EVALUATION OF SMALL BRIDGE DYNAMIC CHARACTERISTICS BY IMPACT VIBRATION TESTAND THE THREE-DIMENSIONAL FEM ANALYSIS
Y. Daiki1, I. Shoji2, and O. Hiroshi3
12Student, Engineering, Iwate University, Iwate, Japan
Professor emeritus, Engineering, Showa civic design, Iwate, Japan
3
Professor, Engineering, Iwate University, Iwate, Japan
g0118048@iwate-u.ac.jp

Keywords: Impact vibration test, Three dimensional FEM analysis, Health evaluation
1. INTRODUTION
For the appropriate maintenance of the existing bridge,
establishment of the technique of an economical,
effective healthy degree evaluation is demanded. Thereis the inspection of structural rigidity that used vibration
properties for something typical as the guideline which
estimates healthy degree. In this paper, the authors foundthe characteristic frequency of the existing portable

Fig. 1. A cross section of the object bridge

bridge by a shock vibration examination using the small
FWD. In addition, the authors reproduced asuperstructure of the object bridge by three-dimensional
FEM model and the vibration analysis which changed
the support condition is performed. The authors
examined a vibration characteristic evaluation of theobject bridge by comparing the result of the three-Fig. 2. The positioning of impact vibration test

dimensional FEM analysis with the impact vibration test.
2. THE OBJECT BRIDGE
Object bridge is the simply supported composite girder
bridge. It’s lengths is 20.0m, and total width of thebridge is 6,300mm. Girder height is 800mm.Tickness ofRC slab is 160mm, and one of concrete pavement is50mm. Figure 1 shows the cross section of the object
bridge.
3． RESULTS OF THE FWD EXAMINATION

Fig. 3. An instance of fourier amplitude spectrum

In this study, the authors executed the impact vibration
test with portable FWD. We let weight of 25kgf dropfrom 1.0m height onto RC slab of the object bridge. It isdropping the weight on six points in 1/2 points and 1/4
points of each girder span. We measured the accelerationof 9 points, in 1/2 points, 1/4 points and 3/4 points ofeach girder span as shown in Figure 2. Eachaccelerometer sampled the reply acceleration data per
0.0001 seconds. We calculated a fourier amplitude
spectrum and a phase difference spectrum by an FFT

Fig. 4.An instance of phase difference spectrum

method from 16384 sequence acceleration data andTable 1. Measured excellent frequency by the impact

looked for excellence frequency. Figure 3 shows thevibration test

fourier amplitude spectrum (obtained from theMeasurement

Excellence

Estimated vibrationpoint

frequency(Hz)mode shape

6.7

The bending mode

7.6

The torsion mode

acceleration wave at that point when dropping 1/2 pointof G3 girder span.) Figure 4 does a one of the phase

1/2, 1/4 points

difference spectrum of same point. The excellenceof spanfrequency of 6.7Hz was seen in the amplitude spectrum

1/4 point of spanof all accelerometer placement positions when a 1/2
point of each girder length was shocked. Excellence
frequency was also seen in 7.6Hz by acceleration waves
on G1 and G3 girders mainly. The authors considered
6.7Hz is the frequency which indicates the first bending
vibration mode, and 7.6Hz is the frequency whichindicates the first torsion vibration mode.
4． RESULTS OF THE NATURAL VIBRATION
The authors made a superstructure model of the objectFig. 5. The bending mode shape of analysis model

bridge using three-dimensional FEM analysis tool. Theauthors used shell element for main girders and cross
beams, and solid element for slabs. The total number ofnodes of the FEM model is 13,482.and the total numberof elements is 9016. Each Young’s modulus of RC andsteel are Ec=23500N/mm2, and Es=205,000N/mm2. Table
2 shows the analysis result that changed support pattern
into 4 patterns.
5． CONCLUSION
The authors measured an acceleration waves pattern by
the impact vibration test using the portable FWD. We

Fig. 6. A cross section of the bending mode shape ofanalysis model
Table 2. Results of the natural vibration analysis
Naturalcalculated a fourier amplitude spectrum and a phasefrequency

Support patterndifference spectrum by a FFT method and found theexcellent frequency of the object bridge. In addition, the(Hz)The

PatternThe

bending Torsion

authors tried identification of the basic vibration mode.
G1

The natural frequency of bending and torsion mode of aG2

G3

certain pattern of supports reaches the excellent frequency

mode

modeshapeshapeof the test.

①

pin-pinpin-pin―

8.3

For the further studies, we will try to insert some spring②

pin-pin pin-roller pin-pin

6.8

7.1

③

pin-roller pin-pin pin-roller

―

6.2

④

pin-roller pin-roller pin-roller

4.5

6.2

pin-pinmodels to consider the degree of restraint of each supports.