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Bolting Galvanized Steel
Bolting Steel Structures
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, sherardised 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
The 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 4680 ‘Hot dip galvanized (zinc) coatings on fabricated ferrous articles’ provides for a standard minimum coating thickness regardless of fastener dimensions:
Requirements for coating thickness and mass for articles that are centrifuged
|Thickness of articles (all components including castings) mm||Local coating thickness minimum μm||Average coating thickness minimum μm||Average coating mass minimum g/m²|
Note: 1. For requirements for threaded fasteners refer to AS 1214
2. 1g/m² coating mass = 0.14μm coating thickness
Oversize tapping allowances for galvanized nuts
To accommodate the relatively thick galvanized coating on external threads, it is usual to galvanize bolts of standard thread dimensions, and to tap nuts oversize after galvanizing. AS 1214 ‘Hot dip galvanized coatings on threaded fasteners’ specifies the following oversize tapping allowances on internal threads:
|Nominal diameter of internal threads||Allowance, mm|
|Up to M22||0.40mm|
To ensure that nut stripping strength is adequate after oversize tapping, galvanized high strength nuts are manufactured from steel with a higher specified hardness than standard high strength nuts.
Galvanized high strength bolts and nuts must be provided with a supplementary lubricant coating for satisfactory bolt tightening.
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||Cost-in place|
* TJ Hogan and A Firkins, 'Bolting of steel structures' Australian Institute of Steel Construction
Zinc plating on 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 galvanizing is usually a more economic 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 Standards 1791 ‘Chromate conversion coating on zinc and cadmium electrodeposits’. Clear, bleached, iridescent or opaque films may be produced, depending on the level of resistance to wet storage staining required.
Australian Standard 1897 ‘Electroplated coatings on threaded components (metric coarse series)’ specifies plating thicknesses which can be accommodated on external threads to required tolerances.
Sherardising 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, sherardising is not used in Australia. The process is characterised by its ability to produce a very uniform coating on small articles.
The thickness of sherardised coatings is generally of the order of 15μm but can vary depending on cycle time from 7.5 to 30μm. Sherardised coatings therefore fall between zinc plated and galvanized coatings in thickness and life.
Although sherardising is an impregnation process there is some build up in dimensions. British Standard 729 ‘Zinc coatings on iron and steel articles, Part 2: Sherardised coatings’ recommends an oversize tapping allowance of 0.25mm on nuts to ensure easy assembly with sherardised bolts.
Mechanical (peen) plating
Mechanical or peen plating offers advantages in the zinc coating of fasteners. Coatings are uniform, and because the process is electroless there is no possibility of hydrogen embrittlement. High strength fasteners not susceptible to embrittlement need not be baked after coating. Lubricant coatings must be applied to ensure satisfactory tightening.
Relevant Australian Standards
Relevant material standards referenced by Australian Standard 4100 are the current editions of:
|AS 1110||'ISO metric hexagon precision bolts and screws'|
|AS 1111||'ISO metric hexagon commercial bolts and screws'|
|AS 1112||'ISO metric hexagon nuts, including thin nuts, slotted nuts and castle nuts'|
|AS 1252||'High strength steel bolts with associated nuts and washers for structural engineering'|
|AS 1275||'Metric screw threads for fasteners'|
|AS 1559||'Fasteners - bolts, nuts and washers for tower construction'|
Bolting Category System
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.
Design for Hot-Dip Galvanizing
For more detailed information about bolting please contact us for a copy of our technical manual CD "After Fabrication Hot Dip Galvanizing: A practical reference"