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Wedge Anchors
A comprehensive engineering reference for structural engineers, EPC contractors, facility managers and procurement teams — covering wedge anchor expansion mechanics, failure mode analysis, ACI 318 design capacity, installation requirements, minimum embedment and edge distance, material grades and full project documentation.
Wedge Anchor Types, Expansion Mechanics
& Failure Mode Analysis
Steel Failure · Concrete Breakout · Pull-Out · Side-Face Blowout
Definition and Engineering Function
A wedge anchor is a post-installed mechanical expansion anchor that develops its holding capacity in hardened concrete by the wedge-expansion action of a stainless or carbon steel expansion clip (sleeve) that is forced outward against the concrete hole wall as the anchor bolt is tightened. Unlike cast-in-place anchor bolts that are embedded before concrete is poured, wedge anchors are installed after the concrete has achieved its specified compressive strength — by drilling a hole, inserting the anchor, and tightening the nut to expand the clip and create bearing contact against the concrete.
The wedge anchor consists of four principal components: a threaded bolt body with a tapered cone at its embedded tip, a serrated expansion clip positioned over the cone, a washer and a nut. When the nut is tightened, the bolt is drawn upward (relative to the hole), pulling the tapered cone into the expansion clip and forcing the clip outward into the concrete. The resulting friction and bearing contact between the clip and the concrete hole wall generates the anchor's resistance to tensile (pull-out) and shear loads. The wedge anchor is suitable only for installation in hardened concrete — it cannot be used in masonry, hollow block, grout, timber or other base materials without specific engineering evaluation.
The wedge anchor develops its tensile capacity through the bearing of the expansion clip segments against the concrete hole wall. As the nut is tightened, the bolt's tapered cone is drawn into the clip, spreading it radially outward. The clip exerts a normal (radial) force on the concrete and the resulting friction force resists the axial withdrawal of the anchor. For a correctly specified and installed wedge anchor in concrete of adequate strength, the governing failure mode should be steel fracture (bolt body tensile fracture) rather than concrete failure — this is the ductile design objective defined in ACI 318-19 Chapter 17. If concrete failure governs (concrete breakout, pull-out or side-face blowout), the failure is brittle and sudden without warning — unacceptable for safety-critical structural connections.
Wedge Anchor Anatomy — Four Key Zones
Wedge Anchor Types
Failure Modes — ACI 318 Chapter 17
ACI 318-19 Chapter 17 defines five possible failure modes for post-installed mechanical anchors in concrete. The design must ensure that the steel fracture mode governs (ductile failure) rather than any concrete failure mode (brittle failure):
Dimensional Data, Minimum Embedment
& Load Capacity Reference
Tension Capacity · Shear Capacity · f'c 20–35 MPa
Critical Installation Dimensions
A wedge anchor requires five parameters to be correctly specified and installed: (1) Anchor diameter — determines the drill bit size and load capacity; (2) Drill-hole diameter — must exactly match the anchor diameter (no oversize allowed — oversize holes prevent the clip from contacting the concrete and drastically reduce capacity); (3) Minimum embedment depth h_ef — the depth from the concrete surface to the bearing surface of the expansion clip; (4) Minimum edge distance c_a — from anchor centreline to nearest concrete edge (prevents side-face blowout and breakout at edges); and (5) Minimum anchor spacing s — between adjacent anchors in a group (prevents interaction between breakout cones).
