Three years ago, an expert structural engineering opinion was sought by a consortium of attorneys on the cause of the tragic Interstate Highway 35 West Bridge (I-35W Bridge) collapse in Minneapolis, Minnesota. The 40-year documented history of the bridge’s structural members and its analytically complex behavior made it difficult to pinpoint one piece of evidence leading to the cause of the failure. To formulate an opinion, it would be necessary to sift through hundreds of thousands of disclosed documents, gather additional documentation, analyze structural members, and review numerous photographs and video recordings of the collapse. Ultimately, a method more powerful and comprehensive than those traditionally used was needed to organize the materials, perform these tasks, and present the resulting findings.
Previously, methods of data management included using spreadsheets and lists to organize and review documents, and creating diagrams, charts, and timelines pertaining to the history of the structure in question. In parallel, analysis models were developed to understand and demonstrate the behavior of the structure. These processes were laborious and static, and offered limited means of presenting a correlation between engineering theories and key pieces of information. Such methods may be inadequate where massive documentation and analysis are required. In those unique situations, a new form of collaboration between attorneys and engineers can be utilized, called Forensic Information Modeling (FIM).
FIM combines the interoperability of Building Information Modeling (BIM)1 with custom-built databases to create a single intelligent model with information that can be referenced at the click of a button. A tool for attorneys and engineers, FIM provides assistance for the review and organization of disclosed documents. Throughout preparations for mediation or litigation, attorneys can use FIM to consolidate documents and develop time histories for structural components, while engineers can use it to search for patterns between disclosed documents and engineering analyses. Ultimately, FIM can be used as a demonstrative exhibit during expert testimony, serving as a presentation tool that clearly illustrates the story of a disputed design or construction process.
Origins of FIM
On the evening of Aug. 1, 2007, the 1,900-foot-long I-35W Bridge collapsed, causing 13 fatalities and numerous injuries. Following the event, a consortium of law firms was assembled to represent victims of the collapse and this structural engineering firm was engaged to investigate the bridge’s failure.
To develop an expert opinion on a catastrophic event, engineers typically conduct on-site observations, document reviews and structural analyses. An immediate challenge faced by the I-35W Bridge team was the restriction of site access by the National Transportation Safety Board (NTSB). With limited physical access to the bridge’s failed structural components, the team had to rely heavily on disclosed materials and structural analysis to understand the cause of the collapse.
The I-35W Bridge was composed of steel trusses that spanned continuously over four supports. Three of the supports were roller bearings designed to carry vertical load from connected members while allowing for horizontal movement should the steel members expand and contract with temperature changes. The main trusses supporting the bridge were composed of steel chord members and smaller diagonal members connected with large flat gusset plates.
Numerous documents were generated over the lifespan of the I-35W Bridge, including a history of inspection reports and retrofit drawings. To consolidate this information in one database, the first FIM was created— a 3D model populated with links that connected the disclosed documents to applicable structural members within the model. Not only did the FIM serve as an organizational tool for the documents, but it also enabled the engineers to intelligently sort through and filter information, revealing trends in how the bridge changed over time. Applying a filter for the members cited in inspection reports (Figure 1), and specifically those reports that mentioned corrosion or rust (Figure 2), revealed that rust and debris had penetrated failed expansion joints and had likely compromised the roller bearings locking them in place so they could not move horizontally. Retrofit drawings revealed the addition of new concrete curbs and resurfacing of the bridge, modifications that resulted in loads that were higher than those of the original construction.
During the review of the disclosed documents, this firm also studied the structural behavior of the bridge. Analysis showed that the lower chord members were particularly sensitive to temperature changes – growing longer with warmer temperatures and shrinking with colder temperatures – a characteristic that required free horizontal movement of the roller bearings to relieve stresses that would otherwise build up in the members.
Figure 1: Members that are mentioned in any inspection report over the life of the bridge are marked in red. (© Thornton Tomasetti, Inc.)
