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188.8.131.52 Check of crack width
Check of crack width
The calculation value wk of the crack width is determined according to Equation (7.8) of EN 1992-1-1, clause 7.3.4.
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)
mean strain of concrete between cracks
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
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:
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.
Furthermore, the maximum crack spacing is analyzed according to EN 1992-1-1, Equation (7.15):
angle between reinforcement in x-direction and direction of principal tension stress
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.
For the calculation value of the crack width wk according to Equation 2.73, we need to determine the factor (εsm - εcm) 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):
The maximum mean strain (εsm - εcm)-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 (εsm - εcm): 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 dγ-α is determined as follows for a selected compression strut inclination:
The length is unitless (the perpendicular to the compression strut was included without unit).
Then, the length sγ-α is determined.
If the reinforcement direction θ1 forms the smallest differential angle with the principal moment m1, we have to insert the previously determined difference in the mean strains (εsm - εcm)θ1 of concrete and reinforcing steel for εα:
If the reinforcement direction θ2 forms the smallest differential angle with the principal moment m1, we have to insert the previously determined difference in the mean strains (εsm - εcm)θ2 of concrete and reinforcing steel for εα:
With the Pythagorean theorem, we can determine the value lγ-α from the lengths dγ-α and sγ-α:
Since all formulas are based on an initial length of 1.0 units of length, the strain ε is determined as follows:
This strain ε = (εsm - εcm) 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.
The calculated value of the crack width wk is determined according Equation 2.73.
In window 1.3 Surfaces, we have specified the maximum allowable crack width wk = 0.3 mm. The following criterion of check for the governing resulting direction is thus obtained: