Bolts: Are you getting what

you specify?

Dr Saman FernandoManager- R & D, Innovations and Engineering

Ajax Engineered Fasteners, Braeside, VIC

Contents• What are structural bolts?• How do structural bolts work?• What are the relevant National and International Standards?• How to identify the correct bolt• What is the Stock range?• The Current Market Environment• Certification requirements• Typical bolt related problems• Typical application related problems• How to specify bolts and tightening methods?• New developments in bolting technology• Technical resources• Legislation to control quality of fasteners?• Discussion

What are Structural Bolts?Structural bolts are a special kind of metric Property Class 8.8 bolts designed specifically for providing high clamp forces for joints (categories TF and TB).

By fully tensioning a structural bolt, a sufficient clamp force is developed to ensure that joint loads are transferred from one joint member to another via the friction between the two contact surfaces (TF) and/or bearing (TB).

This is possible because structural bolts have bigger heads than standard Property Class 8.8 bolts, providing a larger bearing area under head to carry the load.

The bolts can also carry loads in Shear (S). These bolts need not be tensioned. Most structural bolts are used in this configuration. Head size is not very important for S type joints

If the joints are subject to significant dynamic loads (eg. Wind, Earthquake) it is recommended that these joints are designed in TF /TB configuration.

How do Structural Bolts Work?

Using one of two possible methods, structural bolts are tightened to a level very close to -or exceeding - their yield points.

The resulting friction at the contact surfaces allows the load to be transferred directly from one joint member to the other.

Bolts may be tightened using either the part-turn method or direct tension indicators as outlined in AS 4100 Section 15

Under no circumstances is it permitted to tension a structural bolt in a TF/TB joints using a torque figure. This is not so for S joints.

Structural bolt should never be used in the TF/TB configuration if the joint is subject to significant tensile loads.

International Standards

There are several standards that cover the manufacture and supply of structural bolts.

•AS/NZS 1252 - 1996 (Australia and New Zealand joint standard)

•AS 1252 - 1983 (Australian)

•ASTM A 325 (USA inch series with UNC threads)

•ASTM A 325M (USA metric)

•ISO 7411 (International)

•AS 4291.1 (Mechanical properties)

None of these standards is exactly the same as any other. It is best to stick to the Australian Standards, of which AS1252 - 1983 is the most widely available.

Two Australian Standards?There are currently two Australian standards to cover structural bolts:

AS 1252:1983 and AS 1252:1996

This is because the 1996 revision brought the standard in line with the ISO standard and a major change had been made to the head size on M20 bolts that was thought to be impractical for use in Australia because

1. Podgier spanners and impact sockets were not readily available

2. No-one else in the world is using the ISO dimensions across flats.

3. Practically no-one else in the world manufactures bolts to this standard. American metric standard is still inline with AS1252-1983.

1996 Standard product is more robust due to larger head. If the above practical issues can be overcome it is better to use the 1996 standard.

32.0 mm

1996

34.0 mm

1983

General Engineering StandardsThere are also several standards that cover non-structural applications. Products made to these standards are used in general engineeringapplications.

AS 1110: Covers metric bolts made to precision tolerances. Generally supplied in either Property Class 8.8 or Property Class 10.9. Not structural.

AS 2465: UNC / UNF bolts made to precision tolerances. Covers SAE J 429 Grades 5 and 8

AS 1111: Covers metric bolts made to commercial (wider) tolerances. Generally supplied in Property Class 4.6 although some Property Class 5.8 may be available.

AS 1112: Covers metric nuts.

AS 2451: BSW bolts made to commercial tolerances. Mild steel only

AS 4291.1: Mechanical properties for metric bolts. Covers the strength requirements for all bolts made to Australian metric standards. This standard is identical to ISO898.1

AS 4291.2: Mechanical properties for metric nuts. This standard is identical to ISO898.2

How to Identify the Correct BoltAll Australian standards have a requirement for a bolt to be head marked. If the bolt is not correctly marked then it should not be used.