Figure 2: Members with mention of corrosion or rust in an inspection report are highlighted in red. The expansion joint locations are shown in blue. (© Thornton Tomasetti, Inc.)
Engineers hypothesized that on the day of the collapse, the added loading from the concrete curbs, resurfacing materials, temperature increases, construction and traffic, coupled with the compromised roller, led to the catastrophic failure. To validate this theory, an animation incorporated the true-to-life analysis model and video footage — captured by a security camera on the day of the collapse — into the FIM (Figure 3) using proprietary translators. The comprehensive model, when animated, compared the movements of the bridge members leading up to the failure. The final results indicated that the analysis model matched the actual collapse behavior (Figures 4 and 5), thereby validating the team’s failure theory – a conclusion that differed from what had been widely reported by the NTSB.
Figure 3: Overlay of the FIM on the security video. (Security video courtesy of MDOT; FIM image © Thornton Tomasetti, Inc.)
Figure 4: Rendering of the FIM showing the initiating member buckle and the buckle visible in the security camera video. (© Thornton Tomasetti, Inc.)
Figure 5: Overall view of the FIM showing the bridge mid collapse. (© Thornton Tomasetti, Inc.)
Challenges to FIM
Despite continual improvements, there are limitations to FIM. It is a useful tool for organizing disclosed documents; however,FIM has not yet led to a reduction in the amount of manpower required for the initial review and categorization of documents to populate the model. While the upfront review time has not changed, a benefit of using FIM is that a team can review each document once, and for the remainder of the project can find the important evidence quickly.
As attorneys and engineers often do not use the same software, the computer platform required for a FIM can be another hurdle. For the I-35W Bridge case, the model was created in the 3D computer-aided drafting program Revit Structure. Although it served as the appropriate platform for the case, to use the program for the FIM required that the attorneys buy a software license and receive training from the engineers. For more straightforward litigation support projects, FIMs have been created using PDFs, enabling attorneys to access the files using Adobe Acrobat Reader. While it would be ideal for all FIMs to be readable in software that is typically used by attorneys, the complexity of a case and the structural elements in question will continue to dictate the software platforms needed for FIM.
FIM after the I-35W Bridge Case
FIM has been used on jobs of every scale. Recently, a FIM was developed to focus on the construction history for a major construction delay claim in the United Kingdom. The model was populated with the dated releases of structural drawings, shop drawings and fabrication records; this made it possible to compose a timeline comparison among the design schedule, the contractor’s published construction schedule and the actual construction schedule. The case settled in mediation, although the intent was for the FIM to be used by the engineering, fabrication, construction management and scheduling experts.
While FIM has yet to be applied within the courtroom, it has been used to generate output and images for expert reports. Illustrations developed from FIM have been presented in expert meetings, and mediations and have helped lead to successful settlements.
FIM is a powerful tool developed to organize disclosed documents and expert engineering analysis in a comprehensive manner to look for patterns and to study structural behaviors leading to a catastrophic event. Since its inception for the I-35W Bridge case, the concept has advanced and has been tailored to suit the needs of other litigation support projects. FIM has helped experts focus arguments and develop figures that explain technical information in an easily interpreted manner. FIM’s organization of documents benefits both experts and attorneys and has been used in more than one dozen litigation support projects over the past several years. Just as BIM modernized communication between architects, engineers and contractors, FIM holds the promise of transforming the way engineers and attorneys collaborate.
1. BIM (inset)
Building Information Modeling (BIM) is the process of making a digital 3D representation of a building and its systems. These models typically include exterior and interior architectural objects (such as the building façade, doors, floor and wall finishes), building structure (including the foundation and superstructure), and pipes, ducts, wiring and controls associated with the mechanical, electrical and plumbing systems. The “Information” part of BIM consists of properties associated with the modeled objects, such as the structural material and size, manufacturer information, construction sequence and cost. The 3D nature of BIM streamlines communication between design professionals and allows the complexities of a building to be better understood. Commonly used BIM programs include Revit Structure, Tekla Structures and Microstation.