Fixings
Anchoring Into Concrete and Masonry
Every concrete anchor drags a cone of substrate with it — edge distance, spacing and embedment are all arguments about that cone.
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The cone you cannot see
An anchor loaded in tension rarely fails by sliding out of its hole. Far more often it tears a wedge of concrete off the member and takes that wedge with it. The wedge starts at the deepest bearing point of the anchor and spreads upward and outward toward the free surface at roughly thirty-five degrees. Design models idealise it as a pyramid whose base runs about one and a half times the effective embedment in every direction, which is why the figure 1.5hef turns up in every anchor table you will ever read and in the concrete capacity provisions of ACI 318, Building Code Requirements for Structural Concrete and Commentary.
Once that shape sits in your head, the whole trade reorganises around it. Edge distance becomes an argument about whether the cone has concrete on all four sides. Spacing becomes an argument about whether two anchors are trying to lift the same lump. Embedment decides how large the cone gets to be, and member thickness decides whether it punches out the underside instead of the top. Hole cleaning determines whether the anchor ever engages the cone at all, or sits in a sleeve of drill dust and pulls free long before the concrete is asked to do anything.
Failure has a short menu and the cone dominates it: steel rupture, concrete breakout in tension, pullout or bond failure, side-face blowout, pryout under shear, edge breakout under shear, and splitting of the member. Only the first is about the bolt. The rest are about the concrete you are borrowing.
Reading the member before the cone is cut
Everything inside the pour changes the cone's shape. Reinforcement congestion, post-tensioned tendons, embedded conduit and cast-in plates all sit somewhere in that wedge, and cutting a tendon in a PT slab is a structural incident rather than a site inconvenience. Scan the area with ground-penetrating radar or an electromagnetic locator, mark what you find, and treat the marks as fixed obstacles that layout has to work around. Composite slabs on steel deck complicate the geometry again: the cone truncates against the deck profile, and anchors set in a flute behave nothing like anchors in a solid slab of the same nominal thickness.
Concrete condition matters as much as concrete geometry. Anchor capacities are published separately for cracked and uncracked concrete, and an anchor in a tension zone — over a support, in a negative moment region, near a shrinkage crack — has to be assessed as cracked unless analysis proves otherwise. Lightweight concrete reduces breakout capacity by a modification factor. Strength at the age of installation, not the specified 28-day strength, is what the cone actually delivers on a fast-track job. Where the design assumption and the field condition disagree, the engineer of record settles it, not the installer.
Ask early what the anchor is holding. A handrail post, a pipe hanger and a moment-resisting baseplate all pull cones, but only one of them will be checked by a special inspector and only one will take somebody with it if the cone lets go.
Where the cone runs out of concrete
Concrete has to exist on every side of the wedge for an anchor to develop full breakout capacity. Cut one side away with a slab edge, a construction joint, a saw cut, a blockout or an existing core hole, and the projected area drops in proportion to the missing portion. That reduction begins as soon as the edge comes closer than about one and a half times the embedment, and it accelerates from there. Load applied off-centre on the plate compounds it, because the effective cone shifts toward the edge you have the least of.
Two edge-related failures behave differently and catch people out. A deep cast-in headed anchor near a face can blow out sideways at the embedded head before any surface cone forms, which is why generous embedment with mean cover is not automatically safer. Shear toward a free edge carves a half-cone out of the face, and that capacity scales with edge distance rather than embedment — drilling deeper buys nothing at all. Splitting is the third: an expansion anchor set close to an edge or in a thin member can crack the member during setting torque, before service load ever arrives.
Minimum edge distance, minimum member thickness and minimum spacing for any specific post-installed anchor come from its qualification, not from a generic chart. In North America that means the product's ICC-ES evaluation report written against the acceptance criteria for mechanical or adhesive anchors, backed by ACI 355.2, Qualification of Post-Installed Mechanical Anchors in Concrete, or ACI 355.4, Qualification of Post-Installed Adhesive Anchors in Concrete. In Europe it means the European Technical Assessment and EN 1992-4, Design of fastened connections. Jurisdiction decides which document governs; the adopted building code decides which edition.
Edge distances get argued about with a tape measure and settled with numbers, so run the cone's actual breakout capacity against the design tension before you commit a hole position within reach of a free edge.
Basic breakout capacity
54 kN
This is the BASIC single-anchor breakout strength only (ANc/ANco = 1, no edge-distance, spacing, or eccentricity reduction factors applied) — it assumes the anchor is far enough from any edge or other anchor to develop a full breakout cone. Real anchor design requires the complete ACI 318 Chapter 17 procedure, verified by a licensed engineer.
- Equivalent in lbs
- 12143.15 lb
Running these inputs gives 54 kN as the basic breakout capacity. Low confidence on these inputs, so use the number to plan rather than to order. Currently reading for United States — pick a different market above and the figures re-cast accordingly.
