Harbor Cay Condo Construction Failure
Hi and welcome to Failurology; a podcast about engineering failures. I’m your host, Nicole
And I’m Brian. And we’re both from Calgary, AB.
Thank you again to our Patreon subscribers! For less than the cost of a reusable face mask, you can hear us talk about more interesting engineering failures!
This week in engineering news, a self powered, stretchable thermometer.
As we develop soft robotics, smart clothing and biocompatible medical devices, we need flexible sensors that can move with the wearer. But most of today's sensors are rigid.
Harvard John A Paulson School of Engineering and Applied Sciences developed a soft, stretchable, self powered thermometer
The device has three parts, an electrolyte, electrode and a dielectric material to separate the two
Ions accumulate between the electrolyte and dielectric and electrons accumulate between the electrode and dielectric. The imbalance of charge changes the thickness of the ionic cloud and generates a voltage which translate to a change in temperature
The thermometer can measure temperatures from 200 to -100 celsius and can be used for a million different applications
If you want to read more on the soft thermometer, check out the links on the webpage for this episode at failurology.ca.
Now on to this week’s engineering failure; the Harbor Cay Condominium Collapse in Cocoa Beach Florida.
I’ve got to say, when we started researching this episode, we had a pretty good idea what we would find. 9 times out of 10, a building collapse is structural. But going in, we weren’t sure if there would be enough information for a full episode; 50/50 chance this would be a mini failure episode for our Patreon. Boy were we pleasantly surprised, there is a lot here. So buckle up folks, this is a good one.
Cocoa Beach is south of the Kennedy Space Center, east of Orlando
Five story flat plate condo building, collapsed around 3pm on March 27, 1981, killing 11 workers and injuring 27
Collapse occurred as they were constructing and pouring concrete for the roof slab. The collapse appears to have originated in the area where they were pouring concrete.
Survivors described building trembles a couple times that day, with a large pop and then a rumble as the roof caved in and the collapse traveled as a wave until it enveloped the entire structure. One worker who “rode down on a dry form near the south end” said he “could see the middle of each floor give way as each floor was collapsing”. Another worker said that “the floor fell straight down with slight hesitation at the fourth and third floors”.
Building was 74m long, 18m wide, and five stories tall
The concrete slabs were constructed using flying forms, or tables, which are pre-built plywood decks with metal frames that they “fly” from one floor to the next as they build typical floors.
The floor slabs were cast in two phases, but the roof they did all in one pour
At the time of the collapse the exterior walls were in place on main and second, masonry or block walls were built on main, second and third, and material was placed on fourth
There is a drawing on our website, the page for this episode, that indicates the approximate location of the workers at the time of the collapse, denoting survivors with hollow circles and workers killed with solid circles.
National Bureau of Standards (NBS) performed an investigation at the request of the Occupational Safety and Health Administration (OSHA)
Ultimate cause - Low punching shear capacity
Concrete slab was only 200mm thick, should have been 280mm as per the American Concrete Institute’s Building Code minimum
Plastic chair spacers to support the rebar were 110mm high, which reduced the effective depth or thickness of concrete below the top layer of rebar
Chairs supporting the top layer of rebar were too short, cover was supposed to be less, with more concrete under the rebar
should have been 19mm coverage, but was 44mm coverage (double)
Shop drawings from the steel supplier, outlining the number and sizes of these chairs were marked as “approved as noted” with the signature of the engineer of record
The columns punched through the fifth floor slab in one location, which caused the entire fifth floor to collapse, and pancake the slabs below it until the entire building had collapsed
Most likely at column G-2, interior column, supported the last bay of fresh concrete
The concrete had been placed on seven of the nine forms when the collapse occurred
Re-shore, or temporary columns that remain in place for a specified number of days following a concrete pour, were in place at the time of the collapse. Concrete sets relatively quickly, maybe a few days, but takes several weeks to completely cure to its full strength. According to the GCs reshore plan, the 2,3,4th floors had full reshore, but based on worker accounts and information uncovered during the investigation, it was believed that only the fourth had full reshore, the main floor has only a few exterior reshores and the second and third floors were unclear
Concrete was provided by a ready-mix plant nearby and an onsite batch plant. The ready-mix plant provided about one third of the concrete for the slabs, with the on site plant providing the rest of the concrete for the slabs and all of the concrete for the columns. The on site batch plant was reported to have non uniform consistency and finishability. As well, the proportion of each “ingredient” did not seem to be consistent across all of the samples they tested.
The developers superintendent oversaw the batch plant operation, but was away from February 2 to March 9th. The second and third floors were placed during this time and appeared to have an erratic slump of 75-225mm. And the general contractors superintendent was away the week of March 2. The third floor was placed while he was away. One of the concrete finishers noted that the texture and slump of the concrete became more consistent once both workers returned.
The masons found some of the concrete difficult to finish and that it set up quickly. This appeared to only be an issue with the site batch plant and alluded to low water content and a lack of superplasticizer which is added to concrete trucks prior to placement and ensures proper consistency of the concrete.
Core samples from the slabs and columns were removed from site for further testing. The slab samples satisfied the American Concrete Institute code, but the column samples were of lower strength than the slab and the fifth floor column samples specifically did not satisfy the American Concrete Institute requirements.
Many workers who were interviewed noted cracks in all of the slabs
The engineer walked the site on March 10th, 17 days before the collapse. The second and third floors were in place by that time. They recommended that additional rebar be added in the midspan of the walkways
When the plumbing sleeves were removed during construction, some of the “spider cracks” could be seen extending 102-127mm into the slabs
Noticeable slab deflections were witnessed as well. They were most evident on the north and south engs. They noted a 45mm deflection on the north end of the building on the second floor.
A water puddle formed during placement of the roof on the east side of the building near the elevator shaft. Workers used extra concrete to fill the depression, but it reappeared after finishing.
A similar problem occurred at the same location when they placed the fourth floor
Workers mentioned that the construction joints were not of good quality
Described them as “ragged”, “cracked” and “honeycomb”
Rebar showing on the faces of some columns. Incomplete filling of the column form was likely due to steel congestion. It was noted that in some cases, the steel congestion was so severe, that the slab steel could not pass through it on the second and third floors. This prevented the development of an adequate bond between reinforcing bars.
Two engineers, an architect, and two contractors were charged with negligence and failing to follow state and local codes. Both engineers surrendered their license to practice in Florida, the primary engineer, who was 59 at the time, paid a $3,000 fine (about $9,000 today), the other paid fines as well and agreed to never practice in Florida again, the contractor folded almost immediately, and the developer settled out of court and continues to develop properties today.
So there you have it, the Harbour Cay condo construction collapse in Cocoa Beach Florida. Two main issues, with thickness of the slab and the height of the chairs supporting the upper rebar, as well as a ton of minor issues, led to a structure so weak, it couldn’t even support itself and collapsed before the roof was complete. And like all of the failures we have covered to date, this one was also completely preventable. There are a lot of lessons to learn from this failure, and I hope that structural engineers around the world were paying attention to prevent something like this from happening again.
For photos, sources and an episode summary from this week’s episode head to Failurology.ca. If you’re enjoying what you’re hearing, please rate, review and subscribe to Failurology, so more people can find it. If you want to chat with us, our Twitter handle is @failurology, you can email us firstname.lastname@example.org, or you can connect with us on Linked In. Check out the show notes for links to all of these. Thanks, everyone for listening. And tune in to the next episode where we will tell you about the Tous Dam, a Spanish dam that failed after unusually severe rainfall.
Bye everyone, talk soon!