PSI - Issue 5

C.A. Prato et al. / Procedia Structural Integrity 5 (2017) 332–339 C.A. Prato/ Structural Integrity Procedia 00 (2017) 000 – 000

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Damping rate for the dominant mode of response excited by these tests (2.83 / 3.16 Hz) was found in the range of 0.73% to 1.31% with a mean value of 0.94% for mid-span section, and 0.49% to 1.72% with a mean value of 0.93% for quarter-span section, which is in close agreement with the 1% value indicated by the IAP-11 code for pre-stressed concrete bridges. Damping of the second dominant frequency identified in the tests at about 7 Hz, only excited with the impulse at quarter-span section, was found to be 0.85%. The frequency of 2.03 Hz of the fundamental mode obtained from the numerical model was only observed with small amplitude in the Series B of the tests where the excitation and recording were done at quarter-span section as shown by the peak of the Fourier spectrum of Figure 4.b, together with other frequency peaks found in the range of 6.7 / 7.1 Hz that correspond to a second symmetric longitudinal bending mode, and 10.0 / 10.7 Hz involving torsion, and both longitudinal and transverse bending.

2.2. Load tests with reference vehicle

The vehicle used for the tests is shown in Figure 5.

Fig. 5. 30 t reference vehicle used for the tests.

Figures 6(a) through 6(c) illustrate the Fourier Amplitude Spectra of the records obtained through passage of the reference vehicle at speeds of 20, 40 and 60 km/h, respectively. These results point to interesting features referring to the influence of the speed of passage of the reference truck. At 20 km/h speed, the response of the bridge is dominated by frequency peaks at about 3.1, 6.5 and 9.7 Hz. These peaks correspond to the bridge loaded with the moving mass of the reference vehicle and approximately correlate with natural frequencies identified through the damping tests of the unloaded bridge at 2.83, 7.04 and 10.64 Hz. The fundamental mode obtained with the numerical model at 2.03 Hz, however, did not appear at low speeds but showed at higher speeds of 40 and 60 km/h. As it can be seen from the amplitudes of the peaks, the largest accelerations of these tests occurred at 60 km/h. The tests were not carried out at the maximum allowed speed of 80 km/h due to limitations imposed by the road design at the location of the bridge. The peak accelerations recorded at both sides of the central span section of the bridge during passage of the reference truck are given in Tables 1 and 2. There is a systematic difference in amplitude between records at both sides of the bridge at mid-span section. Such difference has been verified not to originate in instrumental errors, and is primarily attributed to the following factors: i) Since the main contribution to the stiffness of the bridge at mid-span originates in the actual stiffness of the stays, it may be due to an uneven force distribution of the weight of the bridge among the sets of stays, and ii) An accidental eccentricity of the path of the truck with respect to the longitudinal axis of the bridge.

Table 1. Average of maximum vertical acceleration recorded at mid-span for speeds of 20 to 60 km/h with both sensors. Speed [km/h] Acceleration [m/s 2 ] Mean value STD 20 0.412 0.113 40 0.617 0.120 60 0.845 0.336