Bolting and mechanical fastening under AS/NZS 5131

Scope: what Section 8 controls and who does it

Section 8 of AS/NZS 5131 governs the installation of the mechanical fasteners whose supply requirements are set in Clauses 5.5 and 5.6 – structural bolts, nuts and washers, thin-gauge fasteners, specialised fasteners, and mechanical and chemical anchors. It applies equally in the fabrication shop and during erection (Clause 8.1.1). Each operation must be run under a quality management system and covered by a documented work method statement before work starts (Clauses 8.1.2–8.1.3).

For an assessor, the first practical question is where the bolting happens. On most export projects the fabricator drills the holes, prepares and protects the contact surfaces and may shop-bolt some sub-assemblies, while the Australian erector installs and tensions the site connections. The construction specification and the contract should allocate each Clause 8 and 13.7 obligation to one party so nothing falls between the shop and the site. Erection-side requirements are covered in the erection chapter.

Bolting categories and fastener basics

AS/NZS 5131 relies on the designer's bolting category, which follows the AS 4100 / NZS 3404 convention. Grade 8.8 high-strength structural bolt assemblies are supplied to AS/NZS 1252.1; see World Steel's note on structural bolts to AS/NZS 1252.

Clause 8.2 sets general rules that inspectors check quickly: structural bolts not smaller than M12; at least one clear thread above the nut and one thread plus run-out clear below it after snug-tightening; nuts that run freely by hand; only steel (no compressible material) within the grip; packers or shims at least 2 mm thick and no more than three in a stack; a washer under the rotated element; tapered washers where the bearing face slope exceeds 1:20; and bolts and nuts never welded. Assemblies delivered as bolt–nut–washer sets stay together until installed and are kept in closed containers protected from dirt and moisture (Clause 8.2.5).

Snug-tightening and contact surface preparation

Every connection is first brought to the snug-tight condition (Clause 8.3): holes must align so a drift pin of bolt diameter passes freely without distorting the steel, and bolts are tightened systematically from the stiffest part of the joint towards the free edges, often in more than one pass. Snug-tight bolts may be reused if threads are undamaged.

Contact surfaces (Clause 8.4.1) must be free of oil, dirt, loose scale and rust and anything that would stop solid seating; burrs up to 1.5 mm may remain, larger burrs are ground. For friction-type (TF) connections Clause 8.4.2 is stricter: the faying surfaces are clean as-rolled steel or equivalent, with no paint, galvanizing or other finish unless that finish has been tested for its friction coefficient. Where coating is kept off, it (including overspray) must be held back at least one bolt diameter – and never less than 25 mm – from hole edges and from the whole bolt group area. This is predominantly a shop responsibility and is closely linked to the masking rules in the corrosion protection chapter.

Appendix G (normative for New Zealand only) describes a double-coverplate tensile slip test to derive a design slip factor for a coated or treated faying surface; for Australia the method in AS 4100 applies, and RCSC or EN 1090-2 methods are also accepted (Clause 8.4.2). If the construction specification calls for coated TF surfaces, ask for the slip test evidence.

Tensioning high-strength bolts: part-turn and direct-tension indication

Tensioning (Clause 8.5) is done only by competent personnel and only after the whole connection is snug-tight. Every bolt must reach the minimum tension in Table 8.5.5 – for example about 145 kN for an M20 Grade 8.8 bolt and 210 kN for M24 – using one of two recognised methods:

1. Part-turn method (Clause 8.5.6): mark the nut and bolt after snug-tightening, then rotate by the amount in Table 8.5.6, which depends on bolt length and whether the outer faces are square or sloped (for a short bolt between square faces, one-third of a turn). Rotation tolerance is over only, never under. Not used for Grade 10.9 unless verified by calibration.
2. Direct-tension indication (Clauses 8.5.7–8.5.8): the device is first proven on at least three assemblies per diameter, length and grade in a load cell, indicating at least 1.05 times the minimum tension. Compressible-washer indicators must meet ASTM F959; over-compressed units are replaced with a new assembly.

Fully tensioned bolts may be re-tensioned only once, in the same hole and grip, and must never be moved to another hole (Clause 8.5.4). Thin-gauge fastening follows AS 1562.1 (Clause 8.6); tension-control bolts, blind bolts, injection bolts and other specialised fasteners must be fully detailed in the construction specification and installed to the manufacturer's procedure (Clause 8.7).

Mechanical and chemical anchors

Clause 8.8 brings post-installed anchors into the standard. The specification either names the exact product and embedment or gives the design actions; no substitution without written authorisation. Installation follows the manufacturer's written instructions, with hole location tolerance of 1.5 mm, depth tolerance of 0 to +5 mm, mandatory depth gauges, thorough hole cleaning, full resin cure time and torque applied with a wrench calibrated within the last 12 months. Anchors must not be shortened, products from different manufacturers must not be mixed, and drilling stops immediately on hitting reinforcement. For cast-in hold-down assemblies see anchor bolt assemblies and chemical injection anchors.

Inspection of mechanical fastening (Clause 13.7)

For every construction category except CC1, a bolting ITP is required (Clause 13.7.1). It names who inspects, at which stage, which connections, and by what method and extent. Clause 8.9 also requires CC2–CC4 installation to be supervised by a competent person. The inspection sequence is:

• Before erection – fastener documentation, packaging and markings checked against Clauses 5.5–5.6; anything that cannot be verified is nonconforming (Clause 13.7.4). Any testing must be statistically based, not single results (Clause 13.7.5).
• Before installation – bolts, nuts and washers checked for cracks, thread damage and lost lubrication; contact surfaces checked against Clause 8.4 (Clause 13.7.6).
• After snug-tightening – correct grade and size, thread projection, plies in firm contact, packing as required; poor fit-up means disassemble, correct and re-inspect the whole connection (Clause 13.7.7).
• After tensioning (CC2–CC4) – part-turn verified from marks, or DTI readings confirmed; no more than 10% of compressible-washer indicators fully closed; sampling by ISO 2859-5 unless specified otherwise (Clause 13.7.8).
• Anchors – product, positions, edge distances, hole condition, torque and cure, with proof testing to BS 8539 where required or doubtful (Clause 13.7.11).

Appendix H (informative) explains the torque wrench check: the inspection wrench is calibrated each shift on at least three bolts tensioned to 1.05 times the minimum tension, the torque to turn the nut a further 5° becomes the job inspection torque, and a suggested sample is 10% of bolts but at least two per connection. Clause 13.7.8 is explicit that this only reveals gross under-tensioning – it does not prove correct tension. Inspection records must show inspector, item, date, requirement and result (Clause 13.7.3). Refer to the current edition of AS/NZS 5131 for the full requirements.