Hurricane Andrew and South Miami-Dade Housing

Drive through Palmetto Bay, Cutler Bay, or the areas toward Homestead and the housing has a discontinuity in it that is unusual for an American suburb. Blocks of houses from the 1960s sitting alongside blocks from the mid-1990s, sometimes on the same street, sometimes on the same lot. There is one reason for it.

What happened

The National Weather Service Miami office records Hurricane Andrew’s eye crossing Elliott Key at 4:40 AM EDT on Monday, 24 August 1992, with landfall on the mainland at 5:05 AM at Fender Point in Biscayne National Park, near Homestead Air Force Base. It came ashore as a Category 5 with sustained winds of 165 miles per hour and a central pressure of 922 millibars.

The housing figures from the same source: approximately 49,000 homes destroyed and around 108,000 damaged, with almost all of it in south Miami-Dade. It remains one of the most destructive hurricanes in United States history, and the rebuild that followed reshaped the southern half of this county.

The structural consequence, three decades on

A building’s age tells you a great deal about how it is likely to behave, and in south Miami-Dade the age distribution has a hole in the middle of it. What that produces on the ground:

Houses that predate 1992 and survived. Older construction, often 1950s and 1960s block, built to earlier reinforcement requirements and thinner concrete cover. These are the properties where corrosion of embedded steel shows up first, and where an addition from a different decade is most likely to be sitting on a footing detailed differently from the original house.

Houses built in the mid-to-late 1990s. Rebuilt in the years after Andrew, and constructed to substantially stronger requirements than what they replaced. Miami-Dade and Broward now sit inside the High-Velocity Hurricane Zone under the Florida Building Code, which imposes stricter wind and product-approval requirements than the rest of the state.

Lots with both. A rebuilt house next to a surviving garage, an original slab with a new structure on it, an addition tied into a wall that has a different history. The join between two structures founded differently is one of the most common places a crack appears in this county, and south Miami-Dade has more of those joins than anywhere else.

Which does not mean newer is automatically fine

The other variable in this part of the county is the ground. South Miami-Dade sits low, well off the higher limestone of the Miami Rock Ridge, with marl, muck, and imported fill in the profile. The USDA soil survey maps roughly 107,000 acres of organic soils across the county, about a sixth of its area, concentrated in exactly this kind of low ground.

A house built to 1990s standards on fill that was placed without proper compaction will still settle, because the structure above and the ground below are independent questions. Building code governs the first. Site preparation governs the second, and the county’s own code recognizes this by requiring an owner-retained architect or engineer as Special Building Inspector to supervise the compaction of fill under slabs on grade.

What to look at on a south Miami-Dade property

• Establish the build history. Original, rebuilt, or partly both, and when. It changes what is likely.
• Look at the joins. Where an addition meets an original structure is where differential movement shows up. A crack there is meaningful but usually contained.
• Shoot elevations rather than trusting the eye. Slab-on-grade floors in this county are rarely laser-flat even when nothing has gone wrong, so a measured contour is worth more than an impression.
• Check the drainage. Flat ground, 67 inches of rain a year concentrated into six months, and permeable soil is a combination that punishes a downspout discharging at a wall.

The general framework for reading all of this is in the main Miami foundation guide, and the specific area pages for Palmetto Bay and Cutler Bay cover the local patterns.

Sources: National Weather Service Miami on Hurricane Andrew’s landfall and damage; USDA NRCS Soil Data Access, Miami-Dade County Area survey; Chapter 8, Code of Miami-Dade County.