In 1976, agreement was reached, and America and Canada agreed to accept the ISO Metric thread system with some modifications. From this time, ISO standardisation has been on metric threaded fasteners only.
2.2 Non-Preloaded Fasteners for Use in Shear and Bearing Connections
The development of ISO metric fasteners commenced in 1949 with the production of a series of reference standards, mainly dealing with materials, mechanical properties, basic dimensions and dimensional standards. The reference standards were used by national standards organisations to produce the first ISO metric fastener standards. In the United Kingdom, these were the standards BS 3692 and BS 4190.
Within the structural steelwork industry, the most commonly used standard for bolts in shear and bearing connections was BS 4190; generally mild steel bolts of strength grade 4.6. The standard was developed, in part, for use with structural steelwork, and included short threaded bolts that enabled structures to be designed with full shank diameter in the shear plane. With changes in the design of structures, increasing use was made of high tensile strength 8.8 bolts. In an attempt to control costs, the structural steelwork industry developed a hybrid; a short-threaded bolt with coarse (black) dimensional tolerances to BS 4190 and mechanical property class 8.8 to BS 3692.
The use of these hybrid bolts continued unchallenged until the development of the 8.8 fully- threaded bolts during the 1990s, again with dimensions to BS 4190 and mechanical properties to BS 3692. This development came about with the publication of BS 5950, which included design for bolt threads in the shear plane. The number of bolt lengths required for a structure could be significantly reduced, speeding the rate of erection and resulting in lower fastener costs with the economies of scale. Increasingly, property class 4.6 bolts have been replaced with 8.8 bolts, especially where both property classes are included in the design. This is to avoid the risk of the lower grade 4.6 being used where an 8.8 bolt is required.
In 2001, a revised version of BS 4190 was published. This included the more generally used high tensile bolt property classes of 8.8 and 10.9 and their associated nut grades. The revision will meet the general structural fastener requirements until the required CEN/ISO standards are produced. The tables below are extracted, as examples, from BS 4190.
The committee CEN/TC185 was formed in 1989 specifically to address the task of converting the International Standards for fasteners to European Standards. By 1993, conversion of the majority of the most commonly used standards had been completed.
In the mid-1990s, a formal agreement was reached between the International and European Standards Organisations to produce "agreed standards" by dual voting. These are published in the UK as BS EN ISO Standards and will gradually replace the existing BS and BS EN Standards.
The changes in fastener standards from the original British Standards to International Standards are as much about the form in which they are published as the technical content. The practice with British Standards has been to publish a standard that includes all the information relating to a subject or product, whereas with International Standards each deals with one aspect of the product. As a result of this, for example, twelve International or ISO Standards replace the single document BS 3692: Precision Bolts, Screws and Nuts.
Compared with the original British Standards, the technical changes that have occurred with the publication of the BS EN ISO fastener standards cover: across flat sizes; head thickness; measurement of thread length; bolt washer-face diameter; under-head fillet radius and head chamfer. These changes may seem to be minor compared with shank diameter and thread form, but they can have a significant impact – eg whether an internally chamfered washer would be needed to ensure that the under-head fillet radius does not prevent the bolt from seating properly. They can also affect design rules if the common mode of fastener failure were to change from rupture across the thread root to, say, splitting of a narrower or thinner nut.
Diameter of Washer Under Depth of Nominal Pitch Unthreaded Width Across Width Across Height of Face Head Washer
Bolt of Shank Flats Corners Head Diameter Radius Face
Diameter Thread d s e k dw r c
p Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Max.
M12 1.75 11.30 12.70 18.48 19.00 20.88 21.9 7.55 8.45 17.2 1.25 0.6 M16 2 15.30 16.70 23.16 24.00 26.17 27.7 9.55 10.45 22.0 1.25 0.8 M20 2.5 19.16 20.84 29.16 30.00 32.95 34.6 12.10 13.90 27.7 1.78 0.8 M22 2.5 21.16 22.84 31.00 32.00 35.03 36.9 13.10 14.90 - 1.78 - M24 3 23.16 24.84 35.00 36.00 39.55 41.6 14.10 15.90 33.2 1.78 0.8 M27 3 26.16 27.84 40.00 41.00 45.20 47.3 16.10 17.90 - 2.28 - M30 3.5 29.16 30.84 45.00 46.00 50.85 53.1 17.95 20.05 42.7 2.28 0.8 M36 4 35.00 37.00 53.80 55.00 60.79 63.5 21.95 24.05 51.1 2.7 0.8
Nominal Pitch Width Across Width Across Nut
Nut of Flats Corners Thickness
Diameter Thread s e m
p Min. Max. Min. Max. Min. Max.
