In the case of subsequently added concrete components, it is necessary to design the transfer of shear force between the different concreting sections. These so-called shear joints occur in concrete components of different ages. Here, for example, connection joints between construction stages in new buildings or renovations or joints between precast elements and in-situ concrete additions need to be considered.
The shear force transfer should be designed as follows. EC 2 [1], Section 6.2.5, Equation (6.23)
There are two options for the calculation of the shear stress in the shear joint:
1. Calculation from Vz,Ed and β-Factor
Here, vEd,i is the design value of the shear force to be absorbed per unit length in the joint. This value is determined by [1] Equation (6.24).
|
z |
Lever arm of the composite section |
|
|
Quotient from the longitudinal force in the concrete topping and the total longitudinal force in the compression or tension zone in the considered cross-section |
|
|
Contact joint width |
|
|
Design value of the acting shear force |
The design value of the shear capacity vRd,i is determined using the following equation [1], (6.25).
|
c, |
Coefficients describing the joint roughness according to [1], 6.2.5 |
|
fctd |
Design value of the concrete tensile strength according to [1], 3.1.6 |
|
|
Smallest stress perpendicular to the joint, which acts simultaneously with the shear force (positive for compression), where σn < 0.6 ⋅ f_cd |
|
|
As/ Ai |
|
|
Inclination angle of the joint reinforcement |
|
|
Strength reduction factor according to [1], 6.2.2 (6) |
2. General Stress Integration
The calculation of the existing longitudinal force difference in the cross-section addition is performed for this option using a general stress integration.
The rigid bond assumed for the design of shear joints in the ULS should be primarily reached by an adhesive bond, that is, adhesion and micromechanical gearing. Hence, the joint reinforcement is responsible for the transfer of forces after overcoming the rigid bond, as well as for the ductility of the connection, while the shear joint would have to be designed solely for the adhesive bond.
Current standards only take this approach into account to a limited extent. A movable bond is allowed, it is conservatively differentiated and complemented by construction rules, to be on the safe side.
For shear joints that are designed for a movable bond in the ultimate limit state, it is necessary to carry out the serviceability limit state design. In this case, the moveable bond must be consistently included in the determination of the internal forces and stresses in the ULS and SLS.
Self-equilibrating stresses normally involving shear stresses in the joint (due to the varying shrinkage behavior of two concrete components of different ages, for example) are generally not considered. The acting shear force vEd,i is calculated exclusively from internal forces at the cross-section.
![Display of shear stresses on joint surfaces in reinforced concrete. (Source [2])](/en/webimage/061617/5148879/2026-02-27_14-16-31.png)
The image above (Source: [2]) displays a section of length dx from a beam with a shear joint parallel to the component axis. Here, the variable bending moment causes a change in the flange forces along the length. This applies, for example, to a compression flange.
There is an equilibrium between the compression force change and the shear stresses in the joint.
Thus, for a constant lever arm z, the stress of the shear joint is in proportion to the shear force VEd, with a constant axial force having no influence on the shear force in the joint parallel to the component axis.
If the shear joint lies within the compression zone, only the portion of the flange force difference between joint and compression flange edge must be transferred. As a result, τEd becomes:
.
![Design Model for Bonded Joint Resistance According to [1]](/en/webimage/009526/2419234/01-en-png.png?mw=760&hash=7ec548270c812e4679005b311ca774da4860d51a)