HOW TO SELECT THE RIGHT POST-INSTALLED MECHANICAL ANCHOR FOR THE REQUIRED APPLICATION?

Fig. 1.1: Different types of mechanical anchors

The selection of mechanical anchors depends on many factors as visualized in Fig. 2.1

Fig. 2.1: Basic criteria for anchor selection

The commonly used mechanical anchors are expansion (sleeve or stud), undercut and screw type for applications with heavy, medium and low load ranges. Though there is no standard rule for the use of any specific type of anchor, in general, for high loading applications, expansion (stud or sleeve) or undercut anchors are preferred. For any temporary or non-structural applications, screw anchors are commonly used. However, since there are lot of factors involved in the selection criteria, it is recommended to check and design anchors for every application using Hilti PROFIS Engineering to help ensure an optimized and reliable solution.

These mechanical anchors derive their load-carrying capacity from the friction generated by the expansion of a sleeve against the sides of the drilled hole. Based on how the expansion of the sleeve is induced, expansion anchors may be further classified into the two types: torque-controlled and displacement-controlled. In this article, we focus only on the first type. Under the torque-controlled expansion anchors, the anchor can be of the stud or sleeve type.

3.1 Stud anchor configuration and working mechanism

A bolt/stud type anchor has a bolt with a conical shaped end with a nut, washer and expansion sleeve (Fig. 3.1). During installation, when tightening the nut to the torque value defined by the manufacturer (known as the installation torque), the torque causes the cone to be drawn into the expansion sleeve, which then deforms and expands against the concrete. Because of the radial pressure generated by the expansion and the friction between the sleeve and the concrete, the tensile strength of the bolt is activated (Fig. 3.2).

Fig. 3.1: Stud anchor configuration (Hilti HST4) Fig. 3.2: Stress field generated by stud expansion anchor in concrete under tension loading

The behavior of stud anchors varies between cracked and uncracked concrete. In cracked concrete, expansion anchors typically slip during crack opening and closing due to reduction in expansion force and friction. Anchors suitable for cracked concrete are capable of developing follow-up expansion forces after a minimal axial displacement. For torque-controlled expansion anchors, the application of the recommended torque is necessary to activate the load-carrying mechanism. Under-torquing results in the cone being in the incorrect position i.e., it is not drawn far enough into the expansion sleeve to achieve the specified holding values and also results in installation with unforeseen displacement. Over-torquing can result in potential damage to the base material or rupture of the anchor steel due to excessive pre-stressing.

3.2 Hilti stud expansion anchor portfolio

Hilti has a range of stud anchors to address the requirements for different loadings and applications. The main Hilti stud anchors with features and advantages are shown (Table 3.1).

Table 3.1: Hilti stud anchor portfolio

3.3 Sleeve anchor configuration and working mechanism

The configuration of sleeve expansion anchors is shown in Fig. 3.3 and stress contour in concrete is shown in Fig. 3.4

Fig. 3.3: Sleeve anchor configuration (Hilti HSL4) Fig. 3.4: Stress field generated by sleeve expansion anchor in concrete under tension loading

Similar to stud anchors, the cone and expansion sleeve are the key elements that determine the pull-out capacity.

The collapsible element mainly has two functions. Firstly, it prevents rotation of the sleeve during the application of torque. Secondly, it ensures the expansion sleeve due to its telescopic deformation is not compressed in any way if the shear sleeve comes into contact with the washer.

The cylindrical steel element, called the distance sleeve, ensures additional steel shear load capacity in some conditions. It can be very helpful in high shear applications.

3.4 Hilti sleeve expansion anchor portfolio

Hilti has a range of sleeve anchors which can be used for different loadings and applications. Main Hilti sleeve anchors are shown in (Table 3.2)

Table 3.2: Hilti sleeve expansion anchors

These mechanical anchors derive their load-carrying capacity from the mechanical interlock provided by the undercutting of concrete with the thread of the screw along the length of the fastener. The screw transfers the load to the concrete through the mechanical interaction between its thread and the concrete. The thread portion of the screw is more vulnerable due to stress concentration at the tip (Fig. 4.2). The load-carrying mechanism is based on the balance of the external tension force, N, with the local bearing pressure between the thread and the concrete, R, along the embedment (Fig. 4.1).

Fig. 4.1: Load transfer mechanism for screw anchor (Hilti HUS4) Fig. 4.2: Stress field generated by screw anchor in concrete

1.1 Hilti screw anchor portfolio

Hilti has a range of screw anchors which can be used for different loadings and base materials. The main Hilti screw anchors are shown in Table 4.1.

Table 4.1: Hilti screw anchors

These mechanical anchors derive their load-carrying capacity from the mechanical interlock provided by undercutting the concrete at the embedded end of the anchor. The Hilti HDA is a type of undercut anchor that has a unique shape and creates the undercut by itself during installation (Fig. 5.1). The anchor features a self-undercutting sleeve, that when set acc. to Instruction for Use (IFU) creates an undercut in the concrete (Fig. 5.2).

Fig. 5.1: Undercut of concrete created by the anchor Hilti (HDA) Fig. 5.2: Stress field generated by undercut anchor in concrete under tension loading

5.1 Hilti HDA undercut anchor portfolio

Hilti HDA anchors are made of carbon steel and stainless steel and available both in pre-set (P / PR) and through-set (T / TR) version (Table 5.1).

Table 5.1: Advantages of using HDA undercut anchor

In conclusion, the right selection of post-installed anchors is important because it directly affects the performance, safety, and durability of the structure or system being anchored. Their failure can lead to serious safety hazards. A well-chosen anchor helps to minimize the risk of repairs, replacements, or failures and ensures adequate performance of the structure.

Please go through Hilti anchor portfolio here: Fasteners - Hilti New Zealand

To start designing post-installed anchor, visit: Hilti PROFIS Engineering

1. S2C Handbook: Steel to concrete connections using Post-installed systems, Schaan: Hilti Corporation, 2024.

2. Fastening Technology Manual; Version 2023, Hilti Corporation, 2023.