We were pleased to have Club member Rick Zimmer of Zimmer Construction Management as our guest speaker who was joined by his nephew David. 
Rick shared with the members why the Champlain Towers South in Surfside collapsed. First, it is helpful to see a visual of the Champlain condo tower complex both before and after the collapse. It is easy to see how significant of a disaster it was looking at the first and then second images:
Now, the entire problem stemmed from water under the pool paver brick decking. You can see an area of the collapsed sundeck slab in the left center of the photo. Once that gave way, the stress applied to the vertical columns to the right. Here is the layout of the pool deck and the related column structure. You can see the problem of the deck column placement being too far apart.
Now, look at the recreation of the pool deck slab, from a side view through the concrete. You can easily see that water was collecting, and then over time, had leaked through the underlying waterproof barrier into the concrete, in which the steel reinforcements rusted and lost their structural integrity.
Finally, you can see how the vertical columns fell one after the other from Building Sections 1 to 2 to 3 and finally 4, resulting in the catastrophic collapse and great loss of life.
In summary, the original construction of the parking garage and overriding deck/pool was done using a formed concrete deck, that was not delivered a crowned or concave shape, to shed water. Rather, it became convex or dished, which enabled water to collect which, in turn, seeped through the underlying concrete which, in turn, corroded the steel rods which, in turn, lost its strength which, in turn, finally fractured and gave way. Once deck failed, there was a domino effect within the vertical layer sections of the condo tower which then failed. The entire process of the building collapse and took less than 12 seconds. 
In 2018, an inspection report prepared by engineer Frank Morabito noted a “major error” in the building’s design: The slab between the pool and the parking garage was flat, not sloped, causing water to puddle and corrode the concrete below. “The failed waterproofing is causing major structural damage. … Failure to replace the waterproofing in the near future will cause the extent of the concrete deterioration to expand exponentially.”
According to the Miami Herald, a pool contractor who surveyed the building days before the collapse photographed seriously degraded concrete in the pool equipment room, and a deep puddle of standing water in a parking spot beneath the pool.
The sad part is that the entire problem could have been eliminated if the owner had followed the recommendations of the inspection report made prior to the required 40-year safety certification process. But that company knew that Miami-Dade County only wanted to see a clean report, so the Owner Group decided to postpone a large $$$ repair assessment, and not proceed with any work to remediate problems. Note that Palm Beach County doesn’t have a 40-year Building Safety Inspection Program, but they along with the City of Boca Raton are now scrambling to develop a responsible inspection process.