PSI - Issue 64

D.B. Zhang et al. / Procedia Structural Integrity 64 (2024) 1997–2004 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

2003

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clouds, which can improve the quality of contour map. Finally, the contour map is drawn by adjusting the contour interval and sampling interval, and checking whether there are problems such as contour line omissions, location errors, contour line intersections and elevation value errors. The contour map for the east facing facade is depicted in Fig. 3(b), while the local contour map for the hollowing region is illustrated in Fig. 3(c). By setting a reasonable elevation value, the maximum height and area of the defect region can be extracted. To analyse the impact of elevation values in the contour map on the recognition of hollowing defects, thresholds of 0mm, 5mm, 10mm, and 15mm are applied to process the point cloud data. The variation of the contour map with different elevation value manifests that the elevation value of 10mm is proper as the threshold for hollowing recognition for the external thermal insulation system of inorganic thermal insulation mortar. 6.2. Analysis of monitoring results To assess the variation law of hollowing characteristics with time, starting from November 10, 2021, the point cloud data of the testing region of east facade was collected once every 30 days based on 3D laser scanning technique to create the contour map, and the maximum height and area of the hollowing region between the 6th and 7th floors would be extracted, as shown in Table 5. Upon initial scanning, the maximum height of the hollowing defect of the external wall was 0.08m, covering an area of 16.141m 2 . Between the 10th and 11th data collection, the super typhoon Hinnamnor landed in Shanghai on September 5, 2022, bringing typhoons and rainstorms, resulting in a decrease of 0.004m in maximum height and an increase of 16.05% in the area of the hollowing region. The typhoon would impose pressure to the hollowing region, causing the defect to expand in all directions. As a result, the maximum height decreased and the area increased. For the last scanning, the maximum height of the hollowing defect was 0.089m, covering an area of 27.113m 2 . Over the course of a year and a half, combined the impact of the super typhoon, the maximum height of the hollowing region of east facade increased by 11.25%, and the area expanded by 67.98%. The continuous hollowing area of the east facade accounts for far more than 0.1%. Based on the risk classification method of falling-off hazards of the building external walls as outlined in Table 3, the east facade is categorized as high risk, and effective measures should be taken immediately to dispose this hazard.

Table 5. Maximum height and area of hollowing region on east facade Number of times Month Maximum height/m Area/m 2 Number of times

Maximum height/m

Month

Area/m 2

1 2 3 4 5 6 7 8 9

November 0.08 December 0.082

16.141 16.416 16.416 16.343 17.181 17.828 18.676 19.726 20.986

10 11 12 13 14 15 16 17 18

September 0.090

22.733 26.382 26.486 26.574 26.761 26.835 26.927 27.071

October

0.086

January March

0.084 0.084 0.084 0.085 0.085 0.085 0.085

November 0.086

December

0.088 0.088 0.088 0.088 0.088 0.089

April May June

January March

April May June

July

August 27.113 To further analyse the variation characteristics of the falling-off hazard over time, a curve representing the changes in hollowing features of the East facade is developed using the measurement data listed in Table 5, as illustrated in Fig. 4. Observing the change curve of the maximum height of the hollowing region in Fig. 4 (a), the maximum height gradually increases. The maximum height of the hollowing defect increases relatively greatly between July and August, with Shanghai experiencing high temperature weather. Examining the variation curve in Fig. 4 (b), the area increases more significantly between May and August compared to other months. The high temperatures in Shanghai during this period (with daily maximum temperatures ranging from 25°C to 39°C) indicate that the hollowing region expands significantly under such hot weather, elevating the risk of a potential falling-off hazard. It should be noted that the recognition results of falling-off hazards of external walls based on 3D laser scanning technique are greatly affected by light, rain, and environmental reflections. Therefore, the ambient light should be uniform, no reflection and consistent colour temperature during data acquisition.