Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael?
Friday, October 12, 2018
Concrete block houses are supposed to be hurricane-proof. And yet a large swath of them were totally destroyed by Hurricane Michael in Mexico Beach, Florida a few days ago. “These were all block and stucco houses—gone,” the former mayor of the town, Tom Bailey, told a New York Times reporter. "The mother of all bombs does not do more damage than this.”
So what went wrong? And does it mean that your concrete block home could suffer the same fate in next Category 4 hurricane? The answer lies in the year the home was built, since older homes do not have the required “continuous load path” of structural connections from the roof down to the foundation—like links in a chain—that has been mandated since the implementation of the statewide Florida Building Code in 2002.
Most people think of the structural failure of a house as collapse. But falling down is only one way a structure can fail, and gravity is not always the culprit. A building can also fail upwards, when a hurricane creates a pressure imbalance that literally sucks off the roof; and it can fail sideways, under the lateral pressure of high winds. It is usually a two-step process during a hurricane: first, the roof is sucked off the walls, then the wind and/or storm surge knocks down the weakened walls.
Newer block homes are required to have concrete poured down in the cells of the blocks at critical locations, such as at corners and around door and window openings, running from the bottom of the tie beam to the foundation. The concrete must be reinforced with embedded steel bars from top to bottom that ties it all together to resist the wind.
Older concrete block homes, espcially ones from the 1960s and earlier, do not have this strong connection between the tie beam and the foundation. While the suction created under the roof during a storm is pulling upward on the roof structure, it is also tugging violently on the tie beam via the truss straps into it. That uplift on the tie beam opens up cracks in the weaker horizontal mortar joints in the blocks directly below the beam, causing the structural connection from the beam to the blocks below to eventually fail. Without the steel-reinforced connection between tie beam and foundation found in post-2002 homes, the walls must face the lateral wind loads as separate components and on their own. At that point, only their weight is resisting the wind and storm surge pressure—so severe in Hurricane Michael that it knocked train cars off the tracks nearby.
Homes built in the last decade in the Panhandle also must be elevated above the estimated maximum storm surge level so that the water can flow under the home. Although a storm surge is a fairly slow, but continuous rise in water level over a brief time period—not a tidal wave—the weight of up to nine feet of water on one side of any wall exerts tremendous lateral force on it. Any wall below the elevated living floor of the home must be break-away type that are not structurally connected to the support columns, and specifically designed to collapse if necessary in a storm surge.
Newer homes have further advantages over pre-2002 construction. They must have more and stronger nails connecting the roof sheathing to the trusses and sturdier straps connecting the trusses to the tie beam, along with storm-resistant windows and doors in the walls. All of these differences are why most of the recently built concrete block homes were damaged, but not destroyed in the hurricane.
Incidentally, the statewide 2002 Florida Building Code that improved the hurricane resistance of new homes was a response to horrific damage caused by Hurricane Andrew in South Florida 1992. The storm was referred to as a “wake-up call” to building officials at the time. The Panhandle has not been required to meet the same "wind-borne debris” standards instituted for South Florida up until now, but will likely be upgraded to tougher standards soon.
To find out how to determine if your older block home has vertical reinforcing in the walls, see our blog post How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?
If you have an older CBS home, there are also retrofit programs to upgrade your home with better hurricane-resistant features. You can learn about them at http://www.flash.org/hurricanestrong/. Also, see our blog post What can I do during a hurricane to reduce the possiblity of roof damage?
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