Bolting has become the most widely used, versatile and reliable method of making field connections in structural steel members. The major advantages of bolting over welding are:
1. Economy, speed and ease of erection
2. Reliability in service
3. Relative simplicity of inspection
4. Fewer and less highly skilled operators required
5. Good performance under fluctuating stresses
6. Ease of making alterations and additions
7. Absence of coating damage
8. No pre-heating of high-strength steels
9. No weld cracking or induced internal stress
10. No lamellar tearing of plates.
Galvanized Steel Structures
In the construction of galvanized steel structures, bolted connections offer further advantages. Damage to the galvanized coating from local heating during welding is eliminated and with it the need for coating repairs to the affected area.
The high cost of maintenance labour and wide use of steel communications towers, exposed industrial structures, steel bridges and power transmission towers, often in remote areas, have made low maintenance corrosion protection systems an essential aspect of design. As a result, galvanizing has become the accepted standard for exposed steel, placing greater emphasis on bolted joints for structural steelwork and leading to development of specialized bolting techniques.
A wide range of galvanized, sherardized and zinc plated structural bolts and related fittings is available to meet any steel construction need.
Zinc Coatings for Fasteners
In bolted steel structures, the bolts and nuts are critical items on which the integrity of the entire structure depends.
For exterior use these critical fasteners must be adequately protected from corrosion. Where steel members of the structure are galvanized it is recommended that fasteners employed should also be galvanized or suitably zinc coated to maintain a uniform level of corrosion protection throughout the structure.
Selection of Zinc Coatings for Fasteners
The zinc coating selected is decided primarily by the period of protection desired which should be equivalent to the life of the protective system selected for the structure.
The zinc coating process selected must also produce a relatively uniform coating over small parts of varying shape. With the thicker zinc coatings, allowances in thread dimensions must be made to accommodate the thickness of the coating.
These requirements dictate that in practice one of four types of zinc coating will be suitable:
2. Zinc plating
4. Mechanical plating
Hot Dip Galvanizing
The hot dip galvanizing of fasteners produces a heavy coating of zinc ideally suitable for long-term outdoor exposure. The coating is applied by the immersion of clean, prepared steel items in molten zinc. The resulting zinc coating is metallurgically bonded to the basis steel, and consists of a succession of zinc-iron alloy layers and an outer zinc layer.
Fasteners are generally centrifuged immediately on withdrawal from the molten zinc of the galvanizing bath to remove excess free zinc and produce a smoother finish and cleaner threads.
Australian/New Zealand Standard 1214 ‘Hot Dip Galvanized Coatings on Threaded Fasteners (ISO metric coarse thread series)’ provides a standard minimum local coating thickness of 40 µm and a batch average coating thickness of 50 µm, regardless of fastener dimensions, and may also be applied to washers.
Thread Dimensions for Hot Dip Galvanizing
Assembling hot dip galvanized nuts and bolts without modifying the thread for the extra coating thickness will result in thread interference. To accommodate the relatively thick galvanized coating on thread components, AS/NZS 1214 provides two different methods; either tapping the nut oversize after galvanizing, or tapping the screw undersize prior to galvanizing, although the former is the most widely adopted method.
The screw thread is always cut prior to galvanizing and the nut thread is always cut after galvanizing, leaving the internal thread of the nut uncoated. The galvanized coating on the bolt and other surfaces of the nut provide cathodic protection to the internal thread, protecting it from corrosion.
Bolts with threads cut undersize before galvanizing should never be mated with nuts tapped oversize after galvanizing due to the high probability of thread stripping.
The recommended maximum coating thickness on the bolt is a quarter of the minimum possible clearance, when the internal and external threads are at the closest point to the nominal thread profile the tolerancing will allow. The one quarter value is derived using trigonometry and represents filling all of the clearance space between the nut and bolt.
Nuts Tapped Oversize
When the nut is tapped oversize after galvanizing, bolts of standard g or h thread tolerance position are galvanized and mated with them. Nuts are tapped oversize to tolerance class 6AZ or 6AX after galvanizing, with the AZ class intended for centrifuged threads and the AX class intended for threads cleaned using other methods. The minimum required nut oversizing is given in AS/NZS 1214 as:
Nominal Thread Diameter (mm)
6AZ Nut Clearance (µm)
6AX Nut Clearance (µm)
Mechanical Properties of Galvanized Fasteners
After galvanizing to AS/NZS 1214, both the nut and bolt are required to individually meet the mechanical property requirements of ungalvanized components for sizes M12 and above. Special requirements are given for M8 and M10 components in Appendix A of AS/NZS 1214, due to the different thread overlapping or stress area after galvanizing.
AS/NZS 1252.1, ‘High-strength steel fastener assemblies for structural engineering—Bolts, nuts and washers’, now allows for bolts from 12 mm to 36 mm of property class 8.8 and nuts of property class 8 with ISO metric coarse pitch series threads and associated washers. It also allows an additional bolt assembly type of property class 10.9 to EN 14399.3. All of these items can be hot dip galvanized.
Galvanized high strength bolts and nuts to AS/NZS 1252.1 must be provided with a supplementary lubricant coating for satisfactory bolt tightening in high strength structural applications. Hop dip galvanized washers tend to bond to each other when tightened and a suitable acceptance criteria for the coating should be agreed upon at the time of ordering.
