Connection Design Calculator
Calculate bolted steel connection capacity: shear, bearing, and overall connection strength for A325 and A490 bolts.
About this calculator
A bolted connection can fail two different ways, and this calculator checks both, then reports whichever is weaker as the governing capacity, following the bolted-connection provisions of ANSI/AISC 360-22, Chapter J. Shear failure happens when the bolt itself shears through; bearing failure happens when the bolt crushes an oversized hole into the connected plate before the bolt ever breaks. For shear, the calculator computes each bolt's cross-sectional area from its diameter, takes 75% of that as an approximate net tensile area (accounting for the thread root reducing the effective section), and multiplies by a nominal shear stress — 0.45 times the bolt's ultimate strength if threads are excluded from the shear plane, or the more conservative 0.35 times ultimate if threads are included, per your Connection Type selection. Ultimate strength itself switches on bolt grade: 830 MPa for ASTM A325, 1040 MPa for the higher-strength A490. A φ = 0.75 resistance factor is applied throughout, in line with AISC 360 LRFD practice.
For bearing, capacity comes from φ × 2.4 × bolt diameter × plate thickness × the plate's ultimate strength, which this calculator fixes at 450 MPa — an A36-steel approximation, so a higher-strength connected plate will understate real bearing capacity here. Both totals scale linearly with the number of bolts. The optional Factored Connection Demand field closes the loop: enter your LRFD factored shear on the connection and the Demand/Capacity Ratio divides it by the governing capacity, so anything at or below 1.00 has adequate strength and anything above it is overstressed. Leave that field at its 0 default and the calculator stays a pure capacity check — the ratio tile reads "Not evaluated" rather than reporting a utilisation of zero that nobody computed.
Inputs
Results
Shear Capacity
264,012 N
Connection Capacity
264,012 N
Figures current as of 2022. Source: American Institute of Steel Construction, ANSI/AISC 360-22, Specification for Structural Steel Buildings
How to Use This Calculator
- Enter the bolt diameter and the number of bolts in the connection.
- Select the Bolt Grade (ASTM A325 or A490).
- Enter the plate thickness for the thinnest connected plate.
- Select the Connection Type (bearing-type or slip-critical).
- Optionally enter the Factored Connection Demand (N) — your LRFD factored shear — to get a Demand/Capacity Ratio; leave it at 0 for a capacity-only check.
- Review Shear Capacity and Bearing Capacity, and the overall Connection Capacity (the minimum of the two).
How the result changes with Bolt Diameter
| Bolt Diameter | Shear Capacity | Connection Capacity |
|---|---|---|
| 12 | 95,044 N | 95,044 N |
| 15 | 148,507 N | 148,507 N |
| 30 | 594,026 N | 594,026 N |
| 36 | 855,398 N | 855,398 N |
What each input means
- Bolt Diameter
- Nominal diameter of each bolt. Common sizes: 16, 20, 22, 24 mm.
- Bolt Grade
- Structural bolt grade per AISC 360 Table J3.2. A325 preferred for economy; A490 for high-strength connections.
- Number of Bolts
- Total number of bolts in the connection.
- Plate Thickness
- Thickness of the thinnest connected plate for bearing check.
- Connection Type
- Bearing-type connections have higher capacity; slip-critical is more conservative.
- Factored Connection Demand (N)
- Required strength (LRFD factored shear) on the connection; leave 0 to show capacity only.
What each result means
- Demand/Capacity Ratio
- Factored connection demand divided by the governing Connection Capacity. At or below 1.00 the connection has adequate strength; above 1.00 it is overstressed. Leave the demand at 0 and this reads "Not evaluated" instead of a number.
How this is calculated
Worked example, using the default values
- Identify Input Parameters6 parametersBolt Diameter = 20, Bolt Grade = 1, Number of Bolts = 4, Plate Thickness = 12, Connection Type = 1, Factored Connection Demand = 0 = 6 input(s) provided
- Calculate Shear CapacityShear Capacity264012 = 264012
- Calculate Connection CapacityConnection Capacity264012 = 264012
- Calculate Bearing CapacityBearing Capacity777600 = 777600
- Calculate Demand/Capacity RatioDemand/Capacity RatioNot evaluated (no demand entered) = Not evaluated (no demand entered)
Figures and sources
- AISC 360-22 Table J3.2 (bolt ultimate tensile strength for A325/A490) and Chapter J bolted-connection design provisions (resistance factor φ=0.75 for bearing-type shear and bearing) (2022) — American Institute of Steel Construction, ANSI/AISC 360-22, Specification for Structural Steel Buildings
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the Demand/Capacity Ratio say "Not evaluated" instead of showing a number?
Because the Factored Connection Demand field is still at its 0 default, which means you haven't told the calculator what load the connection actually has to carry. In that state the tool is a pure capacity check, and printing a ratio of 0.000 would wrongly suggest the connection is unstressed rather than unchecked. Enter your LRFD factored shear and the tile switches to a real utilization figure — demand divided by the governing Connection Capacity.
How does switching Connection Type from bearing-type to slip-critical change the numbers?
It changes which nominal shear stress factor is applied to the bolts: bearing-type uses 0.45 times the bolt's ultimate strength (threads excluded from the shear plane), while slip-critical uses the more conservative 0.35 times ultimate (threads included). That lower factor for slip-critical directly reduces the calculated Shear Capacity, though it leaves Bearing Capacity — which depends only on bolt diameter, plate thickness, and plate strength — unchanged.
Why is my Bearing Capacity possibly overstated for a high-strength connected plate?
The bearing formula fixes the connected plate's ultimate strength (Fu) at 450 MPa, an approximation for common A36 steel plate. If your actual plate is a higher-strength grade, the real bearing capacity would be higher than what this calculator reports, and if it's a lower grade, the real capacity would be lower — either way, the tool doesn't ask for plate grade, so double-check this assumption against your actual material.
Why does the reported Connection Capacity sometimes equal Shear Capacity and sometimes equal Bearing Capacity?
The calculator takes whichever of the two failure modes is weaker — shear failure through the bolt itself, or bearing failure where the bolt crushes the plate — and reports that as the governing Connection Capacity. Which one governs depends on your specific inputs: thinner plates or lower-strength plate material tend to push bearing capacity down below shear capacity, while smaller bolt diameters or lower-grade bolts tend to make shear govern instead.
Where do the A325/A490 strength values and the φ=0.75 factor come from?
Both are from ANSI/AISC 360-22, the American Institute of Steel Construction's Specification for Structural Steel Buildings. The 830 MPa (A325) and 1040 MPa (A490) ultimate tensile strengths this calculator uses come from AISC 360's Table J3.2, and the φ=0.75 resistance factor applied to both the shear and bearing checks is the LRFD factor AISC's Chapter J assigns to bolted connections.
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