Take Your Top of Slab Façade Anchorage to a New EDGE

Hilti Anchor Channel with the new rebar edge confinement plate (HAC EDGE) is a solution for Curtain Wall applications that offers superior concrete edge breakout performance in shear. HAC EDGE changes the traditional concept of anchoring to concrete. Instead of relying on the low capacity of the concrete in tension, it takes advantage of the tensile strength of the reinforcement attached to the anchor channel. HAC EDGE optimizes the shear load transfer from the channel profile into the reinforcing bars and overcomes the challenges with traditional anchor channels with welded reinforcing bars.
HAC EDGE is a new anchoring system that brings value innovation. It copes with today’s fast track construction demands and requirements of the curtain wall industry such as installation tolerance, high wind loads, thin concrete members, pockets, close edge distances, and lightweight concrete. In such adverse conditions, HAC EDGE provides more than 2 times the capacity of traditional top of slab anchor channels where the reinforcing bars are welded to the back of the channel and outperforms standard anchor channels without reinforcing bars by up to a factor of 5. 


Figure 1 HAC EDGE overview

Cast-in Anchor Channel Systems
Anchor channel systems are a cost-effective anchoring-to-concrete technology. This technology provides a needed solution for job site tolerances and on-site adjustability when connecting structural and non-structural elements.
Anchor channel systems provide added savings by not requiring skilled labor to weld the fixture to the embed, as welding is not required. Moreover, they reduce the installation time per connection and the potential for incorrect placement, improve the safety of the installer, and increase the structural reliability of the connection.

Design Standards for Anchor Channels with Round Headed Anchors
Cast-in anchor channels can receive recognition under the International Building Code (IBC) for design with strength design provisions. The International Code Council Evaluation Service (ICC-ES) has developed the Acceptance Criteria for Anchor Channels in Concrete Elements (AC232) to qualify anchor channel systems. AC232 provides amendments to ACI 318 Anchoring-to-Concrete provisions that permit the design of anchor channel systems as if they were included in ACI 318 Anchoring-to-Concrete provisions. For additional information about AC232, visit http://www.structuremag.org/?p=10443.

Code Compliance of HAC EDGE
HAC EDGE is a system consisting of a rebar edge confinement plate (EDGE plate) non-structurally attached to a Hilti Anchor Channel (HAC). The anchor channel itself is covered in the ESR-3520. Because the EDGE plate is not structurally connected to the anchor channel, the system is decoupled and the EDGE plate does not change the behavior of the anchor channel. Therefore, the design model and technical data can be taken from ESR-3520 for most of the failure modes.

The scope of the current version of AC232 (June 2017) is limited to anchor channels with round headed anchors or I-anchors. Anchor channels with reinforcing bars attached to the anchor channels are not explicitly covered in the criteria. However, the testing protocols of AC232 were used to determine the resistances of HAC in combination with the EDGE plate. The design provisions of AC232 are valid to design 19 out of the 20 failure modes of an anchor channel required per ESR-3520. Only the verification for concrete edge failure in shear is modified, based on rigorous in-house testing to benefit from the improved load-bearing behavior provided by the confinement plate with reinforcing bars. The anchorage length is designed in accordance with ACI 318.

Figure 2 HAC EDGE Components

The Weakest Link in Top of Slab Applications
The design of cast-in anchor channels requires the verification of steel and concrete failure modes for tension, shear and combined tension and shear. An anchor channel is only as strong as its weakest link. For curtain wall applications, the weakest link in shear is typically concrete edge breakout failure. This failure mode is controlled by the geometrical dimensions and material characteristics of the concrete.

Standard Anchor Channels
The verification of concrete edge breakout of anchor channels consists of the basic concrete breakout strength of one anchor of the anchor channel multiplied by a series of modification factors that account for the anchor spacing, member thickness, corner effect, state of concrete (cracked or uncracked) and reinforcement in the concrete member.

The basic concrete breakout strength of one anchor without the influence of a corner, member thickness or adjacent anchors in unreinforced concrete is determined as follows: 



For standard anchor channels covered in ESR-3520 the shear loads acting on the fixture are transferred through the anchor channel into the concrete via the t-bolts and the anchor elements (1). The failure occurs in front of the channel profile (2). After reaching the ultimate shear load the concrete in front of the channel is completely separated from the rest of the structure. The shear load decreases with increasing deformation of the channel (3).

Figure 3 Behavior of anchor channels loaded in shear (left). Shear load vs displacement curve (right).

