For many anchors, the minimum spacing and the minimum edge distance cannot be chosen independently. The Hilti KB-TZ2 lists two permitted combinations, and a layout must satisfy both numbers of one combination.

Conditional Manufacturer Limits

Case A favors a smaller edge distance; Case B favors a smaller spacing. Hilti advises following the other parameter in whichever case you use, and mixing the cases is not recommended.

Hilti KB-TZ2 Examples

Source: Hilti KWIK BOLT TZ2 (KB-TZ2) carbon steel. Data edition: ICC-ES ESR-4266 (reissued December 2023, revised October 2024), Table 3, and Hilti North American Product Technical Guide, Volume 2, Edition 22, Table 5.

Interpolation Requirements

ESR-4266 permits additional combinations by linear interpolation between the two boundary values, and Hilti’s technical guide gives the same method. The straight-line calculation below shows how.

Effective vs. Nominal Embedment

Spacing formulas use effective embedment hef, which is not the same as nominal embedment hnom or the drilled depth. Using the wrong depth shifts every breakout calculation.

Source: Hilti KWIK BOLT TZ2 (KB-TZ2) carbon steel. Data edition: ICC-ES ESR-4266 (reissued December 2023, revised October 2024), Table 3, and Hilti North American Product Technical Guide, Volume 2, Edition 22, Table 5. (Table 1, setting information).

Critical Spacing Formula

The standard tension breakout model projects a region of about 1.5hef from the anchor axis. Two anchors whose projected regions just meet are 2 × 1.5hef = 3hef apart.

Worked Example

An anchor can meet its spacing and edge-distance requirements and still be wrong for a thin member. The minimum thickness comes from the product data or the governing design, never from the spacing chart.

Minimum Member Thickness

For the KB-TZ2, ESR-4266 lists minimum thickness by diameter and embedment and requires it in place of the default ACI value. A larger spacing does not substitute for adequate thickness.

Hole Depth

The hole is deeper than the embedment, so the concrete left below the hole is smaller than the slab thickness suggests. For a 3/8 in. anchor at 2 in. embedment in a 4 in. slab, the hole is 2-3/4 in. deep and 1-1/4 in. of concrete remains.

Source: Hilti KWIK BOLT TZ2 (KB-TZ2) carbon steel. Data edition: ICC-ES ESR-4266 (reissued December 2023, revised October 2024), Table 3, and Hilti North American Product Technical Guide, Volume 2, Edition 22, Table 5. Remaining concrete is calculated.

Thin Concrete Members

Check reinforcement, post-tensioning and embedded services before drilling a thin slab or wall. Cover values alone do not show where it is safe to drill. See the Concrete Cover Chart for reinforcement cover context.

Concrete may be treated as cracked or uncracked for anchor design depending on the structure and analysis. The assumption changes which products can be used and how much resistance they have.

Qualification

The KB-TZ2 is evaluated for cracked and uncracked concrete and Seismic Design Categories A through F. Some products are qualified for uncracked concrete only, so approval cannot be inferred from the anchor’s diameter or appearance.

Design Resistance

The KB-TZ2’s minimum spacing and edge-distance table is the same for both conditions, but its design strength uses separate cracked and uncracked factors. The concrete must reach its minimum design strength before anchors are installed; see the Concrete PSI Chart for strength classes.

Seismic Considerations

Seismic loading requires ACI 318 Section 17.10 provisions and a seismic-qualified anchor.

Torque-controlled anchors, such as wedge and sleeve anchors, expand when the nut is tightened. Displacement-controlled anchors, such as drop-in anchors, expand when a setting tool drives an internal plug. Both push outward on the hole wall.

Wedge Anchors

The KB-TZ2 is a torque-controlled expansion anchor. Its spacing and edge distance come from its evaluation report, as shown in the main chart. See the Wedge Anchor Size Chart for dimensions and installation.

Sleeve Anchors

Sleeve anchors spread load along a sleeve and use product-specific spacing. Check the Sleeve Anchor Size Chart and the manufacturer’s data.

