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.
The effective embedment depth of the anchor into the concrete.
The concrete's specified compressive strength.
Whether the anchor was cast into the wet concrete or drilled and fixed afterwards.
Basic breakout capacity
8.6 kips
This is the BASIC single-anchor breakout strength only (ANc/ANco = 1, no edge-distance, spacing or eccentricity reduction, and no Psi-c,N cracking modifier — ACI 318's basic equation is on a cracked-concrete basis, and an anchor verified to sit in uncracked concrete earns a further increase this figure does not take) — 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
- 8,601.4 lb
They open the calculator with your figures already in it
Concrete Anchor Bolt Breakout Capacity Calculator (ACI 318): 8.6 kips — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 8.6 kips — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- This is a nominal strength, not a design capacity. No strength-reduction factor is applied and no factored load is checked against it, so the figure cannot stand as a code compliance check on its own.
- Only concrete breakout in tension is modelled. Anchor steel fracture, pullout at the head, side-face blowout near a free edge, shear breakout and combined tension-plus-shear interaction are not calculated, and any one of them can govern the anchor instead.
- The full unrestricted breakout cone is assumed. Member thickness, group behaviour where several anchors share a cone, and anchor reinforcement designed to carry the load past a breakout failure are all outside this calculation, on top of the edge-distance, spacing and eccentricity factors the result note already flags.
- Normalweight concrete in ordinary service is assumed. There is no lightweight-concrete modification, no cracked-versus-uncracked modification, and no seismic provisions — an anchor in a seismic force-resisting system is subject to extra requirements this page does not touch.
- For post-installed anchors the generic coefficient of 17 stands in for the actual product. It carries nothing about adhesive bond strength, elevated-temperature or sustained-load performance, hole cleaning, drilling method or installation torque, all of which the manufacturer's qualified evaluation report governs.
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 (5/8 in) 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.
The baseplate's overall length (long side).
The baseplate's overall width (short side).
The maximum allowable spacing between anchor bolts around the perimeter.
How far the centre of a corner rod is held back from the plate edge.
Total anchor bolts
10 anchor bolts
- Bolts per long side
- 4
- Bolts per short side
- 3
- Pitch along each long side
- 0.56 ft
- Pitch across each short side
- 0.5 ft
They open the calculator with your figures already in it
Baseplate Anchor Bolt Pattern Layout Calculator: 10 anchor bolts — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The pattern assumed here is a uniform run of rods along all four plate edges. Many column bases instead put rods only outside the flanges in two rows, and a rod that this layout places behind the column section, a stiffener or a shear lug may have no room for a socket or spanner. The calculator does not check wrench access or clashes with the column profile.
- The only rule applied is the maximum spacing you enter, but anchor rod groups are normally governed by the opposite limit. Rods placed too close together have overlapping concrete breakout cones, which cuts the tension capacity of the group well below the sum of the individual rods, and distance to the concrete edge controls the same failure. Enter a small maximum spacing and this page will return a dense pattern with nothing checking that the rods are far enough apart, or far enough from the concrete edge, to develop their capacity; those minima come from the anchor design, typically ACI 318 Chapter 17.
- The edge distance is taken as given and is never checked. It is the figure that has to satisfy the plate — an oversized hole, a washer covering it, and a socket that can reach the nut — and separately the concrete, where distance to a pier face is what stops a breakout cone forming. Those are different numbers from different codes, and the larger of them governs; this page divides by whichever one you type.
- The two pitches are even divisions of each side, so they sit at or below the maximum rather than on it, and the long and short sides will usually differ from each other. Neither is a gauge line across the plate: the rods on the two long sides face each other at the plate width less twice the edge distance, and that cross dimension, along with which rod is the datum, still has to be put on the drawing by hand.
- Nothing about the rods themselves comes out of this calculation: no diameter, embedment depth, projection above the grout line, thread length, baseplate hole size or washer type. The number is also a pattern count rather than a purchase quantity, so it carries no spares for bent or misplaced rods, no template rods, and no levelling or double nuts.
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.
Anchor reinforcement only counts once each bar is developed on the far side of the assumed failure surface, so the tail beyond that plane is the length to establish before the steel fixer starts tying.
The rebar bar size being developed.
The rebar's specified yield strength.
The concrete's specified compressive strength.
A bar with a lot of fresh concrete under it develops less bond.
Epoxy is a bond breaker as well as a corrosion barrier.
1.0 for normal-weight concrete; 0.75 for all-lightweight concrete.
Development length
23.7 in
This is ACI 318's simplified Case 1 equation, valid only when clear spacing and cover meet the code's minimum thresholds and minimum ties/stirrups are provided — tighter spacing or less cover requires the full (longer) development length equation with additional modification factors.
- Equivalent in mm
- 602.41 mm
- Formula constant (K) used
- 25
They open the calculator with your figures already in it
Rebar Development Length Calculator (ACI 318): 23.72 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 23.7 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Case 1 spacing and cover are assumed, never checked. There is no input for clear spacing, clear cover or transverse reinforcement, so the page cannot tell whether your bar actually qualifies for the simplified equation — you confirm that off the drawing. A bar that falls outside Case 1 needs ACI's full development length equation, which returns a longer number than this one.
- This covers straight deformed bars in tension only. Standard hooks, headed bars and bars developed in compression follow different ACI expressions and are shorter; a lap splice is longer than the development length returned here. Do not carry this figure into any of those details.
- Only US bar designations #3 through #11 are offered, and only ACI rules are applied. #14 and #18 bars are not in the list, and neither are soft-metric or European bar sizes — the soft-metric figure is an ACI answer restated, not a Eurocode 2 or CSA A23.3 anchorage length.
