Post-installed anchors offer a reasonably reliable method of fastening and are relatively easy to install. Post-installation of both adhesive and mechanical anchors into masonry base materials, such as grout-filled CMU, hollow CMU, or brick have been done for many years. In fact, connections into the sides of CMU walls for joist and diaphragm connections are often easier to construct using post-installed rather than cast-in-place anchors. While the relevant design codes for post-installed anchors into concrete base materials have evolved (i.e. ACI and CSA), the masonry design codes and test criteria have remained unchanged for decades.

Current testing and evaluation criteria for post-installed mechanical and adhesive anchors is based on the International Code Council Evaluation Services (ICC-ES) Acceptance Criteria for Expansion Anchors in Masonry Elements (AC01), ICC-ES Acceptance Criteria for Predrilled Fasteners (AC106), and Acceptance Criteria for Adhesive Anchors in Masonry Elements (AC58). ICC-ES is a subsidiary of the International Code Council (ICC) which is the company that publishes the International Building Code (IBC). After completing a test program per any of these criteria a manufacturer can then obtain an evaluation report from a 3rd party evaluation service, such as ICC-ES, or IAPMO UES. These evaluation reports contain the information about the building product so that a building design engineer can design the anchor for that base material and so that a building official can verify it is appropriate for the relevant building code. For example, anchor evaluation reports in concrete base material, like ICC-ES ESR-4266 for the Hilti KB-TZ2 Expansion Anchor, contains k factors, minimum edge and spacing distances, shear capacities, etc. all necessary information for anchorage design in concrete. Design software, like PROFIS Engineering Suite, may use these evaluation reports for their calculations. The acceptance criteria, like AC01 and AC58, are documents that outline the testing to be performed and how the results should be analyzed to publish an evaluation report. Simply put, to get an evaluation report for a product, a manufacturer must perform the same tests and analyze them the same way ensuring that the end user, generally the engineer, is given correct data for their designs.

In 2020, AC01, AC106 and AC58 were considerably revised. The many changes include:

• The consolidation of AC106, the separate acceptance criteria for mechanical screw anchors into masonry base materials, into AC01, creating a single acceptance criterion for all mechanical anchors installed into masonry base materials.
• The clarification of the types of masonry systems included in the acceptance criteria – fully grouted, partially grouted, ungrouted CMU and brick façade.
• The modification of the evaluation method – adopting a statistical evaluation approach that will result in more consistent measurements of an anchor’s performance. 
• The adoption and adaptation of ACI 318’s Concrete Cone Design (CCD) method for the design of post-installed anchors into masonry.
• The inclusion of cracked masonry testing and evaluation.

The first major revision engineers should understand is the adoption of ACI 318’s anchor design method. Regarding cast-in and post-installed anchors, the failure modes of anchors installed in masonry are similar. Comparable to the post-installed anchoring provisions for concrete, the failure modes for anchors in grout-filled CMU will be steel failure, masonry breakout, pullout (for mechanical anchors) or bond failure (for adhesive anchors), shear pryout, etc. Aspects of the CCD method have been adopted for anchor design into masonry including the Load and Resistance Factor Design (LRFD) model and the calculation of various failure capacities (steel, breakout, pullout, etc). Now instead of a single, allowable design load for a given embedment, the evaluation report will provide k factors, bond stresses, pullout values, partial safety factors, and other variables to be applied into this methodology. A highlighted benefit of the new acceptance criteria is the ability to use variable embedment depths for masonry design with adhesive anchors. Akin to post-installed anchors into concrete, maximum (and minimum) embedment depths will be given for anchors, but generally most applications will be limited by the masonry thickness.

How will the new design methodology for masonry differ compared to concrete? Engineers will observe a few small differences. First, the k factors given for anchors designed into masonry will be lower than what is used for concrete to account for the inhomogeneity of masonry materials in breakout. Breakout cones for masonry base materials can be greatly influenced by the presence of head joints (vertical joints between blocks) and bed joints (horizontal joints between blocks) of the wall. Second, for anchors loaded in shear, engineers will need to consider masonry crushing in their design. The masonry crushing equation is currently used by The Masonry Society (TMS) 402 anchor design provisions for cast-in anchors. Masonry is often a softer material than concrete and when exposed to high shear loading, the steel anchor may crush and sink into the block of the masonry, thus the calculation of the crushing capacity.

The other major revision engineers should understand is the inclusion of cracked masonry to testing criteria of AC01 and AC58, thus allowing engineers to design their masonry anchors for cracked conditions, as necessary. Prior to the acceptance criteria revisions in 2020, cracked masonry fell outside of the scope of the document. Thus, post-installed anchors tested per AC01 or AC58 were meant to only be used in areas expected to remain uncracked. In comparison to anchor design into concrete, which has included cracked design for 20 years now, masonry anchor testing, and design had wholly ignored cracking as an issue until now. In comparison to concrete, where there are multiple applications where cracking may occur, modern CMU construction has more limited applications to where an engineer may wish to consider cracking in their anchor design. That said, cracking can certainly occur particularly in the mortar joints.
     
The key changes to the new AC01 and AC58 include the introduction of cracked masonry as well as the new strength design methodology. While the introduction of the new design methodology may bring additional complexity to design, software like PROFIS Engineering Design Suite will be updated to make the transition easier. Compliance dates for the updated acceptance criteria are listed as April 15th, 2023, for AC58 (adhesive anchors) and February 16th, 2024, for AC01 (masonry anchor). Manufacturers must complete testing and evaluation of their products before these dates to retain their Evaluation Service Report (ESR). Hilti plans on releasing additional resources as the new evaluation service reports are released such as: a masonry webinar series, design examples, and a new masonry anchor technical design guide.