12.4 Low-Frequency Cyclic Loading Test
413
specimens, while it was about 60 mm for C-11 specimen. Therefore, when the
loading direction is parallel to the cratering face, the position of the critical
section or the plastic hinge is not affected by the previous blast induced damage.
(ii) The bending failure mode was also observed in C-12, C-13 and C-14 post-blast
specimens. The location of the critical section (weak section) or plastic hinge
in the column was shifted to the position of the blast induced crater, and the
steel tube tended to fracture at the crater finally. By comparing C-11 with C-12
specimens, it is found that under the identical explosion scenario, the loading
direction had a great influence on the occurrence of the critical section and the
plastic hinge.
12.4.2.2 Hysteresis Curve
The experimental hysteresis curves of the lateral load and its corresponding displacement (P-) of the present specimens are shown in Fig. 12.14. From Fig. 12.14a, it
is found that the hysteresis curve of II-1 specimen showed the unexpected pinching
phenomenon at early loading stage. This might be attributed to the excessive tightness
of the anchor bolt during the test. Hence, in the following part, the hysteresis behavior
of II-1 specimen was not discussed. From Fig. 12.14b to Fig. 12.14f, it is observed
that the hysteresis curves of these specimens exhibited fusiform shape, implying
that the column had good ductility and energy dissipation capacity. In general, the
seismic behavior of each specimen can be divided into three stages, i.e., elasticity,
elastoplasticity and plastic failure.
(i) When ≤ 30 mm, all specimens were in the elastic state, in which the Pcurves showed linearly increasing trend.
(ii) With elevating the lateral displacement loading level, the specimen performed
plastic deformation accordingly. The upper and lower ends of the three
hysteresis loops at the same lateral displacement level were no longer coincided. It is postulated that the accumulated damage of the specimen under
cyclic loading increased, resulting in the large area of yielding. During the
three cycles at the same lateral displacement level, the peak load corresponding
to the latter cycle was lower than that of the previous one, indicating the degradation of the stiffness of column. However, the unloading stiffness of the
column exhibited slight variations, which was almost kept unchanged as its
initial stiffness. The hysteresis curve of each specimen exhibits slight pinching
phenomenon, indicating the occurrence of debonding and slippage between
the steel tube and the core concrete.
(iii) As the lateral displacement continuously increased, the specimen stepped into
the stage of plastic failure, in which an abrupt breakage failure of the steel
tube was occurred.
Among three specimens of C-12, C-13 and C-14, it is found that II-2 specimen
sustained more loading cycles than that of C-12, C-13 and C-14 column, which
indicates that the pre-damage induced by the blast had significantly impact on the
413
specimens, while it was about 60 mm for C-11 specimen. Therefore, when the
loading direction is parallel to the cratering face, the position of the critical
section or the plastic hinge is not affected by the previous blast induced damage.
(ii) The bending failure mode was also observed in C-12, C-13 and C-14 post-blast
specimens. The location of the critical section (weak section) or plastic hinge
in the column was shifted to the position of the blast induced crater, and the
steel tube tended to fracture at the crater finally. By comparing C-11 with C-12
specimens, it is found that under the identical explosion scenario, the loading
direction had a great influence on the occurrence of the critical section and the
plastic hinge.
12.4.2.2 Hysteresis Curve
The experimental hysteresis curves of the lateral load and its corresponding displacement (P-) of the present specimens are shown in Fig. 12.14. From Fig. 12.14a, it
is found that the hysteresis curve of II-1 specimen showed the unexpected pinching
phenomenon at early loading stage. This might be attributed to the excessive tightness
of the anchor bolt during the test. Hence, in the following part, the hysteresis behavior
of II-1 specimen was not discussed. From Fig. 12.14b to Fig. 12.14f, it is observed
that the hysteresis curves of these specimens exhibited fusiform shape, implying
that the column had good ductility and energy dissipation capacity. In general, the
seismic behavior of each specimen can be divided into three stages, i.e., elasticity,
elastoplasticity and plastic failure.
(i) When ≤ 30 mm, all specimens were in the elastic state, in which the Pcurves showed linearly increasing trend.
(ii) With elevating the lateral displacement loading level, the specimen performed
plastic deformation accordingly. The upper and lower ends of the three
hysteresis loops at the same lateral displacement level were no longer coincided. It is postulated that the accumulated damage of the specimen under
cyclic loading increased, resulting in the large area of yielding. During the
three cycles at the same lateral displacement level, the peak load corresponding
to the latter cycle was lower than that of the previous one, indicating the degradation of the stiffness of column. However, the unloading stiffness of the
column exhibited slight variations, which was almost kept unchanged as its
initial stiffness. The hysteresis curve of each specimen exhibits slight pinching
phenomenon, indicating the occurrence of debonding and slippage between
the steel tube and the core concrete.
(iii) As the lateral displacement continuously increased, the specimen stepped into
the stage of plastic failure, in which an abrupt breakage failure of the steel
tube was occurred.
Among three specimens of C-12, C-13 and C-14, it is found that II-2 specimen
sustained more loading cycles than that of C-12, C-13 and C-14 column, which
indicates that the pre-damage induced by the blast had significantly impact on the