3.2 Compatible Nuts, Washers & Coating Requirements

A high-strength structural bolt is useless without its properly matched nut and washer. The entire grouping is technically referred to as a "fastener assembly." If an inspector only verifies the bolt and ignores the nut, washer, or applied coatings, the integrity of the structural joint is severely compromised. The RCSC Specification mandates strict compatibility rules for these components.

Heavy Hex Nuts: ASTM A563 and ASTM A194

Standard hex nuts are prohibited in structural steel connections; only "heavy hex" nuts are permitted. Heavy hex nuts are slightly wider and thicker, providing the necessary thread engagement length and bearing surface area to withstand the massive clamping forces generated by structural bolts.

The primary specification for carbon and alloy steel structural nuts is ASTM A563. Depending on the bolt grade and whether the assembly is plain or galvanized, different grades of A563 nuts are required: RCSC 2014 Table 2.1 sets the acceptable combinations:

The table's footnotes permit an ASTM A194 Grade 2H nut in place of an A563 Grade DH nut, and the washer column applies only where Section 6 requires a washer (a twist-off assembly always has one under the nut). Two points to remember: plain A325 bolts accept the non-heat-treated C, C3 and D grades, but A490 bolts require DH or DH3 nuts; and galvanized A325 bolts require galvanized, lubricated DH (or 2H) nuts, because hot-dip galvanizing requires overtapped nut threads, which reduces the nut's stripping strength.

An acceptable alternative for high-strength applications is the ASTM A194 Grade 2H nut, originally designed for high-pressure and high-temperature piping, but structurally robust enough to substitute for A563 Grade DH nuts.

Nut identification markings are just as critical as bolt markings. A Grade DH nut will be stamped with "DH", while a Grade C nut will feature three circumferential arcs (or a "C"). Weathering steel nuts (Type 3) will feature a "3" in their designation (e.g., DH3, C3).

Hardened Steel Washers: ASTM F436

Washers in structural steel joints are not merely spacers; they distribute the immense clamping load over a wider area and provide a smooth, hardened surface for the nut or bolt head to rotate against during tensioning. Hardened washers must conform to ASTM F436. The structural-grade plate washers or continuous bars that RCSC Table 6.1 requires over long slots need not be hardened.

Standard F436 washers are hardened to prevent galling (the tearing of metal) when the bolt or nut is turned under high stress. Like bolts, they come in Type 1 (standard) and Type 3 (weathering). Type 3 washers are marked with a "3" to signify their atmospheric corrosion resistance.

Beveled Washers Structural shapes like American Standard beams (S-beams) and standard channels (C-shapes) feature inner flanges that are sloped, typically at a 1:6 ratio (roughly 16.7%). If a flat washer and bolt are installed on a sloped flange, the bolt head or nut will not seat flush. This induces severe bending stresses into the bolt shank, which can lead to premature failure.

To correct this, RCSC requires the use of a beveled washer whenever the slope of the outer face of the bolted part exceeds 1:20 (a 5% slope). The beveled washer compensates for the angle, allowing the bolt to remain perfectly perpendicular to the connection plane and ensuring pure axial tension.

Coatings and Lubrication Requirements

Structural steel is often exposed to harsh environments, requiring protective coatings. The two most common zinc-based coatings for structural fasteners are:

1. Hot-Dip Galvanizing (ASTM F2329 / A153): The fastener is dipped in a bath of molten zinc, providing a thick, durable sacrificial coating.
2. Mechanically Deposited Zinc (ASTM B695): Zinc powder is cold-welded to the fastener in a tumbling barrel with glass beads.
3. Zn/Al inorganic coating (ASTM F1136 Grade 3): RCSC 2014 Table 2.1 lists this coating for both A325 and A490 bolts when it has met the requirements of IFI-144, with matching coated DH nuts.

The Galvanizing Prohibition on A490 / F2280 One of the most critical safety rules a Special Inspector must enforce is the prohibition of zinc coatings on Grade A490 and Grade F2280 bolts. ASTM specifications permit galvanizing A325 bolts but not A490 bolts, and the RCSC commentary adds that applying zinc to A490 bolts by metallizing or mechanical coating is not permitted either. The concern is hydrogen embrittlement: A490 production strengths can approach the range in which high-strength steel becomes susceptible, hydrogen can be introduced during the pickling step of hot-dip galvanizing and then sealed in by the zinc, and corrosion of the zinc in aggressive environments can drive more hydrogen into the steel. The result can be delayed brittle fracture in service. If you encounter galvanized A490 bolts on a job site, they must be completely rejected and removed.

Lubrication of Galvanized Assemblies When a Grade A325 bolt is hot-dip galvanized, the added zinc thickness on the bolt threads means the nut must be manufactured with "overtapped" threads to fit. However, zinc rubbing against zinc under immense pressure creates enormous friction, leading to thread galling and the fastener seizing before proper tension is achieved.

To prevent this, the ASTM fastener standards require galvanized nuts to be supplied lubricated, and the supplier tests the lubricated assembly for rotational capacity before shipment. The lubricant is normally colored so its presence can be seen. If galvanized nuts arrive without visible lubricant, hold them until the supplier resolves the condition and pre-installation verification passes. RCSC Section 2.2 bars field crews from cleaning or modifying components from the as-delivered condition; cleaning, lubricating and retesting are allowed only to resolve a failed pre-installation verification, with every assembly treated the same way (Section 7.2), and twist-off assemblies may be relubricated only by the manufacturer. Plain (uncoated) assemblies also require lubrication, but it is typically a light residual oil from the manufacturing process and does not require a colored dye.

Protected Storage (RCSC Section 2.2)

The S1C outline specifically lists verifying protected storage. RCSC Section 2.2 requires that:

• fastener components be protected from dirt and moisture in closed containers at the site of installation;
• only as many components as are anticipated to be installed during the work shift be taken from protected storage;
• components not incorporated into the work be returned to protected storage at the end of the shift;
• components not be cleaned or modified from the as-delivered condition;
• components that accumulate rust or dirt not be used unless they are requalified by pre-installation verification (Section 7); and
• twist-off assemblies and alternative-design fasteners not be relubricated except by the manufacturer.

The RCSC commentary adds that some galvanized-nut lubricants are water soluble, so galvanized assemblies should be shipped and stored in plastic bags or sealed containers. Open kegs left in the rain, buckets of mixed lots, and loose bolts left on the deck at the end of the day are storage discrepancies. AISC 360 Table N5.6-1 lists protected storage as an Observe task for both quality control and quality assurance.