The drill-hole diameter for a wedge anchor must exactly match the specified anchor diameter — drill bits of the correct size must be used, and must be verified as not worn beyond their usable tolerance limit. A hole drilled even 1–2 mm oversize (e.g., using a worn 10 mm bit for an M10 anchor) will prevent the expansion clip from achieving contact with the concrete hole wall under the specified installation torque — resulting in a pull-out capacity that may be 30–80% lower than the published design value. Oversize holes cannot be corrected — the anchor must be relocated. Verify drill bit size before each hole; replace worn bits. Rotary hammer drill (not core drill) with the manufacturer-specified SDS or SDS-Max bit is required for wedge anchor holes.
| Anchor Dia. | Drill Dia. (mm) | Min h_ef (mm) | Max h_ef (mm) | Min Edge Dist. c_a (mm) | Min Spacing s (mm) | Min Concrete Thick. (mm) |
|---|---|---|---|---|---|---|
| M8 | 8 | 60 | 80 | 60 | 60 | 120 |
| M10 | 10 | 70 | 100 | 70 | 70 | 140 |
| M12 | 12 | 80 | 125 | 80 | 90 | 160 |
| M16 | 16 | 100 | 170 | 100 | 110 | 200 |
| M20 | 20 | 125 | 200 | 130 | 140 | 250 |
| M24 | 24 | 150 | 250 | 160 | 160 | 300 |
| M30 | 30 | 190 | 300 | 200 | 200 | 380 |
| M36 | 36 | 230 | 380 | 240 | 250 | 460 |
Indicative minimum dimensions based on standard wedge anchor manufacturer data and ACI 318-19 Ch.17 requirements for uncracked concrete, f'c ≥ 20 MPa. Actual minimum embedment and spacing requirements depend on the applied loads, concrete strength and ICC-ES evaluation report for the specific anchor product. Always use the product-specific ICC-ES report (ESR number) dimensions for design — not generic table values. Minimum concrete thickness = h_ef + h_ef/3 (minimum) to prevent breakout through the underside of the concrete element.
| Anchor Dia. | h_ef (mm) | Steel Tension (kN) | Concrete Breakout Tension (kN) | Pull-Out (kN) | Steel Shear (kN) | Governing Mode |
|---|---|---|---|---|---|---|
| M10 | 70 | 28.5 | 18.2 | 22.0 | 16.8 | Concrete breakout |
| M10 | 100 | 28.5 | 33.6 | 32.0 | 16.8 | Steel (tension) |
| M12 | 80 | 41.4 | 22.5 | 28.0 | 24.4 | Concrete breakout |
| M12 | 125 | 41.4 | 55.1 | 44.0 | 24.4 | Steel (tension) |
| M16 | 100 | 73.6 | 33.6 | 48.0 | 43.2 | Concrete breakout |
| M16 | 170 | 73.6 | 106 | 80.0 | 43.2 | Steel (tension) |
| M20 | 125 | 115 | 55.1 | 72.0 | 67.4 | Concrete breakout |
| M20 | 200 | 115 | 140 | 112 | 67.4 | Steel (tension) |
Indicative characteristic values only — for design guidance. Actual design values must be taken from the product-specific ICC-ES Evaluation Report (ESR) or ETAG 001 assessment for the exact anchor product, concrete grade and configuration. Design loads = characteristic × φ (ACI 318: φ = 0.65 for cast-in; φ = 0.65×0.75 = 0.49 for post-installed in cracked concrete). Steel tension capacity based on ASTM A36 / Grade 4.6 bolt. "Steel governs" (ductile) requires adequate embedment AND minimum edge distances.