Property class 10.9 bolts require special controls to ensure the product is not damaged by the hot dip galvanizing process. These controls are widely understood and practiced in the galvanizing industry, where class 10.9 bolts have been hot dip galvanized in Europe for many years.
AS 1559, ‘Hot-dip galvanized steel bolts and associated nuts and washers for tower construction’, provides the requirements for hot-dip galvanized tower bolts, including step bolts, with ISO metric coarse pitch series threads for normal and low temperature applications in tower construction, and associated nuts and washers. These are used in transmission towers.
AS/NZS 1252.1 and AS 1559 use AS/NZS 1214 as the base hot dip galvanizing requirement for the bolts, nuts and washers included in the Standards.
Economics of Galvanized Coatings on Bolts
Corrosion protection on bolts should match the rest of the structure and in most circumstances economics favour the use of galvanized bolts rather than painting after erection. The following table * gives indicative cost-in-place relationships for unpainted, painted, and galvanized M20 bolts in structural applications:
Bolt strength grade/Bolting procedure
* TJ Hogan and A Firkins, ‘Bolting of steel structures’ Australian Institute of Steel Construction
Using zinc electroplating to coat fasteners produces relatively light, uniform coatings of excellent appearance which are generally unsuitable for outdoor exposure without additional protection.
There is in general an economic upper limit to the coating mass which can be applied by plating, although certain specialized roofing fasteners are provided with zinc plated coatings up to 35 to 40 μm thick. Where heavy coatings are required hot dip galvanizing is usually a more economical alternative.
Zinc plated bolts having a tensile strength above 1000 MPa must be baked for the relief of hydrogen embrittlement.
Zinc plated high strength bolts and nuts must be also provided with a supplementary lubricant coating to provide for satisfactory bolt tightening.
Australian Standards for zinc plating require that one of a range of chromate conversion coatings be applied in accordance with Australian Standard 1897 ‘Electroplated coatings’ which also specifies plating thicknesses which can be accommodated on external threads to required tolerances.
Sherardizing produces a matt grey zinc-iron alloy coating. The process impregnates steel surfaces with zinc by rumbling small components and zinc powder in drums heated to a temperature of about 370°C. The least known of the various processes for zinc coating steel, sherardizing is rarely used in Australia. The process is characterised by its ability to produce a very uniform coating on small articles.
The thickness of sherardized coatings is generally of the order of 15μm but can vary depending on cycle time from 7.5 to 30μm. Sherardized coatings therefore fall between zinc electroplated and hot dip galvanized coatings in thickness and life. The durability of sherardized coatings is covered in ISO 14713 and is considered to be the same as a hot dip galvanized coating when the coating thickness is the same.
Although sherardizing is an impregnation process there is some build up in dimensions. ISO 17668 ‘Zinc diffusion coatings on ferrous products – Sherardizing – Specification’ and ISO 14713-3 ‘Zinc coatings – Guidelines and recommendations for the protection against corrosion of iron and steel in structures Part 3: Sherardizing’ provide more information on design and use of these products.
Mechanical (Peen) Plating
Mechanical or peen plating process consists of a tumbling barrel, with spherical beads used as an impact medium to peen metallic powders onto the steel’s surface. The process offers advantages in the zinc coating of fasteners, with coatings being uniform and having no possibility of hydrogen embrittlement as the process is electroless. High strength fasteners susceptible to embrittlement when other coating methods are used do not need not to be baked when using the mechanical coating process. Lubricant coatings must be applied to ensure satisfactory tightening.
Relevant Australian Standards
Relevant material standards referenced by Australian Standard 4100 for hot dip galvanized fasteners are the current editions of:
ISO metric hexagon precision bolts and screws
ISO metric hexagon commercial bolts and screws
ISO metric hexagon nuts, including thin nuts, slotted nuts and castle nuts
Hot dip galvanized coatings on threaded fasteners (ISO metric coarse thread series)
High strength steel fastener assemblies for structural engineering – Bolts, nuts and washers
Metric screw threads for fasteners
Hot dip galvanized steel bolts and associated nuts and washers for tower construction
The following bolting category identification system is based on that used in AS4100:
Category 4.6/S – Commercial bolts used snug tight
Category 8.8/S – High strength structural bolts used snug tight
Category 8.8/TF – High strength structural bolts fully tightened in friction type joints
Category 8.8/TB – High strength structural bolts fully tightened in bearing type joints
This category designation system is derived from the Strength Grade designation of the bolt, for example 8.8, and the bolting design procedure which is based on the following supplementary letters:
S represents snug tight
TF represents fully tensioned, friction type joint
TB represents fully tensioned, bearing type joint
Category 4.6/S refers to commercial bolts of Strength Grade 4.6 tightened snug tight (Snug tight is the final mode of tightening for bolting categories 4.6/S and 8.8/S, and the first step in full tensioning for bolting categories 8.8/TF and 8.8/TB).
Category 8.8/S refers to high strength structural bolts of Strength Grade 8.8 used snug tight.
Category 8.8T refers to both categories 8.8/TF and 8.8/TB
Category 8.8/TF refers to high strength structural bolts Strength Grade 8.8 used in friction type joints, fully tensioned in a controlled manner to the requirements of AS 4100.
Category 8.8/TB refers to high strength structural bolts Strength Grade 8.8 used in bearing type joints, fully tensioned in a controlled manner to the requirements of AS 4100.