AS 1110 AS 1111AS 1252

AS 2451AS 2465 Grade 5 AS 2465 Grade 8

How to Identify the Correct NutAll Australian standards have a requirement for a nut to be marked. If the nut is not correctly marked then it should not be used.

AS 4291.2

Stocked Range

Off-the-shelf structural bolts are supplied generally as an assembly of bolt, nut and washer. This ensures compatible quality among components and is easier to use on site.

M16: 40 to 100 mm long

M20: 40 to 150 mm long

M24: 50 to 150 mm long

M30: 75 to 200 mm long

M36: 90 to 200 mm longSome nuts and washers

may be available as separate components

Manufacturing Methods: Cold HeadingCold heading is an manufacturing process that allows bolts to be formed without the steel having to be heated above its recrystallizationtemperature. This produces a cleaner bolt made to close tolerances.

Heading machines known as boltmakers are used to shape the metal in stages by securing a blank in a stationary die and striking it with a moving header die. In the graphic below, the blank is fed into the boltmaker from at the right and transferred progressively through each station, moving in the direction of the yellow arrows.

Stationary die

Moving header die

Cold HeadingThe picture below shows an open Formax cold header.

Slide holding moving header dies

Blanks in the processof being formed

Hot ForgingSome Australian-made structural fasteners can be obtained from small hot forging operations, made to a specific order.

This product is not kept on the shelf and usually takes up to one month from the date of the order for the product to be delivered. Hot forging is only practical for large diameter bolts or bolts over 100 mm long.

• EJ Hadaway (Vic)

• Greg Sewell Forgings (Vic)

• Westray (NSW)

• Epitech Boltmakers (NSW)

• Donhad (WA)

• FEA (WA)

Hot forging does not produce bolts that are as clean as cold forging but they should still comply with the standard.

The Current MarketBecause of their large load-carrying capacity, structural bolts were once the flagship product of the bolt industry. They carried a higher value than comparable sizes of standard 8.8 bolts and were used in larger volumes.

In the late 1980’s, structural bolts were placed under huge price pressure as the need to reduce construction costs became paramount. Introduction of large volumes of imported structural bolts was the result.

To combat this, Ajax Fasteners began a campaign of quality awareness, which forced the importers to match the quality and certification standards set by Ajax.

Eventually, Australian manufacturing became unviable as bolts from China, India and Korea were dumped on the Australian market. In 2000, Ajax Fasteners withdrew from the structural bolt market.

The Current MarketThe loss of Australian manufacturing has resulted in a decline in the discipline of supplying test certification: without a benchmark the standard tends to lower.

Requests for test certification are often refused by suppliers or manufacturers. When certification is supplied, any of the following things has been found to occur.

• Inspection certificates - not test certificates - are substituted

• One or more of the requirements of the certificate is missing. In these cases, Ajax has found that the missing aspect subsequently fails testing

• Refusals to supply certification are accompanied with an admission that the product does not conform and a request to accept it anyway

This does not mean that all product being imported into Australia is defective, but it does mean that we have no way of knowing whether it is or not.

Supplied in response to a request for test certification

No mechanical properties

Plating is below the average 52 micron standard

Certification RequirementsAS 1252: 1996 states A test certificate shows such results as may be required by agreement between the purchaser and the manufacturer relating to

(i) tests performed by the manufacturer for the purpose of establishing compliance with this standard

(ii) additional tests as agreed between the purchaser and the manufacturer.

Ajax suggests the purchasers demand

• Tensile strength

• Proof load - Bolts and Nuts

• Hardness for nuts and washers (also for very large bolts)

• Chemical composition from original heat certificate

•Statement of conformance to the standard

Certification RequirementsTensile Strength: the maximum load the bolt can withstand before fracture. Will be measured in kN or lbf and must show specification and the test result.

Proof Load: The maximum demonstrated load a bolt can carry without permanent deformation. Will be measured in kN or lbf and must show the specification and “Pass” or “Fail”

In the case of nuts, proof load is the only method of measuring the load-carrying capacity. There is no tensile strength for a nut. A good nut shall not fail in any joint under any circumstances.

