09/02/2011
APA Issues Construction Recommendations for High Wind Resistance
Two new publications include recommendations following observations of damage caused by 2011 tornados.
Damage observations conducted by APA after the April 16, 2011 tornados in Eastern North Carolina and the powerful EF-4 and EF-5 storms that struck Tuscaloosa, Alabama on April 27 found that a lack of attention to detail along the uplift load-path often leads to weakness on the route through which high-wind forces must travel within the framing and into the foundation. Following a review of the findings, the Association has published a set of construction recommendations for improving tornado or hurricane resistance in light-frame wood construction. The damage assessment report, Tornados of the South: Structural Performance of Newly Constructed Homes in North Carolina, Alabama, and Georgia, Form SP-1154 and the design recommendations, Building for High Wind Resistance in Light-Frame Wood Construction, Form M310, are both available for free download from APA.
The investigation by APA Engineered Wood Specialist Bryan Readling, P.E., focused on the performance of homes constructed within the last 10 years. The most common roof-to-wall framing failures were attributed to the use of toe-nailed connections, still prescriptively allowed in many non-hurricane areas. In addition, many homes observed were poorly anchored to the foundations. This was especially true in Alabama where nails were used, instead of anchor bolts, to attach the bottom plate of walls to the concrete or masonry foundation.
Another common theme observed along the tornado paths is that homes constructed with non-structural exterior wall sheathing failed at wind speed much lower than called for in the building codes. Walls that are fully sheathed with OSB or plywood and constructed with proper connections have stronger resistance to the damaging forces of high winds.
Summary of Field Observations
In a field survey of damaged structures, Readling found that structural failure along the critical load-path was often located at the roof-to-wall connection. Most of these connections were made using toe-nails through the roof framing and into the top plate of the exterior walls. Toe-nail connections are weak because they rely upon the withdrawal capacity of nails, which is limited. Light-gauge metal connectors provide good performance in wood framing because load is applied perpendicular to the nail shank, instead of pulling the nail straight out. Toe-nail rafter connections are still prescriptively allowed in most non-hurricane areas by modern building codes. It is generally recognized that these connections do not provide the capacity to resist the range of pressure requirements of the International Building Code.
Metal roof-to-wall connectors should be installed in-line with the load-path. Failure was observed when metal roof framing connectors were installed on the inside face of the wall top plate instead of on the exterior of the wall and in-line with the load-path in the structural wall sheathing.
A common observation, especially in hardest hit areas, was the loss of the exterior walls due to poor attachment to the foundation. In many cases hand-driven cut masonry nails were used to attach the bottom of support walls to the concrete or masonry foundations, but in a few locations the nails were observed to be pneumatically driven framing nails. Modern building codes generally require anchor-bolts to be embedded into concrete or grouted concrete masonry units.
Breaches or openings in the building envelope, and the resulting pressurization of the building interior caused catastrophic failure of homes in many examples. Openings in walls due to failure of doors, windows, and non-structural cladding systems were common. These often resulted in heavy damage to home interiors due to rainwater infiltration and flying debris penetration. Larger breaches from loss of weak garage doors and exterior cladding systems often acted to initiate catastrophic failure and exacerbated deficiencies along the aforementioned load-path.
In homes with gable roofs, failures were most notable at the bottom of the gable-end roof and ceiling framing where they are connected to the top of the gable-end wall below. This joint is often not well connected laterally to the rest of the building and is weak to resist negative pressure on the exterior gable-end surfaces.
The gable-end is also a vulnerable location for non-structural cladding systems because the walls within the roof cavity are not backed by drywall like the exterior walls within the living space. Material failure was commonly observed when non-structural foam wall sheathing was used in conjunction with low-strength siding. Besides being vulnerable to wind pressure, these products provide little resistance to the flying debris.
In most observations where buildings were at least partially intact, wood wall and roof sheathing loss could be attributed to improper attachment. When nails were used as prescribed in the building codes good performance was observed. Staples performed poorly since they generally offer less resistance to pull-out than nails and must be used in greater quantity. Greater structural failure often resulted from poor roof sheathing attachment to the last rafter or gable-end truss.
Building for Greater Tornado or Hurricane Resistance
Readling noted that in many cases engineers can point to one of several common weak links as the cause of catastrophic failure in homes that were on the periphery of these powerful tornados. The following APA recommendations address these weak links and provide tips for building a wind-resistant shell:
- Nail wall sheathing with 8d common (0.131 in. x 2-1/2 in.) nails at 4 inches on center at end and edges of wood structural panels as roof sheathing and 6 inches on center in the intermediate framing. This enhanced nailing will improve the resistance of the wall sheathing panels to negative wind pressure. The use of deformed shank nails will even further improve the performance of the sheathing nailing at a minimal cost. Staples offer less resistance to blow-off than nails and so a greater number of them are required to achieve the same level of resistance.
- Sheath gable end walls with wood structural panels, such as plywood or oriented strand board (OSB). In the 2011 tornados, gable end wall failures were frequently observed when non-structural sheathing was used.
- Tie gable end walls back to the structure. One of the weakest links in residential structures during high wind events is the connection between the gable-end and the wall below.
- For the roof framing to wall connection, use a hurricane/seismic framing anchor or equivalent connector, attached on the exterior (sheathing side) of the exterior walls. The roof-to-wall connection under high wind loads is subject to both uplift and shear due to positive or negative wind pressure on the walls below.
- Nail roof sheathing with 8d ring shank (or deformed shank) (0.131 in. x 2-1/2 in.) nails at 4 inches on center along the edges of the wood structural wall sheathing and 6 inches on center along the intermediate framing.
- Nail upper story sheathing and lower story sheathing into common wood structural panel Rim Board®. The most effective way to provide lateral and uplift load continuity is to attach adjacent wall sheathing panels to one another over common framing.
- Continuously sheath all walls with wood structural panels including areas around openings for windows and doors.
- Extend wood structural panel sheathing to lap the sill plate. The connection of the wall sheathing panel to the sill plate is important because this is where uplift forces are transferred into the sill plate and into the foundation through the anchor bolts.
- Space ½ in. anchor bolts 32 inches to 48 inches on center with 0.229 in. x 3 in. x 3 in. square plate washers with slotted holes.
These design recommendations are illustrated in the guide, Building for High Wind Resistance in Light-Frame Wood Construction, Form M310, available for free download from the APA Publications Library. The damage assessment report, Tornados of the South: Structural Performance of Newly Constructed Homes in North Carolina, Alabama, and Georgia, Form SP-1154, is also available for download.
