Center-to-center distance and edge distance: this is how you calculate the reduction factor

To what extent is the load capacity of an anchor influenced by a small edge distance, or by a small center-to-center distance between anchors? That question is asked regularly.

If we assume fixings in concrete, the center-to-center distance (Center distance) and edge distance can indeed play a role in the load capacity. A possible form of failure in case of overload is concrete cone fracture or concrete edge fracture. If two anchors, eg chemical anchors , are placed so close to each other that the theoretical concrete cones overlap, there is on balance less concrete to fail than in a situation where the cones do not overlap. And that obviously has a negative influence on the load capacity.

In order to determine to what extent the load capacity is reduced, it is important to distinguish between two concepts:

1. The characteristic HOH distance or edge distance. This is the distance at which no correction to the load capacity has to be made. The anchors are therefore far enough apart (or far enough from the edge) not to be 'affected' by it.
   
   
2. the minimum HOH distance or edge distance. This means exactly what the term suggests: it is not allowed to fall below this distance, not even with the application of a reduction factor. One possible reason is, for example, that at small edge distances cracking can occur in the concrete with expanding anchors, making the load capacity unpredictable. And if there's one thing we hate about constructive anchors, it's unpredictability.

Calculate the reduction factor for a reduced edge and center-to-center distance

If the actual HOH distance or edge distance is greater than or equal to the given characteristic value (Scr and Ccr respectively), then the load capacity does not have to be reduced. However, if the actual HOH distance or edge distance becomes smaller than this characteristic value, the permissible load capacity must be reduced. Keep in mind that the actual HOH distance or edge distance may therefore never be lower than the minimum value (Cmin and Smin).

The reduction factor for a reduced HOH distance or edge distance can be calculated by summarizing the relevant reduction factors from the ETAG001 Annex C in one simple formula:

Reduction factor Rand Fc: 0.35 + (C/Scr) + 0.6 * (C ² /S ² cr)

Reduction factor heart to heart Fs: (1.0 + S/Scr) * 0.5

Whereby applies:

• Scr: characteristic centre-to-centre distance
• Smin: minimum centre-to-centre distance
• Ccr: characteristic edge distance
• Cmin: minimum edge distance

A calculation example
Let's assume a Sympafix STAM Sleeve Anchor M8x55mm in the pressure zone of concrete. The technical documentation states that this anchor can take up to 6.5 kN tensile force. It can also be found here that the characteristic HOH distance is 180mm and the characteristic edge distance is 90mm (the minimum values are respectively 90mm and 60mm). Suppose that in an anchoring situation there are two anchors with an HOH distance of 150mm.

The load capacity can then be calculated as follows:
(1.0 + S/Scr) * 0.5 = (1.0 + 150/180) * 0.5 = 0.92

For two anchors with a mutual distance of 150 mm, the load capacity is as follows:
2 * 6.5 * 0.92 = 11.9 kN

Would you like to know more about the center-to-center distance or the edge distance?

Sympafix likes to think along with you. We have a constructive solution for every construction project. Feel free to contact us at sales@sympafix.com or 072 303 0500.