Head style affects the fixture hole and the finished look.

• Hex washer head: driven with a hex socket (Tapcon 3/16 in. uses 1/4 in., 1/4 in. uses 5/16 in.).
• Flat Phillips head: Tapcon uses #2 for 3/16 in. and #3 for 1/4 in.
• Star flat head, Maxi-set and stainless Scots heads: available on Tapcon products.
• Titen HD: wrench sizes of 3/8, 9/16, 3/4, 15/16 and 1-1/8 in. for 1/4 through 3/4 in.

A hex-head anchor often needs a clearance hole in the fixture. Washer and nut hardware for larger fixtures is covered in the Washer Size Chart and the Nut Size Chart.

Spacing s is the center-to-center distance between anchors. Close anchors share a stress zone and can reduce capacity.

• Minimum spacing: the closest allowed distance, usually with a reduced capacity.
• Critical spacing: the distance at which full capacity applies.
• Group effects: checked in anchor design.

These are labeled CMU examples, not concrete values. In ESR-1671, critical distances relate to full capacity and minimum distances can involve reduced capacity. The report version reviewed was reissued September 2021; check ICC-ES for the current status.

For cast-in anchor bolts, see the Anchor Bolt Spacing Chart; screw anchors have their own evaluated values.

Edge distance c is measured from the anchor centerline to the free edge.

Anchors close to a free edge can reduce capacity or cause splitting and breakout. In the ESR-1671 CMU example the minimum edge distance is 2 in. and the critical edge distance is 4 in., while Hilti lists 4 in. as the minimum for KH-EZ on the face of grout-filled CMU. That spread shows why there is no universal “2-inch minimum.”

There is no valid universal “screw diameter holds X pounds” chart. Capacity depends on:

• Anchor model and diameter.
• Embedment and concrete strength.
• Cracked or uncracked condition.
• Edge distance, spacing and member thickness.
• Tension, shear and combined loading, plus seismic conditions.
• Installation quality.

One illustration: for the same 1/2 in. Hilti KH-EZ in uncracked grout-filled CMU at f’m 1,500 psi, the tabulated tension design strength is 800 lb at 1.52 in. effective embedment and 2,475 lb at 3.22 in. In cracked CMU the values are 565 and 1,745 lb. That is a data example, not a design value.

DEWALT likewise publishes Screw-Bolt+ design data by diameter, embedment and concrete strength, so the numbers change when any input changes.

Tension

Force pulling the anchor out of the concrete.

Shear

Force acting parallel to the concrete surface.

Combined loading

Structural anchor design checks tension and shear together. ACI 318 Chapter 17 covers anchoring to concrete, including tension, shear, interaction, spacing, edge distance, member thickness and seismic provisions.

Concrete compressive strength f’c affects anchorage performance. DEWALT’s evaluated Screw-Bolt+ is qualified for concrete from 2,500 to 8,500 psi, and its tables change across that range.

Higher strength does not always let you use a smaller anchor without a design check. For strength classes see the Concrete PSI Chart, and for how strength develops see the Concrete Strength Gain Chart.

Approval and allowable-load data must match the base material.

• Normal-weight concrete: the base case for most structural screw anchors.
• Grout-filled CMU: evaluated separately (Tapcon ESR-1671, Hilti ESR-3056, DEWALT ESR-4042).
• Hollow CMU: thin face shells limit embedment; do not use concrete values.
• Brick: some products list solid clay brick as a suitable base material.

Do not transfer concrete values into masonry.

Structural anchor qualification distinguishes cracked and uncracked concrete.

• Simpson states the Titen HD is for use in cracked and uncracked concrete and masonry.
• Hilti and DEWALT publish cracked-concrete data for their evaluated screw anchors.
• Tapcon’s ESR-1671 limits its anchors to uncracked masonry and states that wind and seismic loads are beyond its scope.

Do not present cracked-concrete qualification as a property of all masonry screws.

• Zinc-plated carbon steel: Simpson’s standard Titen HD is intended for dry, interior, non-corrosive environments.
• Stainless steel: Titen HD stainless is for severe environments; Hilti offers a corrosion-resistant KH-EZ CRC and a 316 stainless line.
• Tapcon ESR-1671: anchors are limited to dry, interior use in that report.

Do not assume blue concrete screws are waterproof. Diameter and length alone are not enough for exterior, wet or chemically exposed applications.

1. Identify the base material.
2. Select an evaluated anchor for that base material.
3. Select the diameter and length (fixture thickness plus embedment).
4. Use the specified drill bit (carbide, hammer drill).
5. Drill to the required hole depth (embedment plus the product’s overdrill).
6. Clean the hole with compressed air, a vacuum or a blow-out bulb as required.
7. Insert the anchor through the fixture.
8. Drive it per the instructions: Tapcon is driven with the drill in rotary-only mode, and Titen HD is tightened until the head contacts the fixture.

Do not install anchors in masonry that has not reached its specified strength, and follow the manufacturer’s instructions where they differ from this summary.

More references are in the Construction Charts hub.