CSA S16 Connection Design — Bolted & Welded Steel Guide

Complete reference for CSA S16:19 bolted and welded steel connection design. Covers bolt shear and tension resistance (Clause 22-24), fillet and groove weld capacity (Clause 13.13), block shear (Clause 22.6), prying action (Clause 23.6), eccentric load analysis (elastic method and instantaneous centre of rotation), and bearing resistance at bolt holes. Includes a worked shear tab connection example.

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CSA S16 Connection Design — Overview

Steel connection design under CSA S16:19 follows limit state principles, with distinct clauses governing bolts (Clause 22-24), welds (Clause 13.13), and the connected elements (Clause 13.10-13.12). Canadian practice is detailed in the CISC Handbook Part 5 and the CISC Connection Design Guide.

The governing resistance factors for connection design are:

CSA S16 φb = 0.80 for bolts in bearing is 7% less conservative than the AISC value of φ = 0.75. However, CSA S16 φw = 0.67 for fillet welds is 12% more conservative than AISC φ = 0.75. A connection optimized for one code may not satisfy the other.

Bolt Shear Resistance (CSA S16 Clause 22.8)

The factored shear resistance of a single bolt in a bearing-type connection is:

Vrb = φb × 0.60 × φn × Ab × Fu

where:
  Vrb = factored shear resistance per bolt (N)
  φb  = 0.80 (resistance factor for bolts)
  φn  = 0.70 (reduction factor for threads intercepted)
  Ab  = nominal cross-sectional area of bolt (mm²)
  Fu  = specified minimum tensile strength of bolt (MPa)

The φn = 0.70 factor accounts for the reduction in shear capacity when threads are in the shear plane — this is the Canadian equivalent of the AISC "threads excluded" vs "threads included" distinction. When threads are deliberately excluded from the shear plane (by using grip length calculations), φn = 1.0 may be used, but Canadian practice conservatively assumes threads are intercepted unless the connection is detailed otherwise.

Bolt Shear Values — A325M and A490M

A325M bolts are the standard for most Canadian structural connections. A490M bolts are used in high-strength applications (heavy truss connections, moment connections in seismic frames). Bolts are installed to a pretension of 70% of Fu for slip-critical connections per CSA S16 Clause 23.3.

Bolt Tension Resistance (CSA S16 Clause 22.9)

The factored tension resistance of a bolt is:

Trb = φb × 0.75 × Ab × Fu

where:
  Trb = factored tension resistance per bolt (N)
  φb  = 0.75 (resistance factor for bolts in tension)
  Ab  = nominal cross-sectional area (mm²)
  Fu  = specified minimum tensile strength (MPa)

For an M24 A325M bolt (Ab = 353 mm², Fu = 830 MPa):

Trb = 0.75 × 0.75 × 353 × 830 = 164,774 N = 164.8 kN per bolt

Combined Shear and Tension

When bolts are subjected to combined shear and tension (common in moment end-plate connections and brace connections):

(Vf / Vrb)² + (Tf / Trb)² ≤ 1.0

where:
  Vf = factored shear per bolt (N)
  Vrb = factored shear resistance (threads intercepted) (N)
  Tf = factored tension per bolt (N)
  Trb = factored tension resistance (N)

Fillet Weld Design (CSA S16 Clause 13.13.1)

CSA S16 fillet weld resistance is based on the electrode strength and the effective throat thickness. Canadian practice uses CSA W59-compatible electrodes (E49xx series for G40.21 350W base metal):

Vrw = φw × 0.67 × Aw × Xu × (1.00 + 0.50 × sin¹·⁵θ)

where:
  Vrw = factored weld shear resistance (N)
  φw  = 0.67 (resistance factor for fillet welds)
  Aw  = effective weld area = effective throat × effective length (mm²)
       = 0.707 × leg size × effective length
  Xu  = electrode ultimate strength (MPa), e.g. 490 MPa for E49xx
  θ   = angle between weld axis and line of applied force (degrees)

The directionality factor (1.00 + 0.50 × sin¹·⁵θ) accounts for the increased strength of transversely loaded fillet welds compared to longitudinally loaded welds. For longitudinal welds (θ = 0°), the factor = 1.00. For transverse welds (θ = 90°), the factor = 1.50.

Simplified Fillet Weld Capacities

Per mm of weld length, for E49xx electrode (Xu = 490 MPa):

Groove Welds (Clause 13.13.2-3)

Complete Joint Penetration (CJP) groove welds develop the full strength of the connected base metal:

Trw = φw × Ag × Fy       (tension or compression)
Vrw = φw × 0.60 × Ag × Fy  (shear)

where:
  φw = 0.90 for CJP groove welds
  Ag = gross cross-sectional area of the connected part (mm²)
  Fy = base metal yield strength (MPa)

Partial Joint Penetration (PJP) groove welds use φw = 0.67 with the effective throat per CSA W59 Table 4.3.

