Wind Damage >> How And Where Hurricanes Are Formed

Florida, South Carolina, and Hawaii together offer an illustrative example of residential construction styles and techniques common in hurricane hazard areas. 

In Florida, the most popular type of home building is slab-on-grade foundation with concrete block walls and wood-truss roofs, though wood framing is turning out to be more popular as a substitute to concrete block wall building. In Hawaii and South Carolina, How And Where Hurricanes Are Formed wood is still the most popular building material. 

Wood-truss roofs are common in all the sample areas. Roof covering failure was the most widespread type of damage observed after Hugo, How And Where Hurricanes Are Formed according to Manning and Nichols (1991). Roof coverings which were not adequately attached, and corner and eaves regions of roofs were frequently damaged. 

Smith and McDonald (1991) note that in the Charleston area probably more than 75% of all roofs had at least minimal damage. Once roofs were How And Where Hurricanes Are Formed breached, house interiors were exposed to further damage from water. Roof failures were also the most frequently observed structural failures from Andrew. 

Cook (1991) estimates that over 80% of losses were related to roof failures and associated water damages. In Dade County, Florida, How And Where Hurricanes Are Formed the most common building failure observed was loss of roof cladding (shingles, tiles, etc.). Ninety percent of all homes in Dade County had some degree of roof damage. 

Roof failures occurred because of lack of proper connection between the roof and the exterior walls (Cook, 1991). Often, rafters were attached by toenails to the top plate, and How And Where Hurricanes Are Formed in other cases hurricane clips attached the rafter to only the top plate (rather than to the wall studs). 

With the roof gone, walls lost the support provided by the roof system and were subject to collapse even when exposed to lesser winds (Manning and Nichols, 1991). Miehe (1991) observed that nearly all wall failures were a result of other failed components, How And Where Hurricanes Are Formed such as roofs and doors or windows. 

In Florida, roofs are constructed using plywood sheathing over wood roof trusses and are covered with tar paper and either extruded concrete tile or asphalt composition shingles. Both roof tile and How And Where Hurricanes Are Formed conventional shingles are common. 

Examination of conventional composition shingle roofs showed evidence of substandard workmanship, How And Where Hurricanes Are Formed such as insufficient staples or incorrectly located or oriented staples (FEMA, 1992). Smith and McDonald (1991) also observed misaligned fasteners while examining roof damage from Hurricane Hugo. 

Further, Reardon and Meecham (1994) noted that the use of staples also provided an inadequate connection in attaching sheathing. In addition, How And Where Hurricanes Are Formed it appeared that many shingles and attachment adhesives used were not adequate for the wind speeds that occurred. 

The most common failure mode was lifting of the tabs due to failure of the self-seal adhesive, and How And Where Hurricanes Are Formed subsequent tearing of the shingles at the fasteners (Smith, 1994). Smith went on to note that nearly all the shingles examined were attached with only four fasteners, the minimum required by the 1988 SFBC, although most manufacturers recommended six fasteners in high wind areas. 

Examination of damage from Hurricane Hugo showed that mis-located fasteners were also a common cause of cladding failure (Smith and McDonald, 1991). Tile roofs, composed of either extruded concrete or clay, How And Where Hurricanes Are Formed showed failures in both nailing and mortar connections. The most common failure was the lack of a bond between the mortar and the tile. 

Many mortar pads appeared to have been applied nonuniformly. Clay tiles seemed more susceptible to damage from flying debris than concrete tiles, How And Where Hurricanes Are Formed but they seemed to have better adhesion to mortar than the extruded concrete tiles. During Iniki, over 90% of all one- and two-family dwellings lost substantial portions of their roof covering (Sheffield, 1993). 

On Kauai, where corrugated roofs were common, large portions of the metal sheathing were removed from most roofs due to inadequate fastenings. Failure of roofing material not only exposed the buildings to How And Where Hurricanes Are Formed water penetration, but also provided a major source of wind-borne debris. 

Water penetration was a major problem whenever roofing material was removed by wind action. For steep roof systems, many roofing failures occurred at the ridge or gable ends where wind-induced forces were the highest. For low-slope roof systems, How And Where Hurricanes Are Formed damage occurred primarily at roof corners (Chiu et al., 1994). 

Figure 11 summarizes roof damage greater than one-third from Andrew and Iniki. Building failure during Andrew was primarily a result of negative pressure and/or How And Where Hurricanes Are Formed induced internal pressure overloading the building envelope. The wood-frame gable ends of roofs were especially failure-prone. 

In addition, many houses had been built with the plywood roof sheathing acting as the sole stiffener of the roof diaphragm and lateral support for the trusses. Once sheathing was blown away from the roof, How And Where Hurricanes Are Formed nothing prevented the roof trusses from collapsing. 

Failure to properly attach the roof sheathing to the top chord of the roof truss and omission of gable end and roof truss bracing left roofs highly susceptible to loss of structural integrity (Oliver and Hanson, 1994). Because the roof sheathing provided the only stiffening of the roof diaphragm, How And Where Hurricanes Are Formed the attachment to the sheathing became critical to the successful performance of the building envelope. 

No truss failures were cited as a primary cause of general roof or building failure, and no trusses failed because of the loads imposed. In fact, How And Where Hurricanes Are Formed when properly anchored, trusses transmitted wind loads to the rest of the structure satisfactorily. HUD (1993) identified roof sheathing as a critical component that locks all other roof members together to form a structural system. 

Loss of roof sheathing led to instability and subsequent failure of the wood-frame gable ends and trusses. Oliver and Hanson (1994) did find instances where debris punctured roofs, How And Where Hurricanes Are Formed but this did not seem to be a significant or direct cause of roof failure. 

Where roof failure did not lead to total structural failure, roof failure allowed rain, often heavy, to penetrate to the interior of the home. This not only resulted in damage to furnishings, but also further weakened the structure when rain-soaked ceilings collapsed, How And Where Hurricanes Are Formed reducing reinforcement of the ceiling joists. 

One of the most damaging classes of failure in economic terms was the loss of gypsum wallboard ceilings (Keith, 1994). This form of damage affected most houses How And Where Hurricanes Are Formed in the path of Andrew to some degree. 

The rain accompanying and following the passage of Andrew was driven in through gable-end vents and roof turbines, through the joints between roof sheathing panels after roofing was blown off, and How And Where Hurricanes Are Formed directly into the attic space of failed roof systems. Rain quickly saturated the insulation and the ceiling. 

The loss of ceiling strength due to water saturation, and the increased weight of the wet insulation, How And Where Hurricanes Are Formed caused widespread collapse of ceilings. The loss of the ceiling also contributed to gable-end wall failures due to the diminished lateral support at the base of the gable-end walls. 

Keith (1994) observed that the most common type of structural damage from Hurricane Andrew in Florida, where over 80% of houses have gable roofs (Crandell et al., 1994), How And Where Hurricanes Are Formed was loss of gable-end walls. 

Keith (1994) further observed that loss of the gable-ends was usually accompanied by loss of between four and 12 feet of roof sheathing immediately next to the gable-end wall. Once the roof sheathing was blown off, the gable-end truss and How And Where Hurricanes Are Formed adjacent trusses collapsed in domino fashion.

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