This past weekend the Southeast region of the United States experienced an outbreak of reportedly more than 40 tornadoes, including a deadly EF-3 event in Hattiesburg, Mississippi. In addition to widespread wind damage, flood-inducing precipitation tore through the area, notably in Georgia, Mississippi, Alabama, Louisiana, and Texas (Figure 1), resulting in water infiltration damage to contents. Social media sites are sharing images of damaged property, including stripped roofs, smashed vehicles, rolled manufactured homes, and uprooted trees.
Due to the unusual nature of this storm, NOAA’s Storm Prediction Center (SPC) issued a "high risk" severe weather outlook for it on Sunday morning—the first since June 2, 2014. Most casualties and damage occurred in Georgia (where a state of emergency was declared in Atkinson, Berrien, Brooks, Colquitt, Cook, Lowndes, and Thomas counties) and Mississippi.
A Different Kind of Wind
The quality and types of construction in an area, local building codes and the effectiveness of their enforcement, and any mitigation features implemented play a vital role in alleviating or exacerbating the damage to buildings from tornado events. Even the Hattiesburg campus of William Carey University—which is supposed to be better engineered than most wood-frame residential houses—sustained major damage from the EF-3 tornado on January 21.
A recent study showed that houses deigned for hurricane wind as per the most up-to-date loading standard, ASCE 71, can sustain peripheral wind gusts from EF-1 to EF-3 tornadoes. However, the unique damage mechanisms at the center of tornadoes are quite different from those of hurricanes; localized swirl and rotation results in uplift wind pressures that can exceed the design capacity of most residential houses and manufactured homes.
The Insurance Institute for Business & Home Safety (IBHS) report Rating the States: 2015 evaluates regulations and processes governing residential dwelling construction in the states most vulnerable to catastrophic hurricanes. It notes improvements since the 2012 report in the states significantly impacted by last weekend’s tornado outbreak, although Texas, Mississippi, and Alabama remain at the lower end of the scale—ranking 15th, 16th, and 17th respectively out of the 18 states studied.
Damage reports from affected areas attest to the poor quality of construction practices with respect to tornado-like winds. Simple mitigation moves, such as securing attached structures properly to the host, tying roof decks adequately to wall framing, and appropriately anchoring walls and supporting columns to foundations, can do much to reduce the impact of severe storms.
The AIR Severe Thunderstorm Model for the United States incorporates an exhaustive list of building attributes that can significantly reduce losses from this kind of event. A cost-benefit analysis can be readily performed through the model to enable companies to outline guidelines for mitigation features that can be used to reduce structures’ vulnerability to this hazard when repairing tornado damage, retrofitting existing structures, or constructing new buildings.
The risk of damage from severe thunderstorms is real, even in hurricane-prone regions with adequate building codes designed to cope with that peril. This weekend’s events are a grim reminder that even in areas where the threat from severe thunderstorms is small, a direct hit by tornadoes can result in substantial losses.
1 The 2016 edition of the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures