## 4 Optimization and Discussion  
Using the 4 h shadow proposed in the \"Technical Rules for Green Building Evaluation\" [33] as the evaluation objective, the curve in Figure 10 describes the relationship between the height-to-width ratio and the street orientation angle, as the daily sunshine hours is 4 h.  
As shown in Figure 10, when the street orientation angle was 60\({}^{\circ}\)-90\({}^{\circ}\), a height-to-width ratio between 1.13-0.95 enables the street to maintain a reasonable sunshine duration. It is shown that when the street orientation angle exceeds 60\({}^{\circ}\), the effect of height-to-width ratio on sunshine hours is greatly increased. When the street orientation angle is less than 30\({}^{\circ}\), the influence of height-to-width ratio on sunshine hours will decrease sharply. A large height-to-width ratio is required to create sufficient shadows. However, an excessively large height-to-width ratio can create a sense of oppression and hence is not beneficial to constructing favorable streets. When the height-to-width ratio is 1, Figure 11 shows \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Residual Sum of Squares** & **Fit** & **Modified Fit** & **Standard Deviation** \\ \hline
0.05501 & 0.9998 & 0.9997 & 0.07417 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fitting results for \(a\) and \(\theta\). Figure 10: Relationship between height to width ratio and street orientation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Residual Sum of Squares** & **Fit** & **Modified Fit** & **Standard Deviation** \\ \hline
0.01876 & 0.9366 & 0.9251 & 0.0413 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Fitting results for \(b\) and \(\theta\). the relationship between sunshine hours and angle of street orientation. When the street orientation angle is less than 30\({}^{\circ}\), the architectural shadow areas provided by variation the height-to-width ratio will reduce sharply. Hence, it is less meaningful to create shadows by adjusting the height-to-width ratio. Considering the effects of street enclosure, street orientation, and height-to-width ratio on the sunshine environment, Qilou commercial streets in Nanning can be classified into three categories: streets spanning east-west to 30\({}^{\circ}\) northeast are defined as streets in the east-west orientation; streets spanning north-south to 60\({}^{\circ}\) northeast are defined as streets in the north-south orientation; streets spanning 30\({}^{\circ}\) northeast to 60\({}^{\circ}\) northeast are defined as streets in the northeast-southwest orientation. This paper will analyze the sunshine-hours prediction models of the streets in the first two categories. For the north-south streets, the height-to-width ratio exhibits an evident influence on the sunshine hours. Favorable shadow area can be formed by adjusting the height-to-width ratio of Qilou street. The height-to-width ratio for the target value of sunshine hours can be obtained using the fitting expression to optimize the design. As shown in Figures 12 and 13, after the street height-to-width ratio were 0.33 and 0.99 respectively, the sunshine hours in Qilou street decreased from 8 h to approximately 4 h. It can be seen that the ratio of height to width has a significant effect on sunshine hours of the north-south street. Figure 11: Relationship between sunshine hours and street orientation. Figure 12: Height-to-width ratio of 0.33 in north–south street: (**a**) street plane; (**b**) distribution of sunshine hours. The effects of an east-west street and an east-west street with a \"recessed area\" on sunshine hours are shown in Figures 14 and 15, respectively. For the east-west streets, the height-to-width ratio affected the sunshine hours less. Hence, it is not appropriate to increase the height-to-width ratio to create more shadow areas. Because the buildings on both sides contribute less to the increase in shadow area, more shadow areas can be created from the architectural pattern.