Steel Connection Design — Bolted, Welded, and Base Plate Methods

Steel connections are the joints that transfer forces between structural members. While members (beams, columns, braces) carry load along their length, connections transfer load between intersecting members. A well-designed connection is safe, fabricatable, and economical — in that order. Connection fabrication and erection typically account for 30-50% of total steel cost, so connection design has a direct and significant impact on project economics.

This guide covers the four main families of steel connections: bolted shear connections, welded moment connections, column base plates, and bracing connections. For each family, we walk through the key limit states, design equations, and common detailing practices.

Bolted Shear Connections

Shear connections are the workhorses of steel construction — every simply-supported beam requires two of them. The goal is to transfer the beam end reaction (vertical shear) to the supporting member while allowing sufficient rotational capacity for the beam to deflect as simply supported.

Single-Plate Shear Tab (AISC Manual Part 10)

The single-plate shear tab is the most common US shear connection: a rectangular plate is shop-welded to the supporting member (column flange, column web, or girder web) and field-bolted to the beam web. It is simple to fabricate, easy to erect, and well-supported by AISC design tables.

Limit states to check:

Design sequence for a shear tab:

1. Select number of bolts based on required shear (use AISC Table 10-1 for quick selection)
2. Size plate thickness such that bolt bearing on plate >= required shear per bolt
3. Check plate shear and block shear
4. Select weld size to develop full plate shear capacity
5. Verify beam web bearing and block shear (especially for coped beams with thin webs)

Typical parameters: 3/8-inch or 1/2-inch plate thickness, 3-6 bolts of 3/4-inch or 7/8-inch diameter, 5/16-inch fillet weld, A325-N or A325-X bolts. A single 3/8-inch shear tab with four 3/4-inch A325-N bolts and a 12-inch-long weld pair can transfer approximately 60-70 kips of shear.

Double-Angle Connection

Two angles, one on each side of the beam web, are shop-bolted or shop-welded to the supporting member and field-bolted to the beam web. Double angles provide greater rotational capacity than single-plate connections and are preferred for deeper beams or for connections to column webs.

End Plate Shear Connection

A partial-depth end plate is shop-welded to the beam web and field-bolted to the supporting member. Similar to a shear tab but the plate is attached to the beam rather than the support. Commonly used when the beam is erected before the column and the connection must be made from one side.

Related calculators:

• Bolted connection calculator — shear, bearing, block shear checks
• Shear tab connection calculator
• End plate connection calculator
• Fin plate connection calculator

Welded Moment Connections

Moment connections transfer both shear and bending moment, providing rotational fixity at the joint. They are essential for moment-resisting frames (MRFs) in both seismic and wind applications.

Flange-Welded / Web-Bolted (FW/WB)

The beam flanges are field-welded to the column flange using complete-joint-penetration (CJP) groove welds, while the beam web is field-bolted to a shear tab. The flanges transfer moment as a tension-compression couple; the web connection transfers shear.

Design sequence:

1. Determine the beam end moment and shear from analysis
2. The moment is resolved into a flange force couple: T = C = Mu / (d - tf)
3. Check the CJP groove weld at the tension flange — CJP welds develop the full base metal strength, so the limit state is typically the flange itself in tension yielding or rupture
4. Check the compression flange — local web yielding and web crippling at the column
5. Design the web shear connection for the beam end shear plus any shear from plastic hinging
6. Check column panel zone shear if connecting to a column web
7. Verify column flange thickness for stiffness (column flange bending / prying)

Extended End Plate (EEP) Moment Connection

A plate is shop-welded to the beam end (using CJP groove welds at the flanges and fillet welds at the web) and field-bolted to the column flange with bolts above and below the flanges. EEP connections are bolted-only in the field, avoiding field welding, which is advantageous for speed and inspection.

Limit states for EEP connections:

• Bolt tension rupture (prying action increases bolt force beyond direct tension)
• End plate flexural yielding (yield line mechanism)
• Beam flange tension rupture
• Column flange bending (yield line mechanism in column flange)
• Web panel zone shear

Related calculators:

• End plate moment connection calculator
• Connection stiffness calculator — semi-rigid joint characterization

Column Base Plates

Base plates transfer axial compression (and sometimes tension and shear) from a steel column to a concrete foundation. The base plate distributes the column load over a larger concrete bearing area.

