Structural Bolt Tensioning to AS 4100: Categories & Washers Explained

Structural Bolt Categories: S, TB and TF Explained

Structural bolting in Australia isn't just about grade (8.8, 10.9). Every structural bolt is also assigned a category, and that category — not the grade alone — determines how it must be tightened. Get the category wrong and you can have a perfectly graded, perfectly certified bolt that still fails to do its job in the connection.

• 4.6/S — commercial-grade bolt (property class 4.6, AS 1111), snug-tightened only. The lowest-cost, most widely available option, used in simple connections where the design doesn't call for higher clamping force.
• 8.8/S — high-strength structural bolt (property class 8.8, AS/NZS 1252), snug-tightened. The most common category in flexible (simple) connections in Australian practice.
• 8.8/TB — high-strength structural bolt, fully tensioned, bearing-type. The standard category for rigid connections and bolted splices. Some slip is acceptable — under load, the connection can shift until the bolt shanks bear against the (nominally oversize) holes.
• 8.8/TF — high-strength structural bolt, fully tensioned, friction-type. Reserved for connections where no slip at all can be tolerated under serviceability loads. This is the only category where the condition of the contact (faying) surfaces matters — they must be clean mill scale or a specifically tested coating, never loose paint or mill scale that hasn't been assessed.

The letter after the grade is doing real work: it's telling the erector which tightening method applies and whether the joint relies on bearing or friction to do its job. This category system is set out by the Australian Steel Institute's technical guidance on bolt types and categories.

AS 4100 or AS/NZS 5131 — Which Standard Actually Covers This?

Both standards get cited around structural bolting, and it's worth being precise about which does what, because they're not interchangeable:

• AS 4100 is the design standard — it sets the structural requirements a connection has to meet, and (since the 2020 edition) it formally references AS/NZS 5131 for how that connection is actually built.
• AS/NZS 5131 is the fabrication and erection standard, introduced in 2016 specifically because AS 4100's older editions gave fabrication and erection only a couple of brief sections — thin compared with the standalone execution standards used in the US, Canada, the EU and the UK. AS/NZS 5131 is where the actual bolt installation and tensioning procedure lives: snug-tightening, part-turn tables, direct tension indicator (DTI) verification, and the paperwork trail (Inspection and Test Plans, traceability, construction categories) that goes with it.

In practice: AS 4100 tells the engineer what the connection needs to achieve. AS/NZS 5131 tells the fabricator and erector exactly how to get a bolt to that condition, and how to prove it on site. The Australian Steel Institute's 2020 update notice covers the formal linkage in detail if you want the full picture.

Snug-Tight: The First Step, Not the Last

Every structural bolt — whatever its final category — starts at snug-tight. AS/NZS 5131 describes this as the tightness achieved "by a few impacts of an impact wrench or by the full effort of a person using a standard podger spanner," bringing the plies of the joint into firm contact — see the Australian Steel Institute's Technical Note TN016 on installing bolted connections for the full procedure this section is drawn from.

Two things matter here that are easy to skip on a busy site:

• Contact surface inspection. Before or during snug-tightening, the plies need to be checked for full, solid contact. A gap in the effective contact area isn't cosmetic — it can reduce the final achieved tension by up to 40%, whatever tensioning method follows.
• Tightening pattern. Bolts in a group are snugged progressively from the most rigid part of the connection out towards the free edges, often over more than one pass, so the joint draws up evenly rather than cocking or trapping a gap on one side.

4.6/S and 8.8/S categories stop here — snug-tight is the finished, specified condition. Everything from 8.8/TB up needs a further tensioning step.

Tensioning Methods: Part-Turn and Direct Tension Indicators

AS/NZS 5131 recognises two methods for taking a snug-tight bolt to full tension. Torque alone is not one of them — and that distinction trips up more sites than any other part of this process.