Hardness: Can be used for testing large bolts when a tensile test is impractical; for testing nuts in lieu of a proof load and for checking the mechanical properties of washers. Will be measured on a hardness scale -usually Rockwell C or B, or Vickers - and must show the upper and lower specifications and the upper and lower test result.

Chemical Composition: Must show the amount of each alloying element as a percentage of the total material. C, P and S have upper and lower limits.

s

Typical Bolt ProblemsTensile Strength: Material is not properly heat treated. As a result it could be too soft, too hard or too brittle(yielding, heads popping off, poor impact resistance)

Manufacturing Defects: Incorrect tooling could cause “cold shuts”, sharp corners, thread laps, wrong dimensions (tools won’t fit ), incorrect thread tolerances (too loose of too tight fit),

Poor Material Quality: Chemistry, hardenability, consistency, inclusions

In a bolt, the failure must always occur on the first or second engaged thread. If the bolt fails in any other way either the bolt or the application is faulty.

Coating: Does not provide adequate corrosion resistance.

Hydrogen Embrittlement: This brittle failure is common in PC 10.9 and higher grades bolts. During manufacturing a de-embrittlement process (bake in an oven for 5-8hours at 205ºC) must be carried out to prevent this failure

Stress Corrosion Cracking: High tensile bolts (>PC8.8) when subject to high stresses and corrosion environment could cause sudden brittle failures (eg. Indoor swimming pools). These bolts pass all tensile test requirements

Typical Application ProblemsNot enough tension: Loosening, fretting, leaking, fatigue, bending, slipping, damage to joint members

Too much tension: Bolt breaking, damage to joint members, leaking, fatigue, galling

It has been reported that over 90% of bolt failures are due to incorrect pre-tension.

Missing or uninstalled bolts: Joint failures.

Corrosion: Reduced bolt strength and tension

Incorrect installation: misalignment, incorrect tightening sequence, incorrect tightening method, fabrication errors, bent bolts, not using correct washers

Insufficient Design: Design did not account for all loading conditions, thermal loading, thermal cycling, fatigue, too narrow design pre-tension window, doesn’t allow for correct installation, incorrect bolt selection

A Bolt in its own right is not a sound structural element!

Specifying BoltsSpecifying an appropriate bolt is the responsibility of the design engineer.

Important to specify:•Bolt size (eg.AS1252-1996)

•Bolt shape (eg Pan head, Hex head, Hex flange head)•Bolt diameter•Thread pitch (Fine or coarse)•Length (to ensure adequate thread engagement)

•Bolt Strength (eg. AS4291.1)•Property Class (eg 8.8, 10.9)

•Bolt Finish•Plating (Electro Zn, Galvanised, Phos-oil, Organic etc)

•Window of design Pre-tension (in critical applications)•Minimum and maximum allowable tension

Relevant standards on top of the above specification will consolidate all other finer details of the bolt (eg. A/F dimensions, Thread profile, proof load)

Tightening MethodsSpecifying an appropriate tightening method is the responsibility of the design engineer.

The design will only be as good as the reliability in achieving the design conditions

The tightening method shall be determined by estimating the extent of the design window for pre-tension. Important parameters are:

•Whether it is a tensile or shear joint•Assumptions made in the design•Criticality of the joint•Design safety margins •Plating/lubrication•Stiffness ratio of the joint

Having a larger safety marginallows for errors in installation

Tightening Methods

• Load indicating washers (20%)

• Torque control bolt (20%)

• Direct bolt tensioning (15%)

• Load indicating bolts (10%)

• Strain gauges (1-5%)

• Ultrasonic (7-10%)

• Turn of the nut method (20%)

• Torque angle signature (15%)

Developments in Bolting TechnologyCorrect tightening of the bolts is paramount to the integrity of critical joints. This can only be achieved by “Direct Tension Monitoring”.

