CSA Bolt Pretension — CSA S16 Slip-Critical Connection Clamping Forces

Complete reference for bolt pretension requirements per CSA S16-19 Clause 23 and CSA W59. Minimum bolt tensions for A325M and A490M bolts in slip-critical connections, installation methods, faying surface preparation, and inspection procedures.

Quick access: CSA bolt capacity tables → | CSA bolt hole sizes → | Bolted connection calculator →

CSA S16 Pretension Requirements

CSA S16-19 Clause 23.1 requires that bolts in slip-critical connections be installed to a minimum pretension (clamping force) to develop the required frictional resistance between connected plies. The minimum bolt pretension Ti is specified for each bolt grade and size.

The minimum pretension values are based on approximately 70% of the specified minimum tensile strength of the bolt:

Ti = 0.70 _ Ab _ Fu

However, CSA S16-19 provides directly tabulated minimum pretension values in Clause 23.3, Table 4.

Minimum Bolt Pretension Table

All values in kilonewtons (kN). Minimum pretension per bolt for slip-critical connections.

Verification

For M24 A325M: Ti = 0.70 _ Ab _ Fu = 0.70 _ 452.4 _ 830 / 1000 = 262.9 kN

But the tabulated value is 205 kN. The discrepancy exists because CSA S16-19 uses a factored pretension value (accounting for variability in installation), not 70% of bolt capacity directly. The tabulated values are the minimum bolt tension that must be verified during installation, calibrated to provide reliable clamping force consistent with slip-critical design assumptions.

For M24 A490M: Ti = 0.70 _ 452.4 _ 1035 / 1000 = 327.8 kN Tabulated: 256 kN

Same conservative reduction applies — the factored pretension used in design is lower than the theoretical 70% of tensile capacity to ensure reliable slip resistance across the population of installed bolts.

Slip Resistance

Per CSA S16-19 Clause 13.12.2, the factored slip resistance of a bolt in a slip-critical connection is:

Vs = 0.60 _ mu _ phi*s * n _ Ti

Where:

• mu = slip coefficient for faying surface (class A = 0.30, class B = 0.50)
• phi_s = 0.80 (resistance factor for slip)
• n = number of slip planes
• Ti = minimum bolt pretension (from table above)

Slip Coefficient by Surface Class

Slip Resistance per Bolt — Class A Surface

Class A surface (mu = 0.30) is the default for most building construction where the steel has clean mill scale.

Calculation for M24 A325M, Class A surface: Vs = 0.60 _ 0.30 _ 0.80 _ 1 _ 205 = 29.5 kN

Compare this to the bearing-type shear capacity: Vr = 143.9 kN (AX). The slip-critical capacity is only 21% of the bearing capacity. Slip-critical connections are therefore governed by slip resistance, not bolt shear strength, in most practical designs.

Installation Methods

CSA S16-19 Clause 23.2 permits three methods for achieving required pretension:

1. Turn-of-Nut Method (Clause 23.2.1)

The most common pretensioning method. After bringing the connection to a snug-tight condition using manual or impact wrench (typically 60-80 Nm for M20 bolts), each bolt is rotated through a specified additional turn:

The turn-of-nut method provides the most reliable pretension but requires calibration for each bolt length group. The additional rotation strains the bolt into the strain-hardening range to achieve the required clamping force.

2. Tension Control (TC) Bolt Method (Clause 23.2.2)

Tension control bolts (Twist-Off type, ASTM F1852M) have a splined end that shears off at the specified pretension:

• Install with a special TC wrench that grips both the bolt head and the splined end
• Rotate the nut relative to the washer face while holding the splined end stationary
• When the splined end shears off (at torque corresponding to required pretension), installation is complete
• Visual inspection is simple: if the splined end is broken off, the bolt achieved pretension

TC bolts are the preferred method in Canadian construction for slip-critical connections because installation is fast and inspection is straightforward. The A325M equivalent is ASTM F1852M.

3. Direct Tension Indicator (DTI) Method (Clause 23.2.3)

Direct Tension Indicators (DTI washers) are specially designed washers with protrusions that compress as the bolt is tensioned:

• Place the DTI under the head of the bolt or under the nut
• Tighten the bolt using any convenient method (impact wrench, torque wrench)
• Check the gap between the DTI washer and the bolt head/nut using a feeler gauge
• When the gap is <= 0.25 mm (measured with a 0.25 mm feeler gauge), the required pretension has been achieved

DTI washers provide direct verification of bolt tension without relying on torque-tension relationships, which can vary significantly with lubrication, thread condition, and installation speed.

4. Calibrated Torque Method (Clause 23.2.4)

The calibrated torque method uses a torque wrench set to a value that correlates to the required pretension:

• Requires torque calibration testing for each combination of bolt lot, washer type, and lubrication condition
• Three bolt samples from the lot must be tested in a Skidmore-Wilhelm or similar tension calibrator
• The calibration test establishes the relationship between installation torque and achieved bolt tension
• Production bolts are then tightened to the calibrated torque value

This method is seldom used in Canadian construction due to the variability of torque-tension relationships and the calibration testing requirement. The TC bolt and turn-of-nut methods are preferred.

Faying Surface Preparation

The slip resistance of a pretensioned connection depends critically on the condition of the faying surfaces (the surfaces of the connected parts that are in contact).

