Connections and Fastening

Assemblies and Fastening

Chapter Introduction

Assemblies and fastening form the core of the structural and ornamental ironworker (metal fabricator) trade. This chapter covers design principles, installation methods, strength calculations, and regulatory requirements applicable to bolted, welded, and riveted connections. For the Red Seal exam, you must master not only the practical techniques but also the Canadian standards that govern these operations, notably CSA S16 (Design of Steel Structures) and CSA W59 (Welded Steel Construction — Welding of Steel Structures). This chapter prepares you to answer questions on connector types, load calculations, installation tolerances, and inspections.

Types of Connections and Their Applications

Bolted Connections

Bolted assemblies are the most common in modern construction sites. They offer rapid installation, possible disassembly, and easy visual inspection. There are two main categories: ordinary bolts (ASTM A307) and high-strength bolts (ASTM A325, A490, or their Canadian equivalents CSA G40.20).

Bolt Type	Minimum Tensile Strength (MPa)	Typical Use
A307 (ordinary)	400	Light structures, secondary connections
A325 (high-strength)	825	Structural frames, shear connections
A490 (high-strength)	1040	High-stress connections, critical assemblies

Key Principle: The strength of a bolt depends on its nominal diameter (d), its effective area (A_b = π × d² / 4), and its grade. For the exam, remember the shear resistance formula: V_r = 0.60 × φ_b × A_b × F_u, where φ_b = 0.80 and F_u is the specified ultimate strength.

Welded Connections

Welding is used when bolted connections are impractical (limited access, dynamic loads, aesthetics). Common processes include SMAW (shielded metal arc welding), GMAW (MIG/MAG), and FCAW (flux-cored arc welding). CSA W59 defines welder qualification requirements, joint types, and acceptance criteria.

Types of welds:

Fillet weld: the most common, triangular cross-section, strength calculated on the effective throat (0.707 × leg dimension).

Full penetration weld: used for butt joints, strength equal to the base metal if properly executed.

Partial penetration weld: economical, but reduced strength depending on the effective penetration depth.

Riveted Connections

Although historical, rivets are still encountered in existing structures and restorations. Button-head rivets are installed hot, with a shank that expands and contracts to create clamping force. Their calculation follows principles similar to bolts, but with lower strength values. For the exam, know that rivets are replaced by bolts in new construction, but their inspection (corrosion, play, missing head) remains a tested skill.

Connection Strength Calculations

Bolt Shear Resistance

The basic formula for a bolt in single shear is:

V_r = 0.60 × φ_b × A_b × F_u

Where:

φ_b = 0.80 (resistance factor)

A_b = cross-sectional area of the bolt (mm²)

F_u = ultimate tensile strength (MPa)

Example: 20 mm diameter A325 bolt (A_b = 314 mm², F_u = 825 MPa):

V_r = 0.60 × 0.80 × 314 × 825 = 124,344 N ≈ 124 kN per shear plane.

Caution: If the bolt is in double shear (two planes), the resistance doubles. Exam questions often include this distinction.

Bolt Tensile Resistance

For a bolt subjected to tension:

T_r = 0.75 × φ_b × A_b × F_u

The 0.75 factor accounts for stress concentration at the threads. Do not confuse this with shear — this is a frequent error.

Fillet Weld Resistance

The resistance of a fillet weld is calculated on the effective throat (a), which is the shortest distance from the root to the weld face. For an equal-leg fillet weld, a = 0.707 × leg (s). The resistance per unit length is:

V_r = 0.67 × φ_w × A_w × F_u

Where:

φ_w = 0.67 (resistance factor for weld metal)

A_w = a × L (effective throat area)

F_u = ultimate strength of the electrode (often 490 MPa for E70XX)

Rule of thumb: A 6 mm leg fillet weld over a 100 mm length with an E70XX electrode provides approximately 0.67 × 0.67 × (0.707 × 6 × 100) × 490 ≈ 93 kN. Always check your units (mm → m to obtain kN).

Load and Resistance Factors (LRFD)

Canada uses the LRFD (load and resistance factor design) method according to CSA S16. Loads are multiplied by load factors (1.4 for dead loads D, 1.5 for live loads L) and resistances by resistance factors (φ). The most common load combination is: 1.4D + 1.5L. For the exam, know how to identify load combinations in given problems.

