The mile-wide EF5 tornado that struck Moore, Oklahoma was part of a succession of 22 tornadoes that spun across six states on Monday. This came just one day after a northeast-moving storm from Texas to Minnesota produced about 300 appearances of hail and high winds.

Though it's too early to draw accurate or justifiable comparisons, current scenes from Moore look eerily familiar to the devastation found in Joplin almost two years ago, when an EF5 tornado ripped through the Missouri town. Far less deadly than the Joplin twister—which claimed 161 lives while injuring hundreds more—Monday's tornado in Moore, Okla. similarly carved a path through a heavily populated swath of the city.

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So what kind of structural damage can insurers expect? Moreover, could they be similar in magnitude to the Joplin disaster, which generated $2.8 billion in claims? We consulted two engineers at EFI Global, Inc. to find out—Shawn Johnson, PE, a senior civil/structural engineer, and David P. Amori, PE, RRC, a senior structural and geotechnical engineer and senior district manager. Here's what they had to say in the immediate aftermath of the tragic events this week.

Explain the general types of damage we can expect to see from the tornado that collided with Moore, Oklahoma on Monday, especially given its width and force.

Johnson: Home or property owners often think their house or building may have been hit by a tornado. In reality, there is typically no doubt when your house or building is hit by a tornado because of the devastating damage. Sometimes, a tornado behaves in a near-sentient way, taking out a long row of buildings and/or homes, lifting off the ground for a moment, effectively skipping over one or more structures, then touching back down to continue its destruction. In the process, the home that was “skipped” suffered minor damage to its roof, while its neighbors' roofs were completely blown away.

Some reports indicate the Moore twister was 1.3 miles wide at its peak and an EF4 or EF5. What are some typical damages—in terms of scale and area affected—insurers could see from such a powerful storm?

Johnson: The general types of damage resulting from a powerful tornado can range from denuded trees and minor displacement of shingles to total destruction of every building in the vicinity. Based on initial reports of the Moore, Okla. tornado being an EF4 or higher, homes and buildings at or near the path of the tornado's eye will likely have suffered total loss. Properties farther out from the eye will naturally have suffered gradually diminishing damage with the distance away from the eye.

Specifically, in or near the path of the eye of an EF4 tornado, one would expect to see devastating damage, including well-constructed houses destroyed, some structures lifted from foundations and blown away, as well as cars and other large debris blown away. Just to the right or left (relative to its direction of movement) of the path of the eye and its eye wall, damages would likely be more similar to an extreme wind event, including but not limited to shingle damage, breaches in the building envelope, impact damage from flying debris, electrical and mechanical failures, and above-ground plumbing failures.

Have building codes evolved since an EF5 tornado struck the same area in 1999? What building codes are in place to mitigate loss from tornadoes?

Amori: There have been significant changes to the Building Code Requirements that are adopted by most U.S. municipalities with respect to how wind forces are calculated and applied to the design of new buildings. This affects new construction but seldom (depending on the local municipality) is the change retroactive; so it does not affect existing buildings.

There is more bad news: Even though in general the building code requirements have been ramped up in recent years, the codes are based on statistical probabilities of peak wind gusts, not actual tornados. For example, the latest code provisions (ASCE 7-10) would require designing for 120 mile-per-hour (mph) wind for structures in central Oklahoma where more than 300 people congregate. Tornados can produce winds as fast as 200 mph (EF4), or even higher. Because forces acting on a structure are exponentially related to the wind velocity, this 80-mph difference would require almost three times more lateral load resistance.

The simple fact is that almost all buildings are not designed to withstand tornados. Tornado resistant construction would be an owner driven requirement and typically not that of a municipality or national building code. From a life-safety standpoint communities in tornado prone areas often have selected buildings designed to withstand tornado related forces that serve as community tornado shelters. This allows for an economically feasible solution for this type of construction.

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After the Storm: Analyzing Tornado Structural Damage

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