Calculation method: combined tensile force and shear force

In the technical documentation of anchoring manufacturers you will often find the maximum load capacities in the tensile and shear direction. What not every user realizes, however, is that these forces should not occur simultaneously. A load in the tensile direction reduces the absorbable force in the shear direction, and vice versa. In this blog we would like to share how you can take this into account. This guideline is a simplified calculation compared to the ETAG calculator, and provides an excellent approximation of reality.

What is combined tax?

Combined loading occurs when tensile and shear forces act simultaneously on the anchor. With such a combined load, the load capacity of the anchor is reduced. In this situation, check that the load capacity of the anchor is not exceeded. This can be done using the formula below:

(Ts/Ta) ^5/3 + (Ss/Sa) ^5/3 ≤ 1

The following applies:

Ts: actual load in the tensile direction

Ta: permissible load in the tensile direction

Ss: Actual load in the shear direction

Sa: permissible load in the shear direction

A calculation example
Let's assume a Sympafix SB2-PLUS screw anchor 8x75mm in the pressure zone of concrete. The technical documentation states that this anchor can take a maximum of 6.7kN tensile force and 10.9kN shear force. Suppose that in an anchoring situation there is a tensile load of 6.0kN and a shear force of 5.0kN. At first glance it seems that this anchor load is allowed. After all, in both directions of force it is amply less than the maximum absorbable force. However, the use of the above formula paints a different picture:

(6.0/6.7) ^5/3 + (5.0/10.9) ^5/3 = 1.10 > 1.0

The anchor is therefore not sufficient in this situation. A choice for, for example, an anchor of a larger diameter is therefore obvious.

Calculation software for combined tensile and shear forces

In most cases, anchor manufacturers have calculation software. For example, with Hilti® this is PROFIS, with Fischer® C-FIX and with Sympafix Designfix. You can download our software for free. With this calculation software you can easily check whether an anchor is suitable for a load situation, without having to make the calculation yourself.

Would you like to know more about combined tax or our calculation software?

Sympafix has extensive experience in the field of structural anchoring. We have a smart, innovative and unique solution for every construction project. Feel free to contact us at sales@sympafix.com or 072 303 0500 .