Hurricanes are among the most powerful natural forces known to man. The increased pace of residential development in coastal areas coupled with the recent increase in the number and severity of these tropical storms has made homeowners and insurers painfully aware of their destructive power. In fact, hurricanes have the potential to damage homes in three distinct yet related ways:
• Through the forces imposed by high speed winds
• Through water damage caused by torrential rainfall
• Through the effects of tidal flooding caused by storm surge.
This course examines types of damage caused by each of these 3 storm effects. Course Modules 2, 3 and 4 will discuss ways to modify existing homes to better resist these forces.
Wind Speed Measurement
In the early 1800’s, Sir Francis Beaufort, a British naval officer, devised a scale ranging from 1 to 12 which described wind forces according to their effect on the rigging of an armed sailing vessel known as a “man of war.” The Beaufort scale was not based on wind speed per se but on the amount of ship’s movement that resulted, up until the point at which the sails were destroyed. Over the years, as steam replaced sail, the descriptions were altered to describe sea state conditions with related land effects instead of effects on a ship’s rigging. In 1923, the numbers on what came to be know as the Beaufort Wind Force Scale were standardized to revolutions of a wind speed measuring device known as an anemometer. In 1946, scale categories 13 through 17 were added to describe the destructive effects of tropical storms.
This “extended” Beaufort scale is still used in many parts of the world to categorize cyclones and typhoons, tropical storms similar to hurricanes. Categories 12 through 16 on the Beaufort Wind Force Scale loosely correspond to Categories 1 to 5 on another wind speed classification system known as the Saffir-Simpson Hurricane Scale.
The Saffir-Simpson Hurricane Scale classifies tropical storms that form in the Atlantic or northern Pacific and threaten North, South and Central America and the islands of the Caribbean. This scale was developed in the late 1960’s by a civil engineer named Herbert Saffir and Robert Simpson, then the director of the US National Hurricane Center. Saffir created the original scale of categories from 1 to 5 based on increasing wind speeds and the expected destructive effects on man-made structures. Simpson expanded these expected destructive effects to include damage from wind-induced tidal storm surge.
Tropical Storm - Winds 39-73 mph
Category 1 Hurricane — winds 74-95 mph (64-82 kt)
No real damage to buildings. Damage to unanchored mobile homes. Some damage to poorly constructed signs. Also, some coastal flooding and minor pier damage.
- Examples: Irene 1999 and Allison 1995 Category 2 Hurricane — winds 96-110 mph (83-95 kt)
Some damage to building roofs, doors and windows. Considerable damage to mobile homes. Flooding damages piers and small craft in unprotected moorings may break their moorings. Some trees blown down.
- Examples: Bonnie 1998, Georges(FL & LA) 1998 and Gloria 1985
Category 3 Hurricane — winds 111-130 mph (96-113 kt)
Some structural damage to small residences and utility buildings. Large trees blown down. Mobile homes and poorly built signs destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland.
- Examples: Keith 2000, Fran 1996, Opal 1995, Alicia 1983 and Betsy 1965
Category 4 Hurricane — winds 131-155 mph (114-135 kt)
More extensive curtainwall failures with some complete roof structure failure on small residences. Major erosion of beach areas. Terrain may be flooded well inland.
- Examples: Hugo 1989 and Donna 1960
Category 5 Hurricane — winds 156 mph and up (135+ kt)
Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required.
- Examples: Andrew(FL) 1992, Camille 1969 and Labor Day 1935
source: National Oceanic and Atmospheric Association
source: Florida Building Code 2004
One shortcoming of the scale is that it does not consider the effects of rainfall as a destructive element. Moreover, the actual amount of damage caused by a storm is directly related to the amount of development in its path. For example, a storm that strikes a densely developed metropolitan area is certain to result in more loss of life and property than a similar storm striking a sparsely populated rural area.
Wind speeds on the Saffir-Simpson scale are measured as 1 minute averages. This means that hurricane force winds actually gust to speeds greater than the reported highest sustained mile per hour rating.
The Florida Building Code divides the entire State of Florida into several Basic Wind Speed zones based on “3-second gusts.”
The Code also presents a Table that translates these 3-second gust wind speeds into “fastest mile” wind speeds which are sustained wind velocities similar to the 1 minute averages on the Saffir-Simpson scale.
Table 1609.3.1
Equivalent Basic Wind Speeds a.b.c
V3s
85
90
100
105
110
120
125
130
140
145
150
160
170
Vfm
70
75
80
85
90
100
105
110
120
125
130
140
150
For SI: 1 mile per hour = 0.44 m/s
a. Linear interpolation is permitted
b. V3s is the 3-second gust wind speed (mph).
c. Vfm is the fastest mile wind speed (mph).
source: Florida Building Code 2004
These “fastest mile” wind speeds occur in the narrow band or “wall” that surrounds the eye of the storm and dissipate toward the storm’s outer edges. However, hurricanes frequently generate locally destructive wind events such as micro-bursts or tornadoes that can even damage homes designed to resist the wind-related forces of the hurricane itself.
Damage to Homes from High Speed Winds
Homes built in areas prone to hurricanes, tornadoes or other severe weather events need to be designed to resist severe wind-induced pressures on the windward sides of the structure and suctions on the leeward sides.
As wind blows against a vertical surface of a home such as a wall or steeply pitched roof, it exerts a positive force or “pressure” against that surface. As the wind flows over or around the home, it exerts a negative force or “suction” on the walls or roof planes parallel to or away from the direction of the wind.
