Field vibration test on evaluation of natural vibration characteristics of an existing cable-stayed bridge

Abstract

In this paper, in order to investigate movement of the natural vibration characteristics of an existing three-span continuous cable-stayed bridge (see, Figure 1) with center span of 250 m constructed in 1988 due to degradation, field vibration test was conducted. This bridge is composed of RC main towers, weathering steel two-box girder, and PC cables. Figure 1 Tokachi Chuo Bridge. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig310_1.jpg/> In this study, an in-house measuring system composed of 40 high-sensitive servo-type vibration meters and a wireless LAN system were used to measure the tremor accelerations excited due to passing a weight controlled vehicle and/or natural wind. Sampling time for measuring acceleration was 5 ms (200 Hz). The natural frequencies and modes were specified by using Fourier and phase spectra. Figure 2 shows mode shapes for 1st and 2nd flexural vibration frequencies which are specified using acceleration response obtained after passing traffic. From these figures, since mode shapes along the whole spans for both cases were symmetrically distributed, the natural vibration frequencies and mode shapes can be evaluated by field vibration test. Figure 2 Natural vibration modes obtained from field test. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig310_2.tif/> The comparisons of the natural vibration frequencies among four results obtained from the field vibration test, micro-tremor measurement, and previous vibration tests at the beginning of the common use are indicated in Table 1. From this table, it is observed that 1st and 2nd flexural vibration frequencies were almost similar to each other. Table 1 Comparison of natural frequencies (Hz). Natural vibration mode Present study Previous study Microtremor Measuring after traffic Higashiyama (1989) Const. report (1989) 1st flexural 0.513 0.513 0.510 0.513 2nd flexural 0.684 0.684 – 0.688 3rd flexural 1.196 1.172 – – 1st torsional 1.489 1.489 1.587 1.763 The results obtained from this study are as follows: 1) natural vibration frequencies and modes can be appropriately evaluated by means of the proposed measuring method; and 2) since natural vibration frequencies and modes obtained from the present study were similar to those obtained from the previous tests at the beginning of service, the present degradation of the bridge may be negligibly small practically.