Steel Connection Calculator Guide — Bolted & Welded Design

Quick access:

• What is a steel connection calculator?
• Connection types covered
• Bolt design fundamentals
• Weld design fundamentals
• How the connection calculators work
• Worked example: bolted shear tab
• Frequently asked questions
• Try the calculators

What is a steel connection calculator?

A steel connection calculator determines whether bolted and welded joints in a steel structure have sufficient strength to transfer the design forces between members. Connections are the most failure-prone elements in steel structures — a properly designed member is of no use if its connection fails first.

Connection design requires checking multiple limit states:

1. Bolt shear and tension — bolt rupture or yielding
2. Bolt bearing and tearout — plate failure at the bolt hole
3. Weld strength — weld metal rupture (fillet or groove welds)
4. Base metal strength — plate yielding or rupture at the connection
5. Block shear — combined tension and shear rupture of a plate segment
6. Prying action — additional bolt tension from plate bending
7. Slip resistance — for slip-critical connections (SC)

The Steel Calculator provides dedicated tools for bolted connections, welded connections, end plates, gusset plates, shear tabs, fin plates, HSS connections, and splice connections. All calculations run client-side via WebAssembly.

Connection types covered

Shear connections (simple connections)

Simple connections transfer shear force only and are assumed to have negligible rotational restraint. They are designed for the beam end reaction and any eccentricity:

• Shear tabs (single plate connections): A plate welded to the supporting member and bolted to the beam web.
• Double-angle connections: Two angles bolted or welded to both the beam web and the supporting member.
• End plate connections (shear only): A plate welded to the beam end and bolted to the support.
• Seated connections: An angle or plate supporting the beam bottom flange (bearing type).

Design requirements per AISC 360 Chapter J:

• Bolt shear capacity: phiRn = phi x Fn x Ab (phi = 0.75)
• Bolt bearing at bolt holes: phiRn = phi x 2.4 x db x t x Fu (phi = 0.75)
• Weld strength: phiRn = phi x 0.60 x FEXX x Awe (phi = 0.75)

Moment connections (FR connections)

Moment connections transfer both moment and shear. The connection must develop the moment capacity of the beam or a specified design moment:

• Flange-welded web-bolted (most common): Beam flanges welded to the column flange, web bolted to a shear tab.
• End plate moment connection: A plate welded to the beam end and bolted to the column flange (extended or flush end plate).
• Flange plate connection: Plates welded to the beam flanges and connected to the column by bolts or welds.

Moment connections require additional limit states:

• Flange compression (local flange buckling, web crippling, web sidesway buckling)
• Column panel zone shear
• Continuity plates (stiffeners on the column web at beam flanges)

Brace connections

Brace connections transfer large axial forces from diagonal bracing members:

• Gusset plate connections: A plate connecting the brace to the beam-column joint.
• Slotted tube connections: HSS brace with a slotted end through a gusset plate.
• WT connections: A WT section connecting the brace to the gusset.

ASCE 7-22 requires brace connections to be designed for 125% of the brace design strength in seismic applications (for SCBF and EBF systems per AISC 341).

Column splices

Column splices transfer axial compression, tension, and sometimes moment between column sections. Common splice types:

• Bearing splices: Column ends prepared for full contact bearing, with partial-penetration groove welds or bolted cover plates.
• Non-bearing splices: Columns not in full contact; splice plates transfer the full load through bolts or welds.

Bolt design fundamentals

Bolt types and grades

Bolt shear strength (AISC 360 Section J3.6)

phiRn = phi x Fnv x Ab

Where:

• phi = 0.75
• Fnv = nominal shear stress (68 ksi for A325 threads excluded, 54 ksi for threads included)
• Ab = nominal bolt area (pi x db² / 4)

Thread condition is critical: bolts installed with threads in the shear plane have 21% lower shear capacity (54 vs 68 ksi for A325). The calculator accounts for thread condition automatically based on the grip length and connection geometry.

Bolt tension strength (AISC 360 Section J3.6)

phiRn = phi x Fnt x Ab

Where:

• phi = 0.75
• Fnt = 90 ksi for A325

Combined tension and shear (AISC 360 Section J3.7)

When a bolt is subject to both tension and shear, the available stresses must satisfy:

fvt <= phi x Fnt x (1 - fv / (phi x Fnv))

where fv is the applied shear stress. Alternatively, from Table J3.5:

• For A325 (N): F'nt = 1.3 x Fnt - (Fnt / phi x Fnv) x fv <= Fnt
• Substituting: F'nt = 1.3 x 90 - (90/54) x fv = 117 - 1.67 x fv

Bolt spacing and edge distance (AISC 360 Table J3.4)

