## 3 Results and Discussion
### Area Reduction of Extreme Pressure Coefficient
#### 3.2.2 Area Reduction Factor of Extreme Pressure Coefficient  
To consider the reduction effect of larger areas on the mean wind pressure coefficient, GB50009 (2012) stipulates that the reduction of the design wind load should be considered through the area reduction method of the local shape coefficient. The local shape coefficient refers to the mean wind pressure coefficient and is given by  
\[\mu_{\text{sl}}(A)=\mu_{\text{sl}}(1)-[\mu_{\text{sl}}(1)-\mu_{\text{sl}}(25)] *\lg A/1.4 \tag{8}\]  
where \(\mu_{\text{sl}}\) (\(A\)) and \(\mu_{\text{sl}}\) (\(1\)) are the local shape coefficients of the area with \(A\) m\({}^{2}\) and not greater than \(1\) m\({}^{2}\) in the cladding, and the area size of \(A\) is less than 25 m\({}^{2}\).  
From Section 3.2.1, it is obvious that there is an area reduction effect on the area-averaged extreme wind pressure. The diameter of the pressure measuring tap provided on the surface of the building model is 1 to 2 mm, and it is generally considered that the measured wind pressure is equivalent to \"point wind pressure\". If converted to the full-scale building, the equivalent area of the pressure tap is usually less than 1 m\({}^{2}\). The minimum area is specified as 1 m\({}^{2}\) in the Chinese code. For cladding with a tributary area  
Figure 11: Comparison of the area-average peak pressure coefficients of each facade for different area size under different wind direction: (**a**) \(0^{\circ}\), (**b**) \(90^{\circ}\), (**c**) \(180^{\circ}\), (**d**) \(270^{\circ}\).  
greater than 1 m\({}^{2}\), the wind resistance design is carried out according to the maximum point wind pressure on the surface of the cladding, which is conservative. The ratio of the extreme pressure coefficient of any area size to that of 1 m\({}^{2}\) is the area reduction coefficient K\({}_{\text{a}}\) of the wind load.  
Figure 12 shows the area reduction factors of the extreme pressure coefficients based on wind tunnel tests and local shape coefficients from the code of the N, E, W, and S directions under different area sizes at 0\({}^{\circ}\), 90\({}^{\circ}\), 180\({}^{\circ}\), and 360\({}^{\circ}\) wind angles, respectively.  
The area reduction factors for the extreme pressure coefficient gradually decrease as the average-areas increase. The results from the comparison show that the reduction effect of the extreme wind pressure is smaller than the area reduction effect of the mean wind pressure. According to GB50009, the extreme wind effect will be underestimated by considering the area reduction of the design wind load of the cladding.  
Figure 13 shows the scatter diagram of the area reduction factor of the area-averaged extreme pressure coefficients with areas of 5 m\({}^{2}\),10 m\({}^{2}\), and 25 m\({}^{2}\) under different wind direction angles. As can be seen from Figure 13, when the subordinate area is 5 m\({}^{2}\),10 m\({}^{2}\), and 25 m\({}^{2}\), the extreme wind pressure area reduction factor is in the range of 0.93-0.99, 0.88-0.98, and 0.85-0.97; the mean value of the area reduction factor is 0.97, 0.95, and 0.93; and the COV is 0.02, 0.03, and 0.04. The normal probability distribution is used to fit the area reduction factor K\({}_{\text{a}}\) of the extreme pressure coefficient. Figure 14 shows the probability plot of the normal distribution of the K\({}_{\text{a}}\) value. The results shown in Figure 14 indicate that the normal distribution is suggested for the probability distribution fitting of the area reduction factor effect.  
Figure 12: The area reduction factors of different area sizes of each facade: (**a**) 0\({}^{\circ}\), (**b**) 90\({}^{\circ}\), (**c**) 180\({}^{\circ}\), (**d**) 270\({}^{\circ}\).  
By transforming Equation (8), the recommended formula of the area reduction factor for the extreme pressure coefficient can be  
\[\text{K}_{\text{a}}(A)=1-0.07*\lg A/1.4 \tag{9}\]  
In the formula, \(A\) is the area size but is less than 25 m\({}^{2}\); K\({}_{\text{a}}\) takes 0.93 if the area size is larger than 25 m\({}^{2}\).