Watts Articles

Bolt Failure

A recently published Cross newsletter (Confidential Reporting of Structural Safety) has once again highlighted problems of failures in structural connections using high- grade steels. In the reported case, there is an alarming reference to bolts flying off explosively some 4m above a lifting frame. Readers may remember last year’s widely reported failures at the Cheesegrater in London which were attributed to a process known as hydrogen embrittlement (HE).


Whilst colourful headlines suggest that HE is a new problem, it has infact certainly been known about for many years and there is a huge volume of research on the subject.  Being a tiny atom, hydrogen is able to penetrate the surface of steel and other metals where it can cause a loss of ductility. The hydrogen is often generated as a consequence of the coating and plating process and for the most part it affects high tensile metals, usually carbon and alloy steels. Hydrogen introduced as a by-product of the preparation for electroplating can be sealed in by subsequent coatings; it will then quickly migrate to areas of high stress concentration and potentially cause failure. 


But we shouldn’t jump to conclusions as soon as ‘bolt failure’ is mentioned as there are a number of other forms of deterioration including mechanisms such as stress corrosion cracking, environmentally induced cracking and liquid metal embrittlement. Alternatively inadequate specification and/or poor quality steel can also be culprits.

A case I am currently working on involves a partial building collapse where the integrity of bolted connections is in question. It is, however, far too early to say whether the bolts failed due to HE or to another reason entirely so we must approach these situations with an open mind. What is does highlight though is the dramatic and possibly fatal effects of failures of fixings. Structural failures such as this are thankfully rare, but as an industry we cannot afford to take “simple things” such as bolts and fastenings for granted.

Scrupulous manufacturing processes, quality control and careful specification are essential; reputable manufacturers are alive to the problem, but cheap imports, whilst outwardly similar may not perform as expected. For obvious reasons much attention is given to high profile buildings, but a scan back through the SCOSS newsletters (Standing Committee on Structural Safety) suggests that concerns have been raised over not just HE but the safety and security of tension fixings in many buildings. 

There are plenty of other things that can go wrong with fixings and to focus on HE in preference to other issues would be wrong. However, it is clear to me that construction professionals need to be aware of the potential for failures in fixings and make sure that they are properly designed, manufactured and sourced. Buying bolts from overseas suppliers carries a risk – it’s very important to ensure that the appropriate manufacturing standards have been met; European Standards are not universal and lesser requirements can exist elsewhere. Avoiding HE, and other potential failures, requires scrupulous manufacturing techniques; compromising on quality for the sake of price or availability can sometimes have serious results.

Trevor Rushton (FRICS FCABE ACI Arb) is Technical Director of Watts Group



Bolt and Fracture

The Watts Bulletin is the technical companion to the Watts Pocket Handbook, the essential guide to property and construction, as used by professionals since 1983.

Watts Bulletin editor: Trevor Rushton.