Damage from Tornadoes and Other Wind Events

Thousands throughout the Midwestern and Southern US are reeling from the devastation caused by tornadoes in the past few weeks.  My heart and hope goes out to those affected by these storms.

In my line of work, I frequently assist insurance adjusters and building owners in determining the extent of storm-caused damage to a building.  This post highlights some of the characteristics of wind forces and how they interact with the built environment.

• Wind is not afraid of heights.  Wind forces are stronger at higher parts of the building; therefore, the most severe damage is typically found on the roof (if there is any roof left).

• Wind does not like to turn corners and throws a fit when it has to go around parts of a building.  In other words, wind damage typically initiates and is most severe at corners of roofs and walls or "obstacles" on an otherwise flat building surface. 

Shingles are missing near the roof ridge and next to a flue vent.  Both locations are high on the structure and near discontinuities where wind forces tend to pull away from the roof, bringing weakly attached items along for the ride.

• Wind is a bully; it damages the weakest components of a building first (roof appurtenances, shingles, siding, loose trim).  Wind-caused structural damage will not be present without significant damage to the weaker components on the building exterior.

Roof shingles and garage windows were damaged by wind forces, but the still-intact garage doors demonstrate that the structural frame was not damaged.

• Wind speeds are not constant.  The intensity of a tornado (or any wind event) varies throughout the duration and along the path of the event.  The wind speed cited for a storm is the maximum wind speed that existed for only a short time, often a 3-second gust, and are not representative of the average wind speeds maintained for the duration of a wind event.  For a tornado, the EF rating given is based on multiple damage indicators (buildings, structures, and trees) and represents the highest estimated wind speed that occurred during the life cycle of the tornado.

• Tornado-caused damage can be relatively precise.  It is possible for houses on one side of a street to be demolished by a tornado while houses on the other side have little or no structural damage.  These houses usually have some degree of wind damage to weaker building features.

Houses in the center of this photograph were leveled by a tornado while some houses on the next street over (on the left) sustained only minimal damage to exterior components.

If you would like more information regarding the EF Scale or wind-related damage, contact Wade Sticht at w.sticht@armstrongforensic.com or (801) 922-0259.

Links:

More info on the Enhanced Fujita (EF) Scale

Scholarly Publication on how the EF Scale was developed

Storm Prediction Center map for May 20, 2013 (Moore, OK tornado)