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A tornado can cause catastrophic and extensive damage to all types of structures. For some, the damage requires a total rebuild. However, for the homes and buildings that are salvaged after a tornado, a detailed assessment by a qualified structural engineer is needed to determine the extent of structural damage, the repairability of the structure, and to assist with other repair recommendations related to the building envelope and required building code upgrades.

A structural engineering assessment is comprised of many components, including an analysis of the property related to the path of the tornado, research of the Enhanced Fujita (EF) Scale rating, and a top-to-bottom visual assessment of the structural components, such as the foundation and superstructure. Often, a local building code official will require a structural engineering report to be submitted before issuing a permit for the repair work.

Tornado Basics

Tornadoes are created when air currents of different temperatures and humidity collide. Tornadoes typically occur in the spring and summer months as warm, moist air travels from the Gulf of Mexico and meets cold, dry arctic air from Canada. Most tornadoes occur in the states located between the Rocky Mountains and the Appalachian Mountains. The winds created are rotational in nature and, in the Northern Hemisphere, are almost always counterclockwise in direction. Tornado paths within the United States generally travel from the west/southwest and move to the east/northeast.

Funnels created by tornadoes typically are as large as 250 feet in diameter and can travel several miles before dissipating. Large tornadoes can be upwards of a half-mile wide and travel across a distance of over 30 miles. Multiple funnels within a system are not uncommon. The speeds of most tornadoes are less than 110 miles per hour; however, they can reach speeds of up to 300 miles per hour. The intensities of tornadoes are estimated by the National Weather Service and other teams of experts using the EF Scale, which uses ratings for probable or expected damage thresholds.

Patterns of Damage

Tornado-force winds can damage a building or residential dwelling in several ways. Wind causes positive (inward bearing) and negative (outward suction) pressures, which are greatest near the corners of roofs and walls. Additionally, roofing along the lower edge will be displaced first from wind. The loss of a roof edge makes adjacent sections more vulnerable to damage from wind. Thus, it is common to see wind progressively work its way up a roof, displacing roofing up the slope of the roof.

As the wind forces push and pull on a building, they can cause the building to lean or twist. This action can cause separations and gaps to occur at the intersection of the wall and ceiling drywall, especially in the upper levels of a structure. Wind forces push and pull on the doors and windows and can break, dislodge, and loosen them.

One of the most catastrophic causes of damage from a tornado is caused by pressurization of interior spaces. If the winds penetrate into the building through openings in the walls or roof, the wind forces are magnified and can loosen building components, possibly separating or displacing them from the building. This occurs when air is able to enter a partially enclosed space (i.e., when windows and doors are open or broken) at a faster rate than air is able to exit these spaces. This creates internal pressure inside of a building, which causes outward (positive) pressure on the walls, similar to blowing up a balloon.

Wind-borne debris also can cause substantial damage during a tornado. As the rotational winds gain velocity, a debris field is created and objects are picked up in the airstream. These projectiles can penetrate the building envelope causing internal pressurization, and if they are large enough, they can cause structural framing members to fracture and fail when impacted.

Visual Assessment and Structural Evaluation

An investigation by a structural engineer for tornado damage typically begins with a visual assessment. Generally, the elements of the building are evaluated in a top-down manner, starting with the condition of roof framing. It always is important to note the framing connections of the roof rafters or trusses to the tops of the exterior walls. If no steel straps or ties are used at these connections, then structural members closer to grade are not likely to be affected because the weak point of the framing system occurs at the roof framing connection. Thus, when the roof framing elements are intact, it is unlikely that the lower members have been exposed to damage-causing forces.

Often the use of levels or other measuring instruments is utilized to identify whether walls are in plumb condition or to detect a pattern of floor surface elevation differential. Depending on the patterns of readings, some may indicate recent movement due to high-velocity winds or a pre-existing condition. There are instances where level readings might indicate horizontal shifting of a structure in opposite directions, which is not related to a tornado. Structural deformation due to tornadic winds would be expected to rack or twist the structure and/or displace the top of walls relative to the foundation in the same basic direction. Brittle interior finishes such as drywall will crack or show distress before structural members do. For example, where no interior cracks are observed and conflicting plumb readings of walls exist, it is not probable that the walls were recently shifted from tornadic winds, but were constructed slightly out of plumb.