Why torque alone doesn't cut it

Torque measures the effort needed to turn the nut. Tension is the actual clamping force inside the bolt. The two are only loosely related, and the relationship is heavily affected by friction at the thread and under the nut face — which is exactly what varies most on a real site: dirt, moisture, and (on galvanised bolts especially) surface condition. A torque reading that would deliver correct tension on a clean bench sample can deliver a fraction of that on a bolt that's picked up grit or a touch of white rust waiting to be installed — a real-world gap documented by structural engineering consultancy Costin Roe Consulting in its review of common structural bolting misunderstandings. Torque-controlled tightening isn't formally banned, but it has limited uptake in Australia for exactly this reason, and it is not one of the two recognised verification methods below.

Part-turn method

Once snug, the nut (or bolt head, whichever is being rotated) is given a further prescribed turn — location marks are scored across the nut, bolt and ply first, so the amount of rotation can be checked afterwards. AS/NZS 5131 Table 8.5.6 sets the exact turn against bolt length, but as a general guide it runs from around a third of a turn up to a full turn depending on grip length, with roughly a half-turn typical for standard lengths (4–8 diameters). One useful property of the method: because the bolt is yielding slightly in the threaded grip zone at this point, the final tension achieved is relatively insensitive to exactly how snug the starting point was.

Property class 10.9 bolts are the exception — AS/NZS 5131 doesn't permit part-turn tensioning for 10.9 unless the method's suitability for that specific bolt has first been verified by testing. In practice, this pushes 10.9 connections towards direct tension indicators.

Direct tension indicators (DTI washers)

A DTI washer carries a ring of raised protrusions on one face. As the bolt is tightened, the protrusions compress and the gap closes; the washer is engineered so that a specified remaining gap (checked with a feeler gauge, or — on AIMS's Durasquirt and Squirter range — read visually as coloured silicone squeezed out through witness holes once the gap closes) corresponds to the bolt reaching its minimum specified tension.

• The washer goes under the part that does not rotate during tightening (usually the bolt head, if the nut is being turned).
• Before use on the job, AS/NZS 5131 calls for sample verification — not less than three bolts tested for each combination of diameter, length and grade from the batch being installed.
• DTIs are single-use. Once compressed, a washer can't certify a second tightening cycle.

DTIs are the more forgiving method where torque-based confidence is lowest — galvanised bolts, dirty or corroded site conditions, or property class 10.9 connections where part-turn isn't an option.

Minimum Bolt Tension — What "Fully Tensioned" Actually Means

Both tensioning methods are aiming at the same target: a minimum bolt tension set out in AS/NZS 5131, calculated from the bolt's proof stress and stress area. Indicative minimum tensions for common sizes:

These are indicative reference values for common sizes, drawn from the Australian Steel Institute's Technical Note TN016, not a substitute for the full AS/NZS 5131 table — always confirm the exact minimum tension for your bolt size and grade against the current standard or your project's engineering specification before signing off an Inspection and Test Plan.

Washer Selection: K0, K2 and Taper Washers

Structural washers aren't just spacers — the washer type is specified for a reason, and the two most commonly confused options in the Australian market aren't even certified to the same standard:

• K0 structural washer — the Australian-standard washer, certified to AS/NZS 1252.1:2016, with a hardness of 33–41 HRC. This is the washer specified by default for AS 4100 structural assemblies in Australia, and it's supplied under mandatory verification testing through an ILAC-accredited laboratory with a Supplier Declaration of Conformance.
• K2 structural washer — certified to EN 14399-6, the European standard, with a slightly wider hardness range (32–45 HRC). AS/NZS 1252.1:2016 permits K2 as an alternative assembly type, but it's a different certification pathway (CE marking and Factory Production Control documentation, not an Australian SDoC) — worth checking your project specification names K2 explicitly before substituting it for K0, rather than treating the two as interchangeable.
• Taper washers — used wherever a bearing surface isn't square to the bolt axis (a common issue on rolled sections like channels and I-beams, where the flange has a natural slope). AIMS stocks an 8° taper square washer, which covers the standard flange-slope range; if the surface slope is minor, a flat washer is normally adequate, but as the slope increases a taper washer is what keeps the nut bearing square and the tension reading (torque or DTI) meaningful.

