Structural analysis of cable supports in insulation

Cable holders are "cantilever arms" in the sense of "engineering mechanics", i.e. projecting members. The cables for the greenery are mounted at the outer end of these cantilever arms. Due to the tension in these cables, the weight of the plants, water on the leaves, storms and, where applicable, vandalism, This generates forces of approx. 50 kg (statically correct: 500 N, as in ‘Newton’). Multiplying this by a wall distance/lever arm of, for example, 10 cm or 0.1 m produces so-called ‘bending moments’ of approx. 50 Newton metres (Nm).

Cable holders for insulation are almost always fixed to the load-bearing wall – that is, behind the insulation layer – using composite mortar. A threaded rod fixed in this way is, from a structural point of view, effectively a ‘clamped cantilever’. As the cable holder is fixed only to the inner, load-bearing wall, the insulation layer can be regarded as air. With a 10 cm wall clearance on the outside of the insulation and 15 cm insulation thickness, the cable holder effectively has a “wall clearance” of 25 cm, and this length is now subjected to bending stress! This results in a bending moment which, due to the longer lever arm, is approximately 2.5 times greater than in walls without insulation (see above).

As already described on the ‘Tips for cable holders’ page, cable holders are always subject to particular stress, e.g. due to the pre-tension of the cables. Holders for metal mesh or wooden trellises generate lower bending moments and are therefore less critical. For this reason, the focus below is primarily on cable holders. Three structural issues arise with these; they are discussed below along with their solutions.

The threaded shaft on our mounts and anchor bolts reaches as far as the cable carrier end (all the way up to the cross-head where the wire rope is being attached). The thicker the insulation panel in which the bolt is mounted, the more it will bend under the load of the cables and climbing plant until it buckles under the weight! The bolt is secured in the masonry under the insulation panels (in the load-bearing wall) and not held by the insulation. So, with a 10 cm insulation and with a distance of 7 cm between the cables and the façade, the length of the cantilever is actually 17 cm in total!

The most obvious solution for more stability is to first shorten the mount (the anchor bolt) to reduce the leverage effect. Choose then reduced wall distances (between the wire rope and the facade). This principle is applied to many of our ETICS wall fixings.

The next measure would be to choose a bolt with a thicker thread diameter (e.g. M16 instead of M12), discouraging the bend-factor. Forming a support cone would also be a way to 'thicken' the anchor by distributing the load better.

The third possibility for more stability is to reduce the forces (loads) acting on the anchor mount by reducing the tension of the cables. For this reason, most of our wall mounts permit only 3 mm and 1.8 mm 'cables' (more like 'wire') for thick insulation. In this way, if an excessive load is applied, the weak point of the system is shifted from mount to rope, and the wire rope will break before the wall mount does.

A fourth measure is to reduce the clamping force on the wire rope. This means the rope is clamped less tightly, so that under load it is more likely to ‘slip’ within the clamping head. This provides overload protection – the wall bracket is subjected to less stress. This measure can be implemented, for example, by tightening the grub screws in the clamping head more gently, using less force or “torque”. As this is difficult to define in practice, FassadenGrün has developed a “safety cross-head” that fits an M12 threaded rod but has only an M8 thread for the crucial grub screw. Small grub screw >>> short Allen key >>> less leverage >>> less clamping force. This effect is enhanced by using a grub screw with a flat clamping surface rather than one with a ‘ring cut’.

The further (in the wall) the insertion point, the less elongation and leverage. With the length of the cantilever shorter, the beding-buckling point is shifted and reduced. We use small und large support cylinders for insertion in the insulation panels.

Instead of reinforcing the entire insulation layer with a solid panel, as shown in the diagram above, you can use individual small or large supporting bodies in the insulation board which will be screwed to the facade. From a mechanical engineering perspective, these supports act as cantilevers. Due to the tension in the wire ropes during storms, etc.. forces of up to 100 kg (statically correct: 1,000 N) arise there. This, multiplied by a wall distance / lever of 10 cm or 0.1 m creates so-called "bending moments" of about 100 Newton metres (Nm). This high bending moment load can be absorbed by the supporting bodies (fixation cylinders) without deformation, and redirected to the facade.

By design our wire rope mounts are "cantilever" in the sense of engineering mechanics. The tensioned  cables put loads of 250 kg or more (2.500 Newton) on the bolts, and with a wall distance of 10 cm the bending moment will be of 250 Nm (newton metres). This heavy bending load must be distributed in the wall without any deformation. This is difficult if the entry point is on the outer border of the insulation panel! The entry point has to be rigid.

A new problem arises by moving the bending point outward. The supporting bloc are pressed and screwed together so that tension is applied to the threaded shaft. The thinner a threaded bolt is, the more it will be elongated through this thension and from the load applied by the wire rope. The elongated shaft deforms and even wide wall mounts will deform and can't be fixed at their base.

To avoid elongation, the threaded shaft can be partially embedded in composite mortar. The thread is cast in a bloc of epoxy resin. Lateral stability is thus increased, the threaded shaft can't move or bend inside the insulation panel. This solution is used in the "XP" wall mount series and described further in the page on supporting cones as well as in the combination method.