Drop-In Anchors

Drop-in anchors have an internal thread and set by driving an expansion plug. See the Drop-In Anchor Size Chart and the product’s setting requirements.

Expansion-Induced Splitting

Expansion pressure can split concrete when edge distance or spacing is too small, which is why products publish minimums. Use the manufacturer’s evaluated numbers, not a diameter multiple.

Concrete screws cut their own threads into a drilled hole, so their geometry, hole size and embedment are product-specific. The Concrete Screw Size Chart covers dimensions.

Installation Requirements

Use the specified bit, hole depth and cleaning method, and follow the manufacturer’s instructions on whether a screw may be removed and reinstalled.

Minimum Spacing

Concrete screws publish their own minimum spacing by diameter and embedment. Do not assume the minimum spacing for a wedge anchor applies to a screw.

Edge Distance

Edge distance, member thickness and concrete breakout also come from the screw’s evaluation report and ACI 318 design.

Adhesive anchors are threaded rods or rebar set in a drilled hole with an approved adhesive.

Bonded Threaded Rods

Use an adhesive system qualified under ACI 355.4 with suitable threaded elements, correct hole diameter and cleaning, and the specified cure time and temperature. Material grades are in the Anchor Bolt Grade Chart.

Group Effects

Adhesive anchors can fail by concrete breakout, by bond failure, or by steel failure. The bond influence area can overlap at spacings that do not overlap the concrete breakout region, so both are checked. Minimum installation spacing is a different matter from calculated group strength.

Installation Conditions

Hole cleaning, moisture, temperature and cure time change performance. Follow the product report and instructions exactly.

Cast-in anchors are set before the concrete is placed. Their spacing and edge distance come from design geometry, load and ACI 318, and reinforcement can change breakout behavior.

Headed Anchors

ACI 318-19 sets minimum geometry for headed bolts, shown below. These are lower bounds, and design usually requires much more.

Anchor Rod Groups

Anchor rod groups under steel columns are designed for tension, shear and breakout as a group. Reinforcement, headed-stud details and anchor material affect the result.

Concrete Breakout

The same 1.5hef breakout geometry applies to cast-in headed anchors. Do not use post-installed anchor installation tables for cast-in anchors.

A group of anchors does not simply multiply one anchor’s capacity. When breakout regions overlap, the shared concrete is counted once, so four anchors do not give four times the strength of one isolated anchor.

Two-Anchor Groups

Two anchors spaced closer than 3hef share part of their projected area, as the earlier diagram shows.

Four-Anchor Groups

Four-anchor patterns overlap in both directions, and base plates add load eccentricity.

Overlapping Breakout Regions

Group Eccentricity

When the load is not centered on the group, anchors carry unequal shares and ACI applies an eccentricity factor.

Tension spacing is mainly a breakout question: how much concrete the anchors engage and whether their regions overlap.

Concrete Breakout

ACI 318 calculates breakout strength from the projected area compared with the single-anchor reference area, then applies modification factors. For the KB-TZ2, ESR-4266 requires ACI 318-19 Section 17.6.2 with its own effectiveness factors for cracked and uncracked concrete.

Projected Area

Strength Limitations

Steel strength and pullout strength can govern instead of breakout. The design strength is the lowest of all applicable failure modes, and spacing is only one input.

Shear is governed by the edge, not by the 3hef relationship. Tension characteristic spacing does not apply as a universal shear-spacing rule.

Shear Toward a Concrete Edge

An anchor loaded toward a nearby free edge can break out a wedge of concrete toward that edge. Edge distance in the load direction is the key variable.

Shear Parallel to an Edge

Load parallel to an edge is treated differently from load toward an edge, and ACI allows a larger shear strength for that direction.

Pryout and Group Behavior

Shallow anchors can pry out a cone of concrete behind the anchor, which is checked with a pryout coefficient. Groups near an edge also share the edge breakout region. For the KB-TZ2, the load bearing length of the anchor in shear is limited to the lesser of hef or 8da.

Anchors often carry tension and shear at once, so spacing is only one check in the anchorage design.