k_c = 7 (post-installed) // ACI 318: post-installed anchors use k_c=7 (vs 10 for cast-in)
λ = 1.0 (NW concrete) or 0.75 (LW) // Lightweight concrete modification factor
// WORKED EXAMPLE: M16 wedge anchor, h_ef=170 mm, f'c=25 MPa, NW concrete, uncracked
N_b = 7 × 1.0 × 25^0.5 × 170^1.5 = 7 × 5.0 × 2214 = 77,490 N ≈ 77.5 kN
// φN_b = 0.65 × 0.75 × 77.5 = 37.8 kN (post-installed, cracked concrete β=0.75)
// For uncracked concrete: φN_b = 0.65 × 77.5 = 50.4 kN
Material Grades, Mechanical Properties
& Surface Treatments
ASTM F1554 · NACE MR0175 · Zinc Plate · Epoxy
| Grade | Standard | Yield (MPa) | UTS (MPa) | Hardness | Corrosion | Application |
|---|---|---|---|---|---|---|
| CS Grade 4.6 | ISO 898-1 | ≥240 | ≥400 | ≤250 HV | Low | Indoor dry structural; light loads |
| CS Grade 5.8 | ISO 898-1 | ≥400 | ≥500 | ≤300 HV | Low | General structural, machinery, racking |
| CS Grade 8.8 | ISO 898-1 | ≥640 | ≥800 | 225–300 HV | Low | Heavy structural, high-load anchors |
| ASTM F1554 Gr.36 | ASTM F1554 | ≥248 | ≥400 | ≤237 HB | Low | Standard US structural; HDG compatible; weldable |
| ASTM F1554 Gr.55 | ASTM F1554 | ≥380 | ≥517 | ≤255 HB | Low | Medium-high load US structural; weldable |
| ASTM A193 B7M (NACE) | ASTM A193 | ≥724 | ≥862 | ≤235 HB | Low | Sour service; NACE MR0175 compliant |
| SS 304 (A2) | ISO 3506-1 | ≥210 | ≥500 | ≤220 HV | High | Outdoor, coastal-adjacent, food, water |
| SS 316 (A4) | ISO 3506-1 | ≥210 | ≥500 | ≤220 HV | Very High | Coastal, marine, chloride, chemical |
| Duplex 2205 | ASTM A182 F51 | ≥450 | ≥620 | ≤310 HB | Very High | Offshore, sour service, high-chloride concrete |
| Finish | Standard | Thickness (µm) | Salt Spray (hrs) | Notes & Application |
|---|---|---|---|---|
| Zinc electroplate | ASTM B633 SC2 | 8–13 | 96–120 | Indoor structural; sheltered; light corrosion protection |
| Mechanically galvanised | ASTM B695 Cl.12 | 12 min | 200 | No HE risk; outdoor sheltered; better than electroplate |
| Hot-dip galvanised | ASTM A153 Class C/D | 45–86 | 500+ | Standard outdoor structural; coastal-adjacent; thread re-tap after HDG |
| Climaseal / Geomet | ISO 10683 | 8–12 | 480–720 | High-strength bolts; no HE; outdoor; alternative to HDG for 8.8 |
| Epoxy coat | ASTM A775 | 175–300 | 1000+ | Coastal and marine concrete environments; bridges; water infrastructure |
| SS 304 passivation | ASTM A380 | N/A | 1000+ | All SS 304 anchors; mandatory post-machining |
| SS 316 passivation | ASTM A380 | N/A | 1000+ | All SS 316 anchors; coastal / marine / chloride environments |
NOTE: The expansion clip of the wedge anchor must be made from material compatible with the anchor bolt and with the concrete environment. A stainless steel anchor body with a carbon steel clip will produce galvanic corrosion at the clip — specify full SS assembly (bolt + clip + washer + nut) for corrosion-resistant applications. For HDG anchors, verify that the HDG coating does not prevent the expansion clip from seating correctly in the hole — some manufacturers specify that HDG wedge anchors use a hole drilled 1 mm oversize to accommodate the HDG thickness on the anchor body.
Installation Requirements, QC,
Applications & Export Documentation
Structural · Industrial · Seismic · Marine · Offshore
Installation Requirements
Drill the hole with a rotary hammer drill (never a core drill — core-drilled holes have a smoother surface and reduced pull-out capacity compared to hammer-drilled holes) using an SDS-Plus or SDS-Max carbide-tipped bit of the exact diameter specified for the anchor size. Drill to the required hole depth (minimum = embedment depth + 6 mm clearance for drill tip). Keep the drill perpendicular to the concrete surface — an angular hole reduces the bearing contact of the expansion clip. Do not use a worn or chipped drill bit — replace bits that have drilled more than their rated number of holes per the manufacturer's wear recommendation.
