Takeaways from 2015 Texas Tornadoes

Construction Details Make a Difference

On December 26, 2015, the first EF4 or stronger tornado ever recorded in December in Texas made landfall in Garland and Rowlett, suburban towns near Dallas. According to the National Weather Service, this tornado was part of a larger winter storm that included 12 confirmed tornadoes, mainly across North Texas, and resulted in at least nine fatalities. Three of the 12 tornadoes were rated by the National Weather Service as EF2 or greater, with the most intense damage occurring with the Garland-Rowlett tornado.

Damage observations were conducted in Garland and Rowlett after the tornado. The observation team from APA focused on the performance of recently constructed homes, because these homes tend to contain newer materials, larger interior spaces, and more open floor plans that may have an effect on structural resistance to wind forces.

It is difficult to learn much, if anything, from homes that are completely or mostly destroyed by EF4 level tornadoes. For this reason, damage assessment was mostly limited to those in the lower EF0 through EF2 tornado wind speed ratings. These wind speeds are typical in the periphery zones of a large tornado.

Weaknesses in common

In most cases, engineers can point to one of several common weak links as the cause of structural failure and damage in this event. Observations of the tornado damage found that structural failure was often due to a lack of adequate connections.

A continuous load path, the flow of forces through a structure and the connections from the roof coverings and siding (exterior cladding) to the framing and to the foundation, must be provided for reliable building performance. Most of the observed failures were a result of poor continuity along the structural load path, often due to improper connections.

There were several commonly observed reasons for failure: these included using toenails without metal connectors for the roof-to-wall connection, using pins without anchor bolts to attach walls to the foundation, poor structural performance of laminate-fiber wall sheathing products, and breaches due to failure of windows, garage doors, or cladding/wall systems that resulted in catastrophic failure.

Missing metal connectors

Failures were often located at the roof-to-wall intersection. Most of the roof-rafter-to-wall connections in this area were made using toenails through the roof framing into the top plate of the exterior walls. These toenail connections are weak because they rely upon the limited withdrawal capacity of nails. Commonly available light-gauge metal connectors provide superior performance; with these, the load is resisted in directions perpendicular to the nail shank rather than acting to pull the nail straight out. Use of these metal connectors was only observed in one case among the homes where loss of the roof structure occurred.

Breaches and catastrophic failure

Pressurized buildings often fail suddenly with devastating results. Breaches in the building envelope and the resulting pressurization of the building interior caused significant failure in many of the damaged homes. Openings in walls due to failure of windows, garage doors, and cladding systems were common. Large breaches from loss of weak garage doors and exterior cladding systems resulted in interior pressurization and exacerbated deficiencies within the aforementioned load path. Catastrophic garage failures were observed in a multitude of locations—even in areas with low to moderate wind speeds.

Poor attachment to foundation

Another common observation across a wide range of wind severity was the loss of exterior walls due to poor attachment to the foundation. In many of these cases, powder-actuated pins of varying lengths were used to attach the bottom of support walls to the concrete-slab foundations. Although equivalent systems are allowed to be substituted, modern building codes generally specify deformed steel anchor bolts to be embedded into reinforced concrete foundations for attachment of wood framing.

Structural performance of laminated-fiber sheathing

Most of the homes observed were sheathed on exterior walls with a laminated-fiber (i.e., paper) product that measured approximately 1/8 inch thick. Common problems in the strength of walls sheathed with laminated fiber included poor load path continuity of framing within wall systems, especially at wall corners, and poor load path continuity within wall systems stacked vertically between stories. These walls also performed poorly in resisting racking forces from lateral wind loads. [Independent laboratory shear wall performance tests of laminate-fiber sheathing showed the product underperformed manufacturer design value claims. For detailed test results, see APA’s Product Advisory SP-1172: Laboratory Tests Evaluate Design Values of Thermo-Ply® Red.]

These walls were either completely or mostly clad with brick veneer on the exterior. Relatively flexible walls sheathed with laminated fiber were found to be largely incompatible with brittle brick veneer. In many cases, brick veneer walls were observed to have been damaged due to excessive out-of-plane (transverse) or in-plane deformation, which was exacerbated by poor installation of brick ties. Falling brick from veneered walls, columns, and chimneys were observed in many cases in the impacted areas across a wide range of wind speeds. Falling brick represents a considerable threat to life safety.

