## 4 Layer Force Coefficient  
The windward, leeward, and resultant drag force coefficients defined in the current codes are equal to the windward (i.e., \(\mu_{x,\mu}\)), leeward (i.e., \(\mu_{x,l}\)), and resultant (i.e., \(\mu_{x}\)) layer force coefficients under the case of normal approaching wind to the building surface in this  
Figure 8: Distribution of the mean wind pressure coefficients at 55 m on the windward and leeward faces (\(\theta=45^{\circ}\)).  
Figure 7: Distribution of the mean wind pressure coefficients on the windward and leeward faces (\(\theta=45^{\circ}\)).  
study. For the models tested (\(H=100\) m, \(D=30\) m, \(B=30\) m) herein, the corresponding layer force coefficients determined by various codes [20, 21, 22, 23, 24, 25, 26, 27] are tabulated in Table 2. It can be observed that the ASCE, AS/NZS, NBCC, and AJJ codes provide the same layer force coefficients while the EN code and HK, China code give the maximum and minimum values, respectively.  
For a wind incidence angle of \(0^{\circ}\), Figure 9 shows the layer force coefficient on the windward and leeward facades along the height under different rib extensional depths. It can be observed that the windward layer force coefficients (i.e., \(\mu_{x,w}\)) of the model with a smooth facade and \(b/B=1\%\) are almost around 0.8, which is consistent with all the codes' provisions presented herein. The \(\mu_{x,w}\) near the building top is relatively smaller than that in other regions due to the three-dimensional flow effect. As \(b/B\) increases from 1% to 4%, \(\mu_{x,w}\) may decrease by about 28%. When it comes to the leeward surface, \(\mu_{x,l}\) for the models with \(b/B=1\%\) and 2% is close to that of the smooth facade. However, when the model has a rib extensional depth beyond 2% of the building width, \(\mu_{x,l}\) increases with \(b/B\), which may exacerbate wind suction on the leeward facade by about 17%. It is worth noting that the absolute value of the leeward layer force coefficient increases with the height, whereas that suggested by the codes remains constant along the height, leading to underestimation of the wind suction at the building top.  
For a wind incidence angle of \(0^{\circ}\), Figure 10 shows the resultant layer force coefficient along the height under different rib cases. It shows that the resultant layer force coefficient along the height presents a shape of \"7\" and reaches the maximum value at the height of about 0.8 \(H\). The resultant layer force coefficients of all testing cases decrease with the increase in \(b/B\), indicating that the use of surface-attached rib is beneficial to reducing the total layer wind load acting on the building. Compared to the smooth model, the model with the largest rib depth can reduce the layer wind load by about 9% at most. The EN and GB50009 codes provide conservative \(\mu_{x}\) for most cases while the HK, China code may lead to an underestimation of the resultant layer force.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Code** & \(\mu_{x,w}\) & \(\mu_{x,l}\) & \(\mu_{x}\) \\ \hline GB50009-2012 & 0.8 & \(-0.6\) & 1.4 \\ ASCE 7-22 & 0.8 & \(-0.5\) & 1.3 \\ EN 1991-1-4-2005 & 0.8 & \(-0.617\) & 1.417 \\ AS/NZS 1170.2:2011 & 0.8 & \(-0.5\) & 1.3 \\ NBCC 2015 & 0.8 & \(-0.5\) & 1.3 \\ AJJ 2004 & 0.8 & \(-0.5\) & 1.3 \\ HK, China 2019 & - & - & 1.23 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Layer force coefficient regulated in different codes.  
Figure 9: Layer force coefficients along building height on the windward and leeward surfaces (\(\theta=0^{\circ}\)).