The Anchor Types and What Each Needs From the Base Material
Why this matters
Anchor selection usually runs the wrong direction. Somebody picks a family they are comfortable with, then goes looking for a way to make the wall accept it. Every anchor family buys its capacity by making a specific demand on the base material, and the base material either can supply that demand or cannot. It does not negotiate. The most reliable way to stop picking wrong is to write down what the base material is before you look at a single product page, because the act of filling in six lines eliminates most of the catalog before preference gets involved.
Before you drill a wall you did not build
- Masonry and concrete dust is a silica exposure. Drilling either releases respirable crystalline silica, which is an inhalation hazard with no contact-route control: gloves and glasses are irrelevant to it. Construction work falls under 29 CFR 1926.1153, whose Table 1 pairs handheld drilling into concrete or masonry with water delivery or a shroud plus a dust collector with HEPA filtration; general industry is 29 CFR 1910.1053. Respirators belong to a written program under 29 CFR 1910.134.
- Painted surfaces in older buildings. Drilling through paint on a pre-1978 building can release lead dust. Lead exposure is covered by 29 CFR 1910.1025 in general industry and 29 CFR 1926.62 in construction, and renovation work in pre-1978 target housing and child-occupied facilities also falls under the EPA renovation, repair and painting rule at 40 CFR Part 745 Subpart E, which requires a certified firm and specific containment. Test the paint or presume lead and control the dust; do not sand or grind it to check.
- What is behind the wall. Scan for conduit, pipe, reinforcement and, in structural slabs, post-tensioning tendons. Where the scan is ambiguous and conduit is plausible, isolate the circuits under 29 CFR 1910.333(b)(2) and prove dead using the live-dead-live sequence in NFPA 70E-2021, 120.5. Do not drill a post-tensioned member without locating tendons and getting the structural engineer's approval.
What each family demands
| Family | How it develops load | What it must have from the base material |
|---|---|---|
| Cast-in headed bolt or channel | Direct bearing on the head, deep in the pour | To have been placed before the pour, in the right spot, at the right depth |
| Torque-controlled expansion (wedge) | Friction and keying from an expansion element pressed outward | Solid, confined material that can resist expansion pressure; a tightly controlled hole diameter; hole depth past the embedment for the tail |
| Displacement-controlled expansion (drop-in) | Expansion driven by a setting tool, not by torque | A flat-bottomed hole set to depth and a setting tool driven fully home; solid material as above |
| Undercut | Mechanical interlock into a machined undercut at depth | Solid material plus a drilling and undercutting operation; performs best where the concrete may crack |
| Concrete screw | A hardened thread cut into the hole wall | Material hard enough to hold a cut thread, and hole diameter and depth held to close tolerance |
| Adhesive (bonded) | Shear at the bond line along the whole embedment | A clean hole, a moisture and temperature condition inside the adhesive's qualified window, and undisturbed cure time before load |
| Screen-tube adhesive | Adhesive keyed into the cavities of a hollow unit | Hollow units the anchor is specifically qualified for, and reduced published values |
| Powder or gas-actuated pin | A driven pin held by friction and material recovery | Base material inside a narrow hardness band: too hard and the pin will not set, too soft and it passes through |
| Through-bolt with backing plate | Bearing on a plate on the far face | Access to both faces, and a far face that can carry the plate's bearing |
| Hollow-wall anchor (toggle, sleeve) | Bearing spread across the back of a thin skin | A skin whose bending and punching strength carries the spread load |
The record that makes the choice
Six lines, written before anything is selected. Not a form for its own sake: each line, filled in honestly, deletes families from the table above.
- Material and how you know. Not "concrete." Poured concrete, hollow concrete masonry, solid grouted masonry, brick, hollow-core plank, precast, or "unknown" with the pilot-hole evidence attached.
- Thickness or member depth, measured. For a hollow unit, the face shell thickness read off a depth gauge in the pilot hole.
- Condition. Sound, spalled, delaminated, previously patched, painted, carbonated at the surface, or with an old anchor hole nearby.
- Moisture and temperature of the base material at the time of installation, not the air temperature.
- Scan result. Reinforcement, conduit, pipe, tendons, and how close.
- Available edge distance and spacing, measured to the nearest free edge and between anchor positions.
Then, and only then, the load case: sustained or short-term, tension or shear or both, overhead or wall, vibration present, fire or seismic requirements.
The record filled in: a wall-hung unit on block
A 90 lb piece of wall-hung equipment goes on an interior wall in a building from the 1960s. The record comes back like this.
Material and how you know: hollow concrete masonry. A pilot hole in a spot hidden behind the equipment broke through and then fell into a void.
Thickness, measured: the depth gauge read 1.25 in to breakthrough, consistent with the face shell of a hollow unit, with a void behind it.
Condition: painted, building predates 1978, so the drilling is done under lead controls with a HEPA shroud, and the pilot itself was drilled the same way.
Moisture and temperature: interior, dry, at room temperature.
Scan: no conduit within 12 in of the intended pattern.