M12 1.75 18.48 19.00 20.88 21.90 9.55 10.45 M16 2 23.16 24.00 26.17 27.70 12.45 13.55 M20 2.5 29.16 30.00 32.95 34.60 15.45 16.55 M22 2.5 31.00 32.00 35.03 36.90 17.45 18.55 M24 3 35.00 36.00 39.55 41.60 18.35 19.65 M27 3 40.00 41.00 45.20 47.30 21.35 22.65 M30 3.5 45.00 46.00 50.85 53.10 23.35 24.65 M36 4 53.80 55.00 60.79 63.50 28.35 29.65
Dimensions for Black Hexagon Bolts and Screws to BS 419
All dimensions in millimetres
Dimensions for Hexagon Nut to BS 4190
2.3 Preloaded Fasteners for Use in Friction Grip Connections
Batho and Bateman first proposed the development of high strength bolts for use in connections in steel structures in 1934. They reported to the Steel Structures Committee of Scientific and Industrial Research of Great Britain that bolts could be tightened sufficiently to prevent slip in structural joints. The work concluded that bolts with a minimum yield strength greater than 25 tonnes per square inch (approx 40 MPa) could be tightened to give enough preload to prevent slippage between the connected parts.
In 1938 at the University of Illinois, Wilson and Thomas reported that the fatigue strength of bolt and nut assemblies, with high preload, was at least as great as well-driven rivets. No further work was done on the concept until 1947 when the Research Council on Riveted and Bolted Structural Joints was formed. The council’s work established the suitability of high strength bolts in structural joints: in 1949 the first standard for high strength bolts to ASTM A325 was approved. In 1951 the first standard for structural joints was approved, which permitted the replacement of rivets by bolts on a one-to-one basis.
During the 1950s in the USA, studies were made on installation procedures and the effect of different surface finishes on the slip values of joints. Both in the USA and Germany, studies were undertaken on the use of preloaded bolts in bridges and the behaviour of joints under repeated loadings.
In 1959 the first British Standard for preloaded bolts [BS 3139] was published. This was followed in 1960 by BS 3294, the code of practice for the use of preloaded bolts. The next major development within the UK was the production of the two metric versions of preloaded bolt standards:
BS 4395-1: 1969 General grade bolts, nuts and washers.
BS 4395-2: 1969 Higher grade bolts, nuts and washers.
This was followed in 1970 by BS 4604-1 and -2, which were the relevant codes of practice for the use and assembly of the fasteners in structural steelwork. Of particular importance was the inclusion of the three main methods for tightening of preloaded bolts (see below).
3 Strength of Fasteners
3.1 Mechanical Properties of Fasteners The mechanical properties of bolts had, until 1999, been prescribed in BS 3692. That standard was superseded by BS EN ISO 898-1: 1999.
Amongst many changes in detail were several important changes in principle; one of these, for example, was the removal of property class 14.9 from the standard, since this is not sufficiently ductile for use in normal structural applications.
There has also been considerable debate across Europe over a number of years concerning the strength of nuts and of over-tapped nuts. It had been demonstrated that nuts could fail by thread- stripping within the nut, a dangerous form of failure since the joint appears to be correctly assembled. The revised European and ISO nut standards have increased nut thickness and hardness, selected such that, in the case of over- tightening, at least 10% of assemblies will fail in bolt breakage rather than nut-thread stripping, giving adequate warning that installation practice is not appropriate.
Further work on over-tapped nuts (to accommodate thick protective coatings) also showed that these would also fail prematurely; this has been overcome by the requirement to select the next higher grade of nut for the assembly (eg a grade 10 nut for an 8.8 bolt).
3.2 Materials for Fasteners
It is important to understand that all fastener specifications provide a wide range of potential materials for bolt manufacturers to work from, with variations in chemical composition, base material and treatment and tempering temperature. It is equally important to understand that different production methods, such as cold forming, hot forging and bar turning dictate the selection of material in the first instance, with a limited selection of materials being appropriate for any one particular production method.
BS EN ISO 898-1 states in the scope of the standard that the fasteners specified are suitable for use in conditions ranging from –50°C to +300°C. However, the standard also states that the mechanical properties quoted for the different property classes are for testing at ambient temperature, between 10°C and 35°C.
Increasingly UK structural steelwork contractors are supplying products to countries where sub- zero temperatures and properties are a critical design constraint. The designer and contractor need to be aware of what materials are used to manufacture structural fasteners. Typically, the
materials used by UK fastener manufacturers to cold-form bolts in diameters up to and including M24 will meet low temperature requirements down to –50°C, and –40°C for diameters including M30. Materials for hot-forged fasteners in diameters greater than M30 will require special material selection to achieve good low temperature properties at –50°C, and this would also require special manufacture. Without particular attention to these points, few specifiers would know whether bolts were sourced from cold-formed, hot-forged or machined-bar stock.
