# Influence of Porosity on Drag Coefficient of Perforated Road Signs
## 1 Introduction  
Under abnormal weather conditions, such as strong gusts of wind, induced by natural phenomena, road facilities, such as road signs, frequently get damaged. This damage accounts for approximately 8% of all damage to public facilities during natural disasters [1]. Being composed of panel structures, road signs are adversely affected by the wind load that accounts for approximately 96% of all effective loads [2]. In addition, road signs have a clearance area between the ground and the sign, and additional vehicle-induced flow occurs in the clearance area when vehicles are passing by [3]. Therefore, to ensure structural stability of road signs against wind load, signs perforated at regular intervals have been integrated into road sign structural design [4]. The primary design factor that determines the airflow and wind load of a structure is the drag coefficient, which is a dimensionless coefficient used to quantify the resistance of a structure in fluid environments such as air and water. For perforated road signs, the drag coefficient is influenced by the holes in the sign. Therefore, the drag coefficient of the perforated sign should be investigated to determine whether perforated road signs remain adequately stable in incumbent weather conditions. ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) is an American standard that specifies the drag coefficient according to the aspect and clearance ratios, which are the width/height ratio of a sign and the ratio of the height of the sign to that of the ground clearance, respectively. Furthermore, it presents a formula to calculate the drag coefficient reduction factor according to the porosity of the sign. However, ASCE 7-22 has the following limitations for perforated road signs [5]. Although ASCE 7-22 presents the drag reduction factor according to the porosity of the sign, it ignores perforated signs with a porosity of less than 30%. However, in practice, a porosity of less than 20% is required to ensure the legibility of characters [6]. Therefore, the drag coefficient of signs with a porosity less than 30% must be considered. In addition, the sign drag coefficient of ASCE 7-22 is based on the wind tunnel test results for the basic wind speed of 10 m/s. However, the drag coefficient in actual conditions depends on the wind speed as well as the cross-sectional geometry of the structure [7]. Thus, the drag coefficient must be investigated at various wind speeds by reflecting the continuous increase in wind speed caused by abnormal climate. Moreover, the drag coefficient is uniformly applied in Structural Design Actions (AS 1170.2) [8], an Australian standard; the Road Sign Handbook [9], a Japanese standard, and the Road Sign Standard [10], a Korean standard, without considering the porosity of road signs and the wind speed. Several studies on the drag coefficient of perforated panel structures have been conducted. [PERSON] et al. [11] conducted computational fluid dynamics (CFD) analysis to analyze airflow affecting highway road and traffic signs. They analyzed the pressure distribution and stress on the front and rear of the panels and proposed criteria for increasing the wind load coefficient of the design standard of the American Association of State Highway and Transportation Officials by 30%. [PERSON] et al. [12] proposed an optimal value of porosity to improve the aerodynamic behavior of low-rise building parapets. They conducted CFD analysis, and the highest aerodynamic performance was observed in the case of parapets with a porosity of 38.2 to 52.3%. [PERSON] et al. [13] proposed an aerodynamic strategy that optimizes the corners of a building to reduce the drag of the building using the large eddy simulation (LES) method for turbulence analysis. They analyzed the wind load reduction performance according to the building geometry. Korea Expressway Corporation [14] analyzed whether the standard wind speed used in the design of windproof facilities installed on roads in strong wind areas was appropriate.