12.4 Low-Frequency Cyclic Loading Test
419
12.4.2.5 Stiffness Degradation
Based on experimental results, it is found that under cyclic loading test, the stiffness
of the post-blast UHPCC-FST column was degraded. According to the Code for
seismic test methods of buildings (JGJ/T101-2015 2015), the secant stiffness K i of
specimen under the ith displacement loading level is expressed as
K i =
m
j=1
(
+P i,j
+
−P i,j
)
m
j=1
(
+ i,j
+
− i,j
)
(12.1)
where m is the mth cycle in the ith lateral displacement loading level, positive and
negative sign represent the push and pull direction in the cycle, respectively. P i,j and
i,j are the lateral load and its corresponding lateral displacement for the jth cycle
at the ith displacement level, respectively.
Figure 12.18a shows the degradation of stiffness of the specimens at each lateral
displacement level. It indicates that, (i) The stiffness of all the specimens degrade
correspondingly with increasing the lateral displacement. The reason lies in that, with
the increase of lateral displacement, the buckling of the steel tube tends to develop at
the plastic hinge, and then the rupture of the steel tube occurs; (ii) Under the identical
loading scenario, i.e., II-2, C-12, C-13 and C-14 specimens, the higher TNT charge
weight causes the lower stiffness of the post-blast column. Under the same TNT
charge weight, the stiffness of the post-blast column with loading direction being
perpendicular to the cratering face (C-12) is lower than that of loading direction being
parallel to the cratering face (C-11). This is because that when the crater located at the
tension and compression sides along the loading direction, the stiffness of column is
0
30
60
90
120 150 180 210
0.0
0.4
0.8
1.2
1.6
2.0
2.4
II-2
C-11
C-12
C-13
C-14
K
i /kN·mm
-1
/mm
0
30
60
90
120 150 180 210
0.0
0.2
0.4
0.6
0.8
1.0
II-2
C-11
C-12
C-13
C-14
K
R
/mm
(a)
(b)
Fig. 12.18 Stiffness degradation of specimens a stiffness versus displacement b relative stiffness
versus displacement, reprinted from Wang et al. (2020b), copyright 2020, with permission from
Elsevier
419
12.4.2.5 Stiffness Degradation
Based on experimental results, it is found that under cyclic loading test, the stiffness
of the post-blast UHPCC-FST column was degraded. According to the Code for
seismic test methods of buildings (JGJ/T101-2015 2015), the secant stiffness K i of
specimen under the ith displacement loading level is expressed as
K i =
m
j=1
(
+P i,j
+
−P i,j
)
m
j=1
(
+ i,j
+
− i,j
)
(12.1)
where m is the mth cycle in the ith lateral displacement loading level, positive and
negative sign represent the push and pull direction in the cycle, respectively. P i,j and
i,j are the lateral load and its corresponding lateral displacement for the jth cycle
at the ith displacement level, respectively.
Figure 12.18a shows the degradation of stiffness of the specimens at each lateral
displacement level. It indicates that, (i) The stiffness of all the specimens degrade
correspondingly with increasing the lateral displacement. The reason lies in that, with
the increase of lateral displacement, the buckling of the steel tube tends to develop at
the plastic hinge, and then the rupture of the steel tube occurs; (ii) Under the identical
loading scenario, i.e., II-2, C-12, C-13 and C-14 specimens, the higher TNT charge
weight causes the lower stiffness of the post-blast column. Under the same TNT
charge weight, the stiffness of the post-blast column with loading direction being
perpendicular to the cratering face (C-12) is lower than that of loading direction being
parallel to the cratering face (C-11). This is because that when the crater located at the
tension and compression sides along the loading direction, the stiffness of column is
0
30
60
90
120 150 180 210
0.0
0.4
0.8
1.2
1.6
2.0
2.4
II-2
C-11
C-12
C-13
C-14
K
i /kN·mm
-1
/mm
0
30
60
90
120 150 180 210
0.0
0.2
0.4
0.6
0.8
1.0
II-2
C-11
C-12
C-13
C-14
K
R
/mm
(a)
(b)
Fig. 12.18 Stiffness degradation of specimens a stiffness versus displacement b relative stiffness
versus displacement, reprinted from Wang et al. (2020b), copyright 2020, with permission from
Elsevier