Articles

24 October 2024

Introduction

When there is a structural failure, and potentially a dispute, it is important to understand the different roles and responsibilities of a ‘design engineer’, a ‘forensic engineer’, and an ‘expert witness’.

These roles share the common foundation of an engineering education and training.  All three roles require a strong understanding of engineering principles, including mathematics, physics, and material science.  This foundational knowledge enables them to analyse problems, develop solutions, and understand the behaviour of materials and structures under various conditions. All three roles demand a high level of attention to detail, analytical skills, communication skills of various types, and some level of contractual and legal understanding.  Above all, engineers in these roles must be dedicated to the protection of the public health, safety, and welfare.

We will start with a comparison of the design engineer and the forensic engineer, and some common pitfalls that can befall inexperienced forensic engineers.  We will then look at how the expert witness brings the skills of these two roles together, as well as the additional skills required, and challenges faced by the expert witness.

Design vs Forensic

The design engineer

The goal of the design process in structural engineering is to produce a structure that meets client expectations from a functional perspective, is economical, elegant, and complies with the relevant codes and standards. 

To ensure compliance from an overall strength perspective – that is, to ensure the structure is at a very low risk of collapse or serious structural failure – a designer typically uses design codes, which are prescriptive documents that specify the loading to be applied to the structure, as well as the methodologies used to determine if the structure’s response to that loading is satisfactory.  The designer will also undertake a range of serviceability checks, such as ensuring the structure does not excessively vibrate or deflect (also often set out in design codes and standards). 

With the appropriate design code(s) to hand, the designer applies the design process. The subtleties of the design process vary across structural engineering industries, but the general process is something like this:

• The designer estimates the design loading, and applies factors of safety to increase those loads, to introduce some conservatism into the process. The use of these factors of safety illustrates how the design process focuses on managing, as opposed to investigating, unknowns.
• The designer makes an educated estimate of the preliminary size of the structural members (that is beams, columns, slabs, etc.) and connections based on experience, rules of thumb or industry guidance. They evaluate by calculating and analysing the performance ‘assumptions’ about how their design will behave once built.  These assumptions are codified or well-known rules of thumb and are generally conservative and deterministic.  That is, their appropriateness has been confirmed over time by trial and error and testing to produce generally safe structures.  These assumptions are a simplified representation of reality.
• The designer estimates how that structure will respond and perform when the loads are applied, before ensuring that the individual members and connections have the required strength and stability to safely resist it. If they don’t, the members are increased in size or stiffness.  If they are ‘too strong’, the members may be reduced, to create a more efficient structure.  There are also checks for the stability of the structure as a whole.
• Computer models, using finite element analysis, are often used in this process, particularly for determining the loads on the individual members/connections.  A virtual model is built of the structure using a piece of software, the loading is applied to the model, and the effect on all the members/connections (bending moments, axial and shear forces, stresses, deflections, etc.) is the output. 

The designer is trying to create an efficient structure.  The elements must have the required capacity, but they should not have excessive over-capacity, which would make them inefficient.   The design engineer’s role is to manage – rather than investigate – design unknowns.

The forensic engineer

In a failure investigation following a collapse or structural failure of some sort, every assumption should be confirmed, if possible, by evidence specific to the failure.  Given the (sometimes significant) differences between the simplifying assumptions used in the design process, and how structures actually behave in practice, a forensic engineer seeks to determine the actual loads on a structure, its actual structural behaviour, and the actual material properties at the time of failure, in order to assess why a failure occurred.

The forensic process is key to determining the root cause of a failure and is essentially the implementation of the ‘scientific method’ that we all learnt at school:  the process focusses on (1) evidence collection, (2) development of failure hypotheses, and then (3) testing of those failure hypotheses.

1. The forensic process generally begins with a forensic engineer collecting and collating physical evidence relating to the failure in an objective manner.
2. During the failure hypotheses development stage, a forensic engineer develops a broad range of theories as to what may have caused the failure. In practice, it is an iterative process, and the forensic engineer may switch between the evidence collection and hypotheses development stages.  New evidence will suggest further hypotheses, which in turn may prompt a further search for evidence.
3. Finally, the hypotheses testing stage involves the evaluation of the likelihood that a particular hypothesis caused the failure. A typical approach with structural failures is to evaluate theoretically how a structure would behave when subject to the actual conditions and loading, as confirmed by evidence, at the time of failure.  If that analysis suggests that failure would occur, then the manner in which the analysis predicts collapse or structural failure can be compared directly to the evidence retrieved from the failure site to confirm the validity of the analysis.  At every stage, evidence takes precedence over assumption, and hypotheses can (hopefully) be ruled in or ruled out to determine the single failure hypothesis that explains the failure.

The successful identification of causation is therefore primarily dependant on the quality of evidence available to test the failure hypotheses.  Comprehensive evidence allows hypotheses to be ruled in or ruled out, whilst sparse evidence relies on the experience of the forensic engineer to analyse the most likely scenarios, and narrow the field of hypotheses, to prevent an investigation from being inconclusive.

Common pitfalls

There are two common pitfalls which engineers can fall into if not experienced in the forensic process.  These are, (1) a lack of attention to the evidence collecting phase, and (2) the misuse of engineering analysis tools.  

