The company I work for specialises in Structural Integrity, which is a niche area of Structural Engineering. At university I never even heard the phrase “structural integrity” let alone learnt anything about it. A google search shows a few references to structural integrity in a mechanical or aeronautical context, a lot of engineering companies claiming to provide structural integrity services and not much else. A search on Amazon shows that there isn’t even an introductory textbook.

Given this lack of information, I thought that this might be a good place to provide a brief introduction to what structural integrity is, and why specialists in structural integrity are required.

What is Structural Integrity

Where google and academic sources let us down, as usual Wikipedia provides a reasonable definition of structural integrity:

“Structural integrity is the ability of an item—either a structural component or a structure consisting of many components—to hold together under a load, including its own weight, without breaking or deforming excessively. It assures that the construction will perform its designed function during reasonable use, for as long as its intended life span. Items are constructed with structural integrity to prevent catastrophic failure”

I think that this is a very helpful definition. It is simple, and easy to understand. Importantly, it also identifies that structural integrity is not just concerned with safe operation of structures in normal conditions, but also that they should be able to fail safely.

For example, is this good or bad structural integrity (again from Wikipedia)?:

This aircraft has obviously suffered a significant loss of structural integrity. Without the vertical stabiliser it is likely unable to carry out any of its original design aims (fly to a target, manoeuvre, drop bombs etc.). The cause of the failure was fatigue caused by turbulence (hardly unexpected for an aircraft). Therefore, in one sense this is bad structural integrity: the aircraft was not able to withstand reasonable design loads for its full intended design lift without breaking or deforming excessively.

However, importantly (especially for the crew) it is still able to maintain the bare minimum functions of an aircraft: remaining in the air and stable. According to Wikipedia the aircraft landed safely and even returned to service after repairs! Even if it could not be landed safely, simply maintaining stability for a few minutes likely would have bought time for the crew to safely eject. So the aircraft had at least a minimal amount of structural integrity.

Most structural engineers will never design an aircraft fuselage, but this distinction between normal operations (where no damage is expected) and extreme load events (where prevention of catastrophic failure is the goal) is still important.

This distinction is also useful to know for non-engineers who are often those responsible for day to day operation of structures. It is often easy to ensure that a structure is safe for normal loads. Not too many people start removing columns out of buildings without engineering advice, as everyone has a concept of gravity and things falling down from their everyday experience. However, it is easy to compromise structural integrity against catastrophic events without realising it. For example, removing bracing from structures usually doesn’t result in immediate catastrophic failure. Structures with significant damage to bracing may remain “safe” until an event such as an earthquake or cyclone exposes the weakness of the structure. Just because a structure doesn’t fall down when it is modified doesn’t mean that it hasn’t had significant reductions in its structural integrity and safety.

Why Specialise in Structural Integrity?

Ensuring structures have structural integrity sounds like something that all structural engineers should be doing. All design codes require design to prevent damage in normal operation, and most include specific clauses that ensure a minimum level of robustness under extreme conditions. Even where codes don’t cover structural integrity a professional engineer should ensure they address these issues as part of their obligations to the profession and society. So why are there engineers who specialise in structural integrity?

The reason is that structures have a life of their own. At a recent conference I heard the quote:

“Concrete doesn’t know about the code”

- Attributed to T. Paulay

And this applies equally to all structures (not just concrete). Structures will only ever have perfect structural integrity in the mind of the designer.

Over their lifetime structures:

• May have design flaws, whether minor calculation errors, fundamental mis-understandings of structural behaviour or errors in documentation.
• May not have been constructed in accordance with the design information specified by the engineer.
• Deteriorate due to corrosion, fatigue, wear, rot, abrasion and general aging of all kinds.
• Are modified, often by those without an engineering background and without consideration of the principles of structural integrity (or even basic structural mechanics).
• Are damaged due to overload, impact, earthquakes, cyclones, explosions and other extreme events.

Also, structures may have been designed before the principles of structural integrity, structural mechanics or material behaviour were well understood, and extreme events may reveal fundamental issues with whole classes of structures. Design to prevent catastrophic collapse as fundamental structural engineering practice is largely the result of a number of singnificant collapses in the 1960s-1980s, and extreme earthquake events often expose flaws in whole categories of structures. Even if a structure complied with the design codes at the time it was constructed, there is a reasonable chance that changes to codes now mean that it does not comply.

This is where engineers who specialise in Structural Integrity find their niche. We:

• Carry out inspections to identify damage and deterioration.
• Assess structures for compliance with design codes and / or for robustness against catastrophic collapse.
• Assist in assessing the risks that issues identified pose to the owners, operators and occupiers of structures (and the public).
• Design repairs for issues identified.
• Help develop systems to ensure that inspections and maintenance are carried out and that modifications do not affect structural integrity.

In a sense many (maybe even most) structural engineers do a little of each of these throughout their career. However what makes an engineer a specialist in structural integrity is the focus on these issues, rather than on design, construction or other areas of engineering. Due to this focus, a structural integrity specialist will have greater experience at identifying issues in existing structures and how to address them given the constraints posed by existing structures. For example:

• Many structural deterioration mechanisms are hard to identify for those without experience looking for them. It takes repeated regular experience to understand what you are looking at and (importantly) where to look.
• Quickly and efficiently determining if identified defects are important for the integrity of structures requires understanding where and how deterioration will occur in a way that design & construction engineers may not have.
• How structures are used in reality by their owners, occupiers and operators often varies greatly from what the original designer may have intended, especially as time passes. A structural integrity specialist with expertise in a particular industry may sometimes understand how the structure is used better than the original designer.
• The constraints posed by the geometry of structures (which may affect how repairs can be carried out), the need to keep operating until repairs made out, limited time available to complete repairs or the particular requirements of given operating environments are all often better understood by an engineer specialised in structural integrity.
• Development of inspection regimes requires experience of carrying out detailed inspections to know where to look and how to look.

For these reasons ensuring structural integrity of structures requires structural engineers who specialise in structural integrity.

This should not be meant to imply that a specialist in structural integrity is all that is required to maintain structural integrity. For example, when assessing whether a structure has appropriate integrity, it may be necessary to engage a whole range of additional expertise. The original designer knows what assumptions were made in the design of the structure. Experts in advanced structural analysis, fatigue behaviour, corrosion chemistry or geotechnical behaviour may all be required. Experts in construction may be required to assist in repairs, and if the structure no longer has enough integrity remaining experts in demolition may be required.

However engineers who specialise in structural integrity bring experience of inspection, knowledge of deterioration and understandng of how structures are used in the real world that is essential for maintaining structural integrity. This is particularly the case where unusual structures, high loads, harsh environments and fast inspection / repair turn-around requirements are present.

Conclusion

Structural integrity is a fundamental part of ensuring that structures do what they are designed for, and are safe when conditions exceed the original design intent. However, maintaining structural integrity is not simply a matter of good design. Structures require inspection and maintenance throughout their life to ensure they continue to have appropriate levels of structural integrity. Engineers who specialise in structural integrity play a vital role in ensuring that structures are safe and reliable throughout their life.