Block Shear (CSA S16 Clause 22.6)

Block shear is a rupture failure along a path of bolt holes combining shear on one plane and tension on a perpendicular plane. The factored block shear resistance is:

Tr = φu × [Ut × An × Fu + 0.60 × Agv × (Fy + Fu) / 2]

where:
  φu = 0.75 (resistance factor for block shear)
  Ut = 1.0 for uniform tension stress, 0.5 for non-uniform
  An = net area in tension (mm²)
  Agv = gross area in shear (mm²)
  Fu = specified minimum tensile strength (MPa)
  Fy = specified minimum yield strength (MPa)

Block shear typically governs gusset plate connections, shear tabs, and coped beam ends. It is especially critical in Canadian practice for bolted brace connections where the gusset plate is the weak link.

Minimum Edge Distance and Spacing (CSA S16 Clause 22.4)

Minimum edge distances for rolled edges are 1.25 × sheared edge values. Maximum edge distance = 12 × t (thickness of the thinner connected part) or 150 mm, whichever is less.

Eccentric Loads — Elastic Method vs Instantaneous Centre

Connections with eccentric loads (bracket connections, gusset plates with out-of-plane moments) require analysis of the bolt group or weld group under combined direct shear and torsion.

Elastic Method (Vector Method)

The elastic method assumes linear-elastic behaviour and superposes direct shear and torsional components:

For a bolt group under eccentric load P at eccentricity e:

Vdi = P / n                    (direct shear per bolt)
Vti = P × e × ri / J           (torsional shear per bolt)
Vri = √(Vdi² + Vti² + 2 × Vdi × Vti × cosθi)  (resultant per bolt)

where:
  n    = number of bolts
  ri   = distance from bolt group centroid to bolt i (mm)
  J    = polar moment of inertia of bolt group (mm²)
  θi   = angle between direct and torsional vectors

The elastic method is conservative for bolt groups with significant ductility (A325M/A490M) and is the standard approach in Canadian practice per CISC Handbook guidance.

Instantaneous Centre of Rotation Method

The instantaneous centre (IC) method accounts for the actual load-deformation behaviour of bolts in the group, recognizing that bolts farther from the instantaneous centre resist less load due to slip. The IC method provides 10-30% more capacity than the elastic method for typical eccentric connections but requires iterative solution.

CSA S16 does not mandate either method — both are acceptable. The CISC Handbook provides pre-solved coefficients for common eccentric load cases.

Prying Action (CSA S16 Clause 23.6)

Prying action occurs in tension connections where bolt tension is amplified by the flexural deformation of the connected parts (end plates, T-stub flanges). CSA S16 Clause 23.6 provides an explicit design method:

The prying force Q is calculated as:
Q = (b / a) × (t² × Fy / 4 - T × b / 4)

where:
  a  = distance from bolt centreline to flange edge (mm)
  b  = distance from bolt centreline to flange/web junction (mm)
  t  = flange or end plate thickness (mm)
  T  = applied tension per bolt (N)

The total bolt tension Tb = T + Q must not exceed Trb (bolt tension resistance). Prying action is typically significant in end-plate moment connections and T-stub connections where the flange or plate thickness is relatively thin.

Worked Example — Shear Tab Connection

Problem: Design a shear tab connection for a W460x74 beam (Grade 350W) to a W310x97 column (Grade 350W). Beam reaction: Vf = 250 kN (factored). Use A325M bolts in standard holes and E49xx fillet welds.

Connection Configuration

• Shear tab: 300 mm × 100 mm × 10 mm (350W)
• Bolts: 4 × M20 A325M in the beam web (double shear)
• Bolts: 4 × M20 A325M in the shear tab to column flange (single shear)
• Weld: 6 mm fillet, E49xx, both sides of shear tab to column flange

Step 1 — Beam Web Bolts (Double Shear)

M20 A325M, threads intercepted, double shear:

Vrb_per_bolt = 2 × 68.3 = 136.6 kN (double shear factor)

Total Vr_bolts = 4 × 136.6 = 546.4 kN

Vf = 250 kN ≤ Vr = 546.4 kN OK, D/C = 0.46

Step 2 — Shear Tab Bolts to Column (Single Shear)

Vrb_per_bolt = 68.3 kN (single shear, threads intercepted)

Total Vr_tab = 4 × 68.3 = 273.2 kN

Vf = 250 kN ≤ Vr = 273.2 kN OK, D/C = 0.91

Note: The shear tab-to-column bolts govern at D/C = 0.91. This is acceptable but marginal — consider using 6 bolts or M24 bolts if other framing connections produce higher reactions.

