In the experiment, a 2,000 kg heavy weight was lifted up to the prescribed height of 20 m by using the track crane, and then dropped freely to the mid-span of girder with a desorption device. A heavy weight is made from steel outer shell with 1 m in the diameter, 97 cm in height, and spherical bottom with 80 cm in radius as shown in Figure 2(a) and its mass is adjusted filling concrete and steel balls. Figure 2(b) shows supporting gigue including load-cells and Figure 2(c) shows gigue for preventing RC girder from jumping up. RC girder was set on the supporting gigues, which are made so as to freely rotate but not to move toward each other.
The ends of RC girder is fixed in the upward direction using steel rods and beams to prevent from jumping up at the time of impacted by a heavy weight as shown in Figure 2(c). In this experiment, impact force wave (P), reaction force wave (R), and displacement waves (D) at six points along the girder were measured. Impact force wave was estimated using a deceleration of heavy weight, which is measured using accelerometers set at the top-surface of weight.
The accelerometer is of strain gauge type and its capacity and frequency range for measuring are
1,0 times gravity and DC through 7 kHz, respectively. Each load-cell for measuring reaction force are of 1,500 kN capacity and more than 1 kHz measuring frequency. For measuring displacements, laser-type variable displacement transducers (LVDTs) were used which are of 200 mm maximum stroke and 915 Hz measuring frequency. Analog signals from those sensors were amplified and converted to digital ones.
Table 3. Material properties of rebar.
Fig. 2. Pictorial views of heavy weight and supporting gigues.
The digital data were continuously recorded with 0.1 ms time intervals by using digital data recorders. After that, impact force wave was numerically filtered by means of rectangular moving average method having 0.5 ms time. After experiment, pictures for views of crack patterns occurred around impacted area and on side-surface of RC girder, and a view of peeling and spalling of concrete cover were took. Figure 3 shows crack distributions occurred in the both side-surfaces of the girder.
2. Analytical Overview