Clean the drilled hole of all loose concrete dust and debris before inserting the anchor. The presence of concrete dust in the hole acts as a lubricant between the expansion clip and the hole wall, significantly reducing the friction-based pull-out capacity. Standard cleaning: blow compressed air into the hole twice (from bottom to top), then brush with a stiff wire brush of the correct diameter, then blow again. For ICC-ES evaluated anchors in seismic applications, the cleaning procedure (number of blow-and-brush cycles) must follow the exact procedure stated in the ICC-ES evaluation report.
Insert the wedge anchor through the fixture (base plate, bracket) and into the hole. Drive the anchor to the required embedment depth by striking the head with a hammer until the clip contacts the bottom of the fixture hole. The fixture must be in contact with the concrete surface — shims or grout under the base plate are permitted provided the gap does not exceed the maximum specified in the ICC-ES report. Then apply the nut and washer finger-tight.
Tighten the nut to the manufacturer's specified installation torque using a calibrated torque wrench. The installation torque draws the bolt upward relative to the expansion clip, pulling the tapered cone into the clip segments and forcing them against the concrete hole wall. Do not use impact wrenches or pneumatic tools for final torquing — these cannot be controlled to the required torque accuracy. Installation torque values vary by anchor size and manufacturer; typical values are M10: 20–25 N·m; M12: 40–50 N·m; M16: 80–100 N·m; M20: 150–175 N·m. Over-torquing can fracture the anchor shank or strip the concrete thread — both are irreversible installation failures.
For safety-critical structural applications (overhead anchors, seismic restraints, equipment supports with dynamic loads), proof load testing of installed anchors is specified. A portable hydraulic pull-test jack is used to apply a specified proof load (typically 80% of the design tension load) to each anchor after installation. Anchors that show measurable displacement under the proof load or fail the test must be removed and the connection redesigned with additional or larger anchors. Proof testing does not substitute for correct design — it verifies installation quality only.
Applications by Industry
Wedge anchors are the standard post-installed anchor for structural steel connections where welding to the existing concrete structure is not possible or is prohibited: steel column base plates added to existing concrete slabs and foundations, steel mezzanine floor column bases, structural bracing knee braces anchored to concrete walls and floors, staircase stringers and handrail posts, and overhead structural steel grid connections to concrete roof and wall panels. For seismic zones (IBC Seismic Design Categories C through F), ICC-ES seismic-rated wedge anchors per AC193 criteria must be specified — standard wedge anchors have not been evaluated for cracked concrete or seismic load reversals.
Heavy industrial machines (CNC machining centres, presses, compressors, pumps, centrifuges, generators) are anchored to concrete pads using wedge anchors that resist the combined static weight of the machine and the dynamic reaction forces generated during operation. The anchor design must account for dynamic load amplification factors — typically 2–4× the static load for rotating machinery with unbalanced loads. Wedge anchors for machinery with continuous dynamic loading should be specified at a higher embedment depth than the minimum required for static tension, to ensure the concrete breakout mode does not govern under cyclic loading.
Pallet racking, shelving systems, cantilever racks and mezzanine storage floors are anchored to concrete warehouse slabs using wedge anchors. Rack anchoring is governed by the rack supplier's design calculations per EN 15512 (Europe) or RMI MH16.1 (USA), specifying anchor diameter, embedment, spacing and minimum concrete slab thickness. The critical design case for racking anchors is often the impact from forklift contact with the rack uprights — a dynamic lateral shear load that must be resisted by the anchor without failure of either the anchor or the concrete slab.
Wedge anchors in stainless steel 316 or duplex 2205 are used in offshore platform concrete structures, harbour quay walls, marine jetty decks and coastal infrastructure for equipment mounting, structural connection brackets, railing anchorage and cable management. The marine concrete environment — high chloride, cyclic wetting and drying, biological fouling — requires SS 316 minimum for exposed anchors; duplex 2205 for long-term reliability in chloride-saturated or submerged concrete. EN 10204 3.1 MTC, PMI and corrosion class certification are standard offshore project requirements.