Step 3 — Weld Shear Tab to Column

Two-sided 6 mm fillet weld, E49xx, total effective length = 2 × (300 - 2 × 6) = 576 mm (return welds at top and bottom):

Vrw = φw × 0.67 × (0.707 × 6) × 576 × 490 × 1.0
Vrw = 0.67 × 0.67 × 4.24 × 576 × 490
Vrw = 0.67 × 0.67 × 4.24 × 282,240
Vrw = 537,241 N = 537.2 kN

Vf = 250 kN ≤ Vr = 537.2 kN OK, D/C = 0.47

Step 4 — Bearing at Bolt Holes

Check beam web (t = 9.0 mm, 350W) at M20 bolts:

Bearing resistance per bolt: Br = φb × 3.0 × db × t × Fu
Br per bolt = 0.80 × 3.0 × 20 × 9.0 × 450 = 194,400 N = 194.4 kN

Total Br = 4 × 194.4 = 777.6 kN > 250 kN OK

Step 5 — Block Shear Check on Beam Web

Critical block shear path at the beam web bolt group (end distance = 40 mm, spacing = 70 mm):

Agv = (40 + 3 × 70) × 9.0 = 250 × 9.0 = 2,250 mm²
An = 40 × 9.0 = 360 mm² (net tension area)

Tr = 0.75 × [1.0 × 360 × 450 + 0.60 × 2,250 × (350 + 450)/2]
Tr = 0.75 × [162,000 + 0.60 × 2,250 × 400]
Tr = 0.75 × [162,000 + 540,000]
Tr = 0.75 × 702,000 = 526,500 N = 526.5 kN > 250 kN OK

Step 6 — Summary

Connection is adequate. The shear tab-to-column bolts govern the design. If future framing includes heavier beams, specify 6 × M20 bolts in the tab rather than 4.

Connection Types — Canadian Practice

Frequently Asked Questions

What is the resistance factor for bolts in CSA S16 connection design? CSA S16 Clause 22.8 specifies φb = 0.80 for bolts in bearing-type connections (shear). For bolts in tension (Clause 22.9), φb = 0.75. These differ from AISC 360 which uses φ = 0.75 for both shear and tension. The CSA values mean bolts are slightly stronger in shear (7% increase) but identical in tension compared to AISC.

How does CSA S16 fillet weld resistance compare to AISC 360? CSA S16 Clause 13.13.1 uses φw = 0.67 for fillet welds, compared to AISC φ = 0.75. This 12% reduction makes Canadian weld design more conservative. The CSA weld resistance formula also includes a directionality factor of (1.00 + 0.50 × sin¹·⁵θ) that accounts for the increased strength of transversely loaded welds — this is the same approach used in AISC but with different coefficients.

What is block shear in CSA S16 and how is it calculated? Block shear (CSA S16 Clause 22.6) is a limit state where a block of material tears out along a path through bolt holes, combining shear rupture on one plane and tension rupture on the perpendicular plane. The factored resistance is Tr = φu × [Ut × An × Fu + 0.60 × Agv × (Fy + Fu)/2] with φu = 0.75. Block shear frequently governs in gusset plate and shear tab connections with short end distances.

What is prying action and when must it be considered in Canadian connection design? Prying action (CSA S16 Clause 23.6) is the amplification of bolt tension caused by flexural deformation of the connected parts (end plates, T-stub flanges). The prying force Q increases the tension in the bolt beyond the applied load T. Prying action must be considered in tension connections where the connected parts can deform — typical cases include bolted moment end plates, T-stub connections, and hanger connections. Thick plates reduce prying effects.

Related Pages

• Canada CSA S16 Guide — Full CSA S16:19 steel design reference
• Canadian Steel Beam Sizes — W, WWF, HSS sections per CISC
• Canadian Steel Grades — G40.21 300W to 480W
• CSA S16 Beam Design — Flexure, LTB, shear checks
• CSA S16 Moment Connection — End plate and flange connection design
• Bolt Torque Chart — Pretension values for A325M/A490M
• Fillet Weld Size Chart — CSA W59 weld capacities
• Weld Group Properties — Weld group geometric properties
• Bolted Connection Checklist — Design quality control

This page is for educational reference. CSA S16:19 connection design must comply with the current edition of CSA S16, CSA W59, and NBCC 2020. All results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification by a licensed Professional Engineer (P.Eng.).