HILTI HST4: PERFORMANCE THAT MATCHES CAST-IN HEADED FASTENERS

Concrete cone failure and pull-out failure are two of the most critical failure modes for mechanical anchors under tension.

Concrete cone failure occurs when the concrete around the anchor cracks and breaks out in a cone shape. This happens due to the stress induced in the concrete by the forces transmitted from the structure through the fastening point. The design method for calculating the concrete cone resistance of mechanical anchors is provided by EN 1992-4 [1], which is the European standard for designing fastenings for use in concrete.

According to EN 1992-4, the characteristic resistance of a group of fasteners in the case of concrete cone failure is obtained by multiplying the characteristic resistance of a single anchor placed in concrete (and not influenced by adjacent fasteners or edges of the concrete member), N⁰_Rk1c, , by a series of coefficients that account for the influence of various factors. These account for variables such as the distance from the slab edge, the spacing between anchors within a group, load eccentricities and bending moments, and the presence of dense reinforcement.

In particular, the characteristic resistance of a single anchor N⁰_Rk1c, , is dependent on both the concrete strength and the embedment depth of the anchor itself, as given by the following formula [1]:

where f_ck represents the nominal characteristic compressive cylinder strength of the concrete, and h_ef is the effective embedment depth of the anchor. The coefficient k₁, which linearly influences the concrete cone characteristic resistance, is, in general, indicated in the European Technical Assessment of the anchor. According to current market standards and also indicated in the EN 1992-4, this factor typically assumes values of 7.7 for cracked concrete and 11.0 for uncracked concrete for post-installed fasteners, and 8.9 for cracked concrete and 12.7 for uncracked concrete for cast-in headed fasteners.

Hilti HST4 is the first stud anchor in the world to achieve the same concrete cone resistance as a cast-in headed fastener, with a k₁ factor 8.9 for cracked concrete and 12.7 for uncracked concrete for select sizes thanks to its innovative design. This means that HST4 can reach the same concrete cone load capacity as other stud anchors with a shorter embedment depth, or can achieve a ~16% higher load capacity with the same embedment depth. What does this mean in design practice? It means that you can now use shorter anchors for the same application, or design for higher loads using the same anchors.

HST4 has 4 key components that contribute to its superior performance (see Figure 1):

1. Turbo-channels and interlock, which provide additional interlock resistance with the sleeve;

2. A highly engineered sleeve with a tip groove design, a longer four-segment shape and a tapered tip, which maximizes the grip to the concrete and helps to prevent slippage;

3. A turbo-mountain-valley shape, which maximizes the sleeve expansion, enabling strong performance in both uncracked and cracked concrete as well as in seismic conditions, and provides more reliability with an anti-spinning sleeve;

4. Tailored and proprietary top-coatings for the bolt and the nut, which are optimized for tension loads and allow for close-to-the-edge installation.

Figure 1: HST4 innovative design

The innovative design of HST4 further provides a higher pullout capacity in both uncracked and cracked concrete, under both static and seismic loads. Compared to our previous generation of stud anchors, for a single anchor with the same length and diameter, HST4rovides a tensile performance increase of +36% to +84% in uncracked concrete under static conditions, +18% to +41% in cracked concrete, and +11% to +63% under seismic C2 loads, depending on the diameter of the anchor.

The embedment depth of an anchor in concrete determines both the load transfer mechanism and failure mode of the anchor. In general, the deeper the embedment, the higher the load capacity of the anchor. The downside is that a deeper embedment also results in longer drilling and setting time, as well as a higher risk of hitting rebars.

Most stud anchors are approved for fixed embedment depths, typically between 1 and 3 values. This limits the flexibility and optimization potential for both the anchor selection and design, as the structural engineer is required to choose from a predefined set of anchor lengths and embedment depths, which may result in over-designing the fastening point.

HST4 is qualified according to the new European assessment documents, EAD 330232-01-0601 v02 [2], which covers the improved resistance to concrete cone failure, and EAD 330232-01-0601 v03 [3], which covers the use of variable embedment depth for mechanical anchors fastened in concrete. This means that, with HST4, the embedment depth can be freely selected within given ranges (as specified in the European Technical Assessment document ETA-21/0878 [4]) according to the selected anchor diameter and load requirements, as shown in Table 1.

Table 1: Effective embedment depth ranges for HST4

With variable embedment depth, structural engineers can select the optimal anchor length and embedment depth for their applications, getting the best value for money for their anchors, which helps saving on material and reducing installation costs.

HST4 can be designed using the Hilti PROFIS Engineering software, which is a cloud-based software that integrates with structural suites and aids in the design and optimization of fastening points and baseplates. With PROFIS Engineering, Hilti provides a user-friendly interface, a 3D modelling environment, a report generator, and technical support service. PROFIS Engineering implements the EN 1992-4 design method and includes all the features and benefits of HST4, such as improved concrete cone resistance, variable embedment depth, and no-cleaning installation.

The higher tensile performance of HST4, combined with the use of PROFIS Engineering software, helps you to optimize your fastening solutions, to reduce the size of your baseplates, lower your costs, save on materials, and make your designs more sustainable. The use of smaller anchors and the higher concrete cone performance of HST4 make it possible to use smaller anchor spacings, which, in several instances, allows for a reduction in the overall size of the baseplate.

As a simple example, let’s consider the 10mm thick baseplate shown in Figure 2.

This is a steel baseplate subjected to a static tensile 50kN force. The concrete member is of the C20/25 concrete strength class, and is 250mm thick, and is considered as cracked concrete.

Figure 2: Baseplate in tension with traditional stud anchoring solution

A ‘traditional’ stud anchor solution would require four M12 stud anchors, with an effective embedment depth of 88mm, an anchor length of 125mm, and an anchor spacing of 165mm, resulting in a baseplate size of 200mmx200mm. Placing the anchors closer to each other, for example at a spacing of 160mm, will require the use of longer and more expensive anchors.

How can you now optimize with the HST4?

OPTION 1: Thanks to the higher pullout capacity of HST4, it is possible to reduce the anchor diameter size from M12 to M10. This already allows for a reduction in both the material and cost of the anchors.

Simultaneously, with the improved concrete cone capacity and the use of variable embedment depth, it is also possible to reduce the anchor length, by using a shorter effective embedment depth of 71mm. This shorter embedment will even provide the same concrete cone capacity as the original design.

OPTION 1 BENEFITS: The use of smaller anchor diameters results in smaller diameter boreholes, and the use of shorter anchors results in shorter drilling depth, helping to reduce both installation time and costs on the jobsite.

OPTION 2: However, to truly maximize your value, it is possible to maintain the anchor embedment and reduce the anchor spacing instead. The same performance can now be achieved with four M10 anchors, with the same effective embedment depth of 88mm as before, but with a smaller anchor spacing of 135mm. The improved spacing allows for a reduction in the baseplate size to 165mmx165mm.

OPTION 2 BENEFITS: The optimized baseplate is 30% smaller which translates to a 30% reduction in the volume of steel used!

Figure 3: Specifications with HST4-R – best value for money with baseplate optimization