Monday, August 16, 2021

Forensic Engineer Discovers Design Flaw in Surfside Condo That Collapsed


On June 24, a thirteen-story condominium building in Surfside, Florida, known as Champlain Towers South collapsed, killing 98 people.  The building was shortly due for a 40-year reinspection, but previous inspections had noted serious cracking and corrosion problems.  The U. S. National Institute of Standards and Technology (NIST) is conducting an extensive investigation into the collapse, and their findings will probably be regarded as definitive.  In the meantime, however, a private Florida firm called Consult Engineering undertook to do an investigation on their own, based primarily on publicly available prints of the building's construction.  And they have discovered a serious design flaw which, combined with aging, may well account for the way the building collapsed.


Consult Engineering is headed by Joshua L. Porter, who specializes in forensic engineering of existing structures.  He used the same design equations and approaches that the original designers used in 1979, when the building was under construction, and studied the prints he obtained to determine which ones were for planning only and which ones expressed the way the structure was actually completed ("as built.")  He also consulted recent videos taken inside the structure and the videos of the actual collapse, and produced a 40-minute YouTube video of his own that presents his conclusions.


While I am an engineer, my specialty is electrical, not mechanical, and I had to pause Mr. Porter's video several times to figure out what he was talking about.  But the video was worth watching, because he has found a significant difference between the original design and a revised design that was issued in January of 1980, during the construction of the building.  What follows is my summary of what Mr. Porter found.


Partly to maximize the space available under a then-existing 12-story zoning limit in place when the condominium was built, an excavation was made below grade for a parking garage beneath the structure.  This meant that the ground or lobby floor, which was also partly designed for parking, had to support the live loads of automobiles driving over the poured-concrete slab floor. 


To support the floor, concrete columns were poured, and in many places these were substantial ones, 24 inches square (61 cm x 61 cm).  Others were smaller, 16 inches square or even 12 inches by 16 inches.  Mr. Porter used the hand-calculation methods that were state-of-the-art for 1979 and discovered that the load of the slab on the columns was in many cases right up to the edge of what he calls the "punching shear" limit. 


Punching shear can be explained this way.  Imagine taking a square piece of cardboard from a corrugated cardboard box, and a sharpened pencil.  With one hand, put the pencil point-downward on the floor and place the cardboard on the eraser end, and mash down on the cardboard.  You will probably break the tip of the pencil first.  That means you have not exceeded the punching shear limit of the cardboard yet.


But then, try the same thing with the eraser end of the pencil on the floor and the pointed end on the cardboard.  It won't take much pressure to make the pencil point punch through the cardboard.  This shows that the bigger the column at the attachment point to the slab, the stronger the system is with regard to the danger of punching shear, which is the column punching through the slab.


In the original 1979 drawings, there was a one-foot step or drop at the ground-level (lobby) floor between two areas.  There were planned planter boxes over part of this area—rectangular concrete tubs in which dirt and shrubs were planted and watered.  And there were cars parked over another part of it.  Partly to support the cars and planter boxes, and partly to allow for the one-foot step, the engineer had placed several concrete beams between the columns underneath these extra loads.  A beam is just a thicker piece of concrete reinforced and cast into the floor, which strengthens the floor in that area and transfers loads to the columns it connects to.  These beams were about a foot thick and would have safely carried the extra vehicle and planter-box loads.  One beam was built into the one-foot drop, which was one reason for the beam to be there.


But in a January 1980 revision of the drawings, the architect decided to eliminate the one-foot drop.  Surprisingly, the engineer also eliminated all the supporting beams that went along with it.  So in one case there were automotive live loads within a foot or two of small 12-by-16-inch columns.  And in a photo of the building's exterior taken before the collapse, Mr. Porter found that several planter boxes had been built in places that no existing print allowed for, further increasing the punching shear load on the smaller columns.


Mr. Porter's conclusions were that several aspects of the building design were executed with no margin of safety—loaded exactly to 100% of their carrying capacity.  This allowed no margin for corrosion damage, which was abundantly evident in recent photographs.  But the most serious flaw was the lack of beams underneath cars and planter boxes, right in the area where the initial failure apparently occurred.  From eyewitness reports and videos, the first failure was apparently when the lobby (ground floor) slab had a punching-shear failure and collapsed into the basement, leaving several smaller columns without support over a 20-foot length rather than their designed-for 10-foot length. 


The building stood for several minutes after that, and a few people managed to escape.  But with the lobby floor gone, the middle of each column formerly attached to the lobby floor was now unsupported sideways.  That, together with forty years of corrosion and cracking, probably induced one of the columns supporting the main building to buckle.  Once it went, the lack of safety margins meant that adjacent columns were overloaded and buckled, and the building was doomed. 


There is nothing official about Consult Engineering's analysis of the Surfside condo collapse.  But coming from a professional whose business it is to do such studies, it seems highly persuasive.  Last-minute changes in construction can be deadly, and this tragedy may be yet another example of this principle.


Sources:  The Consult Engineering presentation on the Surfside condo collapse can be viewed at  I also referred to Wikipedia's article "Surfside condominium collapse." 


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