SOFA, How to optimise the design to try and avoid concrete edge failure.

Anchor configurations according to EN 1992-4 arranged close to the edge of a concrete member can be an issue, so in this article we consider how to optimise the design in PROFIS Engineering?

Fastenings consisting of more than one anchor are often installed close to the edge of a concrete member, such as the rectangular anchor group with 4 anchors in Figure 1. Due to the influence of a concrete edge, the controlling failure mode is typically concrete edge failure. Depending on the edge distance, spacing of the anchors, member thickness and clearance hole in the baseplate, the breakout can originate from either the front or the back anchors.

Figure 1 – Illustration of concrete edge failure for an anchor group with 4 anchors arranged close to the edge of a concrete member

EN 1992-4 provides a simplified approach on evaluating the resistance of concrete edge failure of anchor groups (Equation 1). The entire shear load acting on the fixture is assumed to be taken up by the front anchors, which often leads to a conservative calculation of the concrete edge resistance.

Verification of concrete edge failure according to EN 1992-4:

VSd ≤ VRd (c1,1)????????????????????????????????????????????????????????(Equation 1)

Another approach is to assume that the back anchors also participate in resisting the shear load, thereby further increasing the concrete edge resistance and reducing the loads on the front anchors. This approach is provided in the fib bulletin 58 (Design of anchorages in concrete) and is the foundation for the implementation of the SOFA method in PROFIS Engineering. All anchors must be activated simultaneously to use the design method provided in the fib guideline, which requires filling the annular gap between anchor and baseplate during installation.

The fib approach is explained in the following for a group with 4 anchors as illustrated in Figure 2.

If all anchors resist the shear loads, the concrete edge breakout can occur by either:

• Failure from the front anchors if half of the shear load acting on the fixture exceeds the combined resistance of the front anchors (Figure 2a);

or

• The group can fail from the back anchors in case all acting shear loads exceed the combined resistance of the back anchors (Figure 2b).

For the verification of steel failure, the steel resistance of a single anchor is multiplied by the number of anchors (in the illustrated case, n = 4). Thus, the verification of concrete edge failure according to fib bulletin 58 is:

VSd ≤ min(2VRd (c1,1); VRd (c1,2))?????????????????????????????????(Equation 2)

???????? ???a) s1/c1,1 large????????????????????????????????????????????b) s1/c1,1 small

Figure 2 – Example of a group of anchors without hole clearance loaded in shear toward the edge (picture taken from fib bulletin 58)

The approach given by Equation 2 and illustrated in Figure 2 is often called the “shear distribution approach”, since all anchors participate in transferring the acting load into the base material. In the scenario where the resistance of the back anchors is larger than twice the resistance of the front anchors, a crack is generated first from the front anchors as they lose their resistance and, consequently, the shear load is redistributed to the back anchors. This approach also is given in the fib bulletin 58. However, when allowing re-distribution of the shear load, several aspects need to be considered:

1. Verification of serviceability limit state (SLS) at front anchors required
2. For the verification of steel failure only the back anchors are assumed to take the shear load (n = 2)
3. In case of combined tension and shear load, the back anchors are assumed to take the tension load with an eccentricity (see Figure 3)

Figure 3 – Example of a group of four anchors loaded in tension and shear toward the edge where reinforcement to limit the crack width has not been provided: a) action and resistance on group; b) failure of front anchors in shear leading to loss of tension resistance; c) premature failure of group due to unbalanced tension (picture taken from fib bulletin 58)

?In real fastening scenarios, the overall utilisation is often higher in cases when considering redistribution of the shear load to the back anchors. Also, a verification of SLS at the front anchors often leads to a reduction of the resistance to avoid excessive crack opening at the front anchors. Therefore, this approach is not implemented into the SOFA method in PROFIS Engineering.

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