One of the Keys for an Engineering Investigation
In one of the adventures of Sherlock Holmes, Dr. John Watson asks Mr. Holmes how he was going to be able to identify which of the suspects was lying. Mr. Holmes answered, “We look for consistency. Where there is a want of it, we expect deception.” This principle is not only applied during the investigative process by the most famous super sleuth, but is a basic principle that is also applied by forensic engineers.
When evaluating a structural failure we observe the scene and gather much physical evidence. Much physical evidence is gathered initially because it is not known what evidence is pertinent to determining the cause of the failure. The gathering of evidence may also continue after the scene has been examined through the interviewing of eyewitnesses or the obtaining of historical facts through various records or documents.
During the interviewing process, propositions are often put forth by eyewitnesses or other interested parties. These are considered by the engineer during the evaluation. Other propositions and possibilities are also developed and considered by the investigating engineer.
To be a viable proposition, that is, one worthy of consideration, it initially must provide an explanation for much, if not all, the physical evidence and the reported observations of others. The proposition that wins out is usually the one that can best explain all the physical evidence and the reported observations that are not inconsistent with each other and/or the physical evidence.
Inconsistency between reported observation indicates that one or more of the reported observations are not true. Inconsistency between the physical evidence indicates that the physical evidence may have been altered or changed by exposure to subsequent activities or events.
As Mr. Holmes stated regarding his investigations, these inconsistencies sometimes indicate inadvertent alteration or advertent deception. And so, as Mr. Holmes said, “We look for consistency. Where there is want of it, we expect deception.”