Sheath Your House Boston MA

Common in this subdivision were large roof sections that had blown off of homes. Most of these were observed to contain only a few toenails into the support walls.

This detail of “house A” shows that the bottom plate of the garage sidewall is missing. Anchor bolts were typically located 8 feet on center at this location. The 16" return walls were typically attached to the foundation with either one or two anchor bolts. Anchor bolts contained minimally sized round washers.

The return wall of this garage was attached with only one anchor bolt. Connection of the sidewalls and return walls to the foundation was inadequate. Better anchorage of the bottom plate to the foundation and better attachment of the wall sheathing to the bottom plate was needed.

Observations

Light-gauge metal connection hardware at the rafter to wall intersection was observed to be missing on the severely damaged homes in one subdivision. Numerous roof rafters and sections of roof were observed as debris scattered within the damage site. Toenails, observed as the only attachment between roof framing and the tops of walls, offered little resistance to uplift forces from negative pressures on roof surfaces. Additional uplift loads were imposed on these connections when breaches in the building envelope occurred.

Garage doors tended to be constructed of lightweight metal and were no match for the pressures and flying debris accompanying the tornado. Breached garage door openings allowed wind pressures to act on the internal surface of structures, instantly combining with external pressures. This sudden combination of pressures resulted in rapid failures of the garage walls and roof. Failures resulting from breaches in the garage often appeared to include the walls and roof enclosing adjacent living spaces. In addition to the vulnerability of garages to breaching, the garage is generally weaker due to fewer structural redundancies that normally accompany conventional wood-frame construction. Narrow return walls (parallel to, and on either side of the garage door opening) were often incapable of resisting the lateral forces exerted during the high wind event. Numerous homes were observed where such narrow return walls had failed. Garage-wall-to-foundation connections (anchor bolts to bottom plates, bottom plates to studs) were often inadequate. In several cases the bottom plates of garage walls were found still attached to the concrete foundation and the studs were missing. Only the nails through the bottom plate and into the bottom of the studs remained. In this scenario, properly lapped and fastened structural panel wall sheathing can be an economical option for transferring forces around the stud-to-plate joint and then into the foundation. In other heavily damaged garages, the bottom plate was no longer attached to the foundation. In these cases, anchor bolts either split or pulled through the bottom plate. In general, minimally sized round washers were used.

The type of wall sheathing largely influenced the ability of walls to sustain impact from windborne debris. Most of the homes in one hard hit subdivision contained non-structural sheathing such as foam or fiberboard and vinyl siding. The few homes observed with plywood or OSB structural wall sheathing sustained far fewer breaches.

Wind-resistant designs should be considered for buildings constructed in tornado prone areas. Too often the accepted convention is that tornado winds are too strong for conventional construction. This is far from reality. About 90 percent of the roughly 1000 tornadoes reported each year are classified as F-0 through F-2 on the Fujita Scale. The wind forces associated with these less powerful tornadoes are similar to those designed for along much of the U.S. hurricane coastline. By APA's observations in Battlefield, Missouri, severe structural damage to homes could usually be linked to some sort of structural inadequacy.

There is cause to construct homes for high-wind resistance. It may be reasoned that a home constructed in compliance with hurricane coastline wind loads could withstand full exposure to tornados classified up to F-2. Since tornado forces decrease dramatically within a short distance of the funnel, a well constructed home would also be less vulnerable to winds on the outer regions of an F-3, F-4 or F-5 tornado.

Minimum wind loads prescribed in the model building codes should be recognized for what they are: minimums. In cases where enhanced safety and security are desired, such improvements can be made with relatively small additional costs and with some additional attention to details on the part of the designer and builder. The value of such structural enhancements could be realized in lower insurance cost and improved occupant safety.