Surface Preparation Requirements per CSA S16-19 Clause 23.4

Critical requirement: Faying surfaces must not be painted unless the paint system has been specifically qualified for the slip coefficient. Standard structural steel paint (alkyd, epoxy) reduces the slip coefficient from 0.30 to 0.15 or lower. If painted faying surfaces are unavoidable, the slip coefficient must be demonstrated by testing.

Surface Inspection Before Assembly

• Surfaces must be free of oil, grease, dirt, and loose mill scale
• Surface rust is acceptable if the rust layer is tight and stable (brush-blast to remove loose rust)
• Moisture on faying surfaces is not permitted — assembled connections must be dry
• Burrs around holes reduce slip coefficient and must be ground flush
• For hot-dip galvanized surfaces, the galvanized coating should be lightly wire-brushed to roughen the surface (improves slip coefficient from approximately 0.20 to 0.40)

Snug-Tight vs Pretensioned vs Slip-Critical

CSA S16-19 Clause 22.6 defines three tightening categories:

A "pretensioned" bolt and a "slip-critical" bolt require the same installation force (Ti). The difference is:

• Pretensioned: bolts are tensioned to Ti but the connection is designed as bearing-type (slip is permitted under factored load)
• Slip-critical: bolts are tensioned to Ti AND slip resistance is checked as a separate limit state

In practice, this means: all connections in seismic force-resisting systems per CSA S16-19 Clause 27 must have pretensioned bolts. Slip-critical connections additionally require faying surface preparation and slip resistance checks.

Inspection and Quality Control

CSA S16-19 Clause 23.5 and CSA W59 Annex A specify inspection requirements for pretensioned bolts:

Daily Inspection (before installation)

• Verify bolts, nuts, and washers are the specified grade and are clean
• Check that the bolt lot has been tested for pretension consistency
• TC bolts: verify that the splined end shears off at the correct torque range

Installation Inspection (during installation)

Post-Installation Verification

Per CSA W59 Annex A, the following are required for slip-critical connections:

• Rotational capacity test: For turn-of-nut installations, a representative sample (2 bolts per 100) must be tested to verify that the nut can be tightened an additional 1/2 turn beyond the installed position without bolt fracture

• Visual inspection: 100% of bolts checked for proper installation. For TC bolts, verify splined end is broken off. For DTI washers, verify gaps are closed

• Rejection criteria: Bolts that cannot achieve required pretension (e.g., splined end shears off below torque, or DTI gap does not close) must be removed and replaced. Damaged or reused bolts are not permitted in slip-critical connections.

Frequently Asked Questions

What is the minimum bolt pretension for M24 A325M bolts per CSA S16? The minimum bolt pretension Ti for M24 A325M in slip-critical connections is 205 kN per CSA S16-19 Table 4. For M24 A490M, Ti = 256 kN. These values are approximately 70% of the specified minimum tensile strength factored by an additional safety margin for installation variability. The actual pretension achieved during installation typically exceeds the minimum by 10-20% in properly calibrated operations.

What is the difference between snug-tight and pretensioned bolts in CSA S16? Snug-tight means the plies are brought into firm contact using a few impacts of an impact wrench — approximately 10% of the specified pretension. Snug-tight bolts are acceptable for bearing-type connections in non-seismic applications. Pretensioned bolts require full installation to the specified minimum pretension Ti using an approved method (turn-of-nut, TC bolt, DTI, or calibrated torque). Pretensioned bolts are required for all connections in seismic force-resisting systems and for slip-critical connections per CSA S16-19 Clause 22.6.

When are slip-critical connections required per CSA S16? CSA S16-19 Clause 22.8 requires slip-critical connections for: (a) connections subject to stress reversal or vibration that could cause slip; (b) connections in which joints are subject to impact or fatigue loading; (c) connections using oversized or long slotted holes; (d) connections in which slip would cause a serviceability problem or misalignment; (e) connections in crane runway girders; and (f) connections in seismic force-resisting systems as specified in Clause 27.

Can bolts be reused in slip-critical connections? No. Per CSA S16-19 Clause 22.8.2, bolts that have been fully tensioned in slip-critical connections must not be reused. The tightening process strains the bolt into the inelastic range, and retightening may not achieve the required pretension. Additionally, the thread surface condition changes after initial installation, altering the torque-tension relationship. Bolts removed during connection inspection must be discarded and replaced with new bolts for reassembly.

What are the three methods for achieving bolt pretension per CSA S16? The three permitted methods per CSA S16-19 Clause 23.2 are: (1) Turn-of-nut method — additional rotation of 1/3 to 2/3 turn after snug-tight depending on bolt length; (2) Tension control (TC) bolt method — splined end shears off at specified pretension, providing visual verification; (3) Direct tension indicator (DTI) method — specially designed washer with protrusions that compress at specified tension. The calibrated torque method is also permitted but requires Skidmore-Wilhelm calibration testing for each bolt lot.

Related Pages

• CSA Bolt Capacity — A325M & A490M Tables
• CSA Bolt Hole Sizes — Standard & Oversized Holes
• Canadian Steel Bolting — A325M Fasteners & CSA S16
• CSA W59 Weld Electrodes — E48XX Selection Guide
• CSA S16 Code Overview
• Bolted Connection Calculator
• Bolt Torque Calculator
• Anchor Bolt Design Guide

This page is for educational reference. All pretension data per CSA S16-19 Table 4. Verify values against the current code edition before design. Bolt installation must follow certified weld and bolt procedures per CSA W59 and the project specification. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.