10.8 Parametric Study
357
Correspondingly, Fig. 10.27c plots the damage index for the present UHPCC-FST
column with varied steel tube thicknesses. It is clearly that with the increase of
the steel tube thickness, the damage degree of the column under contact detonation
reduces significantly. This demonstrates that the thicker steel tube leads to higher
residual axial capacity of UHPCC-FST column after enduring the contact explosion.
However, it is also observed that, within the present discussed scenario, the strengthening effect of steel tube thickness is reduced when the steel tube thickness is greater
than 6 mm.
10.8.1.2 Strength
Four various yield strength levels for steel tube are further considered, i.e., 235, 500,
800 and 1100 MPa, and the remaining parameters are kept identical with those of
column C-2. Similarly, Fig. 10.28 further shows the blast formed crater depth, the
residual axial load bearing capacity, as well as the corresponding damage index of
present UHPCC-FST column with varied steel tube strength. It indicates that, (i)
the crater depth decreases with the increase of steel tube strength, when the steel
tube strength increases from 235 to 318.5 MPa, the corresponding crater depth is
reduced by 17.4%, while such tendency becomes smoother beyond the threshold
value of 318.5 MPa. This indicates that further increasing the steel tube strength
cannot significantly reduce the crater depth; (ii) an approximately linear increasing
curve in Fig. 10.28b indicates that higher strength of the steel tube enhances the
axial load bearing capacity of UHPCC-FST column effectively; (iii) the damage
index of the present column under contact detonation drops significantly within the
strength range from 235 to 318.5 MPa. However, the effect of the steel tube strength
on reducing the damage degree of UHPCC-FST column gradually weakens with
further increasing the steel tube strength.
10.8.2 Core Concrete Strength and Cross-Sectional Diameter
10.8.2.1 Compressive Strength
Five strength levels of the core concrete are further selected, i.e., 30, 45, 60, 75 and
170 MPa, while the other parameters are kept identical with those of C-2 column. It
should be noticed that the strength levels of 30–75 MPa represents the NSC, whereas
the compressive strength of 170 MPa is considered as UHPCC. As mentioned above,
UHPCC performed superior compressive and tensile fracture strain energy compared
to the NSC. Therefore, the fracture strain energy of 0.12 MPa (i.e. the area under
the modified numerical post-peak compressive stress-stain curve in Fig. 10.17a) is
assigned to the UHPCC. Besides, according to CEB (CEB-FIP 2008), the fracture
strain energy of the NSC is decreased from 0.041 MPa to 0.011 MPa when the
concrete strength increases up to 110 MPa, respectively.
357
Correspondingly, Fig. 10.27c plots the damage index for the present UHPCC-FST
column with varied steel tube thicknesses. It is clearly that with the increase of
the steel tube thickness, the damage degree of the column under contact detonation
reduces significantly. This demonstrates that the thicker steel tube leads to higher
residual axial capacity of UHPCC-FST column after enduring the contact explosion.
However, it is also observed that, within the present discussed scenario, the strengthening effect of steel tube thickness is reduced when the steel tube thickness is greater
than 6 mm.
10.8.1.2 Strength
Four various yield strength levels for steel tube are further considered, i.e., 235, 500,
800 and 1100 MPa, and the remaining parameters are kept identical with those of
column C-2. Similarly, Fig. 10.28 further shows the blast formed crater depth, the
residual axial load bearing capacity, as well as the corresponding damage index of
present UHPCC-FST column with varied steel tube strength. It indicates that, (i)
the crater depth decreases with the increase of steel tube strength, when the steel
tube strength increases from 235 to 318.5 MPa, the corresponding crater depth is
reduced by 17.4%, while such tendency becomes smoother beyond the threshold
value of 318.5 MPa. This indicates that further increasing the steel tube strength
cannot significantly reduce the crater depth; (ii) an approximately linear increasing
curve in Fig. 10.28b indicates that higher strength of the steel tube enhances the
axial load bearing capacity of UHPCC-FST column effectively; (iii) the damage
index of the present column under contact detonation drops significantly within the
strength range from 235 to 318.5 MPa. However, the effect of the steel tube strength
on reducing the damage degree of UHPCC-FST column gradually weakens with
further increasing the steel tube strength.
10.8.2 Core Concrete Strength and Cross-Sectional Diameter
10.8.2.1 Compressive Strength
Five strength levels of the core concrete are further selected, i.e., 30, 45, 60, 75 and
170 MPa, while the other parameters are kept identical with those of C-2 column. It
should be noticed that the strength levels of 30–75 MPa represents the NSC, whereas
the compressive strength of 170 MPa is considered as UHPCC. As mentioned above,
UHPCC performed superior compressive and tensile fracture strain energy compared
to the NSC. Therefore, the fracture strain energy of 0.12 MPa (i.e. the area under
the modified numerical post-peak compressive stress-stain curve in Fig. 10.17a) is
assigned to the UHPCC. Besides, according to CEB (CEB-FIP 2008), the fracture
strain energy of the NSC is decreased from 0.041 MPa to 0.011 MPa when the
concrete strength increases up to 110 MPa, respectively.