Designing post-installed anchor connections with supplementary reinforcement

Post-installed anchors are widely used in structural applications to connect steel elements to existing concrete members. Typical applications include connection between new steel member to existing concrete (Figure 1). The anchors are often subjected to significant tensile and shear forces, especially near concrete edges, where failure is commonly governed by concrete cone breakout or edge breakout. These limitations may result in conservative design outcomes particularly where edge distances or spacing are restricted.

Figure 1: Typical steel to concrete structural connections

Supplementary reinforcement, such as stirrups, can be considered as one possible design measure to influence anchorage behavior under specific boundary conditions. Alternatively, the transverse reinforcement present in the concrete member can be taken into account in zones, where it is partially or completely unloaded. When properly detailed, this reinforcement intercepts developing crack surfaces and contributes to force transfer after cracking. Traditional design provisions for supplementary reinforcement in anchorages, as defined in EN 1992‑4:2018[1], treat the concrete cone and reinforcement as independent contributors. This simplification can lead to conservative outcome, particularly in multirow anchorages where realistic cracking patterns mobilize more reinforcement than assumed by this design model.

Supplementary reinforcement function through the strut‑and‑tie mechanism, which describes how forces are transmitted through concrete when load paths are non-uniform. In anchorage zones, where concentrated forces are introduced into the member, the internal force flow cannot be represented by simple beam theory and instead follows discrete compression and tensile paths. As shown in Figure 2,

1. The applied anchor load is carried into the concrete through struts (1), which are the compressed concrete zones that fan out from the anchor head.

2. These struts connect to nodes (2), which are the force‑transfer points where compression forces change direction. However, the load spreading induces transverse tensile stresses that concrete alone cannot sustain.

3. Ties (3), provided in form of supplementary reinforcement intercepts and resist these tensile stresses, completing the force path and maintaining equilibrium within system. Thus, supplementary reinforcement may contribute to load distribution within the concrete member, increasing the concrete breakout resistance.

Figure 2: Example of strut-and-tie mechanism under tension loading

Over the years, supplementary reinforcement design has advanced from simplified early assumptions to models that better capture actual anchorage behaviour. While EN 1992‑4 provides a clear framework, several of its simplifications such as assuming breakout from only the front anchor row and limiting the contribution of reinforcement after cracking tend to produce conservative results for multi‑row anchor groups and common layouts.

Recent research shows that cracking often begins at the back row, engaging more reinforcement than the standard model anticipates and resulting in larger breakout surfaces and higher ultimate capacities[2]. These findings highlight the need for a method that reflects realistic load transfer, including the interaction between concrete breakout, reinforcement activation, and concrete strut behaviour.

The Hilti method builds directly on these insights. It accounts for load sharing between active stirrup legs and concrete, incorporates a portion of the concrete breakout resistance, and limits the design by concrete strut capacity to reflect observed behaviour in multirow groups.

The Hilti method reflects the load transfer mechanisms observed in experimental research [3], within the scope and limitations described in the referenced publication, and explicitly accounts for interaction between concrete breakout, reinforcement activation and concrete strut resistance. Once the governing breakout crack forms, closed stirrups enclosing the surface/edge reinforcement are engaged and share load with the concrete. The design resistance is therefore taken as the sum of –

1. the anchorage resistance provided by the active stirrup legs (hook + bond resistance),

2. a conservative fraction of the unreinforced concrete resistance

The design anchorage resistance is limited by an upper limit governed by the concrete strut mechanism. This reflects observed behavior in multi-row groups where the decisive crack frequently initiates at the back row and expands to intercept multiple stirrup legs.

Table 1 and graph below (Figure 3) summarize the key differences between EN 1992‑4 and the Hilti method.

Figure 3: Graphical representation of anchorage resistance[3]

Table 1: Summary of key differences between EN 1992-4: 2018 and Hilti design method

Hilti’s PROFIS Engineering software helps the designer to consider the effect of the reinforcement if present in the concrete member, as per EN 1992-4 and Hilti method (Figure 4). Different detailing of reinforcement can be modelled and the influence on different failure modes is taken into account.

Figure 4: Supplementary reinforcement design option in PROFIS

_{Note: Results generated by PROFIS Engineering are based on the input provided by the user and the selected calculation models. They do not replace engineering judgment. The designer remains responsible for verifying the suitability of the assumptions, results, and compliance with applicable standards, approvals, and local regulations.}

The results of state-of-the-art research over the years have enabled the development of a more realistic approach for designing anchorages with supplementary reinforcement. Integrated into PROFIS Engineering’s Concrete module, this method provides a structured way to consider the interaction between concrete breakout, reinforcement activation, and concrete strut behavior, based on current research findings and within defined assumptions.

• Post-installed anchors in concrete are often governed by concrete breakout failure, especially near edges or under high tension and shear demands.

• Supplementary reinforcement improves anchorage performance by engaging a strut‑and‑tie mechanism, where ties (stirrups) intercept cracking and contribute to load transfer after concrete cracking.

• The Hilti method incorporates more realistic behaviour by considering load sharing between stirrup legs and concrete, adding a portion of concrete breakout resistance, and applying limits based on concrete strut capacity.

• PROFIS Engineering implements both EN 1992‑4 and the Hilti method, allowing designers to model reinforcement detailing and understand its influence on different failure modes.

Together with our differentiated hardware systems, productive and safe installation methods, and our technical content and support in our Engineering Center, this design method is a part of our SPEC2SITE solutions. Our Field Force supports you in the office and in the field and we help connect with the contractor so the installation is done as per your specifications.

To start designing, visit https://profisengineering.hilti.com/