The 2017 hurricane season was record breaking on several levels, and Hurricane Irma was one of the major players. Hurricane Irma first made landfall as a Category 4 on Cudjoe Key, Florida, then made a second landfall on Marco Island, Florida, as a Category 3 storm. Tropical force winds impacted the entire state of Florida, as well as Alabama, Georgia, South Carolina, North Carolina and Tennessee. According to the National Hurricane Center, Irma was the 5th costliest hurricane in U.S. history, the most powerful hurricane to make landfall in Florida since 2005, and the most catastrophic hurricane since Hurricane Andrew in 1992.
Given all those records, what were the major property damage impacts from Irma and what can we learn from this storm?
Hurricane Irma Impacts
Hurricane Irma had three major impacts: wind, storm surge, and rain. These affected different areas and caused varying degrees of damage. Here’s a quick summary of what we saw.
- Wind speeds were higher along the coasts and caused structural damage primarily in the Florida Keys and to Florida’s southwest coast.
- Storm surge. The storm surge caused significant structural damage in the Florida Keys and to oceanfront properties around St. Augustine, Florida. Flooding in southwest Florida and in Jacksonville, Florida, was primarily from rising water.
- Some areas received between 10 and 15 inches of rain which caused some inland flooding.
Lessons from Hurricane Irma
We inspected many properties impacted by Hurricane Irma, and here’s what we observed.
- In general, building materials performed as expected. Building code changes since Hurricane Andrew have improved the performance of building materials, especially roof coverings. The improvements reduced damage from cascading failures (windborne debris from one building failure resulting in failure of downwind structures). As would be expected, older buildings were more easily damaged. In some areas with high winds, significant damage occurred to one building while neighboring buildings sustained minimal damage.
- Damage occurred from the top down, inside out, as expected. Roof structure failures often led to the failure of walls and other components.
- Roof covering failures typically initiated at roof edges, hips, and ridges, often as a result of inadequate attachment. Failure typically started on the windward side. Tile roofs were damaged in high-wind areas.
- Most of the structural damage from the surge was to ground-level buildings (slab foundations) on the waterfront facing the incoming surge. Elevated structures performed well during the hurricane.
- As a general rule, structures even several hundred feet inland did not experience significant structural damage from the storm surge, consistent with the flood zone designations.
- The main cause of damage was impact from waves and waterborne debris.
- Scour was a minimal problem (in contrast to Hurricane Sandy).
- The primary issue with water intrusion was determining whether the water intrusion was the result of a storm-related opening and determining whether the water intrusion was from rain or flood.
Conclusions from Hurricane Irma
There are a lot of lessons to be drawn from powerful storms like Hurricane Irma, but here are some takeaways from forensic engineers who worked in the field after the storm:
- There are often common problems that can be recognized in specific areas, such as vertical cracks and spalling in concrete columns in the Keys.
- It’s important to understand how wind and water forces act on a structure. This can help explain why the damage is or is not the result of the storm.
- If damage is not from the storm, discovering the actual cause and being able to support that with science, is important.
- The storm revealed underlying deficiencies, such as flashing and inadequate nailing of roof coverings, that may never have been a problem before the storm. Window installation defects that resulted in water intruding around the windows was a common problem.
John Miller, P.E., is a forensic engineer, principal consultant, and CAT manager at Donan. He spearheaded Donan’s catastrophe response efforts after Hurricane Harvey and Hurricane Irma. For more on this topic, visit donanuniversity.com where you’ll find free webinars, as well as a recording of Beyond the Webinar – The Official DU Aftershow, focusing on case studies from Donan’s work after Hurricane Irma.