How the Sand Palace Survived Hurricane Michael

November 29, 2018

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Memorably, one beachfront home survived Hurricane Michael’s intense winds and surging waters intact—the now-famous “Sand Palace,” located on 36th Street in Mexico Beach. This got us thinking—could we somehow quantify the impact of the Sand Palace’s mitigating features using a catastrophe model? The answer is a resounding “yes.”

 

Widespread Destruction

Hurricane Michael made landfall near Mexico Beach in the Florida Panhandle on the afternoon of October 10, 2018. The intensity of the storm, which made landfall as a Category 4 hurricane with maximum sustained windspeeds of 155 mph, was unprecedented. After the skies cleared, the evidence of widespread destruction in this small beach town was obvious. Aerial imagery and photos from this stretch of beachfront revealed rows of flattened homes and scattered debris. Several structures closest to the coastline were washed away by storm surge, leaving behind only wooden pilings. Homes located beyond the area of storm surge inundation were reduced to splintered wooden frames by Michael’s winds. As AIR’s Dr. Tim Johnson noted in his blog, “a combination of storm surge and wind damage reduced a quaint beach town into an apocalyptic wasteland.” According to the New York Times, of 440 identified buildings, 54% were destroyed, and 23% “severely damaged.”

The Sand Palace was left relatively unscathed. Individuals covering the destruction—including structural engineers at AIR—quickly set out to understand how this single structure survived Michael’s record-breaking intensity. The answer lies in its structural design. Most homes located in the Florida Panhandle were built prior to the introduction of building codes put in place following Hurricane Andrew. Although newer structures performed better, many still sustained significant damage. In contrast to its neighbors both new and old, the Sand Palace was built with mitigating features beyond those required by Florida’s stringent building codes.

Mitigating Features

While the most recent building codes prescribe that structures in Mexico Beach be designed to withstand 120-130 mph wind speeds, the owners of the Sand Palace indicated that the structure was built to withstand 240-250 mph winds. The structure itself was built of reinforced concrete—a construction type that is seldom used for single-family homes in the United States. Furthermore, additional care was taken with other building features to make the structure more resilient in the face of hurricane-force winds:

  • Small overhangs were used, reducing the amount of potential uplift of the overhangs by wind
  • The number of windows was limited, reducing the risk that projectile debris would cause a breach in the building envelope
  • A balcony on the east side of the house was removed from the original design

These features, or lack thereof—small overhangs, low glass percentage, a small number of attached structures—all contributed to the home’s reduced vulnerability to extreme winds.

While Michael’s winds were intense, the bigger threat for beachfront homes on Mexico Beach was storm surge, which exceeded 10 feet at many locations. The Sand Palace was built on 40-foot pilings with a first-floor height 15 feet above the ground, which significantly reduced the potential for substantial flood damage resulting from Michael’s surging waters. Surge-related damage was ultimately minor compared to that of neighboring houses. Given the Sand Palace’s performance in the face of Hurricane Michael’s destructive forces, it’s safe to say that this structure was built to withstand storms of even greater intensity than Michael’s.

AIR’s Analysis

Using AIR’s detailed catastrophe modeling platform, Touchstone®, we conducted sensitivity analyses to understand the impact of capturing the many mitigating features associated with the construction of the Sand Palace.

In one of our tests, we compared two homes: the first, a two-story single-family home made of wood frame construction with a year built of 1990 and no explicitly coded mitigating features; and the second, a two-story single-family home made of reinforced concrete and other various secondary risk features associated with those of the Sand Palace. Using the Hurricane Michael simulated ALERT event set we calculated a roughly 90% reduction in wind losses after explicitly accounting for the various primary and secondary risk features associated with the Sand Palace compared to those of a typical home in Mexico Beach.

AIR’s Touchstone platform also allows users to explicitly code the first-floor height of a given structure. As discussed previously, the Sand Palace stood atop 40-foot pilings, raising the first-floor height of the structure 15 feet above the ground elevation. We modeled the structure twice: once using a default assumption for the first-floor height, and a second time, explicitly entering a first-floor height of 15 feet. The test revealed that setting the first floor height to 15 feet resulted in a significant reduction in storm surge related damage compared to modeling the structure using the default first floor height. The modeled damage, after setting the first floor height to 15 feet, was negligible, which is consistent with the observations from AIR’s own damage survey.

The Sand Palace is an excellent case study of the impact of mitigating features for use in risk reduction. Given the expense associated with the Sand Palace’s enhanced mitigating features, it would also be worth studying the return on investment for these associated features. Touchstone users can explicitly capture the impact of individual risk features on loss reduction compared to the associated cost. Beyond the most common uses of a catastrophe modeling platform—portfolio management, rate making, individual risk modeling—Touchstone offers clients the capability to study and quantify the impacts of many mitigation features to make more-informed decisions about risk reduction.


Visit Hurricane Michael: Lessons in Risk Management and Resilience to learn more about this historic storm

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