## 4 Results and Discussion  
### Effect of Different Extension Time Differences on the Collapse Process  
A reasonable time difference in the extension has a crucial influence on the blasting effect of the demolished object [26; 27], on the basis of this acid-station workshop model. Only the time difference was changed appropriately, without changing other elements of the model, and the nine time-difference schemes between each row of columns of this small-aspect-ratio reinforced concrete structure were set as follows: 100 ms, 200 ms, 300 ms, 400 ms, 500 ms, 600 ms, 700 ms, 800 ms, 900 ms. Among these scenarios, the blasting effect under a 500 ms time difference was verified. The remaining eight schemes were calculated and simulated separately, and the interfaces of the calculation results of the first, third, seventh and ninth representative schemes were output at an interval of 1 s. Finally, the collapse process ended when the vertical velocity of the structure was less than 0.19 m/s. The four schemes were compared and analyzed, and the collapse processes of the buildings with different delay-time differences are shown in Figure 10.  
From Figures 7 and 10, it can be seen that, according to the numerical analysis results, all nine time-difference design schemes can complete the directional collapse according to the preset direction. These figures show the four stages of the blast-incision formation, destabilization overturning, incision closure, and structural touchdown disintegration of this small-aspect-ratio reinforced concrete structure, and each scheme has different collapse patterns in each stage. The time-course collapse attitude of this small-aspect-ratio reinforced concrete structure blasting-demolition numerical simulation is in high agreement with the actual collapse, and the simulation results are more satisfactory. According to engineering practice and relevant blasting guidelines [28], the plastic hinge is an important parameter in blasting construction and plays a crucial role in the structural-blasting effect. In the short time difference of 0.1-0.3 s, the plastic hinge is not formed between the beams and columns of the A, B, C and D axes in time to make a similar in situ collapse motion in a whole form. Because the beams and columns of the plant are long, after the blast incision at the A, B, C and D axes is detonated, a long cantilever structure will be formed, so that the resisting moment of the beam end at the D and E axes is smaller than the overturning moment generated by the gravity of the upper structure. Because of the short extension time, after the blast incision at the E axis is formed and touches the ground, it will be subject to the vertical upward impact reaction force from the ground, so that plastic hinges will be formed at the ends of the beams at the D and E axes, forming a plastic-deformation zone. As shown in Figure 11a, the bottom of the rear row of columns undergoes a fixed-axis rotation. After entering the collapse stage of the touchdown, the structure will not completely disintegrate because the energy accumulated by the building itself is low, and part of the structure will lose energy after the touchdown. In the time difference of 0.4-0.8 s, with the gradual increase in the postponed time difference, the beam-column nodes have sufficient action time to form plastic hinges, and the rotation of the structure between the rows of columns will provide the horizontal direction velocity component, thus intensifying the overall forward tilt phenomenon of the structure and leading to the flexural plastic hinge at the top of the last column layer, as shown in Figure 11b. In the time difference of 0.4-0.8 s, as the time difference increases, the plastic hinge also has sufficient rotation time to accumulate enough kinetic energy after reaching the state of incision closure, and the plastic hinge dissipates more energy to the extent that the structure disintegrates more completely by touching the ground.