequency is respectively larger 10.9% and 64.6% than test results.(2) When using heavy hammer hammering method,the horizontal and vertical maximum pick acceleration of beam are respectively 1.58~4.02 m/s2 and 5.47~11.82 m/s2, the horizontal and vertical maximum pick acceleration ratio of beam mid-span and heavy hammer are respectively 0.00323~0.00789 and 0.01314~0.02679,the peak acceleration of beam follows the law of exponential decay. When using the road roller vibrating fixed-point method, the horizontal maximum pick acceleration is 0.5101 m/s2, the vertical is 0.2375 m/s2,correspond to 6°and 5°seismic intensity.(3) Using FBG sensing techniques to real-time monitor a brick structure. Through processed the monitoring data of a month, pulsating signals are obtained on monitored location and thresholds are set for different monitoring points to assessed its reliability. Setting threshold of three strain gauge on beam is ±4με, maximum failure probability is 6.9%, occurred in the middle of the beam.