Plug Weld Strength Calculator

Calculator Inputs

Formula Used

This calculator estimates strength using the filled-hole weld area for each plug: Aw = π·d²/4.

Nominal strength per plug is checked for two simplified limit states:

• Weld metal: Rn,w = 0.60·FEXX·Aw
• Base material (simplified): Rn,b = 0.60·Fu·Aw

Governing nominal per plug is min(Rn,w, Rn,b), multiplied by n for total nominal strength.

Factored/allowable capacity is computed as: φ·Rn (LRFD-style) or Rn/Ω (ASD-style). Combined loads use (V/Vcap)² + (T/Tcap)² ≤ 1.

Note: Real projects may require additional checks (edge distances, minimum spacing, thickness limits, and procedure qualification). Use governing project criteria.

How to Use This Calculator

1. Enter hole diameter d, plate thickness t, and number of plugs n.
2. Provide electrode strength FEXX and base metal ultimate strength Fu.
3. Select LRFD-style (φ) or ASD-style (Ω), then set the factor value.
4. Enter applied shear V and tension T if needed.
5. Press Calculate to see results above the form.
6. Use Download CSV or Download PDF for records.

Example Data Table

Values shown are based on the formulas in this page and rounded for display.

Professional Notes on Plug Weld Strength

1) What a plug weld does in construction

Plug welds transfer shear through a filled hole when a lap joint cannot use continuous fillets. They are common in repairs, stiffener attachments, retrofit plates, and access‑limited connections. Capacity depends on the effective fused area, weld metal strength, and the connected material strength.

2) Key input data you should verify

Confirm hole diameter (d), plate thickness (t), and the number of plugs (n) from shop drawings. Verify electrode strength (FEXX) from consumable certificates and base metal ultimate strength (Fu) from mill test reports. Typical FEXX values are 490–620 MPa, while common structural Fu values are roughly 400–550 MPa.

3) How area drives strength

The governing area is the hole area, Aw = π·d²/4. Because area scales with d², increasing diameter from 16 mm to 20 mm raises Aw by about 56%. That single change can materially increase nominal plug capacity, but only if detailing, access, and procedure controls remain acceptable.

4) Limit states represented in this calculator

This calculator compares a weld‑metal shear estimate (0.60·FEXX·Aw) and a base‑material shear estimate (0.60·Fu·Aw). The smaller value governs each plug, then multiplies by n for a connection total. Use project specifications if they require different coefficients or additional checks.

5) LRFD‑style versus ASD‑style results

For LRFD‑style output, a resistance factor φ is applied to the nominal strength; φ often falls near 0.65–0.80 depending on criteria. For ASD‑style output, the nominal strength is divided by a safety factor Ω, frequently 2.0–3.0. Keep one method consistent within a design package.

6) Interpreting the interaction check

Many plug welds see combined shear and tension. The interaction equation (V/Vcap)² + (T/Tcap)² ≤ 1 provides a conservative combined‑effects screen. If tension is negligible, the equation reduces to shear utilization. If combined demands exceed 1.0, redesign or redistribute forces.

7) Practical detailing and quality controls

Beyond strength, performance depends on hole preparation, fit‑up, cleaning, and access for complete fusion. Very small holes can be difficult to fill soundly, while very large holes may increase distortion and heat input. Maintain qualified procedures, inspection hold points, and rework criteria.

8) Documenting results for submittals

Use the CSV and PDF exports to capture inputs, governing limit state, calculated capacity, and utilization. Attach records to weld maps and inspection logs so field verification aligns with calculations. These steps support traceability, reduce disputes, and improve turnover quality for every connection.

FAQs

1) What units should I use?

Enter millimeters for geometry, MPa for strengths, and kN for applied loads. The calculator converts internally and reports capacities in kN. Keep units consistent to avoid scaling errors.

2) Why does the calculator use 0.60 times strength?

A 0.60 factor is a common shear basis used in simplified strength checks. Your governing code or project specification may use different coefficients or additional reductions, so treat this as an engineering screening tool.

3) Which value should I use for Fu?

Use the specified or tested ultimate tensile strength for the connected base material. If multiple grades are present, use the weaker material for a conservative check unless the design documents justify otherwise.

4) How do I choose φ or Ω?

Select values required by your design standard or project criteria. Common φ ranges around 0.65–0.80 and Ω around 2.0–3.0, but you must follow the contract documents and engineer of record.

5) Does thickness t change the calculated strength?

In this simplified model, strength is driven by the filled hole area. Thickness is included for detailing awareness and warnings. Real designs may impose thickness limits, minimum fusion requirements, or different effective areas.

6) How is combined shear and tension handled?

The tool applies an interaction check: (V/Vcap)² + (T/Tcap)² ≤ 1. If the result is above 1.0, the combined demand exceeds the calculated capacity and the connection should be revised.

7) Can I rely on this result for final design?

Use it for preliminary sizing and documentation, then confirm with the governing welding/design standard, edge distances, spacing, workmanship requirements, and inspection criteria. Always coordinate final acceptance with the engineer of record.