Shear-Induced Pull-Out: A Critical Re-examination of the Bond Term in the EN 1992-4 Pry-Out Verification

EN 1992-4:2018 verifies concrete pry-out of a post-installed bonded fastener as VRk,cp = k8 · min{NRk,c; NRk,p}, with a default factor k8 of 1 or 2. Clause 7.2.2.4(1) gives the reason for the bond term in as many words: pull-out caused by a tension force that the shear load introduces into the fastener is “not verified explicitly, but implicitly accounted for in the verification for pry-out failure”. With k8 = 2 the provision therefore assumes that half of the applied shear reaches the bond as tension; with k8 = 1, all of it. This paper tests that assumption against the mechanics of a laterally loaded anchor in solid concrete. Because concrete is roughly seven times less stiff than steel, bearing stresses at the mouth of the hole exceed the local crushing strength well before ultimate load, the rotation pivot migrates to 1.5d–2d below the surface, and the shank tilts. Resolving the shear along the tilted shank gives an induced axial force N = V tan θ; for plastic tilt angles of 8°–15° that is 14–27 % of the shear — one-half to one-quarter of what the code implies. A closed-form credibility criterion is derived: shear-induced pull-out can precede steel shear failure only above a threshold bond strength that depends on the embedment ratio alone, not on anchor diameter. For an 8.8 rod the threshold is 4.7 N/mm² at hef = 4d and 2.3 N/mm² at 8d. In a parametric study of 76 single-anchor configurations, restricted to the embedment depths the design rules actually admit, the bond term governed the shear resistance in 21 cases under the code — and in none when a factor kp = 4 was applied to NRk,p. The code’s own asymmetry is examined. The same shear-induced pull-out that is ignored for mechanical fasteners — including concrete screws and expansion anchors with a tested pull-out resistance — caps the pry-out resistance of bonded ones. The point is made with a single product: the Hilti HUS4 screw, assessed as a mechanical fastener under one ETA and, set with a mortar capsule, as a bonded screw under another. For anchor groups the bond term is reduced a second time by the tension-derived area and group factors, although for anchors in line with the load the bearing zone of the rear anchor compresses the concrete in which the front anchor’s crater must form. The author has found no documented case of an anchor plate failing in shear by bond pull-out. A separate factor kp ≥ 4 on NRk,p is proposed as a hypothesis for experimental validation, together with a test programme. Author: Yves De Lathouwer, structural engineer, Adit Ltd — ORCID 0009-0005-8802-1686.Related material: chemical anchor systems, standards library, free anchor-design software.