The aftermath of the building collapse in Florida resembles the aftermath of the Oklahoma City Bombing and a 1993 earthquake. Did building construction play a role?
By Evan Reis, PE, SE, Executive Director of the US Resiliency Council
Citation: Reis, E., 2021, Op-Ed: Florida building collapse bears similarities to other tragedies, Temblor, http://doi.org/10.32858/temblor.185
The collapse of the Champlain Towers South condominium in Surfside, Fla., last week was a tragedy. The disaster will likely be measured in scores of lives lost. Though speculation is rampant, the cause of the collapse will not be known for weeks to months — not until engineers are able to inspect the site and perform forensic analysis of the building’s structural components.
The short video that captured the moment when the building began to fall immediately reminded me of another building collapse that occurred 26 years ago: that of the Alfred P. Murrah Federal Building in Oklahoma City. The images of both buildings looked eerily similar, despite the clear difference in initiation. The pictures of the aftermath also reminded me of the destruction of buildings in a 1993 earthquake in Guam. Could the structural components in these three building collapses be similar and be failing in similar ways?
Progressive collapse — comparison to Oklahoma City
The Murrah building was brought down by a domestic terrorist attack, but not all of the damage was caused directly by the impact of the blast from the truck bomb. Rather, the blast’s direct impact primarily destroyed a single column on the face of the building (Hinman and Hammond, 1997).
What occurred afterward was a case of “progressive collapse.” The building’s floor construction consisted of relatively thin, flat plate concrete slabs spanning between concrete columns. The loss of the single column at the first two stories meant that the gravity loads in the columns directly above lost their primary support. The flat slabs were not strong enough to transfer the loads to adjacent columns, resulting in their collapse. The floor collapses caused eccentric loading and instabilities in the adjacent columns, resulting in them becoming overloaded and failing as well. The failures “progressed” outward and back toward the center of the Murrah Building, ultimately destroying almost half of the structure. (A better, albeit more expensive, construction practice would be to place beams around the perimeter of each floor to strengthen the floor slabs, and to place more reinforcing steel in the column/floor joints to better resist localized damage.)
While it is too soon to judge definitively whether the same type of progressive collapse brought down the Champlain Towers South building, it too was constructed of concrete columns with flat plate slabs, and the buildings were completed within four years of each other in the late 1970s and early 1980s. The 1997 Hinman and Hammond report recommended that federal buildings of the future be designed with more redundancy, such as placing perimeter beams at each story that have the capacity to support floors if one column fails below. The report also recommended that floor slabs and the steel reinforcement within beams and columns be similar to the type of designs regularly required for building in high seismic areas like California. The federal government adopted these recommendations.
Unfortunately, the Florida Building Code has no such progressive collapse design requirement.
Progressive collapse — comparison to Guam earthquake
If progressive collapse was the cause of the building’s destruction, what could have caused a column to fail? Again, forensic studies will have to be performed to determine for sure. However, looking at the site, again I was reminded of another disaster, which occurred in 1993: the magnitude-7.8 earthquake off the island of Guam. Dozens of buildings collapsed, including many older concrete structures built in the decades before the event.
As a young engineer I traveled to Guam to view the damage. One of the most striking observations I made was that the reinforcing steel in many of the collapsed or damaged buildings was rusted. In the exterior columns and walls of concrete buildings, I could see cracks in the surface concrete that covered and protected the rebar. Guam is subject to hurricane-force winds, which, in the marine climate, often drive saltwater against buildings and through these cracks. The corrosivity of saltwater oxidizes the rebar, causing it to expand and increase the size of cracks in the concrete. A negative cycle ensues that can ultimately cause the beams, columns and floor slabs to lose integrity.
Importance of structural engineering
Tragedies like the collapse of the Champlain Towers are thankfully rare in the United States, but older structures, designed before modern building codes were in place, may be at risk. In California, structural engineers have recognized that several types of older structural systems are particularly vulnerable, such as unreinforced masonry, soft story apartments with parking on the bottom, tilt-up warehouse type structures, and older steel and concrete buildings, including those with designs similar to that of the Champlain Towers. Several cities, like San Francisco and Los Angeles, have adopted ordinances to retrofit these older structures.
The cost can be considerable, and it is difficult to gather the public will to invest in protecting our older building stock. But the cost will never be as high as the loss of lives when tragedy strikes.
References
Department of Defense, 2009, Design of Buildings to Resist Progressive Collapse. https://www.wbdg.org/FFC/DOD/UFC/ufc_4_023_03_2009_c3.pdf
Hinman, E.E. and Hammond, D.J., 1997, Lessons from the Oklahoma City Bombing: Defensive Design Techniques. ASCE Press Series, American Society of Civil Engineers, 60p.