Preserving the Building
The severe and complex nature of hurricane-force winds presents special problems for protecting a building. Under normal circumstances, a building is designed to allow wind to flow over the structure. During a hurricane, a fully intact building envelope, or “outer skin,” allows wind to flow at, over and past the structure without damage. Preserving the envelope of the building is one the most important steps in preserving the integrity of the structure and ensuring non-destructive wind flow during hurricane-force winds.
During a hurricane, intense positive pressure is created on a structure as the wind impacts the building (windward face). As the wind flows over or around a structure it can cause “lift” on the roof or “suction” on the opposite side (leeward face). This “suction” is normally referred to as negative pressure. Negative pressure is always higher than the positive pressure acting upon a building during hurricane conditions. If the building envelope is breached, wind also enters the building, thereby causing a dramatic increase in internal pressure. This internal pressure, summed with the external pressure, can effectively double the force acting to lift the roof and push the walls outward (see Figure 2). One of the most susceptible components of the building envelope is a glazed opening. Glazing that is designed to accept the impact from wind-borne debris without evacuating the opening and causing a breach in the building envelope is desirable.
Wind and Debris
Sustained hurricane-force winds can last for several hours; gusting, turbulent winds repeatedly buffet a building; and, as a hurricane passes, winds slowly change direction, altering the direction of pressure being exerted on the building. During the sustained and high-speed winds of a hurricane, debris can be lifted from its at-rest position and become an airborne missile or wind-borne debris. Preservation of glazed openings and therefore the integrity of the building envelope is made difficult by the presence of wind-borne debris. In a hurricane, windborne debris can become a damaging missile. Large missiles, such as roof shingles and other construction materials, are common at heights up to 9 m (30 ft.) above grade; smaller missiles, such as roof gravel, are most prevalent at heights above 9 m (30 ft.). Following the impact from one of these missiles, building materials need to maintain the building envelope as they continue to be subjected to sustained and gusting winds until the hurricane passes.
Requirements for Protection
Hurricanes will occur; wind-borne debris will be present; and the result is an uncontrollable natural event. To prevent a hurricane from turning into a disaster, people and buildings need to be protected. To protect a building during a hurricane, the building envelope must be maintained. To prevent the building envelope from being compromised, windows and doors, typically the weakest link in a building’s envelope, must be safeguarded from penetration by wind-borne debris, and they must also remain in place throughout the remainder of the storm. Laminated glass with Saflex interlayers can be designed to handle the damaging impact from wind- borne debris and remain in the opening during cyclical wind loading. In the past, temporary or permanently attached shutters were used to protect residential and commercial buildings in hurricane-prone areas. Not all shuttered systems are capable of enduring impact and continuing to provide protection to the opening. Laminated glass with Saflex, in properly designed windows and doors, eliminates the need for shuttered systems because the glass will stay intact even if cracked.