One further requirement applies across all of the above: on hot-dip galvanised assemblies, a washer must always be placed under whichever part is actually being rotated during tightening (bolt head or nut) — it protects the coating from being scored and keeps the friction behaviour, and therefore the tensioning result, consistent. The Galvanizers Association of Australia's design manual covers this requirement, along with the extra care galvanised bolts need generally, in more detail.

Matched Assemblies: Why the Bolt, Nut and Washer Come as a Set

AS/NZS 1252.1:2016 requires high-strength structural bolt assemblies to be supplied as a complete matched set — bolt, nut and washer from the one certified batch, packaged together with lot and heat traceability back to test certification. Mixing components from different batches or brands isn't a paperwork technicality; the nut and washer are toleranced and heat-treated specifically to perform with that batch of bolts, and a mismatched set can't be relied on to reach its rated tension by either method above.

This is exactly what a pre-packaged structural assembly kit is for — AIMS's K0 Structural Assembly (Class 8.8, hot-dip galvanised) supplies bolt, nut and K0 washer together as one certified, traceable unit, removing the risk of a mismatched combination being assembled on site.

Construction Categories (CC1–CC4): Why the Paperwork Varies by Project

AS/NZS 5131 grades every structure into one of four construction categories, based on a risk assessment of structure importance, service category and fabrication category. If a project doesn't nominate one, CC2 applies by default. The Australian Steel Institute's guidance note sets out how traceability and testing requirements scale across the four categories.

• CC1 — low-risk structures: gates, handrails, agricultural buildings, greenhouses.
• CC2 — the default category, covering most commercial, residential and industrial buildings up to 15 storeys, and warehouses.
• CC3 — bridges, fatigue-designed structures, high-rise buildings above 15 storeys, and buildings like hospitals with surgical facilities.
• CC4 — structures where failure has extreme consequences: long-span bridges, power stations, and similarly critical infrastructure.

The documentation, inspection and testing requirements around bolted connections step up from CC1 to CC4 — a CC1 gate isn't going to need the same Inspection and Test Plan as a CC3 bridge connection. If you're not sure which category your project sits in, that's a question for the engineer of record, not something to assume on site.

Structural Bolting Range at AIMS Industrial

AIMS stocks the structural fastener range this guide covers, in both plain and hot-dip galvanised finishes:

• K0 Structural Hex Nut Class 8.8 Hot Dip Galvanised
• K0 Structural Hex Nut Class 8.8 Plain
• K0 Structural Washer 35-41 HRC Hot Dip Galvanised
• K0 Structural Washer 35-41 HRC Plain
• K2 Structural Washer EN14399-6 Hot Dip Galvanised
• Taper Square Washer 8° Hot Dip Galvanised
• Taper Square Washer 8° Plain
• K0 Structural Assembly Class 8.8 Hot Dip Galvanised — matched bolt, nut and washer set

For direct tension indication, AIMS stocks the Durasquirt and Squirter DTI washer range across Class 8.8, Class 10.9 and imperial Grade 5, in mechanical galvanised and plain finishes:

• Durasquirt DTI Washer Class 8.8 Mechanical Galvanised
• Squirter DTI Washer Class 8.8 Mechanical Galvanised
• Squirter DTI Washer Class 10.9 Mechanical Galvanised
• Squirter DTI Washer Class 10.9 Plain / Black
• Squirter DTI Washer Grade 5 Mechanical Galvanised

Browse AIMS Industrial's Durasquirt & Squirter DTI Washer range →

Frequently Asked Questions

Need help specifying the right structural bolt category, washer or assembly for your project? Get in touch with the AIMS Industrial team — we're happy to help you find the right fit.