Add the equipment this sizes
This result is a specification — 54 kN — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Cones that overlap
Two anchors close together do not pull two cones. They pull one shared lump of concrete, and the combined projected area is smaller than the sum of the individual areas. Full separation needs roughly three times the embedment between anchors; below that, group capacity per anchor falls steadily. Crowding four bolts onto a small baseplate is one of the most common ways to lose capacity while appearing to add it — the pattern collapses toward a single cone barely bigger than one anchor would have produced on its own.
Fixing an overlap is a layout problem, almost never a bolt-diameter problem. Spread the pattern on a larger plate. Rotate the group so its long dimension runs parallel to the nearest edge rather than toward it. Stagger rows so no two cones stack directly. Where a corner leaves the cone short on two sides at once, moving the whole assembly a few hundred millimetres inboard usually recovers more capacity than any change of hardware. Every one of those moves has to happen before the plate is detailed, because bolt holes, gusset positions and weld access are fixed by then.
Hole tolerance belongs in the same conversation. Oversized plate holes ease erection and demand plate washers to spread bearing; templates hold the pattern during a pour but only if they are braced against concrete pressure and checked after vibration. A pattern that arrives on site 15 mm out is an argument you will have with the fabricator while the crane stands idle.
Since overlapping cones are solved by moving bolts rather than upsizing them, work the pattern geometry here — before the template goes to the fabricator and the holes become permanent.
Total anchor bolts
10 anchor bolts
- Bolts per long side
- 4
- Bolts per short side
- 3
Running these inputs gives 10 as the total anchor bolts. Currently reading for United States — pick a different market above and the figures re-cast accordingly.
Add the equipment this sizes
This result is a specification — 10 anchor bolts — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Buying cone with depth
Effective embedment is measured from the concrete surface to the load-bearing point of the anchor, and that is not the same as the length of steel you bought. A wedge anchor bears at the expansion clip, a screw anchor along its engaged thread, an adhesive anchor along the bonded length, a headed stud at the underside of the head. Confusing nominal length with hef is a standing source of undersized cones. Drill deeper than the required embedment to leave room for cuttings, set the depth gauge, and use the embedment mark on the anchor rather than counting exposed thread.
Cleaning decides whether the depth you drilled is depth you can use. Adhesive systems specify a brush-and-blow cycle — a wire brush of the correct diameter, oil-free compressed air, repeated the number of times the manufacturer's printed installation instructions require. A worn brush that no longer contacts the wall is a failed clean that looks identical to a good one. Hollow drill bits with vacuum extraction remove that variable and are worth the hire. Diamond-cored holes are a separate case entirely: the smooth wall cuts bond drastically, and only adhesives qualified for cored holes, usually with roughening, may be used in them.
Adhesive behaviour is time and temperature dependent in both directions. Cold substrate extends cure time; hot substrate shortens working time to less than the time it takes to place a long bar. Sustained tension raises the creep question, which is why overhead and horizontal adhesive anchors carrying sustained load attract installer certification requirements and continuous special inspection under most adopted codes.
Designing the cone out of the problem
Sometimes the concrete simply cannot produce the wedge you need — a thin slab, a narrow beam, a corner condition with edges on two sides. The way out is anchor reinforcement: bars developed on both sides of the assumed failure surface so that tension crosses the cone into the body of the member rather than relying on plain concrete in tension. Hairpins around baseplate bolts and U-bars in beam faces do exactly this. The design belongs to the engineer, but the placement belongs to whoever is tying steel, and a hairpin at the wrong depth is decoration.
Cast-in anchors beat post-installed anchors for the same reason: their bearing point sits deeper, their cone is bigger, and no drilled hole is required through reinforcement that is already congested. The cost is coordination. Templates must be located, braced, surveyed and re-checked after the pour begins, because vibration walks bolt groups out of position. Wet-setting anchors into fresh concrete by pushing them in and wiggling them is not an installation method; most product approvals exclude it outright, and it leaves a void down one side of the shank.
Seismic and ductility requirements change the target. Where the code demands ductile behaviour, the connection has to be governed by yielding steel rather than by concrete breakout — which means either a bigger cone or reinforcement across it, deliberately, not by accident of what fitted.
Masonry: the cone stops at the face shell
Hollow concrete masonry does not offer a cone in the concrete sense. The face shell is thin, the cell behind it is void, and any wedge that tries to form truncates at the back of the shell within the first inch or so of embedment. What carries load instead is bearing: a screen tube filled with adhesive keying into the cell, a toggle bearing on the inside face, a sleeve expanding against the shell. Failure mode changes accordingly — units break out as whole pieces along mortar joints rather than as neat cones.