- Seismic detailing is outside this calculation. ACI 318's provisions for special moment frames and special structural walls carry their own anchorage and splice requirements in the higher seismic design categories, and none of them are in this equation.
- This returns a required length, not a design. It does not check that the column, footing or wall you are anchoring into is deep enough to provide that embedment, does not locate the critical section the length is measured from, and does not replace the engineer of record's stamped detail.
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 (25 mm) 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.
Block capacity is scaled off the depth the manufacturer actually tested, so a proposed embedment has to be converted into what that depth is worth before anybody prices the fixing.
The anchor's rated capacity at its tested reference embedment depth, from the manufacturer's ICC-ES report.
The embedment depth the rated capacity was tested at.
The embedment depth you're actually planning to use.
Whether there is solid material behind the face shell where the anchor lands.
Scaled capacity estimate
1.1 kips
The embedment you entered is deeper than the one the capacity was rated at, so this reports the RATED capacity rather than scaling it up. Linear scaling past a tested depth would be extrapolation, and masonry anchor capacity does not behave linearly there — it flattens off as the failure moves from the anchor into the block. The evaluation report will have a row for the deeper embedment; use that figure, not this one. Establish whether the cell is grouted before relying on this. The same anchor at the same depth carries very different loads in a grouted cell and a hollow one, and the evaluation report tabulates them separately. Linear scaling is a rough approximation, not a substitute for actual manufacturer test data at your specific embedment depth — masonry anchor behavior is governed by the specific product's ICC-ES evaluation report, which should be consulted directly whenever the exact embedment you're using has its own tested/rated value.
- Rated capacity entered
- 1.12 kips
- Embedment ratio applied
- 1
They open the calculator with your figures already in it
CMU Anchor Embedment Scaling Calculator: 1.12 kips — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1.1 kips — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- No safety factor is applied, and nothing here knows what basis your rated figure was on. Enter an ultimate test load and you get a scaled ultimate load back; enter an allowable working load and you get a scaled working load. This is not a design capacity and it does not replace the manufacturer's tabulated value for your actual embedment, signed off by whoever is responsible for the connection.
- Only the two depths and the rated number reach the arithmetic. Edge distance, spacing between anchors, block and grout strength, mortar joints, cracked masonry and the anchor's own steel capacity are not inputs, so a fixing set near the end of a block or hard against its neighbour returns the same figure as one in the middle of a sound wall.
- Tension and shear are scaled identically and a fixing loaded in both at once is not checked against any interaction rule. The page has no input for load direction and cannot tell which one you are asking about.
- Scaling down assumes capacity falls in step with depth. Where the failure is a cone breaking out of the block rather than the anchor pulling through, capacity drops off faster than that below the tested depth, so a shallow-embedment answer here can read higher than the anchor will actually reach.
- The wall condition you select changes the warnings, not the number: a hollow cell and a grouted one at the same two depths return the same capacity. There is no wall thickness or block size input either, so nothing checks that the embedment fits inside the unit, and nothing accounts for installation — a hole landing in a joint or a web, spalling, dust left in the hole, or setting torque.
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.
What sits between a baseplate and the concrete
- Baseplate, washers and nuts — the piece everything else is detailed around — hole size, gusset clearance and washer bearing are all frozen the moment it is fabricated Baseplate Anchor Bolt Pattern Layout Calculator
- Non-shrink grout bed — placed after plumb is confirmed so bearing under the plate is continuous instead of resting on four steel packs Baseplate Anchor Bolt Count & Grout Pocket Volume Calculator
- Shim pack or levelling nuts — carries the plate at its set elevation while grout is worked under it, and is the thing final torque must never be pulled against
- Anchor rods — load enters at the far end of the rod, so the length on the delivery note and the embedment you actually get are two separate figures Concrete Anchor Bolt Breakout Capacity Calculator (ACI 318)
- Concrete member and its bars — supplies the wedge the whole connection borrows; steel crossing that wedge is what carries tension where plain concrete has none to give Rebar Development Length Calculator (ACI 318)
Bolt count for the uplift and the pocket of grout under the plate are the two quantities this sequence turns on — fix both while the plate is still on the trestles.
The total factored uplift (tension) load the baseplate connection must resist.
The lowest governing rated tensile capacity for a single anchor, per ACI 318 Chapter 17.
The baseplate's plan (footprint) area.
The thickness of the leveling grout pocket beneath the baseplate.
Anchor bolts needed
4 anchors
Use the lowest governing rated tensile capacity for your specific anchor (steel strength, concrete breakout, or pullout per ACI 318 Chapter 17) — not an assumed value. AISC Design Guide 1 covers full baseplate/anchor rod design.
- Grout pocket volume
- 3.62 gal
They open the calculator with your figures already in it
Baseplate Anchor Bolt Count & Grout Pocket Volume Calculator: 4 anchors — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Dividing one total by one anchor's capacity assumes every rod pulls the same. A column base carrying moment does not work that way: the axial load and the moment together decide the tension, and it lands almost entirely on the rods to one side of the plate, so a group sized on a shared total can be two rods short where it counts and two rods idle where it does not.
- Says nothing about setting the rods. Anchor rods are cast before the column exists, located off a template, and the tolerance they have to land within is tight — a group placed out of position is the commonest problem on a steel job, and the remedy (oversized plate washers, a replacement plate, field welding) is an engineering decision rather than a site one, because it changes the load path the count assumed.
- The grout figure is a pocket volume, not an order. Bagged non-shrink grout is sold against a yield quoted at a stated consistency, and a flowable mix yields differently from a plastic one, so the volume converts to bags only through that product's own table — and the pocket has to be filled from one side with a head behind it, because a void left under a plate is exactly the bearing area the column was sized on.
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 (1-1/2 in) 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.