Wedge anchors in overhead applications (HVAC duct hangers, fire sprinkler pipe hangers, electrical cable tray supports, suspended ceilings) carry their full design load in sustained tension — the most severe loading condition for a concrete anchor. All overhead wedge anchors must be designed with a generous safety margin, must be proof-tested after installation, and must use ICC-ES evaluated anchors where seismic ground motion could impose additional load. Undetected improper installation of overhead anchors (oversize holes, insufficient embedment, improper torque) is a life-safety hazard — these applications require contractor verification and inspection records for every installed anchor.
Export Packaging and Preservation
- Wedge anchors packed by size and coating in sealed polypropylene bags or cardboard boxes, labelled with anchor diameter, length, material grade, coating, batch/lot number and quantity
- Each anchor assembly must include all components (bolt, expansion clip, washer and nut) — missing components discovered on site cause installation delays; verify component count per bag before dispatch
- VCI (Volatile Corrosion Inhibitor) poly liner for carbon steel zinc-plated anchors for sea freight or storage exceeding 3 months
- SS anchors in clean sealed poly bags — segregated from carbon steel anchors to prevent ferrous contamination of stainless surfaces
- Boxes in double-wall corrugated cartons, palletised on ISPM-15 heat-treated timber with stretch wrap
- For ICC-ES evaluated anchors: the ICC-ES Evaluation Report (ESR) number must appear on the product label and the MTC, enabling the installer and inspector to download the current version of the ESR for the installation requirements
- MTC, dimensional inspection report, mechanical test report (proof load, installation torque), corrosion test certificate (salt spray), PMI report (SS/duplex), ICC-ES compliance statement (if applicable), and all project documents in waterproof sealed envelope on each pallet
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 | 3.1 for EPC structural; 3.2 for offshore, NACE, safety-critical applications |
| 02 | Dimensional Inspection Report | Manufacturer / ICC-ES ESR | AQL 1.0; anchor diameter, length, clip dimensions mandatory |
| 03 | Mechanical Test Report | ASTM F606 / ISO 898-1 | Proof load, tensile strength, installation torque per batch |
| 04 | Pull-Out Test Report (In-Situ) | ACI 355.2 / ETAG 001 | Required for safety-critical structural applications; on-site pull test records |
| 05 | Corrosion / Salt Spray Certificate | ISO 9227 / ASTM B117 | Required for all coated anchors; confirms coating class and hours-to-rust |
| 06 | Hardness Test Report | ASTM E10 / E18 | Mandatory for NACE MR0175 sour service; ≤22 HRC full cross-section |
| 07 | PMI Report (XRF / OES) | Project specification | 100% of SS, duplex and all exotic grade wedge anchors |
| 08 | ICC-ES Evaluation Report Reference | ICC-ES AC193 / ESR | ESR number + current report version; required for seismic and cracked concrete |
| 09 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include wedge anchor manufacture |
| 10 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports wedge anchors in all types — standard, heavy-duty, seismic/cracked concrete, through-bolt and custom — in carbon steel grades 4.6, 5.8 and 8.8, ASTM F1554 Gr.36/55, A193 B7M (NACE), stainless steel SS 304 and SS 316, and duplex 2205. Sizes M8–M36 metric and 3/8"–1-1/4" inch UNC. Coatings: zinc plate (ASTM B633), mechanical zinc (ASTM B695), hot-dip galvanise (ASTM A153), Climaseal/Geomet, epoxy coat, SS passivation. EN 10204 3.1 MTC, mechanical test reports, salt spray certificates, NACE hardness mapping, PMI, ICC-ES compliance statements. 48-hour express dispatch on standard in-stock sizes.