During the inspection, the structural engineer should make a list of suspect or noncode compliant conditions. Code-deficient conditions also will be documented through the course of a damage investigation. Building code requirements have changed significantly over the years; often a structure built more than 20 or 30 years ago will not meet current code requirements. With most structural framing members and connections, code upgrade requirements typically are required only for areas of buildings that have sustained substantial damage. Each state, city, and county has local ordinances, adaptations, and amendments of and to the International Building Code (IBC), which governs the specific requirements.

In addition to identifying the structural damage and scope of repairs, licensed engineers also are required by most state laws to report any unsafe structural conditions if they are observed. Verbal concerns, warnings, and restrictions should be provided to the property owner, and recommendations for temporary/emergency shoring sometimes are needed. However, in many instances, the local building code official and other emergency management personnel will inspect a building immediately after a catastrophic tornado and place a notice on the building to identify if it has been vacated, if dangerous conditions exist, and if an inspection by a structural engineer is required.

Once the site inspection has been completed by a structural engineer, additional analysis and evaluation is performed based on the pattern of damages and research of available tornado weather data. If suspect conditions are covered with debris or building finishes, then a second inspection may be required after initial debris removal and shoring has taken place. An engineering report should include a summary of the tornado-related damages, pre-existing conditions, a scope of repairs, and any code-related issues. Depending on the jurisdiction where the loss occurred, a signed and sealed report prepared by a structural engineer may be sufficient for a contractor to obtain a permit to perform repairs to a residential dwelling. For commercial buildings, repair drawings typically are required.

Analysis in Action

Let's take a look at a real-life example of a tornado loss. The National Weather Service (NWS) reported that high winds and tornadoes were present in Washington (Tazewell County), Illinois, on Nov. 13, 2013. The approximate intensity of the tornado registered as an EF4, with a documented maximum wind speed of 190 miles per hour. The path had a length of 46.2 miles, a width of a half-mile, and it traveled directionally through the area from the southwest to the northeast. The subject property in Figure One was located in the path of the tornado, and several adjacent structures were displaced from their foundations.

The damage to the subject dwelling was unique because most of the structure remained intact, except for a portion of the roof truss framing and the garage. Fortunately, the homeowners took cover in the basement when the tornado came through the area and there were no significant injuries reported. However, when the garage door was displaced due to wind pressure, the garage pressurized and the framing became displaced from the foundation.

The home was relatively new at just nine years old, so the structural framing was assumed to meet the minimum requirements of the International Residential and Building Codes. Thus, the foundation walls were constructed with reinforced concrete, the wood framing was anchored to the foundation, and the framing members were of sufficient size and strength. Based on visual observations it was determined that the wood structure generally had remained intact, except for the roof framing and garage structure.

A digital level was used to confirm that the balance of the exterior walls and ceilings were plumb and level. Specific attention was made to joints and changes in geometry where structural distress would be expected and visible. There were typical hairline cracks and separations present in the sheetrock finishes, but nothing indicated that there was displacement of the structural framing system.

Experts concluded that, based on the extent of damage to the roof framing, the roof should be reframed with new pre-engineered wood trusses. While it may have been possible to replace only a portion of the roof framing, a full replacement of the roof trusses and roof sheathing was recommended to ensure that the roof diaphragm (lateral wind-force resisting system) was restored and structurally sound. Additionally, recommendations were made to reconstruct the garage and replace all of the siding and windows.

This example illustrates how a home could have possibly been demolished if an experienced and qualified structural engineer had not performed a thorough investigation. To the untrained eye, the damage was substantial, making repairs seem impossible. Instead, a repair alternative was recommended. A photograph was taken of the subject property on July 2015 (Figure Two). The house had been repaired, and the attached garage was expanded. The community of Washington, Illinois, had come a long way in a short time since the devastating tornadoes occurred in November 2013.

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