Interaction

ACI 318 Section 17.8 allows full tension strength when the shear demand is no more than 20% of the design shear strength, and full shear strength when the tension demand is no more than 20% of the design tension strength. Otherwise, the sum Nua / φNn + Vua / φVn must not exceed 1.2.

Governing Failure Modes

Steel, breakout, pullout and pryout are all checked in tension and shear, and the lowest governs.

Design Requirements

Use the ACI 318 interaction requirements and the product report. Do not invent a universal allowable load.

A steel column base plate over a concrete pedestal connects two systems. The plate geometry controls where the holes are, and the concrete controls how close the anchors can be to each other and to the edges.

Bolt Patterns

The hole pattern must match the anchor layout. Fixture hole diameters are product-specific; the KB-TZ2 lists 7/16 in. for the 3/8 in. anchor and 9/16 in. for the 1/2 in. anchor. Plate material, plate thickness and member shape come from the structural design.

Concrete Edges

Check the distance from every anchor to every free edge of the pedestal, not to the plate edge.

Structural Loads

Four equally spaced anchors do not automatically give enough resistance. The group must be designed for the applied loads and moments.

This page does not publish a code-approved bolt pattern. The pattern comes from the structural design.

Equipment anchors sit under dynamic and vibratory loads that static anchor tables do not cover.

Equipment Patterns

The equipment manufacturer sets the base hole pattern, and the engineer sets the anchors.

Vibration

Some mechanical anchor evaluations state that fatigue and shock loading are outside their scope. ESR-4266 says this directly for the KB-TZ2.

Engineered Design

Do not use residential sill-plate spacing or any universal rule for pumps, motors, generators or machinery. Concrete thickness and installation tolerances also need checking.

Guardrails and post bases are safety-critical, and loads are applied at the top of the post, which creates large moments at the anchors.

Edge Locations

Anchors near slab edges and corners have the least concrete to resist shear and breakout. Such locations may need a specialized design or a different connection detail.

Lateral Loads

Guardrail loads come from the governing code and must be carried by a qualified anchorage design.

Safety Requirements

The 2024 IRC prescriptive rule for sill-plate anchorage is a maximum on-center spacing, normally 6 feet, together with end-distance, bolt-count, diameter and embedment requirements and additional rules for higher wind and seismic conditions.

IRC Rules and Sill Plates

That limit controls how far apart bolts may be. It says nothing about how close two post-installed anchors may be.

Difference from ACI Minimum Spacing

ACI minimum spacing and edge distance protect the concrete. For complete foundation anchorage, use the Anchor Bolt Spacing Chart.

Many anchor products publish separate tables for concrete and masonry. A spacing value from a masonry table is not a generic minimum for cast concrete.

Cast Concrete

Use the concrete evaluation report. For the KB-TZ2 that is ESR-4266.

Grout-Filled CMU

The KB-TZ2 has a separate evaluation for grout-filled concrete masonry, with its own installation and spacing tables.

Product Qualifications

Masonry values depend on the wall construction and the stated conditions.

Three of these formulas are geometry only (center-to-center, clear spacing and edge distance). The critical spacing formula relates to concrete breakout analysis and is not an installation limit.

Center-to-Center Formula

Clear Spacing Formula

Critical Spacing Example

Confirm the Approved Anchor System

1. Review the approved plans and loads.
2. Identify the exact anchor product.
3. Confirm the concrete condition and thickness.
4. Obtain the product’s minimum spacing and edge-distance requirements.

Mark Anchor Centerlines and Check Edges

5. Mark anchor centerlines.
6. Check all relevant concrete edges.
7. Verify base-plate hole geometry.

Check Reinforcement and Services

8. Locate reinforcement and embedded services.
9. Confirm drilling or cast-in-place installation requirements.
10. Inspect the layout before installation.

Select the drill size from the Concrete Anchor Drill Bit Size Chart and the product instructions. Do not drill through reinforcement or post-tensioning tendons without an approved procedure; the Concrete Cover Chart does not show where it is safe to drill.