How Hurricanes, Tornadoes and Windstorms Cause Buildings to Fail
The “envelope” of a building consists of the walls, roof, doors and windows. Damage from high winds increases rapidly as the envelope of a building begins to break down. From the first failure, whether it’s a roof panel, a broken window or a door, the internal pressure within the building increases. This force, in combination with other forces can cause the roof to lift from the building. Wind and rain can then enter the building and, if the winds are strong enough, cause structural failure of the frame of the building.
In recent years work has been done with computer models to predict the failure of various types of buildings in high winds. The structure of a building envelope is composed of various components such as walls, studs, roofing, doors, etc. The types and strength of both the components and the connections between these components is critical to predicting the performance of the building envelope.
In the computer model, each connection is assigned a strength depending on the construction of the connection. “Toe-nailed” strength and “hurricane clip” strength, for example would apply to a rafter-to-wall connection. In addition, the strengths of individual components, such as windows, skylights, vents, etc. are also defined.
This data is then entered in the computer model. The end result of all this effort is to run thousands of simulations under a variety of wind conditions, for every type of building. Results show how the failure of one connection or component affects other connections and components. A series of such failures can cause the envelope to fail and, ultimately cause failure of the structure itself.
Much envelope damage is cause by wind born debris. Smaller debris travels at greater heights and greater speeds than larger objects such as lumber, which travel at slower speeds near the ground. Some surface components such as roofing tiles, glass or vinyl siding can withstand high winds alone but not with the added impact of debris.
Researchers now understand the full importance of component strength, both preimpact and postimpact. The preimpact strength of components had been the only consideration previously. Many envelope cladding materials, like glass even tempered glass are severely damaged by small wind born debris.
Wind born debris can also have a devastating effect on external insulation and finish systems. Hard cladding materials such as vinyl siding or asbestos cement may shatter from impacts. Soft external insulation may be pulled from the building envelope. Building officials have recognized the need for improved impact resistance in the newer building codes created after Hurricane Andrew. Testing standards for research and wind resistant products have also changed to reflect the reality of an impact followed by cycles of higher and lower pressures.
Research into windstorm damage, especially damage as a result of wind born debris, has led to a number of important changes in how buildings are designed. First, while the structural frame of the building is important, the envelope of the building is equally important. Second is that the design process must address the issue of wind born debris. Third is that both preimpact performance and postimpact performance must be considered equally. Taken together, these design considerations will help building envelopes survive high wind damage and wind born debris damage.