I was 30 years old in 1981 when I moved to South Florida. I was a carpenter there to validate the extreme economic pressures of the Mid West and celebrate the drug fueled economy of South Florida. Buildings were popping up everywhere. Also significant was the new legislation passed referred to as the Threshold Act. This law stemmed from the collapse of a 6 story building under construction that fell to the ground. Later determined the failure was not poor construction but rather a design flaw called "punch through". Punch through describes the shearing of the slabs at column locations. In addition to the Threshold Act this was the time that Post Tensioned Concrete Slabs became popular in Condo Design. Architects were attracted to the increased spans between supports that allowed greater spans without increases in slab thickness. To me the carpenter I witnessed the difference between a typical rebar slabs where reinforcing steel of various diameters cross up in a basket type application near the top and bottom of the slabs. Known as top mat and bottom mat. Bars would often be spaced 8 to 10 inches as structural engineers calculated. The advent of Post Tensioned Slabs eliminated the need for much of this rebar and spacing between cables would often be 4 feet. The cables were very strong 7 stranded and impossible to bend by hand. The cables stretched the length of the building with tails extending 4ft beyond the edge of building. After concrete testing indicated strength of 3000 psi a team of ironworkers 2 usually would attach a hydraulic jack to the tails of each tendon noting the length of elongations along with lift off pressure recorded. This information was sent to the structural engineer for his review. With the blessing of the Engineer the forms could now be removed and moved to the next floor. The tightening of cables would often raise slabs at mid span 1 inch providing the required supports. Amazing that a cable so hard could be elongated 2 feet or more under pressures in thousands of pounds, depending on the length of cables. The cables would often remove the need for rebars at 8 to 10 inches. Structurally this makes the cables much more important and a failure of a single cable being 10 times more significant than a missing rebar. I was in a building under construction in 1985 when a cable broke loose. It sounded like thunder and the entire structure shook in the released energy. My inspection reveled the pathway of the cable as it ripped loose from the building and tossed a chunk of slab with it 300 yds outside the building. If this were in your living room it would kill everyone in it's exit.
The building at surfside was built in 1981, I was not there, I cannot attest to the construction means and methods of the day. I can say the building did use cables. The fact that half of the building did not fall is all the testament I need to validate means, methods and engineering of the structure. The fact that half of the building withstood this amount of stress and still standing is telling.
What I did see from the video is the event begins in the lower portion of the structure. It fails in three pieces. Center then left then right video. You can see the building designs working as the structure tries to stop the collapse. It finally wins at half way.
So what caused the failure? It was punch through around the columns in the lower floors. The slabs fall the columns are knocked down and the upper floors fell. The investigation will reveal the reasons for the punch through. I suspect corrosion of cables or their anchors.
It was not settlement in a building or concrete piles sinking. It could be a failure of the piling but very unlikely. These piles are load tested and blow count data recorded to prove bearing pressures.
I'm not an engineer. I was there when cables took over as the industry standard in the 1980's. If your looking for the problems look at the cables first. Cables compose large portions of the buildings integrity. When one cable breaks it can and has killed people. When many break it's collapse.
Tom Palmer
