Anchor Capacity Calculator

Calculator

Example Data Table

Formula Used

Steel tensile strength (per anchor):

Nsa = 0.75 × As × fu / 1000 (kN)

Steel shear strength (per anchor):

Vsa = 0.60 × As × fu / 1000 (kN)

Concrete breakout in tension (per anchor, simplified):

Ncb = kNc × √fc × hef^1.5 / 10^6 (kN)

Concrete breakout in shear (per anchor, simplified):

Vcb = kVc × √fc × hef^1.5 / 10^6 (kN)

Pullout (per anchor, simplified):

Np = kPull × fc × d × hef / 10^6 (kN)

Group reduction factors (bounded):

psi_edge = clamp(0.7 + 0.3 × c / (1.5hef), 0.2, 1.0)
psi_sp = clamp(0.7 + 0.3 × s / (3hef), 0.2, 1.0)
Group factor = n × psi_edge × psi_sp

Design strengths:

phiN = min(phi_s*Nsa_total, phi_c*Ncb_total, phi_p*Np_total)
phiV = min(phi_s*Vsa_total, phi_c*Vcb_total)

Interaction check: (Pu/phiN)^alpha + (Vu/phiV)^beta ≤ 1

How to Use This Calculator

1. Enter anchor diameter, embedment depth, and number of anchors.
2. Provide concrete strength and anchor steel ultimate strength.
3. Input edge distance and spacing to reflect your layout.
4. Enter the applied tension and shear demands for the anchor group.
5. Adjust strength factors and coefficients to match your method.
6. Click Calculate and review capacities, utilization, and PASS/FAIL.
7. Download the CSV or PDF report for records if needed.

Always verify assumptions, installation quality, and governing code provisions before finalizing design.

Professional Guide

1) Why anchor capacity matters

Anchors transfer equipment, façade, handrail, and baseplate forces into concrete. Capacity checks reduce the risk of sudden brittle failures, limit serviceability movement, and support safe installation planning. For preliminary sizing, compare factored tension and shear demands to design capacities and verify interaction limits.

2) Main limit states considered

Common limit states include steel yielding or fracture, concrete breakout in a cone-shaped failure surface, and pullout governed by bond or bearing. Shear checks also consider steel shear strength and concrete breakout in shear. The governing capacity is the smallest design strength among the applicable modes.

3) Steel strength interpretation

Steel capacity is driven by the tensile stress area and the anchor’s ultimate strength. Larger diameters increase area rapidly, so small diameter changes can meaningfully raise steel resistance. However, steel rarely governs when embedment is shallow or when the anchor is close to an edge.

4) Concrete breakout behavior

Concrete breakout typically grows with concrete strength and embedment depth. Deep embedment increases the effective failure surface and improves resistance. Edges and closely spaced anchors reduce the effective breakout area, so capacities are commonly reduced using edge and spacing factors for realistic group performance.

5) Pullout and bond considerations

Pullout capacity depends on bond quality, hole cleaning, installation torque, and adhesive curing for bonded anchors. Moisture, temperature, and cracked concrete can affect performance. Use manufacturer data when available and apply conservative factors where installation conditions are uncertain or variable.

6) Layout effects: edge distance and spacing

Increasing edge distance reduces the chance of edge breakout and improves reliability. Increasing spacing reduces overlap of stress fields between anchors and helps the group behave closer to independent anchors. When constraints force tight layouts, compensate with larger embedment, larger diameter, or additional anchors.

7) Combined tension and shear interaction

Anchors often see combined actions from eccentric baseplates or lateral loads with uplift. Interaction formulas provide a practical way to evaluate combined utilization. If the interaction exceeds unity, reduce demand through load path changes, increase capacity, or revise the connection geometry.

8) Practical workflow and reporting

Start with verified loads, select a trial diameter and embedment, then confirm edge distance and spacing from drawings. Run the check, review the controlling mode, and iterate. Save outputs using CSV for calculations and PDF for submittals, then finalize with detailed code checks. Record assumptions, check deflections, and coordinate drilling tolerances before site installation begins always.

FAQs

1) What inputs most strongly affect capacity?

Embedment depth, diameter, and concrete strength dominate. Edge distance and spacing can significantly reduce breakout capacity for groups, even when steel strength is high.

2) When does steel usually control?

Steel often controls for deep embedment, good edge distance, and strong concrete. In shallow or edge-limited layouts, concrete breakout or pullout typically becomes governing.

3) How should I choose the strength factors?

Use factors from your governing standard or project specification. If you are uncertain, apply conservative values and confirm with a qualified engineer and manufacturer documentation.

4) Can I use this for cracked concrete?

Yes as a preliminary screen, but cracked concrete often reduces bond and breakout performance. Use cracked-concrete ratings and design provisions specific to your jurisdiction for final decisions.

5) Why do edge distance and spacing reduce capacity?

Edges truncate the potential breakout cone and reduce effective concrete area. Close spacing causes overlapping stress zones, limiting group efficiency and reducing breakout resistance.

6) What does the interaction value mean?

It combines tension and shear utilization into one check. Values at or below 1.0 indicate acceptable combined demand for the selected exponents and factors.

7) Should I always rely on the PDF report?

Use the PDF for documentation of inputs and results, not as a substitute for final design. Confirm installation, detailing, and code compliance before construction.