Lessons learned from tornado damage investigations
June 3, 2008
Written by Bogusz Bienkiewicz, a civil engineering professor at Colorado State and a past president of the American Association for Wind Engineering.
In the wake of the Windsor tragedy and as we approach the heart of tornado season along eastern Plains, it's a good opportunity to review the major strides scientists have made in improving lives through structural engineering - and the work still ahead.
New solutions to reduce wind-induced losses
In addition to being an innovator in renewable energy, Colorado has a potential to become regional (and with appropriate institutional support) national leader in providing science-based and sustainability engineering-oriented solutions to reduce wind-induced direct and indirect losses. Organizations such as Colorado State University are working on advancing fundamental and applied research focused on new solutions to reduce wind-induced property and human losses.
Performance of structures during extreme wind events is an ultimate validation of effectiveness of advances in design and construction practices. These real-world full-scale tests provide information that would be very difficult to duplicate in laboratory experiments and it seems logical to study the damages that take place. From this information and from inferred wind speeds occurring during such events it should be possible to develop countermeasures to eliminate or reduce the levels of damage.
Tornadoes pose a particularly difficult problem
Another aspect of collecting perishable damage information is to determine the tornado-generated wind speed and to relate it to the inflicted damage. Tornadoes pose a particularly difficult problem, as much of the meteorological data is reported from radar observations and ground level measurements are usually very scarce. The wind is quite varied and structures located away from the tornado core will be subjected to a much lower wind velocity and forces.
Synthesis of the tornado damage documentation and information on the tornado strength (rating on Fujita scale, wind speed) allow formulating general observations regarding tornado resistance of buildings and structures and safety of occupants. A brief summary of these findings is presented next.
Older structures, many of them not constructed in compliance with building standards or codes of practice, are especially vulnerable. Damage investigations carried out in various parts of the country have showed nearly complete destruction of old-town districts, in direct path of moderate in strength tornadoes. With destroyed historic buildings, significant part of community heritage is lost forever.
Advances have decreased tornado fatalities
Advances in weather forecasting, improved tornado warning protocols and communication infrastructure, as well as greater awareness by the general public of tornado hazards have resulted in significant decrease in fatalities attributed to tornadoes. Interviews with members of communities in tornado-prone areas have indicated that in response to tornado warnings they use tornado shelters (either community shelters -designated buildings) or "in house" shelters (basements, cellars, inner space of houses, etc.). However, post-tornado investigations have revealed several cases of the buildings designated as shelters that did not perform as expected - underwent partial or total damage, leading to injuries and fatalities.
Overall, the degree of tornado damage of business, communal and residential buildings varies and it depends on the tornado strength, distance from the tornado track, age of the structure, as well as quality of construction. Debris from this damage typically leads to tornado missiles, impacting houses, contributing to damage of building envelopes and presenting a significant hazard to building occupants.
Manufactured homes are especially prone to tornado damage
Especially prone to tornado damage are manufactured homes. Dislodging and subsequent destruction of these homes typically results from inadequate tie-downs or lack of anchoring. At times, such homes are placed on loosely-laid light concrete blocks and in such situations homes are totally damaged, even in presence of relatively weak tornadoes.
Overall, the wind resistance of new residential houses has been significantly better than that of older houses. However, they are also prone to significant damage when exposed to weak tornadoes (even if they are not in a direct path of a tornado) when connections between the foundation, wall and roof are not properly designed or constructed. The structural components of a house must ensure proper path for wind-induced loading and its transfer to the house foundation. This issue is especially critical when the house garage door or windows break and pressure inside a building suddenly changes. Frequently, the damage of a house is initiated in the roof corner area(s) where the combination of high wind suction and pressure build-up inside the building lead to large wind force. Similar failures have been observed for houses under construction, with incomplete walls and/or uninstalled garage doors or windows.
Appropriate design and construction practices are key
The wind force exerted on houses and structures in direct path of tornadoes of moderate rating is relatively large and a certain level of damage is expected to be inflicted in such cases. However, by appropriate design and construction practice the property damage and probability for human toll can be significantly reduced. Improved tornado resistance of houses and structures make them also more resistant to other high wind phenomena, e.g. Chinook winds and thunderstorms in Colorado.
Visit the College of Engineering: http://www.engr.colostate.edu.
Contact: Bogusz Bienkiewicz
Phone Number: (970) 491-8232