Weld design fundamentals

Fillet weld strength (AISC 360 Section J2.4)

phiRn = phi x 0.60 x FEXX x Awe

Where:

• phi = 0.75 (LRFD)
• FEXX = electrode classification strength (70 ksi for E70XX)
• Awe = effective area = effective throat x length

Effective throat for fillet welds:

• 0.707 x weld leg size (for 90-degree fillet)
• For unequal leg fillets, direction-dependent

Weld directional strength increase

Per AISC 360 Section J2.4, fillet weld strength depends on the angle of the applied load relative to the weld axis:

• Longitudinal (load parallel to weld): Fw = 0.60 x FEXX
• Transverse (load perpendicular to weld): Fw = 0.60 x FEXX x (1.0 + 0.50 x sin^1.5(theta))

For a pure transverse weld: Fw = 0.60 x FEXX x 1.50 = 0.90 x FEXX (50% stronger than longitudinal)

The calculator accounts for this directional increase when computing weld group capacity.

Minimum fillet weld size (AISC 360 Table J2.4)

Groove weld strength

Complete joint penetration (CJP) groove welds develop the full base metal strength — no weld strength check is needed (the weld metal is stronger than the base metal). Partial joint penetration (PJP) groove welds are designed like fillet welds using the effective throat.

How the connection calculators work

Bolted connection calculator

The Bolted Connection Calculator checks:

1. Bolt shear: For each bolt or bolt group, the shear stress is computed from the applied load and eccentricity. For eccentric loads on bolt groups, the instantaneous center of rotation (ICR) method is used (elastic method for Service Load Design, ICR for strength design per AISC Manual Part 7).
2. Bolt bearing: The bearing stress at each bolt hole. For plates, both bolt-to-plate bearing and net section rupture are checked.
3. Block shear: Per AISC 360 Section J4.3, the combination of tension on one plane and shear on a perpendicular plane.
4. Slip resistance: For slip-critical connections, phiRn = phi x mu x Du x hf x Tb x Ns where mu = 0.30 (Class A surface), Du = 1.13, hf = 1.0 (standard holes).

Welded connection calculator

The Welded Connection Calculator checks:

1. Weld group strength: Using the instantaneous center of rotation method (AISC Manual Part 8) or the elastic (vector) method for simpler groups.
2. Base metal strength: Checks the connected parts for yielding and rupture at the weld interface.
3. Weld size limits: Minimum and maximum fillet weld sizes per AISC 360.

End plate calculator

The End Plate Connection Calculator checks:

1. Plate bending: The end plate is treated as a series of T-stubs at tension bolt rows.
2. Bolt prying action: Additional bolt tension from plate bending deformation.
3. Weld between beam and end plate: Fillet or groove welds at the beam flanges and web.

Worked example: bolted shear tab

Problem: Design a single-plate shear tab connection (AISC Manual configuration 10) for a W18x35 beam (A992) framing into a W14x90 column (A992). Beam reaction (LRFD): Ru = 40 kips. Use A325-N bolts (threads included) and 3/8 in plate (A36).

Step 1: Bolt configuration

Use (3) 3/4 in A325-N bolts in a single vertical row (standard for W18 beams): Bolt shear strength per bolt: phiRn = 0.75 x 54 x 0.4418 = 17.9 kips Three bolts: 3 x 17.9 = 53.7 kips > 40 kips → OK

Step 2: Bolt bearing strength (plate)

Plate thickness tp = 3/8 in, Fu_plate = 58 ksi (A36): Bearing per bolt: phiRn = 0.75 x 2.4 x 0.75 x 0.375 x 58 = 29.4 kips Three bolts: well above 40 kips → OK (bearing rarely governs for 3/8 in or thicker plate)

Step 3: Plate shear yielding

Plate dimensions: 3/8 in x 8 in (assumed length for 3 bolts at 3 in spacing + edges): Agv = 0.375 x 8 = 3.0 in²

Shear yielding: phiRn = 1.0 x 0.60 x 36 x 3.0 = 64.8 kips > 40 kips → OK

Step 4: Plate shear rupture (net section)

Net area (3 bolts at 7/8 in bolt hole diameter): Anv = 0.375 x (8 - 3 x 0.875) = 0.375 x 5.375 = 2.02 in²

Shear rupture: phiRn = 0.75 x 0.60 x 58 x 2.02 = 52.7 kips > 40 kips → OK

Step 5: Block shear

For a single-row bolt group, the block shear failure plane goes through the bolt holes in shear on the vertical plane and tension on the horizontal plane at the bottom bolt:

