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9 Dynamic Responses of Reinforced UHPCC Members Under …
As shown in Table 9.5, under the same impact energy, the plateau impact force
on UHPCC specimens increases as axial force increasing. Thus, the global bending
stiffness can be strengthened with a larger axial force. Typically, the confinement
effect induced by the presences of axial force and spiral stirrups can improve the
impact resistance of RC columns. Considering that the stirrup of present specimens is constructional reinforcement, which is deficient for confinement effect, the
compressive membrane action (or called arching effect) may be the mechanism of
axial force (Fan et al. 2019). With the increase of axial force, the horizontal force of
the arch increases, and the dynamic bending stiffness of the specimen is improved. It
is generally considered that the impact resistance of specimens can be reflected by the
values of the plateau impact force, and thus the values of impact force platform are
related to the ultimate bearing capacity of static loading. According to the Chinese
Standard GB 50010-2010 (2010), the ultimate shear capacity of NSC and flexure
capacity UHPCC specimens are 273 and 446 kN, and the corresponding plateau
impact forces listed in Table 9.5 are slightly larger due to the strain rate effect.
9.3.3 Deflection-Time History
The deflection-time histories of all specimens are shown in Fig. 9.9. Generally, the
deflection-time history curve can be divided into two stages in Fig. 9.9c. (i) Forced
vibration stage under impact, in which deflection of tracing points goes through the
maximum value and decreases until the end of impact process; (ii) Free vibration
stage after impact, in which the specimen vibrates freely and then maintains a stable
position. Besides, considering that the tracing points are symmetrical about middle
span of specimens, i.e., points B and D, points C and E, and thus only the deflectiontime histories of tracing points A, B and C are shown in Fig. 9.9. In addition, there are
two characteristic values on the deflection-time history curve, i.e., the maximum and
residual mid-span deflections given in Table 9.5, which are usually used to describe
the impact response of the specimen.
Figure 9.10 illustrates the instantaneous deformation shape of four typical specimens without the axial force. As for the NSC specimens, due to the larger fragmentation close to fixed end, both the specimens “N-2-AF0” and “N-3-AF0” exhibit
larger deflections at the fixed support side. With the increase of impact energy from
16.7 to 25.0 kJ, the maximum mid-span deflections of NSC specimens increase to
72.6 from 50.6 mm correspondingly. The relatively slight influence of support on
specimen deformation can be found with larger impact energy, and the deflection of
UHPCC specimens are comparably symmetric.
Figures 9.11 and 9.12 further show the maximum and residual mid-span deflections of specimens according to various concrete type and axial force levels, respectively. It can be concluded that, (i) the deflection of UHPCC specimen is almost half
that of NSC specimen under the same impact energy; (ii) with the impact energy
increasing, the deflection of specimens with two concrete types increases and the
ratio of the maximum to the residual mid-span deflection ( 1 // 2 ) is almost kept
9 Dynamic Responses of Reinforced UHPCC Members Under …
As shown in Table 9.5, under the same impact energy, the plateau impact force
on UHPCC specimens increases as axial force increasing. Thus, the global bending
stiffness can be strengthened with a larger axial force. Typically, the confinement
effect induced by the presences of axial force and spiral stirrups can improve the
impact resistance of RC columns. Considering that the stirrup of present specimens is constructional reinforcement, which is deficient for confinement effect, the
compressive membrane action (or called arching effect) may be the mechanism of
axial force (Fan et al. 2019). With the increase of axial force, the horizontal force of
the arch increases, and the dynamic bending stiffness of the specimen is improved. It
is generally considered that the impact resistance of specimens can be reflected by the
values of the plateau impact force, and thus the values of impact force platform are
related to the ultimate bearing capacity of static loading. According to the Chinese
Standard GB 50010-2010 (2010), the ultimate shear capacity of NSC and flexure
capacity UHPCC specimens are 273 and 446 kN, and the corresponding plateau
impact forces listed in Table 9.5 are slightly larger due to the strain rate effect.
9.3.3 Deflection-Time History
The deflection-time histories of all specimens are shown in Fig. 9.9. Generally, the
deflection-time history curve can be divided into two stages in Fig. 9.9c. (i) Forced
vibration stage under impact, in which deflection of tracing points goes through the
maximum value and decreases until the end of impact process; (ii) Free vibration
stage after impact, in which the specimen vibrates freely and then maintains a stable
position. Besides, considering that the tracing points are symmetrical about middle
span of specimens, i.e., points B and D, points C and E, and thus only the deflectiontime histories of tracing points A, B and C are shown in Fig. 9.9. In addition, there are
two characteristic values on the deflection-time history curve, i.e., the maximum and
residual mid-span deflections given in Table 9.5, which are usually used to describe
the impact response of the specimen.
Figure 9.10 illustrates the instantaneous deformation shape of four typical specimens without the axial force. As for the NSC specimens, due to the larger fragmentation close to fixed end, both the specimens “N-2-AF0” and “N-3-AF0” exhibit
larger deflections at the fixed support side. With the increase of impact energy from
16.7 to 25.0 kJ, the maximum mid-span deflections of NSC specimens increase to
72.6 from 50.6 mm correspondingly. The relatively slight influence of support on
specimen deformation can be found with larger impact energy, and the deflection of
UHPCC specimens are comparably symmetric.
Figures 9.11 and 9.12 further show the maximum and residual mid-span deflections of specimens according to various concrete type and axial force levels, respectively. It can be concluded that, (i) the deflection of UHPCC specimen is almost half
that of NSC specimen under the same impact energy; (ii) with the impact energy
increasing, the deflection of specimens with two concrete types increases and the
ratio of the maximum to the residual mid-span deflection ( 1 // 2 ) is almost kept