In curtain wall applications, the design of the anchor channel typically takes place at one of the last stages of the building design. Oftentimes, the design of the anchorage comes as an afterthought. By the time the anchor channel is sized, the variables that can significantly influence the performance of the anchor channel have been defined. The edge distance is the variable that has the largest impact on concrete edge breakout failure. However, even this variable may have already been defined leaving designers with minimal options. Typically, the edge distance can only be varied in a very narrow band due to the size of the bracket. 

Overcoming the Limitations of the Concrete Edge Breakout Strength

Anchor Channels with Welded Reinforcing Bars
To overcome the limitations of the relatively low concrete edge breakout strength close to an edge, often anchor channels with reinforcing bars structurally welded to the back of the channel are used.
In analogy to standard anchor channels, the shear loads are transferred into the concrete via the channel profile (1). The failure also occurs in front of the channel profile (2). However, compared to standard anchor channels the shear loads are redistributed to the reinforcing bars. The higher ultimate resistance of the system is reached only with increased deformations and large cracks in front of the channel (3).  
  

Figure 4 Behavior of anchor channels with reinforcing bars loaded in shear (left). Shear load vs displacement curve (right).

Welding reinforcing bars to an anchor channel changes the behavior of the anchor channel in tension and combined tension and shear. When tension forces are applied to the anchor channel, the concrete above the reinforcing bars fails and reduce the concrete cone resistance of the anchor channel. Also, due to the large cracks in front of the anchor channel in case of shear loading, the interaction with concrete cone failure is negatively influenced. These effects are even more pronounced in lightweight concrete structures. Therefore, the negative effect of the welded reinforcing bars should be considered in the design of these types of anchor channels.

HAC EDGE
Utilizing the basic principles of reinforced concrete structures has lead Hilti to develop a new anchor channel system that copes with the curtain wall market needs. Rather than relying on the relatively low tensile capacity of the concrete for the failure mode “concrete edge breakout”, HAC EDGE brings superior concrete edge performance by optimizing the load transfer for small edge distances, thin concrete members, low concrete compressive strengths, and lightweight concrete.
The first loading stage of HAC EDGE is equal to the one of standard anchor channels and anchor channels with welded reinforcing bars (1). After shear loads are applied to the anchor channel, the loads are first transferred from the channel profile towards the free edge of the concrete. The EDGE plate which is held back by the welded reinforcing bars confines the edge, allowing for the formation of compression struts (2). The shear load is transferred back to the reinforcing bars. The ultimate resistance of the system is reached after the reinforcing bars are fully activated (3).  
  

Figure 5 Behavior of HAC EDGE loaded in shear (left). Shear load vs displacement curve (right).

The EDGE plate is not structurally connected to the anchor channel. This allows decoupling of the shear and tension load transfer mechanism. The concrete cone resistance of the anchor channel can be conservatively calculated according to AC232.
For small edge distances HAC EDGE provides more than 2 times the capacity of traditional top of slab anchor channels where the reinforcing bars are welded to the back of the channel. Moreover, depending on the geometry of the concrete member and location of the anchor channel, HAC EDGE can outperform standard anchor channels without reinforcing bars up to a factor of 5. 

Additional Benefits of HAC EDGE
In addition to the high concrete edge breakout in shear performance, HAC EDGE comes with the specified/ordered edge distance to reduce errors in placement, and provide a safer and quicker installation. This goes in hand with Hilti’s mission of building a better future. HAC EDGE reduces the probability of installing the product at the wrong edge distance. The edge distance is the most sensitive variable that has the highest impact on the concrete edge breakout strength in shear.
Moreover, the installation of the anchor channel is simplified as the product can easily be secured to the formwork by nailing or screwing the EDGE plate to it. Moreover, HAC EDGE brings overall material savings as it allows the use of smaller facade brackets. The superior concrete shear performance for small edge distances allows the curtain wall bracket to be concealed by the stool trim or gypsum wall, if they are at least 4”-5” away from the edge of the building. This eliminates the need of so-called pockets, bringing substantial savings.

Take your façade to a new EDGE. Start designing now.

HAC EDGE is now available in North America and fully covered in PROFIS Anchor Channel. Do you have a project where you are challenged to find a solution? Download the software at https://www.hilti.com/content/hilti/W1/US/en/engineering/software/profis/profis-channel.html or feel free to email your condition to US+CA.HAC@Hilti.com.