1. In can be tempting for an engineer to start with hypotheses development. This approach has risks: it can lead to a lack of focus on the evidence collection during a site inspection or when reviewing documents, and it can lead to prematurely developed failure theories.  In essence, this is allowing a theory to drive the evidence, as opposed to evidence driving the theory, with investigators becoming susceptible to ‘confirmation bias’ to support their failure theories.
   
   For experienced forensic engineers, once disciplined separation of the evidence collection and hypotheses development is established, the two phases tend to inform and support each other; that is, certain failure hypotheses will prompt a search for specific evidence to prove or disprove a particular theory, which in turn may suggest previously unconsidered failure hypotheses.
   
   In general, though, good forensic engineers resist the urge to develop hypotheses until evidence collection is reasonably well advanced, although this can be difficult in the face of a client’s desire for quick answers and hasty repairs immediately following the damage / loss / incident.
   
   
2. Engineering analysis, particularly finite element analysis, can play a very important role during the hypotheses testing phase for certain loading conditions. However, engineering analysis can be misused in failure investigation if it is relied on too heavily or used in the wrong way.  Used in design, it is based on assumptions with respect to loading, material properties, and structural behaviour.  In a failure investigation, the appropriateness of each of these assumptions needs to be investigated and confirmed (where possible) with evidence specific to the failed structure.  The validity of such engineering analysis is therefore largely dependent on the validity of the assumptions it is based upon.

Bridging the gap – the expert witness

Just because a structure does not comply with a design code does not necessarily mean it will fail.  Conversely, just because a structure is designed properly does not mean it will not fail; for example, it could be subject to loads greater than designed for, or it may not have been built in accordance with the design.  So how does an expert witness determine causation?  And how do they determine the relative apportionment of responsibility of various issues? 

Expert witnesses must possess a thorough understanding of both design and forensic engineering principles.  They must draw on their design engineering knowledge to evaluate whether the actions (or inactions) of the designer met the standard of care expected.  Most expert witnesses have practical experience as designers or contractors, which provides the background and necessary context to understand the practical constraints faced during the design and construction process, and to evaluate whether industry standards were met.  Simultaneously, an expert witness’ role can involve analysing failures from a forensic perspective, identifying the root causes, and understanding the mechanics of the failure.  In the section below, we will talk more about how the expert witness assesses the adequacy of a design, before discussing the expert’s report. 

Assessing design

Depending on the circumstances of the matter, the relevant legal requirements might typically include those imposed by a contract, a standard of care in tort, an obligation or duty imposed by statute or a combination of the these. The most common formulation, in its simplest form, is ‘reasonable skill and care’.   

An expert witness might receive a Statement of Claim setting out the alleged shortcomings in the designer’s performance and be asked whether the designer met the required standard.  There is no manual, textbook, article, or code, that provides explicit guidelines on evaluating whether a specific error constitutes a breach.  The expert, therefore, commonly relies on their own judgment and accumulated experience to form their opinions.

Anyone with reasonable technical knowledge can check a design against a code or standard and demonstrate compliance or otherwise.  But determination of a breach of a standard of care typically requires an understanding that can only really come from personal experience of similar practice.  To form an opinion, an expert witness should understand the complexity of the work, the conditions in which it was carried out, and the quality of deliverable that would normally be expected in those circumstances.  The expert should evaluate this by considering the level of information available at the time, the contractual context, any external constraints, and the reasonable expectations of the client.  Furthermore, the assessment should reflect the time and place at which the services were provided – expectations vary between jurisdictions and over time.  All these elements must be framed within the terms of the agreement between the parties, and then benchmarked against the expert’s own knowledge and experiences of technical standards and industry practice.

The expert’s report 

The expert’s report should first and foremost communicate complex technical concepts to non-technical readers, who have no specialised knowledge.  It should use simple language, diagrams, or analogies where appropriate.  Each section should build upon the previous one, allowing the reader to follow and see the expert’s reasoning process.  This is an important skill, and one that often sets good experts apart from many design and forensic engineers.    

In preparing their report, it is also important that the expert sets out their considered opinion why a particular action (or inaction) fell short of the required standard.  The report should detail the process by which their conclusions were reached, including the factors considered, the experiences referenced, and the rationale.  Too many experts set out to demonstrate failures in their reports but overlook the ‘reasonable practitioner’ benchmark, leaping to the conclusion that, having proved an error, they’ve proved a breach. 

Conclusion

The design engineer’s skills emphasise creativity, technical knowledge, and problem-solving. 

The forensic engineer prioritises technical knowledge combined with analytical skills.  A good forensic engineer understands how structures fail in practice (as opposed to how they are designed), understands the importance of obtaining and assessing evidence before generating hypotheses, and understands the application and limitations of the technical tools available for forensic analysis.

The expert witness is often asked to opine on both (1) the cause of a failure, and (2) the technical aspects relating to whether or not the structure was designed and constructed in compliance with the relevant legal/contractual requirements.  Addressing the causation question requires forensic expertise, whilst the compliance question requires design expertise, which are quite distinct. The expert witness provides insight based on first-hand experience of the design process and construction practicalities, and a thorough understanding and application of the forensic process. 

The expert's personal knowledge of the specific issues and processes at the heart of the dispute is compared against expected performance standards, and the value the expert provides lies in their honest judgment of their peers against reasonable and competent practice.  

The expert witness relies heavily on effective communication and report writing, and their ability to explain complex issues clearly, with some legal understanding thrown in and an emphasis on objectivity.

Contributed by:

Nick Barham - Director & Head of Technical Expert Services (Asia), Rimkus