Issue 53

D. Wang et alii, Frattura ed Integrità Strutturale, 53 (2020) 236-251; DOI: 10.3221/IGF-ESIS.53.20 242 The new cracks were 16-18 cm away from the original cracks. The cracks 1# and 3# on the east side and the cracks 2# and 3# on the west side propagated to the center of the column, while the original cracks became wider. On the edge of the plate, multiple cracks grew deeper into the plate. The crack width on the edge of the plate fell between 0.12 and 1.40 mm. From 38 to 39 min, the load climbed up from 150 to 175 kN. During this process, four new cracks emerged on the east long side, and four on the west long side. The new cracks were 16-22 cm away from the original cracks. The original cracks quickly widened and grew deeper into the plate, giving off a clear cracking sound. On the east and west sides, several cracks penetrated the edges of plate-column nodes. The areas of plate-column nodes were cracked and developed into a closed diamond shape with wide cracks. The crack width on the edge of the plate fell between 0.18 and 2.2 mm. From 49 to 50 min, the load continued to rose from 175 to 200 kN. During this process, two new cracks emerged on the east long side, and one on the west long side. In the areas of plate-column nodes, cracks developed quickly and became rather wide (10 mm). The original cracks widened quickly. The crack width on the edge of the plate fell between 0.38 and 2.8 mm. Fig. 8 presents the crack development at room temperature. The furnace was ignited at 105 min. From 114 to 118 min, the furnace temperature was between 654 and 727 °C. Six popping sounds were heard consecutively, due to concrete spalling. At 119 min, the furnace temperature increased to 736 °C. Water stains started to appear simultaneously in the cracks on the north and south sides of the plate, and seeped to the short sides. Meanwhile, two concrete spalling sounds were heard. At 121 min, the furnace temperature was 763 °C. Four sounds of concrete spalling were heard; lots of water stains appeared in the cracks on plate surface, and seeped towards the short sides; water vapor escaped from the relatively width cracks (Fig. 9). One concrete spalling sound was heard respectively at 131 and 133 min. At 140 min, the furnace temperature rose to 857 °C. The plate height was distorted at the top of the column. At 252 min, the furnace temperature was 1,078 °C, and the water stains on the plate surface were basically evaporated. At 315 min, the fire was turned off. Fig. 10 displays the distribution of cracks in the thickness direction on the long sides of the plate. It can be seen that, in the thickness direction, longitudinal cracks developed and extended to 3/4 of plate thickness, without penetrating the plate. The spacing of the cracks was basically the same as the steel bars on the plate. After flameout, the cracks on the top of the plate radiated from the center to the edges (Fig. 11). (a) S1 (b) S2 (c) S3 Figure 8: The crack development at room temperature. Figure 9: Water stains and water vapor on the plate surface.