The Forensic Engineering Process for Structural Failures

Forensic structural engineering involves the investigation and determination of the causes of a failure of structures such as bridges, buildings, industrial structures, and metro stations. (Figure 1). Along with understanding the legal procedures and giving testimony, forensic structural engineers use their knowledge and experiences in these investigations to act as a detective, investigator, and expert witness when confronted by attorneys and other opposing experts during the litigation process.

1) Safety and Structural Stability Assessment

Safety is the first priority after a structural failure. Safe routes through the debris should be identified for the investigation. There may be areas to avoid until stabilized and components and elements that are in danger of further collapse. The structure should be investigated for stabilization and any protection required for public access should be implemented to provide safe public traffic. In addition, alternatives for demolition phasing should be examined and evaluated (Figure 2).

2) Preserve Destructible and Perishable Evidence

Forensic engineers know that after a collapse, any condition and circumstances of the site could be potential evidence; therefore, they document the evidence that could possibly change. Durable evidence may remain unchanged for a period of time, and perishable evidence must be documented immediately after collapse. An example of perishable evidence is the weight of the snow accumulation on the roof or other horizontal areas of the structure such as balconies. This is very important as it may indicate whether the failure was due to a design or construction error, or any unforeseen overload (Figure 3).

3) Reserving Samples

In large size structural failures, it’s not practical to reserve the entire structure; therefore, the forensic engineer needs to take samples of both failed and non-failed elements and key components of the structure.

4) Documentation of Conditions

The documentation of the failure can be in the form of field notes, photographs, video, or other methods of recording.

5) Interviews

Interviews with witnesses and other persons on site can provide valuable information for the forensic engineer. The interviews should be performed as soon as possible after the collapse as they help to identify and locate the witnesses, receive fresh information, and assist in formulating the scenarios for investigation.

6) Cooperation with Other Forensic Engineers

When multiple specialties are involved in the investigation, establishing a common system or program so all parties can use the resources can avoid and minimize misidentification and debates. Pooling resources can avoid duplication of efforts and establish a common knowledge base. For instance, different parties could agree at the beginning that destructive testing should be performed on certain components of the structure and all parties use the same testing protocol during the investigation process.

7) Initial Document Gathering

Forensic engineers need to collect and review the project documentation such as design drawings, specifications, boring logs, engineering calculations, shop drawings, submittals, inspection reports, daily and weekly reports, test results, correspondence, and any other pertinent information related to the structure.

8) Preliminary Evaluation

After the initial information is collected during the above steps, the forensic engineers may be able to provide a preliminary evaluation and develop failure scenarios, a testing program, and perform the preliminary structural analysis. The engineers may also identify the missing documents, additional required expertise, and more individuals to interview after the first steps are completed.

The legal process may simply consist of assembling the investigative and legal response team, developing an action plan, establishing a plan to protect confidentiality, cooperation and dealing with public agencies such as the Occupational Safety and Health Administration (OSHA) and Federal Emergency Management Agency (FEMA), dealing with media, and providing for special consideration of interested parties.

Project Initiation and Assembling the Investigation Team

The project objective, scope of work, and the investigative plan will be established by all parties to start the investigation process. To avoid conflicts of interest, the investigative team should not have any relation with the parties involved on the loss such as contractors, designers, or other initial project interests.

Investigative Process

The structural analysis of a new design is different than the analysis in a structural failure investigation. Passing the yield point, nonlinear behavior, reaching out to ultimate capacity point, and load redistribution should be taken into account by the forensic engineer. A common mistake by the forensic engineer may be not examining and considering all failure scenarios due to their experience with similar investigations in the past. They may jump to that the failure is the same as a previous investigation and may ignore other hypotheses and scenarios for the failure. The loads and capacity of the structure should be evaluated and calculated through the structural analysis with hand calculations or computer software.

Document Review

Forensic structural engineers may need to review the following documents throughout the investigation process:

Field Investigation and Laboratory Analysis

Further field investigations, sampling of the materials and components, field tests, interviews, and laboratory tests may be needed in the investigation process and may be performed per the forensic engineer’s request (Figure 4).

Structural Analysis

From a simple hand calculation to the complicated finite element calculations using computer software, various computations are used by the engineers to investigate a failure. Determining the loads and strength of the structure is the main task in this stage. In many cases, there may be parameters regarding the strength of the structure that are not precisely known. In these cases, sensitivity analysis may be performed, and the engineer may use the probable low and high values as the input for the unknown parameters to evaluate the sensitivity of the calculation result.

Determining Structural Failure Causes and Report

As the investigation moves forward and the results, facts, and calculations advance, the failure scenarios and hypotheses are dropped and rejected or approved. New scenarios may emerge through the investigation process.

In some cases, all failure scenarios may be eliminated except one and in other cases, the results are not straightforward. Multiple causes may lead to a critical combination of the load and capacity that finally cause the failure. After narrowing down the potential causes of failure to one or a few, all evidence should be examined to determine whether it does or does not support the finding cause(s). Finally, the investigation team provides a report including an introduction and background, description of the structure, field investigation, laboratory tests, calculation results, discussion, conclusion, and recommendations.

This article, all or in part, was previously published in the Concrete
Repair Bulletin, March 2022. It is reprinted with permission.