Connection Design Methods Compared — Bolted, Welded & Base Plate Across 5 Codes

A comprehensive comparison of steel connection design methods: bolted shear connections, moment-resisting connections, welded connections, and base plates. Covers when to use each method, governing failure modes, code-specific design provisions across AISC 360, AS 4100, EN 1993, and CSA S16, with worked comparisons and practical guidance.

Overview

Steel connections are where theory meets practice. The most elegant member design is worthless if the connections cannot safely transfer the design forces. Yet connection design is often the most error-prone part of structural steel design — the failure modes are numerous (bolt shear, bearing, block shear, weld rupture, base metal failure, prying action), the load paths are three-dimensional, and each code handles the same physical problem with different assumptions and safety factors.

This guide systematically compares the four major connection families — bolted shear connections, bolted/welded moment connections, fillet and butt welds, and column base plates — across the four major steel design codes. For each connection type, we explain the governing failure modes, show the key design equations, and present a side-by-side comparison of capacity for a standard connection detail.

The guide is intended for practicing engineers who need to work across multiple design codes, and for students who want to understand why connection design provisions differ between AISC, AS, EN, and CSA standards.

1. Bolted Shear Connections

1.1 Classification

Simple / shear connections are designed to transfer shear only, with minimal moment restraint. They are assumed to behave as pins in the structural analysis model (AISC 360 §B3.4a). Common types:

1.2 Failure Modes (All Codes)

Every bolted shear connection must be checked for:

1. Bolt shear — single or double shear depending on number of shear planes
2. Bolt bearing — bearing of bolt against plate material
3. Block shear rupture — tear-out combining tension and shear
4. Plate tension yielding / rupture — gross and net section of the plate
5. Weld (if shop-welded to beam) — fillet weld connecting plate to supporting member

1.3 Bolt Shear — Code Comparison

AISC 360-22 §J3.6:

Rn = Fnv x Ab                            (per shear plane)
phi = 0.75 (standard holes)

For A325 bolts:  Fnv = 54 ksi (threads included, i.e., N)
                 Fnv = 68 ksi (threads excluded, i.e., X)
For A490 bolts:  Fnv = 68 ksi (N), 84 ksi (X)

AS 4100-2020 Cl. 9.3.2.1:

Vf = phi x 0.62 x fuf x kr x (nn Ac + nx Ao)
phi = 0.80

For Grade 8.8 bolts:  fuf = 830 MPa
For Grade 10.9 bolts: fuf = 1040 MPa
kr = reduction factor for long joints (kr = 1.0 for Lj < 300 mm)

EN 1993-1-8 Table 3.4:

Fv,Rd = alpha_v x fub x A / gamma_M2
gamma_M2 = 1.25

For Grade 8.8: alpha_v = 0.6 (shear plane through thread)
               alpha_v = 0.5 (shear plane through thread, for 4.8, 5.8)
For Grade 10.9: alpha_v = 0.5 (thread in shear plane)
                alpha_v = 0.6 (thread outside shear plane)

CSA S16:24 Cl. 13.12.1.2:

Vr = 0.60 x phi_b x 0.70 x Fu_b x m x Ab
phi_b = 0.80

Where m = 1 for single shear, m = 2 for double shear (in bearing-type connections).
Fu_b = 830 MPa for ASTM A325M, 1040 MPa for ASTM A490M.

1.4 Worked Comparison — M20 / 3/4" Bolt in Single Shear

Common bolt: A325 (US) / Grade 8.8 (metric), threads in shear plane.

The range is 79-94 kN — a 19% spread driven primarily by different phi/gamma values and different assumptions about effective shear area. AS 4100 gives the highest single-bolt capacity; EN 1993 is close behind. AISC and CSA give similar lower values due to more conservative phi factors.

1.5 Block Shear — AISC 360-22 §J4.3

Block shear is a fracture mode combining tension on one plane and shear on the perpendicular plane:

Rn = 0.60 Fu Anv + Ubs Fu Ant <= 0.60 Fy Agv + Ubs Fu Ant
phi = 0.75

This is the most commonly missed failure mode in connection design and frequently governs bolted shear connections with small edge distances or tight bolt spacing.

2. Moment Connections

2.1 Types

2.2 Design Philosophy by Code

2.3 The Component Method (EN 1993-1-8 §6.2)

EN 1993-1-8 uses a unique "component method" where the joint is decomposed into individual components (column web in shear, column web in compression, column web in tension, column flange in bending, end plate in bending, bolts in tension), each with its own stiffness and resistance. The overall joint stiffness Sj,ini and moment resistance Mj,Rd are assembled from these components.

This is the most rigorous analytical framework for moment connection design and has been adopted (with modifications) in the upcoming AISC 360-28 provisions for moment connections.

3. Welded Connections

3.1 Fillet Welds

Fillet welds are the workhorse of structural steel connections. They transfer force through the weld throat, which must resist shear regardless of the direction of the applied load.

