# RF-CONCRETE Surfaces – Online Manual Version 5

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## RF-CONCRETE Surfaces – Online Manual Version 5

# 2.6.4.12 Check of crack width

#### Check of crack width

The calculation value w_{k} of the crack width is determined according to Equation (7.8) of EN 1992-1-1, clause 7.3.4.

${w}_{k}={s}_{r,\text{max}}\xb7\left({\epsilon}_{sm}-{\epsilon}_{cm}\right)$

where

s | maximum crack spacing in final crack state (see Equation 2.74 or Equation 2.75) |

ε | mean strain of the reinforcement under governing action combination, including the effects of applied deformations and taking the concrete's effect of tension between the cracks into account (only the additional concrete tensile strain beyond the zero strain at the same level is considered) |

ε | mean strain of concrete between cracks |

_{r,max}

If the spacing of the rebars in the bonded reinforcement is not larger than 5 ⋅ (c + φ/2) in the tension zone, the maximum crack spacing for the final crack state may be determined according to

${s}_{r,\text{max}}={k}_{3}\xb7c+{k}_{1}\xb7{k}_{2}\xb7{k}_{4}\xb7\frac{\varphi}{{\rho}_{p,\text{eff}}}$

If the spacing of the rebars in the bonded reinforcement exceeds 5 ⋅ (c + φ/2) in the tension zone or if no bonded reinforcement is available within the tension zone, the limit for the crack width may be determined with the following maximum crack spacing:

${s}_{r,\text{max}}=1.3\xb7\left(h-x\right)$

The depth of the compression zone x in state II therefore has to be calculated for the check of the crack width. It is determined with the neutral axis depth ξ that is related to the depth of the structural element.

$x=\xi \xb7h=\frac{0.5+{\alpha}_{e}\xb7{\displaystyle \frac{{a}_{s,\text{exist}}}{b\xb7h}}\xb7{\displaystyle \frac{d}{h}}}{1.0+{\alpha}_{e}\xb7{\displaystyle \frac{{a}_{s,\text{exist}}}{b\xb7h}}}$

Furthermore, the maximum crack spacing is analyzed according to EN 1992-1-1, Equation (7.15):

${s}_{\mathrm{r},\text{max}}=\frac{1}{\frac{\mathrm{cos}\theta}{{s}_{\mathrm{r},\text{max},x}}+\frac{\mathrm{sin}\theta}{{s}_{\mathrm{r},\text{max},y}}}$

where

θ | angle between reinforcement in x-direction and direction of principal tension stress |

s | maximum crack spacing in x- or y-direction |

This equation is important if the first method, *By assuming an identical deformation ratio of the longitudinal reinforcement* for determining the design internal forces in the serviceability limit state, has been selected in the *Settings for Analytical Method of Serviceability Limit State Design* dialog box (see Figure 2.89).

In the third method (*By taking into account the deformation ratio of the longitudinal reinforcement*), on the other hand, the direction of the compression strut is determined according to Baumann.
The limit angle of 15° is ignored because the crack width in this area is not governing.

_{sm}- ε

_{cm})

For the calculation value of the crack width w_{k} according to Equation 2.73, we need to determine the factor (ε_{sm} - ε_{cm}) for each reinforcement direction and for the direction of the resulting strain.

The difference in the mean strain of concrete and reinforcing steel is determined according to , clause 7.3.4, Equation (7.9):

${\epsilon}_{sm}-{\epsilon}_{cm}=\frac{{\sigma}_{s}-{k}_{t}\xb7{\displaystyle \frac{{f}_{ct,\text{eff}}}{{\rho}_{\text{eff}}}}\xb7\left(1+{\alpha}_{e}\xb7{\rho}_{eff}\right)}{{E}_{s}}\ge 0.6\xb7\frac{{\sigma}_{s}}{{E}_{s}}$

The maximum mean strain (ε_{sm} - ε_{cm})_{-z,res} is obtained as the resulting mean strain of the individual reinforcement directions with 1.291 ‰.

To simplify the expression, we introduce symbols for the sought mean strain (ε_{sm} - ε_{cm}): *s* for the side length in the reinforcement direction, *d* for the partial length of the compression struts, *l* for the perpendicular to the compression strut, and ε.

The partial length d_{γ-α} is determined as follows for a selected compression strut inclination:

${d}_{\gamma -\alpha}=\frac{1}{\mathrm{tan}\left(\gamma -\alpha \right)}$

The length is unitless (the perpendicular to the compression strut was included without unit).

Then, the length s_{γ-α} is determined.

${s}_{\gamma -\alpha}=\frac{1+{\epsilon}_{\alpha}}{\mathrm{tan}\left(\gamma -\alpha \right)}$

If the reinforcement direction θ_{1} forms the smallest differential angle with the principal moment m_{1}, we have to insert the previously determined difference in the mean strains (ε_{sm} - ε_{cm})_{θ1} of concrete and reinforcing steel for ε_{α}:

${s}_{\gamma -\alpha}=\frac{1+{\left({\epsilon}_{sm}-{\epsilon}_{cm}\right)}_{{\theta}_{1}}}{\mathrm{tan}\left(\gamma -\alpha \right)}$

If the reinforcement direction θ_{2} forms the smallest differential angle with the principal moment m_{1}, we have to insert the previously determined difference in the mean strains (ε_{sm} - ε_{cm})_{θ2} of concrete and reinforcing steel for ε_{α}:

With the Pythagorean theorem, we can determine the value l_{γ-α} from the lengths d_{γ-α} and s_{γ-α}:

${I}_{\gamma -\alpha}=\sqrt{{s}_{\gamma -\alpha}^{2}-{d}_{\gamma -\alpha}^{2}}$

Since all formulas are based on an initial length of 1.0 units of length, the strain ε is determined as follows:

$\epsilon ={I}_{\gamma -\alpha}\xb7-1.0$

This strain ε = (ε_{sm} - ε_{cm}) is checked again by means of the intermediate angle (β - γ).

For the determination of the SLS design internal forces according to the *By assuming an identical deformation ratio of the longitudinal reinforcement* method, the strain ratio of the reinforcements can significantly deviate from the assumed geometric strain ratio.
To correctly determine the resulting strain ratio, the program therefore uses the strain of the reinforcement that is closer to the main action.

_{ k }

The calculated value of the crack width w_{k} is determined according Equation 2.73.

In window 1.3 *Surfaces*, we have specified the maximum allowable crack width w_{k} = 0.3 mm.
The following criterion of check for the governing resulting direction is thus obtained: