Masato TAZAWA
Vibration Measurement of Existing Steel Truss Bridges for Structural Health Monitoring
Masatsugu NAGAI, Eiji IWASAKI, Takeshi MIYASHITA
This paper presents vibration measurement of existing steel bridges in order to understand their dynamic characteristics for the purpose of developing structural health monitoring technique. Focused bridge is a steel Gerber truss bridge located in Nagaoka City. Total length and the number of spans are 850.8m and 13 respectively. About 70 years have passed after its construction.
At first, measurements of ambient and traffic-induced vibration were carried out using MEMS-based tri-axial accelerometers implementing wireless LAN. They were installed on stringers around the bearing supports and the expansion joints below Gerber hinges. As a result, a characteristic acceleration response vibrating only on one side was confirmed. From laboratory experiment, it was found that the response resulted from impact force based on the capacitive-typed MEMS sensor. The response frequently appeared in around an expansion joint having large gap. Although the response shows the occurrence of impact force, it is impossible to evaluate the magnitude of acceleration under traffic-induced vibration.
Therefore, in order to evaluate the magnitude in each span, re-measurement was conducted using a high-performance servo-typed accelerometer. It was attached on accessible vertical member in the middle of each span. Comparison of the maximum vertical accelerations in each span indicates that the response at suspended spans is 2 to 13 times larger than that at anchor spans under traffic passage. Moreover, the response at a vertical member near the expansion joint with large gap reached about 2000gal. From visual inspection, a crack at the lower flange of a stringer under the joint has been detected. The reason of the crack has not clearly determined. However, the correlation between the damage and the large magnitude seems to be large.
Then, for the comparison of dynamic characteristics, vibration measurements were conducted at other 4 steel truss bridges at Nagaoka City other than Gerber type. About 50 years have passed after their construction. As a result, the followings are found; 1) Acceleration response at the Gerber steel truss bridge becomes 10 times larger than other bridges. 2) First natural frequencies of the bridges can be predicted from Shimada's formula, which calculates first natural frequencies from span length and structural forms based on statistical analysis. Also, local damage such as corrosions of members can not be reflected in first natural frequencies.
At present it is not possible to directly diagnose the integrity of existing bridges from measurement. However, there is little information on the dynamic characteristics of existing bridges. Therefore, this paper is expected to provide valuable information.
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