Wind Damage >> Hurricane Roof Damage

Experts describe the following progression during gable-end collapse: typically, the gable-end popped out due to suction on the leeward side of the building and the loss of sheathing, or Hurricane Roof Damage to a combination of suction and increased pressure resulting from breached openings in the shell. 

When the gable-end was on the windward side of the building, collapse was caused by the withdrawal of the fasteners connecting the sheathing to the gable end top chord. This caused the gable-end overhang to peel up, Hurricane Roof Damage causing a cascading loss of additional sheathing downwind. This led to more sheathing loss and the eventual toppling of the adjoining trusses. 

Diagonal cross-bracing of end trusses was rarely present in roofs that failed in this manner. Keith (1994) observed that gable-end trusses were often only attached to the top plate of the end walls by infrequent toenailing, Hurricane Roof Damage only four to six feet on center, and inadequate to transfer shear forces from the gables to the walls. 

Sanders (1994) concurs that gable-ends were Hurricane Roof Damage especially problematic. Sanders observed that one of two failure modes accounted for almost all gable-end failures: 

Either the connection of the top chord to the roof diaphragm was not able to resist the combination of horizontal reaction from the truss combined with the uplift on the sheathing at the roof edge (i.e., nailing patterns used on roof Hurricane Roof Damage sheathing were not designed for both shear and uplift acting simultaneously), or the bottom chord was not supported adequately to resist lateral loads. 

Manning and Nichols (1991) examined damage from Hurricane Hugo and Hurricane Roof Damage concluded that roofs had been tied to walls with hurricane clips that were inadequately sized to support the design wind load. 

Hoover (1993) examined gable-end collapses from Hurricane Andrew and concluded that, in every case, the collapse was due to lack of proper connections, either between the gable-end and the roof, or Hurricane Roof Damage the gable-end and the end-wall. Hoover noted the following problems: 

Nail Spacing did not meet the code minimum of 6 inches o.c. [on center] in the roof panel edges, and Hurricane Roof Damage 12 inches o.c. in the interior of the panels. 2. Staples were not installed at the correct spacing and orientation. Staples must be spaced closer than nails, and installed parallel to the truss rafter chord.  

Fastener spacing over the gable probably had been incorrectly considered as interior spacing rather than edge spacing. 4. In general, Hurricane Roof Damage there seemed to be a reliance on the code minimum nail spacing as opposed to the specific connections being designed. 

It was the opinion of the FEMA assessment team that reliance on sheathing for truss-roof bracing, coupled with the corresponding loss of sheathing, Hurricane Roof Damage was a major cause of the total damage of the building systems. 

Cook (1994) regards this as the most costly aspect of the damage and notes that loss of sheathing was usually the result of inadequate nailing; either nails were spaced too far apart according to building code, or Hurricane Roof Damage nails missed the underlying rafter altogether. 

Damage to Walls Walls are not as vulnerable to hurricane damage as roofs, windows and door openings, but failures did occur during Hugo, Andrew, and Iniki. Damage from flying debris was not a significant factor, Hurricane Roof Damage although there were cases where debris penetrated walls. Wall failures were caused mainly by to poor connections (Sanders, 1994). 

The most common residential building methods are concrete block and stucco (CBS) and wood-frame. CBS construction is popular in Florida, Hurricane Roof Damage while wood-frame is common in Hawaii. In any region, elevated homes in flood or erosion zones are typically wood-frame. 

In both Florida and Hawaii, wall damage was not as common as damage to roofs as a significant cause of building failure, Hurricane Roof Damage but it did occur. Sanders (1994) notes that in South Florida reinforced hollow concrete block masonry is by far the most common building material used in wall construction in both residential and commercial structures. 

The South Florida Building Code prescribes a method of reinforcing block walls called in Florida "tie beam-tie column." In this building method, Hurricane Roof Damage unreinforced block walls are first erected. Reinforcing bars are then inserted through the blocks at intervals of no more that 20 feet, and the reinforced column of blocks then filled with mortar. 

The top ends of the reinforcing bars are then attached to a cast-in-place tie beam at the top of the wall. When properly constructed, the result is a hollow block wall laced with a strengthening network of steel bars and Hurricane Roof Damage poured concrete columns and cross-members. Improperly reinforced masonry walls failed because of a combination of uplift and pressure forces. 

These forces combined to lift up the edge of the roof and bond beam, Hurricane Roof Damage separating the bond beam from the rest of the roof system. Curry (1991) believes that good inspection practices could have prevented this type of failure. Failures in this type of wall construction were observed when the reinforcing bars were omitted at wall intersections or corners. 

These intersections provided continuity of tie beam reinforcing. When this deficiency existed in combination with the failure of the tie beam to roof connection, the wall collapsed. In general, Hurricane Roof Damage when the tie beam to roof connection failed, or was not present, the tie beam was then subjected to lateral stresses for which it was not designed (Sanders, 1994). 

Many total failures of CBS houses were the result of lack of tie down for the tie beam. Once uplift forces on the roof overcame the mass of the roof and tie beam, Hurricane Roof Damage there was only the tension strength of the mortar to prevent total building collapse (Reardon and Meecham, 1994). 

Curry (1991), and Cook (1991) observed that most wall and roof system failures from Hugo began at the roof corners and eaves of buildings Hurricane Roof Damage regardless of wall type. The FEMA team observed that masonry-wall buildings tended to fair better than wood-frame homes. Where failures did occur, the primary reason was lack of vertical wall reinforcing. 

The lower rate of masonry wall failure was attributed to the heavier mass of the masonry wall, and Hurricane Roof Damage the tendency of a continuously constructed system to be less prone to failure from lack of attention to design and construction details. 

Where failures did occur, the team observed the following conditions: poor mortar joints between walls and slab; lack of tie beams, Hurricane Roof Damage horizontal reinforcing, tie columns, and tie anchors; and misplaced or missing hurricane straps between walls and roof. 

Khan and Suaris (1994) observed some cases of failure where block walls collapsed completely because of inadequate anchorage to resist uplift and lateral forces. In these cases, the deficiencies (and code violations) were common and included lack of tie downs, tie downs in unfilled cells, Hurricane Roof Damage missing hooks from tie downs to the tie beams and foundation, and lack of corner bars. 

Woodframe walls suffered few component failures, except damage from missile impact. When failures did occur, Hurricane Roof Damage connectors were usually the cause. Although nearly all wood structures had hurricane straps to transfer tensile forces across framing joints, shear connections were nonexistent or inadequate. 

This was especially problematic with multistory buildings (Sanders, 1994). In wood-frame construction, the SFBC requires that either board or Hurricane Roof Damage plywood storm sheathing cover all exterior walls. Kahn and Suaris (1994) and Cook (1994) report that sometimes, however, the only sheathing was hardboard siding applied directly over the studs of some homes, leading to their collapse. 

In other cases, products such as Masonite and Thermax were used. Although approved by the SFBC, Masonite had a raking shear strength of only 120-125 pounds per square foot, Hurricane Roof Damage compared to 430 pounds per square foot for 15/32 inch plywood sheathing with a wider stud spacing. 

The actual raking shear strength was often even less, Hurricane Roof Damage since contractors did not usually follow the recommended nailing and stud spacing requirements. Kahn and Suaris observed other cases where the SFBC had been violated: 

Corner studs constructed with less than three studs or improperly constructed, 2. no overlapping of plates at intersections, 3. inadequately nailed connections, 4. improper splicing of members 5. improper notching of members, 6. missing hurricane straps in stud-plate connections, Hurricane Roof Damage and 7. missing sill plate anchors.

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