## 4 Discussion  
During the measurement period, the processes such as construction of frames, curtain walls, and lightweight walls were performed according to the as-built construction schedule in Figure 12. The increase in loads during the construction processes could be a major factor affecting strain values measured by sensors. From Figure 11b, it is observed that the measured strain data corresponding to the magnitude of shortenings in a vertical member increased as the construction progressed.  
To investigate the correlation between the measured strains and the increasing weight, the strain data for elapsed days 339, 375, and 414 in Figure 11b were selected. The construction process for the three stages is shown in Figure 13. It was shown that the strains measured on the 10 th floor in tower 2 are approximately 77% higher than the elastic strains, which are calculated on the structural weights, on the 10 th floor in tower 2 for the period from elapsed day 339 to elapsed day 375. In addition, the strains are 69% higher than the elastic strains for the period from elapsed day 339 to elapsed day 414. The measured strains can be larger than the elastic strains because of the influences of creep, shrinkage, load uncertainty, and simulation inaccuracy. Thus, field measurements of the shortening of vertical members in high-rise buildings under construction are necessary.  
As can be seen in Figure 10b, for measurement of differential shortenings between the adjacent vertical members, sensors 43-1 and 45-1 were installed on shear walls, and sensors 43-2 and 45-2 were installed on columns adjacent to sensors 43-1 and 45-1. Using the measured strain data during the period in Figure 11b, magnitudes of accumulated axial shortenings from of the shear wall with sensor 43-1 and the column with sensor 43-2 are 12.15 mm and 11.80 mm (Figure 14), respectively. Then, the differential shortening between the wall and column is found to be 0.35 mm. Similarly, as can be seen in Figure 14, the differential shortening between the wall with sensor 45-1 and the column with sensor 45-2 is obtained. The maximum differential shortening for adjacent walls with sensors 43-1 and 45-1 is shown to be 3.92 mm (Figure 14). Thus, the magnitude of differential shortening, the degree of slab tilt due to the length changes in vertical members, can be controlled by applying compensation methods [7,8].  
Figure 12: As-built construction schedule for tower 2.  
Figure 15 represents the data history for the sensors on the 10 th floor of towers 1 and 2. The amount of measured data fluctuated during the monitoring period because the wireless sensing units in this study employed unidirectional communication and the time synchronization was not performed during the wireless communication. The number of days for which no data are measured during the monitoring period for sensors 41-2, 41-3, 42-1, and 42-2 in tower 1 are as follows: 13 (14.77%), 13 (14.77%), 0 (0%), and 0 (0%). The number of days for which no data are measured during the monitoring period for sensors 43-1, 43-2, 45-1, and 45-2 in tower 2 are as follows: 0 (0%), 0 (0%), 8 (9.76%), and 9 (10.59%). The data loss may occur because of the sleep-action mode of the wireless sensing units, unstable power, breakdowns in the sensing system, or obstacles such as workers and construction materials.  
Figure 14: Differential shortenings for walls and columns in tower 2.  
Figure 13: Simulation models of tower 2 for different days.