The complex range of available materials also delivers significantly different properties at high and low temperatures, as illustrated in the paragraphs below:
High Temperature properties:
• Final material properties are dependent on both steel composition and higher tempering temperatures.
• The properties of carbon and alloy steels and their behaviour at elevated temperatures are summarised in the elevated temperature requirements table of BS EN ISO 898-1.
The effect of increasing temperature is to reduce the mechanical properties, particularly the yield stress. Extended time at elevated temperatures has an even greater effect in reducing pre-load in a bolt and nut assembly.
Low Temperature properties:
• The composition (analysis) of the steel used has a major effect on the impact properties and transition temperature obtained with carbon content having the most significant influence on these properties.
• Increasing fastener strength grades results in a reduction in ductility and, hence, impact strength.
BS EN ISO 898-1 does not specify properties at low temperature; the wide range of materials permitted result in too many variations. The transition temperature, the temperature at which failure changes from ductile to brittle, is affected by the chemical composition, particularly carbon content, as shown in the following table:
4 Installation of Fasteners
4.1 Matching Combinations
Whilst the tables and the Standards referred to in the earlier paragraphs dealt with individual elements of fasteners – the bolt/screw, the nut and the washer, the table below reproduces an extract from Table 2 of BS 5950-2 – matching bolts, nuts and washers.
This table is extremely important, as it illustrates a way through the plethora of individual standards, to give a tabular summary of what combinations of bolt, nut and washer are appropriate for structural applications. The table is also reproduced in the National Structural Steelwork Specification for Building Construction.
Furthermore, in the extensive notes at the bottom of the main table, guidance and requirements for increasing the class or grade of the nut where the assembly has been specified with a protective coating is given.
As far as the designer or specifier is concerned, all the necessary knowledge and background information necessary is summarised in this table, as are the appropriate BS and BS EN ISO Standards.
4.2 Non-Preloaded Fasteners in Shear Connections with Clearance Holes This is the typical situation for the majority of connections in ‘normal’ building frame structures, and is usually referred to as a ‘simple connection’, where the main fastener loads are in bearing and shear. The ratio of shear stress to tensile stress is approximately 62%; this value was determined experimentally with ASTM A325 and A490 bolts in the USA in 1965.
The fasteners are required to bring the structural members into contact with each other and maintain stability of the structure. Tightening of the bolt assembly is also required to ensure that loosening of the joint does not occur; this requires the maintenance of a level of tension in the joint or the provision of some preventative measure (see below). Where the fastener is relied upon to resist ‘tension’ alone, non-preloaded fasteners are not suitable for use in situations where fatigue or stress reversal occur.
Property Yield Stress MPa at Temperature °C Class +20ºC +100ºC +200ºC +250ºC +300ºC
8.8 640 590 540 510 480
10.9 940 875 790 745 705
12.9 1100 1020 925 875 825
Transition Temperature °C for Materials of Property Class 8.8 Source BS 3111 BS 3111 BS 970 BS 970 BS 970 BS 970 BS 970 BS 970 Material Type 9/3 Type 10/2 709 605 503 150 080 070 Specification M40 M36 M36 M36 M40 M55 Range of Carbon 0.17/0.23 0.32/0.39 0.36/0.44 0.32/0.40 0.32/0.40 0.32/0.40 0.36/0.44 0.50/0.60 Content % Transition -95°C -80°C -74°C -100°C -80°C -62°C -30°C +4°C Temperature
Additionally, the axial tensile stress should not be high enough to result in a reduction of the shear capacity of the bolt. Some work carried out in the USA indicates that the axial tensile stress needs to exceed 20-30% before a reduction occurs in the bolt shear capacity, and thus this level of tension is satisfactory when achieving a "snug tight" condition for an assembly used in a shear/bearing mode. For these fasteners, it is generally accepted that tightening with a "podger" spanner will develop the level of tension required;
alternatively, a percussion wrench can be used up until the point it begins its ‘hammering’.
In the tightened condition there should be at least one full thread and the point visible protruding beyond the nut face; in addition, the dimensions selected should ensure a minimum of one thread and the thread run-out between the loaded nut face and the full shank diameter (although it is obviously impossible to visibly check this once the assembly is tightened).
