## 1 Introduction
\(k_{2}\) is equal to 0.5 for members under bending and 1.0 for those under pure torsion (Figure 3). Figure 2: Calculation of the effective area to determine the flexural crack spacing, according to NS 3473E 2003 [54]. The crack spacing \((W_{k})\) is calculated as follows:  
\[W_{k,EC2}=\beta_{EC2};W_{m,EC2}=S_{m,EC2}\xi\varepsilon_{\xi 2}, \tag{6}\]  
where \(W_{k,EC2}\) is the design crack spacing, \(S_{m,EC2}\) the crack spacing according to Equation (3) and \(\xi\) is a dimensionless coefficient between 0 and 1. \(\varepsilon_{\xi 2}\) is the average strain under external load combinations. \(\beta_{EC2}\) is the average cracking ratio for design purposes  
CEB-FIP [56] suggests a different formula for predicting the crack spacing. According to this standard, the crack spacing can be obtained utilising the formula below:  
\[S_{m,CEB}=\frac{2}{3}l_{s,max}, \tag{7}\]  
where \(l_{s,max}\) is the length of tension rebar's slip, which can be calculated using Equations (8) and (9) for the complete cracking and first crack, respectively:  
Figure 1.0  
Figure 1.1  
where \(\sigma_{s2}\) is the rebars' tension stress at cracking (MPa); \(d_{b}\) is the rebar's diameter (mm); \(\tau_{b}\) is the mean value of the boundary stress (MPa), which is equal to \(1.8f_{ctt(t)}\), where \(f_{ctt(t)}\) is the average value of concrete's tensile strength at cracking (MPa). \(\rho_{l_{CEB}}\) is the effective rebar's area, equal to \(A_{st}/A_{ct}\), where \(A_{st}\) and \(A_{ct}\) are the longitudinal rebar's area and the concrete's effective cross-sectional area, respectively. Furthermore, to simplify Equation (9), the value of \((1+\alpha)\)can be considered equal to 1.0, where \(\alpha\) is the ratio between the moduli of the elasticity of steel and concrete \(\left(\frac{E_{s}}{E_{c}}\right)\). CEB-FIP [56] offers the following equation to determine the flexural crack spacing:  
Figure 1.2  
where \(\varepsilon_{cs}\) is the free concrete shrinkage strain, \(\varepsilon_{s72}\) is the rebar's stress due to the cracks under an equivalent load \(f_{ctt}\), which can be calculated as \(A_{cef}\varepsilon_{s72}=f_{ctt}/A_{cef}\Big{(}1+\alpha\rho_{l_{CEB}}\Big{)}\). Furthermore, \(\beta_{CEB}\) is a dimensionless constant and \(\varepsilon_{s2}\) is the steel's strain, which can be ignored in the transition section. Figure 1.3 The effective area [55]. Using ACI 224R-14 [57], the maximum crack spacing in the beams and slabs is calculated based on static analysis. Thus, the maximum bending crack spacing can be calculated using the following formula:  
Figure 1.4  
where \(W_{k,ACI}\) is the maximum crack spacing (mm), \(f_{s}\) is the steel rebar's stress (MPa), \(d_{c}\) is the cover of the outer tensile rebar's layer (mm) and \(A\) the symmetrical concrete's cross-sectional area (mm\({}^{2}\)). \(\beta_{ACI}\) is a ratio equal to 1.2 times the space between the horizontal axis and the concrete's outer tensile layer divided by the distance between the horizontal axis and the longitudinal rebars in the RC beam, as shown in Figure 1.5  
According to ACI 224R-14 [57], the moment crack spacing restricts the rebars stress \(f_{s}\) at the cracked section under a load of 2/3\(f_{y}\). Furthermore, the distance between the tensile rebar's and concrete's outer tensile layer (\(S\)) should not exceed the following value:  
Figure 1.6  
where \(C_{c}\) is the minimum distance between the surface tensile rebar's or prestressed rebar's and the concrete's outer tension layer.