Ant = 0.375 x (1.25 edge distance - 0.875/2) = 0.375 x 0.813 = 0.305 in² Anv = 0.375 x (8 - 3 x 0.875 + 0.875/2) ≈ 0.375 x 6.0 = 2.25 in² (approximately)

Un = 0.60 x Fu x Anv + Ubs x Fu x Ant (per AISC 360 Eq J4-5) phiRn = 0.75 x (0.60 x 58 x 2.25 + 1.0 x 58 x 0.305) = 0.75 x (78.3 + 17.7) = 0.75 x 96.0 = 72.0 kips > 40 kips → OK

Step 6: Weld design (shear tab to column flange)

Use 1/4 in fillet weld (E70XX) on both sides of the plate, full height: Effective throat = 0.707 x 0.25 = 0.177 in Weld capacity per inch = 2 sides x 0.75 x 0.60 x 70 x 0.177 = 11.2 kips/in

Weld length required: 40 / 11.2 = 3.6 in Available: 8 in full height → OK

Frequently asked questions

What is the difference between bearing-type and slip-critical bolted connections?

In bearing-type connections, the bolts bear against the plate to transfer shear. Small slip is acceptable. In slip-critical connections, the clamping force from pretensioned bolts creates friction between the faying surfaces, and the load is transferred by friction. Slip-critical connections are required for: (1) connections subject to fatigue, (2) connections with oversized or slotted holes, (3) connections in bridges subject to impact/vibration, (4) connections at column splices in high-seismic zones. AISC 360 Section J3.8 covers slip-critical design.

When should I use fillet welds vs groove welds?

Fillet welds are used when the parts lap or when welding from the outside of a T- or corner joint. They require no edge preparation and are cheaper per pound of deposited weld metal. Groove welds are used when parts align in the same plane (butt splices) or when full-strength welds are needed in T-joints. CJP groove welds develop the full base metal strength but require edge preparation (beveling) and are more expensive. Rule of thumb: use fillet welds whenever possible; use groove welds only when fillet welds cannot develop the required strength or when the connection is subject to tension through the thickness.

How does eccentricity affect bolt group capacity?

Eccentric loads on bolt groups create a moment that adds to the direct shear. The ICR method (instantaneous center of rotation) finds the exact capacity by iterating the rotation point until equilibrium is achieved. The magnitude of the eccentricity effect depends on the load eccentricity distance (e) relative to the bolt group dimensions. For a bolt group with 12 in eccentricity and 6 in vertical spacing, the ICR capacity may be only 30-50% of the concentric capacity. The elastic (vector) method is a conservative approximation that becomes increasingly conservative as eccentricity increases.

What is block shear and when does it govern?

Block shear is a rupture failure where a block of plate material tears out along a path through the bolt holes — tension on one plane and shear on a perpendicular plane. It governs for connections with: (1) thick plates with relatively few bolts, (2) short edge distances (less than 1.5 x db), (3) close bolt spacing (less than 3 x db), (4) low-ductility steel. Block shear must be checked for every bolted connection per AISC 360 Section J4.3.

How do I design a moment connection per AISC 360?

Moment connections in seismic frames follow AISC 358 (Prequalified Connections). The most common prequalified moment connection is the welded unreinforced flange-welded web (WUF-W) connection. For non-seismic moment connections: (1) determine the design moment (typically at least 50-70% of the beam Mp), (2) design the flange welds (CJP groove welds for full-strength), (3) design the web connection for beam shear plus a portion of the moment through the web, (4) check the column panel zone for shear capacity, (5) add continuity plates if the column flange thickness is inadequate.

Try the connection calculators

Use the free connection design tools on Steel Calculator:

• Bolted Connection Calculator — bolt group analysis with ICR method
• Welded Connection Calculator — weld group analysis with directional strength
• End Plate Calculator — flush and extended end plates
• Fin Plate Calculator — fin plate shear connections
• Gusset Plate Calculator — brace gusset plate design
• HSS Connection Calculator — HSS-to-HSS and HSS-to-beam connections

For reference and guidance:

• Bolted Connection Design Reference — bolt design procedures
• Connection Types Reference — connection classification and detailing
• Block Shear Reference — block shear and limit state checks
• Weld Symbol Chart — weld symbol identification
• Steel Connection Design Examples — detailed worked examples

Disclaimer

This guide is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the governing building code, project specification, and applicable design standards. The Steel Calculator disclaims liability for any loss, damage, or injury arising from the use of this information. Always engage a licensed structural engineer for connection design on actual projects.

See Also

• Beam Capacity Calculator
• Beam Deflection Calculator
• Steel Beam Sizes Reference
• Beam Design Guide
• Beam Span Reference