In an area with widespread use of brick in single-family homes, cracked and collapsed brick were major contributors to residential property damage in this event.

Misconceptions about tornado strength

A common myth is that all tornadoes are so powerful that structural failure is unavoidable no matter how well a building is constructed. This unfortunate belief has resulted in construction of homes that lack adequate attention to important structural details.

In truth, homes can easily be built to survive a majority of tornadoes. Weaker tornadoes rated as EF0, EF1 and EF2 comprise 95 percent of all tornadoes. These smaller, less-violent tornadoes produce winds which a carefully constructed home can be expected to withstand. Stronger tornadoes with a maximum rating of EF3, EF4 and EF5 are statistically much rarer, with only 5 percent falling into this category. Although the maximum wind forces in stronger tornadoes are harder to resist, improvements in design can still help. This is especially the case when a building is located along the periphery of a strong tornado path. Stronger building components combined with more intentionally constructed connections can mean the difference between homes that withstand tornadoes and those that don’t.

Wind-Resistant Construction Recommendations

A house constructed with a wind-resistant shell that can protect the building and contents against catastrophic loss is the first line of defense against high wind events. After the April 2011 Tornado Super Outbreak, the largest, costliest, and one of the deadliest tornado outbreaks ever recorded, APA developed Building for High Wind Resistance in Light-Frame Wood Construction, Form M310, containing recommendations for wind-resistant building.

Since publication of these guidelines in 2011, they have been incorporated into local building codes such as the Georgia Disaster Resilient Building Code. Estimates by the Georgia Department of Community Affairs concluded that the additional cost represented by these enhancements is less than $600 for an average 2,100-square-foot, one-story, single-family home.

This type of construction can improve building performance and safety for occupants in areas susceptible to tornadoes. Builders who incorporate these details can improve marketability of their products, resulting in better peace-of-mind for their customers. When homeowners are made aware of these options, demand will increase for better safety provisions in single-family home construction.

Nine Techniques for Wind-Resistant Construction

The following APA recommendations, some of which exceed the minimum requirements of current residential codes, address the most common weak links and provide guidance in constructing a wind-resistant shell. Besides labor, the additional expense for materials is largely from additional nails, roof-to-wall metal connectors, anchor bolts and larger plate washers at anchor bolt locations. All of these are commonly available and are compatible with standard prescriptively constructed homes based on the International Residential Code. These recommendations include:

  1. Nail roof sheathing with 8d ring shank (or deformed shank) (0.131" x 2-1/2") nails at 4 inches on center along the ends of the sheathing and 6 inches on center along intermediate framing.
  2. 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.
  3. Sheath gable-end walls with wood structural panels, such as plywood or oriented strand board (OSB). In the 2011 tornadoes, gable-end wall failures were frequently observed when non-structural sheathing was used under vinyl siding.
  4. For the roof framing to wall connection, use an H1 or equivalent metal 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.
  5. 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 over common framing, such as rim board.
  6. Nail wall sheathing with 8d common (0.131" x 2-1/2") nails at 4 inches on center at end and edges of wood structural panels 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. Staples offer less resistance to blow-off than nails, so a greater number of them are required to achieve the same level of resistance.
  7. Continuously sheath all walls with wood structural panels including areas above and below openings, such as windows and doors.
  8. Ensure that wood structural panel sheathing overlaps and is properly fastened to the sill plate. When the first story floor is framed over a basement or crawlspace, extend wood structural panel sheathing over the rim board to lap the sill plate The connection of the wall sheathing panel to the sill plate is important, because this is where lateral forces are transferred from the wall into the sill plate and then into the foundation through the anchor bolts.
  9. Space 1/2" anchor bolts 32 inches to 48 inches on center with 0.229" x 3" x 3" square plate washers with slotted holes.

To read the entire report, which includes approximately 40 exemplars of damaged structures, download Damage Assessment Report: Texas Tornado, Form SP-1177.