Shallow Embedded Anchors Load-carrying capacity of mechanical anchors under tension

ACI 318-14, Chapter 17,1 provides design requirements for anchors in concrete used to transmit structural loads between: • Connected structural elements; or • Safety-related attachments and structural elements. Provided that they are used correctly, these provisions result in safe and economical solutions for fastenings and connections. However, Chapter 17 provisions are restricted to the design of structural anchors. In fact, the Commentary to the Code explicitly states that specialty inserts—devices that include those used for anchoring nonstructural elements—are “not within the scope of this Code.” Modern buildings require the installation of cable trays, fire sprinklers, air conditioning equipment, and pipes. The installations frequently are made using shallow fastenings consisting of an individual anchor or groups of two or four anchors embedded in the concrete cover (Fig. 1). This means that significant loads are carried by anchors that are embedded in the zone between the reinforcement and the outer surface of reinforced or post-tensioned concrete slabs, precast hollow core concrete slabs, or concrete slabs on metal decks. ICC-ES AC4462 covers the prequalification of cast-in specialty inserts and refers to ACI 318 as the design method resource for these nonstructural applications. The critical parameter is embedment depth hef (Fig. 1 and 2). Postinstalled anchors with hef ≥ 1.5 in. (40 mm) are prequalified according to ICC-ES AC193,3 which is based on ACI 355.24 and also refers to ACI 318 for design provisions. ICC-ES AC193 also allows anchors to have hef < 1.5 in., but the lower limit for hef is 1.0 in. (25 mm) and the anchors must be used in redundant anchorages in interior applications only. ICC-ES AC193 also provides a special design method for these redundant anchorages. In the event that one anchor fails or exhibits excessive deflection, the method relies on having a fixture that is capable of redistributing the load of the insufficiently behaving anchor to the neighboring anchors. European design provisions for anchors are provided in EN 1992-4.5 In this standard, the unfavorable effect of the concrete properties in the concrete cover is taken into account by the capacity reduction factor ψre, even for anchors with hef ≥ 1.5 in. Smaller embedment depths are allowed only in statically indeterminate (redundant) nonstructural systems subjected to static loads only. They are designed using an approach similar to the one in ICC-ES AC193. The minimum hef is 1 in. for anchors subject to internal exposure conditions, and the minimum hef is 1.25 in. (30 mm) for anchors in all other applications. These applications raise the question: Can ACI 318 be used to calculate the concrete break-out capacity of cast-in specialty inserts or post-installed anchors with hef < 1.5 in.? This paper presents background information on the ACI 318 design procedure, parameters influencing the concrete tension capacity in the cover concrete, test results used to develop the European design approach, and a proposal for modifying the ACI 318 design concept to yield a conservative result for mechanical anchors with shallow embedment.