Type of Bolts Nuts Washers
Grade Standard Class or Grade c Standard Class Standard Non pre-loaded bolts
4.6 BS EN ISO 4016 Class 4 d BS EN ISO 4034 100 HV BS EN ISO 7091
BS EN ISO 4018
BS 4190 Grade 4 BS 4190 — BS 4320 i
BS 4933 a
8.8 BS EN ISO 4014 b Class 8 e BS EN ISO 4032 b 100 HV BS EN ISO 7091
BS EN ISO 4017 b
BS 4190 Grade 8 f BS 4190 — BS 4320 i
10.9 BS EN ISO 4014 b Class 10 g BS EN ISO 4032 b 100 HV BS EN ISO 7091
BS EN ISO 4017 b
BS 4190 Grade 10 h BS 4190 — BS 4320 i
Non pre-loaded HSFG bolts
General grade BS 4395-1 General grade BS 4395-1 — BS 4320 j
Higher grade BS 4395-2 Higher grade BS 4395-2 BS 4320 j
Preloaded HSFG bolts
General grade BS 4395-1 General grade BS 4395-1 — BS 4395-1 k
Higher grade BS 4395-2 Higher grade BS 4395-2 BS 4395-2 k
Holding down bolts
4.6 BS 7419 Grade 4 BS 4190 BS 4320 i
8.8 BS 7419 Grade 8 f BS 4190 — BS 4320 i
8.8 preloaded BS 7419 General grade BS 4395-1 BS 4395-1 k
a BS 4933 has been declared obsolescent, but should still be used for 90º countersunk head bolts and cup head bolts until corresponding BS EN standards are available.
b Grade 8.8 and 10.9 bolts to the strength grades of BS EN ISO 4014 or BS EN ISO 4017 but with the dimensions and tolerances specified in BS EN ISO 4016 or BS EN ISO 4018 may also be used, with matching nuts to the strength classes of BS EN ISO 4032 but the dimensions and tolerances of BS EN ISO 4034.
c Nuts of a higher class or grade may also be used. d Class 5 nuts for size M 16 and smaller.
e Nuts for galvanized or sherardized 8.8 bolts shall be class 10.
f Nuts for galvanized or sherardized 8.8 bolts shall be grade 10 to BS 4190. g Nuts for galvanized or sherardized 10.9 bolts shall be class 12 to BS EN ISO 4033. h Nuts for galvanized or sherardized 10.9 bolts shall be grade 12 to BS 4190. i Black steel washers to section 2 of BS 4320, normal diameter series. j Black steel washers to BS 4320:1968, Section 2, large diameter series.
k Direct tension indicators to BS 7644 may also be used with preloaded HSFG bolts. Matching Bolts, Nuts and Washers
4.3 Fasteners in Preloaded Connections In this form of connection, fastener loads are principally axial tension; the bolts should not be subject to shear and bearing stresses. The fasteners act to bring the structural members into close contact with each other; the high axial loads generate friction between the so-called "faying" or contact surfaces of the structural members, which enables shear loads to be carried through assembled connection, without slip. This results in a rigid structure that is resistant to both movement and fatigue. Use of slotted or over-sized holes may also be permitted in such connections which then allows for adjustment of dimensional fit-up to be made using such connections.
In UK practice, tightening is required to generate a final axial prestress load of at least 70% of the minimum tensile load of the bolt. However, there are initially two components of ‘load’ experienced by the fastener – the axial tension generated and the torsion from the wrench as the bolt assembly is tightened against the internal friction between the parts of the fastener assembly. Thus, the assembly is under its maximum load during and immediately before completion of the tightening process.
From that initial ‘tightened’ position, the bolt assembly undergoes some relaxation and, therefore, a loss of pretension force, which arises from:
• Elastic recovery with the removal of the tightening wrench. This ranges between 2 and 11% with a typical average value of 5%.
• A further 4 to 6% relaxation found to occur in the period from immediately after tightening to in excess of 20 days – factors contributing to this include:
– Grip length – relaxation increases as the grip length is reduced.
– Number of plies – increasing the number of plies for a constant grip increases relaxation.
– Galvanized assemblies are found to have twice the level of relaxation as a self-colour assembly – thought to be due to the creep or flow of the zinc coating.
When tightened, at least one full thread and the point should be visible beyond the nut face, and dimensions should be selected to ensure a minimum of four threads and the thread run-out between the loaded nut face and the full shank diameter.
For obvious reasons, it is very important that the assembly should be correctly and fully tightened, delivering the expected amount of prestress action. This requirement demands special care
where certain bolt protective coatings are in place, as these can adversely affect the tightening process. So, for example, if the bolt and nut have zinc coatings (especially when both the threads are coated) failure to lubricate them can result in pick-up (galling) and ultimately failure by torsional shear during tightening. Lubrication may be achieved using the traditional tallow or beeswax, although any good high-pressure lubricant is satisfactory, including Molybdenum Disulfide.
5 Installation Methods for Preloaded Bolts There are three primary methods of tightening preloaded bolts, namely:
• Torque control
• Part-turn method
• Direct tension indicator
5.1 Torque Control
This method is explained in BS 4604, but it should be remembered that the calibration required on the equipment and on the bolts is specific to each batch of bolts and nuts – a new batch should immediately require a re-calibration of the equipment and bolt assemblies.