While preparing a structure for exposure to natural disasters isn’t always top of mind in the construction industry, research is continually under way to find ways to limit the damage caused by hurricanes, tornadoes and earthquakes. 

Standing Up to Violent Winds
Tornadoes are classified by the National Weather Service in very much the same way as hurricanes: by the most extreme damage they can find anywhere in the storm’s path, even if only a small percentage of the area affected reaches that extreme. In most cases, the wind speed in each tornado’s path has been an EF-0 (65-85 mph), EF-1 (86-109mph) or EF-2 (110-137 mph), which are comparable to hurricane wind speeds up to a Category 3. As a point of reference, new homes along large portions of the Texas coast are built to withstand hurricane force-winds up to 130 mph. 

William Coulbourne is a national expert in wind and flood mitigation and has been involved in FEMA Mitigation Assessment Teams and natural hazard damage assessments for close to 20 years. He has become intently focused on building design for tornadoes after the recent devastating touchdowns in Alabama, Missouri and Oklahoma. 

Based on his research, Coulbourne suggests that the same practices used for hurricane wind resistance can be applied in tornado prone areas of the country. Translating the same awareness of natural disasters and application of stringent building codes during the design process has the potential to significantly improve the safety of homes, businesses and human lives in the event of a tornado. Practices such as creating load path continuity from the roof-to-wall connections through to the wall-to-foundation connections and installing laminated windows to minimize the effects of airborne objects should be used when rebuilding a structure damaged by a tornado and when considering new construction in a high-risk area. 

The two most common failures during a tornado are roofs lifting off their frames and homes being pushed off their foundations.  According to Coulbourne, preservation of buildings and homes in tornado alley could be greatly improved with a focused approach to the mechanical connections that attach roofs to walls and walls to foundations. Current building codes only require roof-to-wall connections to be made with nails, but Coulbourne recommends that metal connectors be used for optimal strength and wind resistance (up to 130-135 mph). