Everyone has heard of the terrible tragedy in Surfside, a suburb of Miami, where a large portion of a twelve-story beachfront condominium building suddenly collapsed. As of July 5, 32 people were confirmed dead, with over 100 still missing and likely dead in the rubble. As an engineer (not a structural engineer) I am interested in what caused this structural failure. I’ll share what seems to be the latest intelligence on that. I will also offer a speculation on possible economic ripples of this event: what if confidence is lost in the structural integrity of other Miami beachfront condos?
Here is the before:
The best video I am aware of which shows the actual collapse is embedded in this CNN article. It shows the central portion of this building crumbling to the ground from front to back, then a pause, and then the right hand (north-east) section simply pancakes straight down.
It has been widely reported that previous inspections warned of serious problems with this building, especially a 2018 report. Some snips from that report are here. It does contain dire language (“…major structural damage to the concrete structural slab… failure to replace the waterproofing in the near future will cause the extent of the concrete deterioration to expand exponentially”), but this is mainly directed at the deterioration of the concrete deck around the pool and over part of the parking garage: “Several sizable [cracks in the concrete] were noted in both the topside of the entrance drive ramp and underside of the pool/entrance drive/planter slabs, which included instances with exposed, deteriorating rebar.” The underlying problem here was that the deck was flat, not sloped, so rainwater just sat on it and soaked into the concrete, as opposed to running off like it should.
I don’t have access to the full report, but I am not aware that it, or any other inspection, warned that the main buildings themselves were in any danger of imminent collapse. However, it appears that the failure of that pool deck was the trigger for an unexpected cascade of further collapses, first of the midsection of the tower, and finally the pancaking of the north-east section:
On June 27, the Miami Herald reported on the consensus of six engineering experts it interviewed. Based on publicly available evidence, the experts believed that a structural column or concrete slab beneath the pool deck likely gave way, causing the deck to collapse into the garage below. This formed a crater beneath the bulky midsection of the tower, which then caved in on itself. This is a type of progressive collapse, in which one structural part gives way, destabilizing and removing support from other parts, which in turn collapse and rapidly remove structural support. Among the evidence supporting this conclusion was the report that moments before the building collapsed, a resident of a fourth-floor unit called her husband to tell him that a crater had appeared in the pool deck. The resident was among those missing in the collapse. A surviving resident also stated that part of the pool deck and street-level parking area had collapsed into the parking garage minutes before the collapse. (per Wikipedia)
Below are before and after photos from a Washington Post article, shot from the north-east, i.e. closest to the final pancaked section. I zoomed in on the pool deck area, and circled in red the deck section that seems to have been the first to collapse, leading to the train of further structural failures.
Source: Washington Post
Source: Washington Post
There will of course be lawsuits, and large settlements for the victims of these horrific events. A larger financial question is whether this building was a unique outlier, or whether many of the other high-rises which crowd the South Florida coast are likewise at risk. We note that this particular area in Surfside is man-made landfill, not solid rock. This type of material tends to be unstable; a study in the 1990’s noted that the land in this area, and other places around Miami, was sinking at about 2 mm/yr, and sea water was encroaching. And the underlying rock in Florida is mainly limestone, which is notorious for dissolving and forming sinkholes. These high-rise structures are made mainly of reinforced concrete. If cracks in the concrete allow water (especially salt water) and air to get to the steel rebar inside the concrete, the rebar can rust and swell and split the concrete.
It is too early to speculate on the outcome of the various investigations, but if it turns out that numerous buildings along the Florida coast are at risk, it is not clear to me where we go from here. If buildings full of bought-and-paid-for condos must be demolished, it seems that the economic impact to Florida would depend crucially on the federal response. These places may become uninsurable, and real estate prices would tank, perhaps ruining the finances of hundreds of thousands of residents and crippling the retirement/resort trade. However, the gigantic but seemingly painless Federal Reserve funded deficits of the past 18 months suggest that there is effectively no limit to spending by the federal government, so maybe Uncle Sam will pay for tear-downs and rebuilds, if needed. This could, ironically, end up as a big boost to GDP. But again, these are just speculations.