Grouted cells restore something closer to concrete behaviour, but only where the grout is actually there and actually consolidated. Confirm grouting by drilling a probe hole or checking the grout lift records; assuming a fully grouted wall on the strength of a drawing note has embarrassed many crews. Position within the unit matters too: webs, cells, bed joints and the middle of a face shell all give different results, and mortar joints are usually the weakest of them. Use rotary drilling rather than hammer action in hollow units so the shell does not blow out around the hole.
Design provisions come from TMS 402/602, Building Code Requirements and Specification for Masonry Structures, and anchor values from masonry-specific evaluation reports. Scaling an embedment straight from a concrete table into block is not a valid substitution — the substrate, the failure mode and the qualification test are all different.
Setting, torquing and grouting
Sequence protects the cone. Survey, scan, mark, drill, clean, set, torque, verify — in that order, with the plate or template used as a drilling guide only where the approval allows it. Torque-controlled expansion anchors need setting torque applied with a calibrated wrench: too little and the clip never expands, too much and the cone in the concrete is damaged or the anchor spins in the hole. Some products require follow-up torque after a period; that requirement lives in the installation instructions, and it exists because concrete relaxes.
Screw anchors and undercut anchors are less forgiving of a reused hole than they look. Once a thread has cut its track or an undercut has been formed, backing the anchor out and re-driving it into the same hole gives a fraction of the original capacity. Treat a removed anchor's hole as abandoned. Abandoned holes inside the projected area of a live anchor reduce that anchor's cone as surely as a slab edge does, so fill them with high-strength non-shrink grout or an approved adhesive and record their positions.
Baseplates add their own sequence. Level on shims or levelling nuts, verify plumb before final torque, then place non-shrink grout under the plate so bearing is continuous. Torquing a plate down onto an unshimmed, unlevel bed bends the plate, pries on the outer bolts and applies tension nobody designed for. Specify anchor material by designation — ASTM F1554, Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength, covers the common grades — and match corrosion protection to exposure rather than to what is in the van.
Proving the cone is there
A finished anchor gives away almost nothing by eye, which is why proof testing exists. On-site proof loads are applied with a calibrated hydraulic ram against a bridge that spans well clear of the anchor's cone — bearing too close preloads the very concrete you are testing and makes the result meaningless. Tests are run to a specified load and held, not to failure, and the sample rate is set by the project specification or the special inspector. Qualification testing, by contrast, follows ASTM E488/E488M, Standard Test Methods for Strength of Anchors in Concrete Elements, and belongs to the manufacturer.
Watch for the cone announcing itself. A faint circular crack on the surface at roughly one and a half embedments from the anchor, dust ejecting from the annulus under load, or a nut that keeps turning at constant torque all indicate the concrete is moving rather than the steel stretching. Stop, do not retighten, and report it. Retightening a creeping anchor destroys the evidence and the concrete at once.
Documentation closes the loop: anchor type and evaluation report number, drill diameter and depth, cleaning method, adhesive batch and cure conditions, torque applied and by whom, plus any relocations and the reason for each. Where an inspector later asks why a bolt sits 40 mm off the drawing dimension, a scan record showing a tendon at that position ends the conversation in a minute. Without it, the same question can cost a day of coring and a remedial design.
Before the first hole
Five things that decide whether the cone you are counting on actually exists.
- Scan record with rebar, tendons and conduit marked — A hole moved on paper is trivial; a tendon cut in a post-tensioned slab is a structural incident.
- Evaluation report or ETA for the exact anchor specified — Edge, spacing, thickness and cracked-concrete values come from the product's own qualification, not a generic table.
- Drill bit and hole-cleaning kit matched to that report — Brush diameter, blow cycles and bit type are part of the approval; a worn brush fails silently.
- Depth gauge and calibrated torque wrench — Embedment is measured to the bearing point, and setting torque is a specified value rather than a feel.
- Non-shrink grout for abandoned holes and plate bedding — An unfilled hole inside a live anchor's cone reduces capacity exactly like a slab edge does.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.
Drawn from
- ACI 318 — Building Code Requirements for Structural Concrete and Commentary
- ACI 355.2 — Qualification of Post-Installed Mechanical Anchors in Concrete and Commentary
- ACI 355.4 — Qualification of Post-Installed Adhesive Anchors in Concrete
- ASTM E488/E488M — Standard Test Methods for Strength of Anchors in Concrete Elements
- ASTM F1554 — Standard Specification for Anchor Bolts, Steel, 36, 55, and 105-ksi Yield Strength
- TMS 402/602 — Building Code Requirements and Specification for Masonry Structures
- EN 1992-4 — Eurocode 2: Design of concrete structures, Part 4: Design of fastened connections
Guidance, not a specification. Local codes, the engineer of record and the product manufacturer’s instructions govern where they differ from anything written here.