Structural integrity: Overview

What is structural integrity?

"Structural integrity is the ability of a component, structure or asset to operate at optimum level under the pressure of a load, including the weight of the asset itself." [1].

Without structural integrity buildings would collapse if subjected to external forces.  Structural integrity is important not only when something is being built, but also throughout its life cycle.

Of course, it's not just buildings that require structural integrity - many man-made and natural constructions need structural integrity - from aeroplanes, to river banks.  The key thing is - if structural integrity fails, the results can be devastating.  It is therefore very important to think about structural integrity not only in the design stages, but also in the long term maintenance phase of a structure's life cycle through to when it's demolished/dismantled.

Material selection

When designing anything that requires structural integrity the choice of materials to build it with is an important element.  You have to choose materials that are strong enough to deal with the loads and external forces they will be subjected to, but that are within your budget, are safe to use and meet regulatory requirements.

For example, while asbestos is a cheap fibrous material resistant to heat and corrosion - and so great for insulation purposes - if it is disturbed, small fibres are released and when inhaled cause serious health problems [2].  The use of asbestos has been banned in the UK since 1999 [3], so while it's cheap and good at what it does - you can not use it due to current regulations.

In addition you may also need to consider the way a material is formed (for example sheet metal, tubes/pipes, I-beams etc.) or joined to other materials (by welding or a fasteners like bolts or rivets).  Joints are particularly susceptible to fatigue and wear especially if they are subjected to repeated loading.

The environment in which a structure is to operate is also a key factor in the choice of materials.  You may find the perfect material for a structural load, but the environment it has to be in means you can not use it. Such as structures built in extreme environments, like the North Pole or inside a nuclear power plant. Extreme temperatures or radiation can have adverse effects on materials making them fail, so you may need to compromise in your design in order to use materials which can survive the environment.

Alternatively - you may need to build structures that are designed to fail - just at the appropriate time, and in a particular way - such as crash barriers on roads.  These are designed to take the brunt of an impact from a car to help slow them down and hopefully avoid fatal injuries to the passengers.

So knowing how structures manage loads and where they are prone to failure, as well as a good understanding of material properties and how they degrade over time is an extremely important for a structural engineer.

Testing

Once a structure has been designed and built it needs to be maintained.  There are many processes that effect materials over time, such as wear, fatigue, creep, to name but a few.  Inspection and testing of structures to ensure they continue to work as designed is an important part of maintaining structural integrity.

Testing of materials falls into two main categories - destructive, and non-destructive testing.

Destructive testing involves taking a finished product or sample and testing it to destruction to see when it fails - for example, increasing the load on a metal bar until it breaks, thus establishing the maximum load it can take.  Destructive testing is commonly used for materials characterisation and fabrication validation, and is carried out either by following a standard or by recreating specific environmental conditions to determine performance prior to building [4].

On the other hand, non-destructive testing (NDT) allows you to test structures without destroying them in the process.  There are many types of non-destructive testing ranging from simple visual inspections through to sophisticated methods such as Neutron Radiographic testing (NR).  The obvious advantage of non-destructive testing is that you can assess the condition of a structure without destroying it in the process.  Should a fault be found it can be repaired rather than replaced, making NDT far more economical [5]. 

NDT is especially useful for testing the condition of welds and joints in-situ.  Many methods also show up hidden faults which can not be seen with the naked eye, such as micro-fractures.  The choice of NDT method will depend on the materials and environment the structure is in.  Several different NDT methods may be used on the same area to give a more rounded picture of the material's condition.

Into the future

There is plenty of research around new materials with different properties - woods that are as strong as steel [6], through to plastics that change colour when under stress [7] which can help with visual inspections.  In addition there are also self-repairing materials - such as concrete which can repair small cracks due to the additions of materials to the concrete mixture.  Some researchers are even using bacteria in concrete mixtures [8].

As humans aim to travel further and faster, structural engineering will play an important role in keeping them safe while they do. What materials and structures would you use to get to Mars and build a habitat? Getting into space costs millions of pounds, and weight is a key factor, so you would either need to look at lightweight materials, or find a way to create what's needed once you get there from the raw materials available.

Where ever we end up - we will always need structural engineers!

References

Picture credits

Example of construction using struts in a triangular arrangement by Toni Pomar is used under Unsplash licence.

A variety of metal work displayed in a rack by Robby McCullough is used under Unsplash licence.

Close up of suspension bridge by Ricardo Gomez Angel is used under Unsplash licence.