- Device and design features of anchor fasteners
- Effort calculation methodology
- Selection of anchors depending on the base material
- Mechanical anchoring rules
- Use of chemical anchors
The choice of a method for attaching critical structures to the bases is not always obvious, and yet there are quite universal options that can be used in different conditions. In the article we will talk about the types of anchoring systems: chemical and mechanical.
Device and design features of anchor fasteners
In a broad sense, an anchor is a two-piece rod, one part of which deforms the other in order to expand and anchor in the hole tightly enough so that the entire structure can withstand a load comparable to its deformation threshold.
Frame anchors can be considered as an example, they are the most popular. The design of the fastener includes a tube with a metal rod inside, at one end of which there is a thread, and at the other – a head for a hex or Phillips screwdriver. The tube has longitudinal slots; a wedge-shaped element with an internal thread and external slots is installed in them, which prevents rotation when tightening. As the rod rotates, the wedge nut shifts axially and crushes the tube, expanding it in the hole.
Bolt anchors have the same principle of operation, but a different design: the wedge-shaped expansion is located on the rod, and the nut is located at its threaded end in the visible part of the fastener. Such anchors are used in cases where the dimensions of the protruding part of the fastening are not critical, because the anchor rod can be pulled out when fastened by 3–6 cm. A subspecies of anchor bolts are collet fasteners in which the spacer part is compressed by two wedges on both sides.
There are also anchors in which the nut is firmly fixed in a mandrel at the end of a tube that has 4 stamped slots along its entire length. Such anchors are called Molly bolts and are used for fastening to sheet materials and hollow walls. When tensioned with a pin, the tube folds into an X-shaped structure and, thus, the mount reliably crimps even a thin rear sight.
Other structural features relate to the shape of the head; it may have an earring or a hook at the end. The anchors have a diameter of 6 to 24 mm and a length of 72 to 300 mm. Elongation occurs mainly due to the non-expansion part of the fastener: the degree of deepening does not matter, so long anchors are used to fasten thicker parts.
Effort calculation methodology
There are two types of load on anchor fasteners – traction and transverse. Although anchors are generally not designed to resist pull-out, they are sometimes used in these conditions, increasing the number of points to obtain distributed fasteners..
For anchors of different types and manufacturers, the permissible loads vary greatly, but in any case they are directly proportional to the static tensile strength of the metal core. When calculating the traction load, the degree of crumbling of the base material also plays an important role, due to which the breakdown of the fastener without destroying it or loosening of the anchor and its partial deformation is possible..
The spacing of the axes of the multi-point attachment should also be taken into account: the distance between the anchors should not be less than 15 diameters of the hole for them. The same rule applies to the distance from the edge of the array at which the anchors are fixed..
Since in most cases anchors are selected with a large margin of safety, the following permissible loads will be a guaranteed guideline in the calculation:
|Nominal diameter, mm||Permissible lateral load, N / kg||Permissible pulling load, N / kg||Recommended tightening torque|
The permissible loads are given for anchors with a soft steel core anchored in B30 concrete, in which minor structural defects are allowed: cracks or overstresses. Ideally, anchors are able to withstand much higher loads (up to 5 times higher than those given), so their reliability directly depends on the characteristics of the base.
Selection of anchors depending on the base material
The main requirement for the base for anchoring is the absence of elasticity, fragility and high hardness of the material. Masonry and concrete are ideal for anchoring. Less reliable, but still acceptable is anchoring in hollow structures – GWP and cinder block. A prerequisite is that the length of the anchor must be sufficient for fastening to the second partition (behind the void). In other cases, Molly bolts should be used, first of all, this applies to false walls and partitions assembled using “dry” technologies.
Bolted anchoring to walls made of aerated concrete, shell rock and similar porous materials is categorically unacceptable. In such cases, either distributed fasteners with steel screws or chemical anchors should be used. The principle of their operation is simple: the hole is injected with two-component glue, and then a steel pin is inserted. When solidified, the substance increases in volume and hardens, providing high resistance to pulling out and locally strengthening the structure of the material due to impregnation.
Chemical anchors for any wall material increase the fixing strength of the steel core by 40%, that is, the effect of crumbling concrete is almost completely absent.
Mechanical anchoring rules
The key moment when fastening with anchor bolts is strict regulation of the tightening torque. Excessive force is no better than insufficient force, very often due to exceeding the deformation threshold of the base material, its pronounced crumbling is observed. Recommended torque values are set by the manufacturer, but if the official specification is not available, the following guidelines can be used:
|Diameter||Recommended torque for frame / bolt anchors, Nm|
There are also subtleties of the installation process: the holes must be cleaned, or better – rinsed from the drill crumbs. If there are several anchoring points for one node, you should first pre-fix the anchors in the holes, and only then final tighten them. In the latter, order is important: the anchors are tightened in pairs from diametrically opposite attachment points.
Use of chemical anchors
The technique of fastening with chemical anchors is generally simpler, but the exact composition of installation operations differs from almost every manufacturer. Correct preparation of the hole is important here as nowhere else: it is first blown out with a hand pump, and then cleaned with a steel brush and dust is blown out again.
For household installation, anchors are used, in which the components of the glue are placed in a sealed capsule, which is destroyed when the stud is screwed in. This is the simplest type of installation, but it is not suitable for the assembly of critical structures due to insufficient mixing of the components.
For a more durable fastening, anchor chemistry compositions are used, supplied in special two-component syringes. In the prepared hole, the composition is injected at half the depth, after which a hairpin or a mortgage rod is inserted into the mass in one motion. This method is distinguished not only by its high strength, but also by a very economical consumption of glue..