Distribution of Torque

41% 38%44%

48%45%

51%

11%17%

5%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Case 1 Case 2 Case 3

Variations in Frictiion

% D

istri

butio

n of

App

lied

Torq

ue

Clamp ForceUnderhead FrictionThread Friction

Typical as supplied bolts and nuts

Some lubrication Applied

FrictionContribution Friction

Contribution reduced by 7%

FrictionContribution increased by 7%

Minor thread damage or dried joints

Clamp Force

Clamp Force increased by 55%

Clamp Force reduced by 55%

WARNING:

A +7% variation in friction will cause a +55% error in tension load !!!.

What is SMARTBOLT™ ?

• A simple device to directly measure the bolt tension

• Very high accuracy ( 1- 3%)• Any Bolt can be fitted with the simple sensor• Applicable for studs as well• A handheld monitor will show the tension• Tension can be measured at any time• Does not require technical labour• Bolt strength not sacrificed• Simply eliminate the problems due to

wrong bolt tension

Applications. . .•Loy Yang B Power Station

•Jaques Gyratory Crusher

•Changi Airport Terminal 3

•Alcoa Sand Classifier

•ANI Rock Crusher Analysis

•ABR Tension Facade•Bombardier Transportation

•Jabiru/Challis Venture Crane Slew Rings

•Rimex Wheels SMARTSTUD•VSL Pre-stressing

•ABB Oil Riser Joint Analysis

• A true structural blind fastener • Full structural strength in both tension and shear

What is ONESIDE™ ?

Features/Benefits

• For structural joints where access to the other side is not possible- eg. Hollow Members (RHS)

• Easy to adjust, tighten, loosen, remove and re-use• Simple hand tool installation does not require power or

technical skill• Comply with ISO and Australian Standards• Available for bolts larger than 6mm (1/2in) diameter

in any strength class • Simplify structural design and construction using

hollow members • Retrofitting to hollow members without welding• Add higher damping to the structure• Architectural finishes available• Lower cost compared to on-site welding

Technical ResourcesAjax Fasteners does not provide free technical support for products that they do not supply. Unfortunately, this includes structural bolts.

Options Include:• The company that supplied the bolts• Australian Standards 1252 (bolts) and 4100 (steel structures).• Ajax Fasteners Bolt Product Handbook 1999 (RRP $10+GST+P.H.). • Gorenc, Tinyou and Syam, Steel Designers’ Handbook, 6th Edition,UNSW Press, 2001• Firkins, A. and Hogan, T.J., Bolting of steel structures, 3rd edition, AISC, Sydney, 1990• Hogan,T.J.and Thomas,I.R., Design of structural connections, 4th edition, AISC, Sydney, 1994• Hogan,T.J.and Thomas,I.R., Standardized structural connections, 3rd edition, AISC, Sydney, 1985• Bickford,J.H.and Nassar,S., Handbook of bolts and bolted joints, Marcel Dekker, New York, 1998

Ajax Fasteners Innovations (AFI), the Consulting arm of Ajax Engineered Fasteners offers technical support on a consulting basis. Services include:Fastening System design (Invent) Joint monitoring and analysisBolt Testing Material TestingFailure analysis Ameliorative measures Fastening problem solving Risk analysis and assessmentExpert advice Expert witness

Ajax Fasteners would manufacture Economic Order Quantities (EOQ) on demand

Legislation to Control Fastener Quality?Fasteners keep structures together. Without fasteners it is only a pile of steel. The fastener quality is of utmost importance for the safety of the community

Fastener Quality Act, USA:• A government authority specifies and monitors the quality of fasteners• Civil action may be taken against the “Final purchaser” in case of loss or damage or simply not meeting the standards

Local Action?:

• Australian Standards covering all critical aspects of fasteners to be developed

• At least the users shall demand compliance to the standards

• Possibility of legislative control should be investigated

If adequate measures are not taken as a matter of urgency, it is just a matter of time before a major disaster would occur! Most failures are covered-up on-site and not reported (near misses)!

end of the presentation

Discussion

Thank you