Installation and Tolerances

Bolt Installation

Surface preparation: Contact surfaces must be free of mill scale, rust, and contaminants. For slip-critical connections, surfaces must be prepared according to Classes A, B, or C defined by CSA S16 (friction coefficients from 0.30 to 0.50).

Tightening: High-strength bolts must be tightened using the turn-of-nut method or calibrated wrench method. Torque tightening requires a certified calibrator. The additional rotation after snug-tightening is typically 1/3 turn for A325 bolts.

Inspection: Verify that bolts protrude at least one full thread beyond the nut. Washers are mandatory under both the nut and head for A325/A490 bolts.

Alignment Tolerances

CSA S16 and CSA S16.1 (tolerances) impose precise limits:

Vertical deviation of a column: ± 1/500 of the height, maximum 25 mm.

Horizontal deviation of a beam: ± 1/300 of the span, maximum 15 mm.

Bolt hole alignment: oversized holes (up to +2 mm) are permitted for adjustment, but slotted holes must be oriented perpendicular to the direction of the load.

Exam trap: Tolerances are often given as fractions (1/500) — convert correctly to millimeters for a given span.

Field Welding

Preheating: Required for thick steels (> 20 mm) or high-carbon steels. The minimum temperature is specified by W59 (often 50 °C for common steels).

Sequencing: Weld in alternating segments to minimize distortion. Continuous welds are preferred over tack welds for structural connections.

Cleaning: Each pass must be cleaned using a wire brush or chipping hammer before the next pass. Weld spatter must be removed before inspection.

Applicable Canadian Standards

CSA S16 — Design of Steel Structures

This standard is the primary reference for connection design. Relevant sections for the ironworker include:

Chapter 13: Resistance of members and connections (bolts, welds, gusset plates).

Chapter 23: Fabrication and erection requirements (tolerances, marking, inspection).

Annex J: Acceptance criteria for weld defects (porosity, lack of fusion, cracks).

CSA W59 — Welded Steel Construction

W59 defines:

Welder qualifications (qualification testing per the standard, periodic renewal).

Joint types and their minimum dimensions.

Inspection methods (visual, magnetic particle, ultrasonic).

Weld acceptance criteria (maximum porosity size, allowable crack length).

Other Relevant Standards

ASTM A325/A490: Specifications for structural bolts.

CSA G40.20/G40.21: General requirements for structural steel.

CSA W47.1: Certification of welding companies (mandatory for shops and field sites).

Rule 8-200 (reference to the Canadian Electrical Code, Chapter V): Although this chapter deals with electricity, ironworkers must know the minimum distances between metal connections and electrical conductors — a possible cross-discipline question on the exam.

Inspection and Quality Control

Visual Inspection of Welds

Visual inspection is the first line of control. Defects to look for:

Cracks: longitudinal, transverse, crater — always unacceptable.

Porosity: gas bubbles in the deposited metal — acceptable if cumulative porosity is less than 6 mm in diameter over 25 mm of length.

Lack of fusion: between passes or at the root — unacceptable for full penetration welds.

Undercut: a notch along the edge of the weld — maximum depth of 0.5 mm for welds subject to fatigue.

Non-Destructive Testing (NDT)

Magnetic particle testing: detects surface and near-surface cracks (up to 6 mm depth).

Ultrasonic testing: detects internal defects, requires a certified Level 2 operator.

Dye penetrant testing: for non-magnetic materials (aluminum, stainless steel).

For the exam: Know which NDT method is appropriate for which defect and which standard requires it (W59 requires ultrasonic testing for full penetration welds in seismic connections).

Marking and Traceability

Each steel piece must be marked according to the shop drawing (piece number, assembly mark). Material certificates (mill sheets) must be available to verify compliance with specifications. Welders must apply their stamp near their welds to ensure traceability.

Practical Field Calculations

Required Weld Length

For a given load P, the weld length L is:

L = P / (0.67 × φ_w × a × F_u)

Example: 200 kN load, 8 mm leg fillet weld (a = 5.66 mm), E70XX electrode (F_u = 490 MPa):

L = 200,000 / (0.67 × 0.67 × 5.66 × 490) = 200,000 / 1,244 ≈ 161 mm.

Always add 2 × the leg dimension for the ends (start and stop) — approximately 16 mm, so total L ≈ 177 mm.

