How a forensic engineer differs from a design engineer

Introduction

When there is a structural failure, it is important to understand the differences between the expertise and skills required to investigate that failure as opposed to the skills required to design and construct a new structure.

For forensic engineers, ‘detective’ skills, in addition to ‘design’ skills, are a key requirement.  A good forensic engineer will generally have extensive design engineering experience, but not all good design engineers will necessarily make good forensic engineers.  Whilst, at face value, it seems that a good designer who regularly designs a certain type of structure is the ideal candidate to investigate why such a structure failed, it does not always follow.  That is due to the different skills and processes involved.

Design

The design process develops a design solution, from a range of potentially viable alternatives, that meets a client’s requirements while respecting the imposed constraints. The designer has to navigate this path using their expertise and experience. The appropriateness of a particular design solution must be evaluated by calculating and analysing performance ‘assumptions’ about how such a design will behave once constructed.

These assumptions are codified or well-known rules of thumb, and are generally conservative: 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, and their role is to manage – rather than investigate – design unknowns.

Failure investigation

When structural failure occurs, knowledge of the design process is required to determine whether or not the original design was prepared with reasonable skill and care. Furthermore, it may also be required if rectification or replacement of the failed structure is necessary.

However, in a failure investigation, 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, the forensic engineer should attempt 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.

Forensics

While the objective of the design process is solution development utilising assumptions, the objective of the forensic process is to establish causation utilising verifiable evidence.  The forensic process is key to determining the root cause of a failure, and is essentially the implementation of the ‘Scientific Method’: 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, the 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 will 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 conditions and loading, as confirmed by evidence, at the time of failure.  If such analysis suggests that failure would occur then the manner in which the analysis predicts collapse can be compared directly to the evidence retrieved from the failure site to establish the analysis’ validity.  At each point in such testing, 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 confidently ruled in or out, while 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.

The importance of these forensic or ‘detective’ skills in failure investigations cannot be overstressed.

Common pitfalls

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

1. Some engineers start with hypotheses development, which 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 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 / event.

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.

Engineering analysis is also often used to assess whether the structure, as designed, complied with the relevant design standards.  This approach is useful in a dispute scenario in determining whether or not the original design was prepared with reasonable skill and care, but it should not be mistaken for being, in and of itself, the cause of failure.  The actual conditions present at the time of the failure should be the governing basis for technical analysis. 

Conclusion

The forensic and design processes are very different.  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.