What is Structural Engineering?

What do structural engineers do?

When asked what structural engineers do I used to reply that we design buildings and bridges. This was often met with a puzzled look followed by another question; isn’t that what architects do? Of course, this also happens to be true. Once this line of thought has been set in motion there is a certain inevitability to the conversation. Shortly the inquirer will utter the phrase ‘so, architect’s design things and engineers make them stand up’.

There is a sense in which this is true, but only to the extent that it is true to say that ‘engineers design things and architects make them look nice’.

In fact engineers and architects are co-authors of design and they exist in a somewhat symbiotic relationship. Normally, though not always, architects are the lead designer of buildings, however these roles tend to reverse when it comes to bridge design.

That said, for most of human history the architect, structural engineer and builder were one and the same person. Indeed the etymology of ‘architect’ derives from the Greek term arkhitekton meaning ‘master builder’. This term encompasses rather more than the modern architect would be comfortable with.

It follows that both architects and engineers can lay claim to history’s renowned designers of pyramids, palaces and gothic cathedrals. The foundation of the Institution of Civil Engineers in the nineteenth century was significant in the story of engineering, although the term ‘Civil Engineer’ had more to do with creating a distinction with military engineers, who had been responsible for fortifications, war machines and infrastructure since the classical period. 

Perhaps the most famous military engineer was Vitruvius, who campaigned with Julius Caesar, and wrote the seminal work ‘De Architecture’, literally ‘On Architecture’.

That said, nineteenth century Civil Engineers tended to focus their efforts on canals, viaducts, bridges, tunnels, railways, ports, harbours and the like. By this time they did not really occupy the same space as architects, who tended to focus on buildings.

It was not really until the early twentieth century that Structural Engineering became a distinct profession with the foundation of the Institution of Structural Engineers. The catalyst for this was the development of new materials like iron, steel and reinforced concrete.

Early reinforced concrete was designed and manufactured in accordance with patented systems. When the patents expired general rules were required to ensure that designs remained safe. Contrary to what you might expect the first standards were drawn up by the Royal Institute of British Architects and the originals are held at their library in London. This is often a surprise to both architects and engineers.

Responsibility was soon passed to the newly founded Concrete Institute, which later became the Institution of Structural Engineers reflecting a broader remit.

Structural engineers and architects therefore spring from the same root and share responsibility for design. In any given project it is not always easy to unpick who was responsible for what. That is not to say that the architect starts to perform engineering calculations or that the engineer takes over ordering of the internal spaces.

Rather what happens is that the structural arrangement defines and influences spaces and forces the architect to reconsider and rework initial proposals. Conversely the architect’s concept of space influences the engineers conception of the structural arrangement. Many small choices and compromises are made by both parties in order to find the most appropriate balance between competing factors. At the end of the process neither the engineer nor the architect can claim exclusive rights to the resulting design. If they have done their jobs properly then the sum is greater than the parts.

Apart from this close collaboration it is of course the role of the engineer to conceive reliable structural load paths for conveying the weight and mass of a building from the roof to the foundations and for resisting forces imparted by nature, for example the force of wind or seismic activity. Each load-path must be checked against potential failure mechanisms prior to the structure being erected. This is of course where maths and science are necessary tools of the engineer. However, the engineer is not a scientist. Justification of the load-path is a subordinate activity to the conception of the load path in the first place.

Nevertheless, it is not just buildings and bridges that structural engineers design, there are many other objects that need designed too. For example engineers are often crucial to the creation of sculptures and other works of art. The Angel of the North and the Kelpies are two well known examples. Structural design is also required in the design of aircraft, ships and all manner of other objects. 

We have thus far talked about the engineer as designer, however this is of course only part, albeit a large part, of an engineers role. Structural Engineers are also called upon to inspect buildings. Sometimes this is to diagnose distress and to specify repairs. Other times it is to establish whether a building is suitable for conversion or alteration. It could also be to establish the best method of conservation, particularly when a building is of some historical importance.

This of course opens a new controversy. I am often asked, isn’t that what surveyors do? In part yes, but mostly no. Surveyors, as their name would suggest, do indeed survey buildings. However, in most cases they are not looking for the same things as an engineer. For example, surveyors include an interest in the internal finishes and servicing of a building in their role, whereas an engineer is only interested in these items in so far as they provide clues about the underlying structure.

Another distinction, and I accept this is a simplification, is that surveyors tend to schedule defects in buildings, whereas engineers are not necessarily interested in building defects. This perhaps needs a little explaining. 

The primary role of the engineer inspecting a building is to establish its existing structural load paths, whether they remain intact and how they would be affected by proposed alterations. A defect is only of material importance to the engineer in so far as it affects or diminishes a structural load path. Despite the fact that surveyors title their reports ‘Structural Surveys’ in all but the simplest cases determination of structural load paths and diagnosis of structural distress is not a surveyors role. The term ‘Structural Survey’ is in this sense misleading.

Another aspect of structural engineering, which is related to building inspection and diagnosis of distress, is engineering forensics. Sadly, sometimes things can and do go tragically wrong and someone needs to determine probable cause so that lessons may be learned. This is a very specialised role, which requires considerable experience, particularly as it may subsequently lead to an appointment as ‘Expert Witness’ to a court. This is of course an important duty, which hints at the Structural Engineer’s role in ensuring public safety in the built environment.

Clearly it is not possible to encompass everything that a structural engineer does in a short post, however perhaps a good summary is to say that Structural Engineering is fundamentally about applying a combination of imagination and science to establish and assess load paths, and the materials from which they are made. This process may be applied to many and varied situations.

Structural Engineering is thus an important and distinguished profession, with an illustrious history. It is enormously satisfying, though on occasion it can be a little frustrating. To do it well requires hard work and a curious mind.