Edge distance and spacing: the nearest free edge is a door jamb 20 in away, which is not a constraint here, but sounding the wall put the block webs on a grid that the four-anchor pattern does not naturally land on.
Load case: sustained tension and shear, wall-mounted, the equipment's center of gravity 8 in off the wall face, anchor pattern 12 in tall by 10 in wide, four anchors.
What the record eliminated
Against a 1.25 in face shell with a void behind it, most of the table is gone before any capacity number is opened:
- Wedge and drop-in expansion anchors are out. Both develop load by pressing outward against confined material. A 1.25 in shell with air behind it provides no confinement, and the shell blows out at a fraction of the anchor's published value.
- Powder-actuated pins are out. A hollow face shell is the wrong material to drive into and the shell will spall.
- Through-bolting is out. No access to the far face.
- Standard adhesive is out unless the anchor lands in a solid web or a grouted cell, because an adhesive anchor in a hollow unit without a screen tube simply pumps adhesive into the void.
- What remains: a screen-tube adhesive anchor qualified for hollow units, a hollow-wall anchor that bears against the back of the face shell, or relocating the pattern so every anchor lands in a web or a grouted cell and a solid-material family becomes available again.
The last option is worth more than it looks. Sounding the wall and shifting the bracket a few inches so anchors land in webs converts a hard problem into an easy one, and it is free. When the pattern cannot be moved, you are in hollow-unit territory and you use hollow-unit published values, which are a fraction of the same anchor's solid-concrete numbers.
The load the record still has to satisfy
Selection narrowed the family. It did not size anything. Run the demand.
The equipment weighs 90 lb with its center of gravity 8 in off the wall face. That overhang tries to rotate the bracket about its bottom edge, and the top row of anchors resists it. With the rows 12 in apart, the tension the top row carries is 90 lb multiplied by 8 in divided by 12 in, which is 60 lb. Split across the two top anchors, that is 30 lb of tension each.
The weight itself is carried in shear across all four anchors, so 22.5 lb of shear each.
Both act at once on the top pair, and tension and shear interact: an anchor at most of its tension capacity has little shear capacity left. Manufacturers publish an interaction rule for this, and ACI 318 Chapter 17 uses one in which a component below roughly a fifth of its own capacity can be neglected while the other is taken at full value. Neither of these components is small relative to a hollow-unit rating, so this joint gets checked on the interaction, not on tension alone.
Those two demand numbers, 30 lb tension and 22.5 lb shear per top anchor, are what you carry to the manufacturer's hollow-unit table with the stated safety factor applied. Not the solid-concrete table. The single most common way this job goes wrong is reading a value off the front of the data sheet, which is the solid-concrete number, and never turning to the hollow-unit page.
The failure mode if that page is skipped. The unit goes up, holds for months, and one day the top of it tips away from the wall with two ragged craters in the face shell where anchors pulled cones out of a 1.25 in skin. The anchors are intact. So is the equipment's mounting flange. Everything failed in the one element nobody looked up.
Where the record flips the answer
Four conditions change the family even when the base material does not:
- Cold base material. Adhesive gel and cure times lengthen sharply as the base material cools, and below the adhesive's qualified minimum temperature the cure is not qualified at all. The controlling temperature is the concrete's, not the room's, and a wall that has been below freezing overnight is still cold at mid-morning. This alone routes many winter jobs to a mechanical family.
- A saturated or water-filled hole. Some adhesives are qualified for water-saturated concrete or water-filled holes and most are qualified only dry, at different values for each. This is a datasheet lookup, not a judgment call.
- Overhead sustained tension. Adhesive anchors installed horizontally or upwardly inclined to resist sustained tension are treated as a special case under ACI 318 Chapter 17, in the edition adopted by the building code your authority having jurisdiction enforces, requiring a certified installer and continuous inspection. If the record's load case line says "overhead, sustained," that requirement is triggered before you compare any capacities.
- Vibration or repeated load. A torque-controlled expansion anchor holds by a preload that vibration can relax, which is the same mechanism a sibling article covers for bolted joints generally. Undercut and adhesive families are less sensitive to it. Where the record notes rotating equipment nearby, that changes the family even in perfect concrete.
References
- ACI 318 Chapter 17, Anchoring to Concrete, in the edition incorporated by the building code your authority having jurisdiction has adopted, which binds the design and reaches you through the permit and the engineer of record
- Anchor manufacturer evaluation reports and installation instructions, including the separate hollow-unit tables, temperature and moisture qualification windows, and hole-cleaning procedures, which own every specific capacity
- 29 CFR 1926.1153 and 29 CFR 1910.1053 for respirable crystalline silica; 29 CFR 1910.134 for respiratory protection programs
- 29 CFR 1926.62 and 29 CFR 1910.1025 for lead; EPA renovation, repair and painting rule at 40 CFR Part 745 Subpart E for pre-1978 target housing and child-occupied facilities
- See related: How an Anchor Transfers Load Into Concrete; What Hollow Wall and Thin Material Change About Anchoring