Axially Loaded Base Plate

Limit states:

1. Concrete bearing: The bearing stress under the plate must not exceed the design bearing strength of the concrete (phi_c × 0.85 × f'c × sqrt(A2/A1) per ACI 318, where A1 is the plate area and A2 is the maximum concrete area geometrically similar to and concentric with A1)
2. Base plate flexural yielding: The plate acts as a cantilever beyond the column footprint, bending under the bearing pressure
3. Anchor rod tension (if uplift is present)
4. Anchor rod shear (if lateral load is transferred through the base)
5. Shear lug design (if anchor rods alone cannot resist the applied shear)

Plate thickness from flexural yielding (AISC Design Guide 1): For a rectangular HSS or wide-flange column with axial load only:

• m = (N - 0.95d) / 2 (cantilever distance parallel to column depth)
• n = (B - 0.80bf) / 2 (cantilever distance parallel to column flange)
• tp_min = max(m, n) × sqrt(2 × Pu / (phi × Fy × B × N))

Base Plate with Moment

When the column base transfers both axial load and bending moment, the bearing stress distribution under the plate is non-uniform (triangular or trapezoidal). The design must account for the eccentricity e = Mu / Pu.

For small eccentricities (e <= N/6), the entire plate is in compression. For larger eccentricities, a portion of the plate experiences tension, and the anchor rods on the tension side engage. The equilibrium equations are nonlinear (quadratic in the neutral axis depth), requiring iterative solution or design aids.

Related calculators:

• Base plate and anchor bolt calculator
• Anchor embedment calculator — concrete breakout, pullout
• Base plate design checklist
• Base plate worked example — step-by-step AISC DG 1

Bracing Connections

Bracing connections transfer axial force from diagonal braces (angles, channels, HSS, or W-shapes) into the beam-column joint. They are critical elements in concentrically braced frames (CBFs) for both wind and seismic resistance.

Gusset Plate Connection for Angle Braces

The brace is connected to a gusset plate (typically bolted), and the gusset plate is welded to the beam and column at the joint. The gusset plate must transfer the full brace axial force into the framing members.

Limit states:

1. Gusset plate yielding on the Whitmore section (effective width at 30-degree spread from the first bolt row to the last)
2. Gusset plate buckling (treat the gusset as a column strip with effective length factor K = 0.65 for corner gussets)
3. Block shear at the bolt group (brace-to-gusset and gusset-to-beam/column)
4. Weld strength (gusset to beam and column)
5. Beam and column local yielding from the gusset interface forces

Related calculators:

• Gusset plate connection calculator
• Steel brace frame design calculator

Connection Type Selection Guide

Bolt Grade Selection

Weld Types and Applications

FAQ — Connection Design

What makes a connection "simple" vs "continuous" in the analysis model?

A simple connection in the analysis is modeled as a pin — it transfers shear but releases moment. In reality, all connections have some rotational stiffness. AISC 360 Section B3.4 permits classifying a connection as simple if its secant rotational stiffness at service loads is less than 2EI/L (i.e., the connection is flexible enough that it does not attract significant moment). Most shear tabs and double-angle connections meet this criterion. A fully restrained (FR) moment connection must have stiffness of at least 20EI/L. Between these limits, the connection is partially restrained (PR), and the analysis must explicitly include connection flexibility.

How do I handle eccentricity in bolted connections?

When the line of action of the applied load does not pass through the centroid of the bolt group, the bolts resist both direct shear and torsional moment from the eccentricity. Use the elastic method (AISC Manual Part 7) or the instantaneous center of rotation method (AISC Manual Tables 7-6 through 7-17). The elastic method is conservative; the IC method provides the true ultimate strength. Our calculators use the IC method for groups with eccentricity.

When should I use a shear lug instead of anchor rods for base shear?

When the factored base shear exceeds the combined shear capacity of the anchor rods (accounting for the concurrent tension from uplift/moment), a shear lug is required. A shear lug is a short length of steel section (typically a WT or flat bar) welded to the underside of the base plate and embedded in a grouted pocket in the foundation. The shear lug transfers lateral load through concrete bearing rather than anchor rod shear — which avoids anchor rod edge distance and breakout concerns. As a rule of thumb, use a shear lug when the design base shear exceeds 30% of the axial compression, or when anchor rod edge distances are tight.

Do I need to check connection ductility for seismic applications?

Yes. For Special Moment Frames (SMF) and Special Concentrically Braced Frames (SCBF) in high-seismic regions, connections must be designed to sustain inelastic deformations without fracture. AISC 341 (Seismic Provisions) requires specific detailing: protected zones where welding and attachments are prohibited near plastic hinge locations, demand-critical welds with notch-tough filler metal, and connections designed to develop the expected yield strength of the connected member (Ry × Fy × Z for beams in SMF). The beam-to-column connection in an SMF must be capable of sustaining an interstory drift angle of at least 0.04 radians without significant strength degradation.

Related Pages

• Connection design hub — Designer Hub S6
• Bolted connection calculator — shear, bearing, block shear
• Welded connection calculator — fillet, CJP, PJP
• Base plate and anchor bolt calculator
• End plate moment connection calculator
• Gusset plate connection calculator
• Bolted connection checklist — verification steps
• Weld design checklist — verification steps
• Bolted connection worked example — AISC 360
• Connection stiffness calculator — semi-rigid characterization
• All tools directory
• All guides directory

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