Are your high-strength structural steel bolts up to spec and correctly installed?
International concern
A recent article in the Australian Regulation Bulletin highlights international concern about bolt failures due to defective and counterfeit bolts. The article discusses a specific structural failure in Australia associated with Property Class 8.8 structural bolts, where 20 and 24 mm diameter bolts did not meet relevant tensile strength requirements. In 1999 the US government enacted the Fastener Quality Act after defective and counterfeit fasteners caused the death of nearly 400 US citizens over 15 years.
New Zealand experience
Dr Charles Clifton of the Heavy Engineering Research Association (HERA) advises that he knows of two cases since June 1983 where correctly specifi ed and marked bolts failed due to the bolt material not meeting the specification. In both instances the bolt shank fractured while the bolts were being fully tensioned.
Given that wider instances of problems with defective and counterfeit bolts have been reported in Australia, it is important that specifiers and construction reviewers follow the quality assurance procedures described in this article to minimise the likelihood of these problems arising in New Zealand.
Why these bolts are special
High-strength Property Class 8.8 structural bolts, nuts and washers are designed to be dependably tightened on installation to the extent that the bolt shank is plastically elongated. This generates a very large clamping force across the steel components gripped by the bolts (over 25 tonnes of clamping force per bolt for an M24 bolt). This ’full tensioning’ allows the bolts to be used in a wide range of structural connections that provide the same rigidity as a welded connection under normal operating
conditions. It also provides a very high resistance to fatigue fracture of bolts in connections subject to high cycle reversing loads.
The Building Code
Using bolts in buildings and structures is covered by Clause B1 Structure, Clause B2 Durability and Clause F2 Hazardous Building Materials. The manufacture and supply of high-strength steel bolts with associated nuts and washers for structural engineering are specified by the Australian/New Zealand joint Standard AS/NZS 1252. Building Code Verification Method B1/VM1 references NZS 3404 Steel Structures Standard, which in turn references AS/NZS 1252.
Identifying Property Class 8.8 bolts, nuts and washers
It is vitally important that all components in these bolt sets (bolt, nut and washer) carry the special identification given in Clause 1.5 of AS/NZS 1252. NZS 3404 recognises that property classes of bolts other than Property Class 8.8 may be used provided they are compatible with the relevant material supply standards listed. These bolts are not covered by AS/NZS 1252. Some of these can be used for fully tensioned applications. Advice on their identification and proper use can be obtained from HERA.
Correct installation and on-site checking
High-strength structural bolts can be either snug tightened or fully tensioned. Procedures for both these methods of installation are specified in NZS 3404 (Clauses 15.2.3 to 15.2.5) and it is important that these procedures are followed. It is equally important that the required level of bolt tensioning is specified for each application.
Inspection of bolted connections on site is in accordance with the Steel Structures Standard (NZS 3404). On-site inspection is typically the task of the construction reviewer.
Quality assurance
The bolt supplier needs to be asked to provide written certification from the manufacturer that the bolts have been manufactured to AS/NZS 1252. AS/NZS 1252 specifies not only the mechanical properties and dimensions of the bolts, nuts and hardened washer but also requirements for their manufacture including testing and quality control. This written certification, together with confirming that the bolt, nut and washer set carry the correct markings, will provide a high degree of quality assurance.
Unmarked bolts must not be used. If there is no certifi cation that the bolts have been manufactured to AS/NZS 1252, a testing programme for these bolts, as specifi ed by Appendix A of AS/NZS 1252, will be necessary.
Bolt source
High-strength structural bolts, nuts and washers of the type covered by AS/NZS 1252 are no longer manufactured in New Zealand. The most common source of supply is through Australia. Regardless of the source of supply, the bolts must be correctly marked, and written certification from the manufacturer that the bolts have been manufactured to AS/NZS 1252 needs to be obtained.
Performance of high-strength structural bolts in practice
HERA advises that there have been up to 10 reported instances since 1983 where the bolt method called for full tensioning but the bolts were installed snug tight. This lead to problems with bolt failure or the nut loosening some time after installation. A greater number of problems have been encountered where the bolt supplied is not the bolt specified, or where there has been insufficient thread engagement by the nut. NZS 3404 Clause 14.3.6.1.2 specifi es that at least one clear thread must protrude above the nut after tightening. Almost all the latter type of problems are construction errors related to holddown bolts being set too low.
Other types of structural bolts
Other types of high-strength structural bolt include the TC (torque controlled) bolt and the DC (direct tensioned) bolt. These are installed on site in a different manner from those manufactured to AS/NZS 1252. The manufacture of TC and DC bolts is covered by different Standards, which include the correct marking to identify these types of bolt. Quality assurance practices should still be followed, such as written certification by the manufacturer that their product is to the relevant standard, and identification of
the correct markings.
Defective and counterfeit bolts in New Zealand
Most problems with bolts in New Zealand relate to incorrect specification, supply or installation. The international concern about the supply of defective and counterfeit highstrength bolts resulting in failures does not appear to be such a concern here. Specifiers and construction reviewers must remain vigilant and continue to follow quality-assurance procedures to minimise the likelihood of such a problem arising in New Zealand.