Throat dimension:

t_throat = leg_size x cos(45°) = leg_size x 0.707

AISC 360-22 §J2.4:

Rn = 0.60 FEXX x A_weld            (per unit length)
phi = 0.75

FEXX = 70 ksi for E70XX electrode (SMAW/FCAW)
Rn per inch of weld, 1/4" fillet: 0.75 x 0.60 x 70 x (0.25 x 0.707) = 5.57 kip/in

AS 4100-2020 Cl. 9.7.3:

Vw = phi x 0.60 x fuw x tt
phi = 0.80 (for SP category welds)
fuw = 480 MPa for E48XX electrode

For 6 mm fillet: Vw = 0.80 x 0.60 x 480 x (6 x 0.707) = 0.976 kN/mm

EN 1993-1-8 §4.5.3:

Fw,Rd = fvw,d x a
fvw,d = fu / (sqrt(3) x beta_w x gamma_M2)
gamma_M2 = 1.25
beta_w = 0.90 (for S355 steel)

For 6 mm fillet, S355 + E42 electrode:
fvw,d = 420 / (1.732 x 0.90 x 1.25) = 215.6 MPa
Fw,Rd = 215.6 x 6 = 1,294 N/mm = 1.29 kN/mm

3.2 Fillet Weld Code Comparison — 6 mm / 1/4" Weld

EN 1993 gives the highest fillet weld capacity per unit length — about 32% higher than AISC for a 6mm fillet — primarily because of its lower implicit safety factor and different treatment of the directional strength enhancement.

3.3 Butt (Groove) Welds

Complete joint penetration (CJP) groove welds are designed to match the base metal strength — no separate weld strength check is required for tension or compression normal to the weld axis (AISC §J2.1). Partial joint penetration (PJP) welds must be checked for the effective throat of the groove, which depends on the groove preparation.

4. Base Plates

4.1 Design Philosophy

Column base plates transfer axial compression, shear, and moment from the steel column into the concrete foundation. The design involves three interacting checks:

1. Bearing — concrete bearing stress under the plate
2. Plate bending — flexure of the plate between column flanges
3. Anchor bolt tension — uplift resistance from moment or net tension

4.2 Code Comparison — Base Plate Bearing

AISC 360-22 §J8:

Pp = 0.85 f'c A1 sqrt(A2/A1) <= 1.7 f'c A1
phi_c = 0.65

A1 = plate area, A2 = maximum area of the supporting surface geometrically similar to A1

AS 4100-2020 Cl. 6.13:

Nc = phi x 0.85 x f'c x A1 x sqrt(A2/A1)  where sqrt(A2/A1) <= 2.0
phi = 0.60

EN 1993-1-8 §6.2.5:

Fc,Rd = fjd x beff x leff
fjd = beta_j x alpha x fcd  (beta_j = 2/3 typically)
fcd = fck / gamma_c = fck / 1.50

4.3 Worked Comparison — Base Plate on f'c = 4,000 psi Concrete

Column: W10x49 (d = 10.0 in, bf = 10.0 in). Pier: 24 in x 24 in.

The different code results reflect different assumptions about the concrete confinement effect (sqrt(A2/A1)), different resistance factors, and different treatment of the effective bearing area. Always check the local concrete design code for compatible bearing strength assumptions.

5. Connection Type Selection — Decision Guide

6. Multi-Code Connection Capacity Summary

Related Calculators

• Bolted Connection Calculator — Bolt shear, bearing, and block shear per AISC 360, AS 4100, EN 1993, and CSA S16
• Welded Connection Calculator — Fillet weld stress and capacity across all 4 codes
• Base Plate Design Calculator — Bearing pressure, plate bending, and anchor bolt design per AISC 360
• Seismic Connection Design — Special moment frame connection detailing per AISC 341 and AISC 358
• Connection Design Flow Guide — Step-by-step workflow for connection design

FAQ

Q: When should I use slip-critical bolts instead of bearing-type bolts?

A: Use slip-critical (SC) bolts per AISC 360 §J3.8 when: the connection is subject to fatigue loading or load reversal, slotted holes are used with the load perpendicular to the slot, the connection is in a structure classified as a slip-critical application (AISC 341 seismic provisions for certain SMF connections), or where slip at the connection would impair serviceability. Slip-critical connections require faying surface preparation (Class A, B, or C) and controlled bolt pretension per RCSC Specification. Bearing-type connections are adequate for most statically loaded building structures.

Q: Why does EN 1993 give higher connection capacities than AISC?

A: The primary driver is the partial safety factor approach: EN 1993 uses gamma_M2 = 1.25 for connections (a 20% reduction from nominal), while AISC 360 uses phi = 0.75 (a 25% reduction). The 5% difference in safety margin, combined with different assumptions about effective stress areas and the directional strength enhancement for welds (EN 1993-1-8 §4.5.3 allows up to sqrt(3) increase for transverse fillet welds), produces the observed 10-30% higher EN 1993 capacities. This does not mean one code is "more conservative" — the load factors (ASCE 7 vs EN 1990) must also be considered to evaluate total reliability.

Q: What is prying action and when must I check it?

A: Prying action occurs in bolted T-stub or end plate connections when the connected plates flex, creating additional tensile force in the bolts beyond the direct applied tension. Per AISC DG4 and EN 1993-1-8 §6.2.4, prying must be checked when the bolt line is positioned between the flange/web and the plate edge (i.e., when there is an unsupported edge beyond the bolt line). If the plate is thick enough, the prying force approaches zero. For thin end plates with large gauge distances, prying can increase bolt tension by 30-50%. AISC DG4 Eq. 3.13 provides the prying check.