Number of Bolts Required

For a shear connection with n bolts:

n = P / (V_r per bolt)

Example: 500 kN load, 20 mm A325 bolts in single shear (V_r = 124 kN):

n = 500 / 124 = 4.03 → round up to 5 bolts.

Rounding rule: Always round up to the next whole number. Never round down, even if the result is 4.01.

Bolt Spacing

CSA S16 requires:

Minimum center-to-center spacing: 2.7 × d (nominal diameter).

Minimum edge distance: 1.5 × d for sheared edges, 1.25 × d for rolled edges.

Maximum spacing: 24 × t (thickness of the thinnest plate) to prevent local buckling.

Trap: Exam questions often give spacings in inches — convert to millimeters (1 in = 25.4 mm) before comparing to requirements.

Special Connections

Slip-Critical Connections

Used in bridges and structures subject to dynamic or reversing loads. Resistance is based on friction between surfaces, not bolt shear. The minimum clamping force is 70% of the bolt proof load. Surfaces must be prepared with a specified friction coefficient (Class A: 0.30; Class B: 0.50).

Seismic Connections

In seismic zones (British Columbia, Quebec), connections must be ductile. A490 bolts are often prohibited in tension connections because they are less ductile than A325 bolts. Welds must be full penetration with low-hydrogen electrodes (E7018). CSA S16, Chapter 27, defines requirements for concentrically and eccentrically braced frames.

Gusset Plates and Connection Plates

Gusset plates transfer loads between members. Their minimum thickness is generally 6 mm, and their size is determined by block shear resistance — a failure mode combining tension and shear. The formula is:

V_r = φ_u × (A_n × F_u + A_g × F_y)

Where A_n is the net area in shear, A_g is the gross area in tension, and F_u and F_y are the ultimate and yield strengths, respectively.

Pitfalls to Avoid

126.Confusing shear and tension: The resistance factors and formulas differ. Always re-read the question to identify the mode of loading.

127.Forgetting resistance factors: φ_b = 0.80 for bolts, φ_w = 0.67 for welds. Forgetting them leads to results that are too high.

128.Using nominal diameter instead of effective area: For threaded bolts, the net section area is approximately 0.75 × A_b. Always use A_b (full section) for shear calculations unless otherwise indicated.

129.Neglecting tolerances: Exam questions often include traps on maximum deviations — read the units (mm vs m) and the fractions.

130.Ignoring preheating requirements: For thick or high-strength steels, preheating is mandatory. A question may ask for the minimum temperature — remember 50 °C for common steels.

131.Rounding the number of bolts incorrectly: Always round up, even if the calculation gives 4.01.

132.Confusing the standards: CSA S16 is for design, CSA W59 for welding, CSA W47.1 for company certification. Do not mix them up.

133.Forgetting weld ends: The effective length of a fillet weld excludes the ends (start and stop). Add 2 × leg to the calculated length.

134.Using imperial units without conversion: Canada uses the metric system in its standards. Convert inches to millimeters (× 25.4) and pounds to newtons (× 4.448).

135.Neglecting surface preparation: For slip-critical connections, the surface class (A, B, C) determines the friction coefficient — a typical exam question.

Summary

Connection types: Bolted (A325, A490), welded (SMAW, GMAW, FCAW), riveted (historical). Each type has specific applications, advantages, and limitations.

Strength calculations: Use LRFD formulas with resistance factors φ_b = 0.80 (bolts) and φ_w = 0.67 (welds). Master the shear, tension, and fillet weld formulas.

Key standards: CSA S16 (design), CSA W59 (welding), CSA W47.1 (certification), ASTM A325/A490 (bolts).

Tolerances: Vertical deviation 1/500, horizontal 1/300, minimum spacing 2.7 × d, edge distance 1.5 × d.

Inspection: Visual first, then NDT (magnetic particle, ultrasonic) as required by W59. Look for cracks, porosity, lack of fusion, undercut.

Practical calculations: Weld length L = P / (0.67 × φ_w × a × F_u), number of bolts n = P / V_r. Always round up.

Special connections: Slip-critical (friction), seismic (ductility), gusset plates (block shear).

Final exam tip: Questions from this chapter represent approximately 15 to 20% of the Red Seal exam. Practice calculations with real numbers and memorize the resistance factors. Read each question twice — the traps are often in the units or forgotten factors. Good luck with your preparation!