The forensics of infrastructure failure
The tragic collapse of the 12-story beachfront condominium in Florida’s Miami-Dade County in June shined a bright light on the issue of infrastructure failure. While extremely rare, there have been several building collapses besides those triggered by earthquakes. Typically, failures have occurred during construction. An investigation into the cause of such a failure is conducted to understand the reasons, carefully evaluating the damage to determine if changes in design and construction procedures are needed to improve future construction.
There are many approaches to these investigations. Drones with cameras can document the entire site to capture the collapsed condition; remote sensing by LIDAR (light detection and ranging) also can document exact locations. Video evidence of the actual collapse, if available, can be extremely helpful. Testimony from witnesses also provides valuable insights.
Depending on the failure, each investigation is different. Most collapses occur due to multiple factors, so a number of things must be considered. Materials are moved from the site to another location for further inspection. The structural members are documented with respect to the location from which they were obtained, and are reviewed to log distress and/or deterioration. Some areas of the structure may be reconstructed for further evaluation.
Forensic engineers obtain samples of the concrete and steel for material tests, to assess if the material strengths are consistent with the design documents. The engineers review the structural drawings to determine if the design met code requirements. They will investigate to determine whether any unusual loads were present at the time of collapse. Depending on the failure, the condition of the foundation may be evaluated to determine if it played a role.
It’s also important to develop analytical models to evaluate the loading demands as compared with the structural capacity. Structural analyses can be conducted in silico via computational simulation. These analytical models may be used to simulate failure sequence, to determine if a specific trigger causes a failure similar to that actually experienced.
There are several well-known building collapses that occurred before the Surfside, Florida, disaster.
The first is one I teach in my design course on two-way slab systems (a construction method in which the slab spans in both directions between columns). This case involves the Harbor Cay condominium, a five-story concrete building in Cocoa Beach, Florida, that collapsed in 1981, costing 11 workers their lives and injuring 27 others.
Two main factors contributed to the failure. First, the slab was designed to be too thin, due to the engineer not checking punching shear (the resistance of the slab to forces that concentrate around the perimeter of the columns). The incorrect-height rebar chair (spacer that vertically locates the reinforcing steel in the slab) also was employed during construction. These two reductions in thickness significantly decreased the punching shear strength of the slab.
Another familiar case is the Kansas City Hyatt Regency walkway collapse, which also occurred in 1981, killing 114 people and injuring more than 200. There was a change in the design when the shop drawings were being prepared. Instead of the original design, a different one was developed during fabrication to simplify construction, which doubled the force on the mechanical fasteners — the nuts — and created other structural issues.
My research focuses on the design and behavior of structural concrete. While not specifically concentrated in forensics, all of my research prepares students for forensic investigations. In fact, several of my former students work in this area.
In the research lab, we test structural concrete and document its behavior, all the way from unloaded to initial cracking, to failure. This provides a full understanding of the complete behavior of a structural system and its capacity. We learn to interpret what the structure is telling us. As loads increase, we document cracking and crack patterns, and we measure crack widths. Cracking in a structural concrete member is a way the structure “talks” to us and tells us what it is experiencing. Students learn through hands-on testing what various types of cracks mean, as well as when the structure is nearing failure.
What makes a good forensics engineer? Learning to see the forest for the trees; in other words, looking at the entire picture. In addition, a strong understanding of the load path — the direction loads take as they transfer through the connected members of a structure — so the engineer can “see” how the loads are flowing in a structure.
Infrastructure failures in buildings and bridges seldom occur. If problems develop in a structure, they usually are isolated and found during inspections, or even noticed by the public and reported. In fact, we design these structures for ductility — the capacity of a material to deform in response to stress — which provides warning signs prior to failure. For example, beams are designed to produce large deflections if they are overloaded, so the public sees there is a problem and gets out of the structure. When problems are found, repair and rehabilitation can be performed. Typically, problems are detected early and fixed.
While bridges are required to be inspected at two-year intervals, there is not a similar requirement for buildings; the time periods most commonly are owner-dependent. In general, bridges are in a much harsher environment than buildings — exposed to weather, deicing salts, etc. — whereas the structure of most buildings is contained within the enclosure. Obviously, however, there can be exposed structural systems in buildings as well; examples include parking garages and balconies. These open-air elements should be inspected more regularly.
To mitigate future building failures, regular inspections should be required. In the wake of the Surfside tragedy, I believe we are going to see requirements for regular inspections. Protection of the health, safety and welfare of the public is a state’s right based on the U.S. Constitution. Therefore, each state will be evaluating necessary changes to protect its citizens.
Robert J. Frosch, PhD, PE, FACI (Fellow of the American Concrete Institute), FASCE (Fellow of the American Society of Civil Engineers)
Professor of Civil Engineering, and Senior Associate Dean for Facilities and Operations, College of Engineering
Executive Director of Strategic Initiatives, Office of the Executive Vice President for Research and Partnerships
Purdue University
