Post A Reply
my profile
login
|
register
|
search
|
faq
|
forum home
»
Allstocks.com's Bulletin Board
»
NASDAQ, AMEX, NYSE Stocks
»
ZENX MORE CONTRACTS
» Post A Reply
Post A Reply
Login Name:
Password:
Message Icon:
Message:
HTML is not enabled.
UBB Code™ is enabled.
[QUOTE]Originally posted by explorer186: [QB] Roof Damage Roof covering failure was the most widespread type of damage observed after Hugo, 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 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, 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 (Doehring et al., 1994). 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 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, 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 conventional shingles are common. Examination of conventional composition shingle roofs showed evidence of substandard workmanship, 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, 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 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, 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, but they seemed to have better adhesion to mortar than the extruded concrete tiles (FEMA, 1992). 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 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, damage occurred primarily at roof corners (Chiu et al., 1994). Figure 11 summarizes roof damage greater than one-third from Andrew and Iniki. Figure 11 Percent of damaged homes surveyed with damage to roofs greater than one-third (From HUD, 1993) Andrew Iniki Cladding 59 63 Sheathing 54 33 Rafters/Trusses 21 18 Soffit/Facia 27 22 Roof-Wall 12 5 Connection Gable End 30 5 Building failure during Andrew was primarily a result of negative pressure and/or 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, 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, 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, when properly anchored, trusses transmitted wind loads to the rest of the structure satisfactorily (Riba et al., 1994). 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, 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, 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 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 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, 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), 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 adjacent trusses collapsed in domino fashion. Riba et al. (1994) 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 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, 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, 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 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 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 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 the gable-end and the end-wall. Hoover noted the following problems: 1. Nail Spacing did not meet the code minimum of 6 inches o.c. [on center] in the roof panel edges, and 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. 3. Fastener spacing over the gable probably had been incorrectly considered as interior spacing rather than edge spacing. 4. In general, 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, 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 nails missed the underlying rafter altogether. [From http://www.colorado.edu/hazards/wp/wp94/wp94.html#roofdamage] [/QB][/QUOTE]
Instant Graemlins
Instant UBB Code™
What is UBB Code™?
Options
Disable Graemlins in this post.
*** Click here to review this topic. ***
Contact Us
|
Allstocks.com Message Board Home
© 1997 - 2021 Allstocks.com. All rights reserved.
Powered by
Infopop Corporation
UBB.classic™ 6.7.2