The combination of these pressure and suction forces can result in the following damaging effects to structures:
Uplift
Sliding
Overturning
Racking
source: Federal Emergency Management Agency
Uplift
source: Federal Emergency Management Agency
The suction effect of wind flowing over a roof is similar to the lift created when air flows over an airplane’s wing. This creates uplift forces that can strip the roof coverings and sheathing or, in extreme cases, overcome the fasteners that connect the trusses or rafters to the top of the supporting walls and destroy the entire roof assembly.
source: Simpson Strongtie
These uplift forces increase dramatically when there are large openings in the walls of the structure. The main example of such an opening is the cavity created when the garage door fails. This allows wind to blow into the structure and increase the pressure inside the building. The combination of increased interior pressure and the suction effect of the wind blowing across the roof plane increases the likelihood that the roof will fail.
source: Federal Emergency Management Agency
Sliding
source: Federal Emergency Management Agency
Sliding occurs when the combination of the positive pressures on the walls facing the wind and the negative pressures on the walls of the home parallel to or away from the wind causes the failure of the fasteners anchoring the floor assembly to the foundation.
source: Simpson Strongtie
The wind forces can shear the fasteners, pull the fasteners out of the foundation material, or pull the heads of the fasteners through the wood members of the sill plate or floor joist assembly. In any case, the result is the same, and, as the picture shows, a home could be blown off its foundation with little other apparent structural damage.
Overturning
source: Federal Emergency Management Agency
When the combination of the positive pressures on the windward walls and roof planes and the suction/uplift forces on the leeward walls and roof planes is great enough, the entire structure can be overturned. This is clearly the worst case scenario resulting in the complete destruction of the home.
source: Simpson Strongtie
Racking
While section 1609.1.3 of the 2004 Florida Building Code requires the anchoring of “structural members and systems and components and cladding . . . to resist wind-induced overturning, uplift, and sliding,” another potential form of wind-induced building failure is Racking.
Racking occurs when the combination of positive and negative (or “suction”) pressures is sufficient to overcome the diagonal bracing in the wall assembly and the walls collapse. As the photo shows, a common failure point due to Racking is at the openings for garage doors. At these points, the width of the opening replaces a major portion of the wall and the remaining studs and sheathing are not strong enough to resist tropical storm wind forces.
source: Simpson Strongtie
Damage to Homes from Wind-borne Debris
The air-borne projectiles accompanying a hurricane are another extremely important cause of wind-induced damage to homes. Tree limbs, pieces of damaged homes, even clay and concrete roofing tiles at 7 to 8.5 pounds per tile can become air-borne missiles capable of breaking through windows, doors, garage doors or even wall assemblies themselves. Once these missiles have created holes in the building envelope, wind can enter the structure threatening the roof, and rain can damage the building components and contents even if the structure remains intact.
The “Basic Wind Speed” map for Florida referred to above identifies special “Wind-borne Debris Regions” along the Atlantic and Gulf coasts in which the Basic Wind Speed for structural design purposes is 120 mph or greater. The map also identifies a second category of Wind-borne Debris Region in which the design wind speed is only 110 mph but which lies within 1 mile of the mean high tide line. A third and final category of Wind-borne Debris Region is an “Exception” specific to 7 counties along the Florida Panhandle. In this “Exception”, the Wind-borne Debris Region is limited to communities within 1 mile of the mean high tide line despite the fact that the Basic Wind Speed for most parts of these counties is 120 mph or greater. The terms “Basic Wind Speed” and “Wind-borne Debris Region” are derived from concepts developed by the American Society of Civil Engineers (ASCE) and published in the ASCE document “Minimum Design Loads for Buildings and Other Structures.” This is the source cited by the FL Building Code when it refers to design load requirements for structures.
In Wind-borne Debris Regions, the basic assumption is that windows, doors and garage doors will be broken by flying projectiles unless they are either impact resistant or protected by some form of impact resistant shutter. Impact resistance standards have been established by several authorities, among them:
The Miami-Dade County Building Code Compliance Office
• PA-201 Impact Test Procedures (Large Missile Test and Small Missile Test)
• PA-203 Cyclic Wind Pressure Loading Test
The American Society for Testing and Materials (ASTM International)
• ASTM E1996-06: Standard Specification for Performance of Exterior Windows, Curtain Walls, Doors and Impact Protective Systems Impacted by Windborne Debris in Hurricanes
• ASTM E1886-05: Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors and Impact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials.
From the titles above, it is clear that tested materials are subjected to cyclic wind pressures as well as high speed impacts by air-borne objects. These cyclic wind pressure loading tests are designed to replicate the variable wind speeds and directions typical of tropical storms to see how well the component holds up after the initial impact. Ratings are reported for both impact resistance and the amount of positive and negative wind-induced pressure that the impacted component can withstand. Whenever an impact resistant component, such as a window, door, garage door or shuttering system, is selected for hurricane protection, it is extremely important that the component has been tested to a Code-acknowledged standard by an independent agency such as ASTM International or the Miami-Dade County Building Code Compliance Office.
To continue with Module 1, please click Section 2.
Racking occurs when the combination of positive and negative (or “suction”) pressures is sufficient to overcome the diagonal bracing in the wall assembly and the walls collapse. As the photo shows, a common failure point due to Racking is at the openings for garage doors. At these points, the width of the opening replaces a major portion of the wall and the remaining studs and sheathing are not strong enough to resist tropical storm wind forces.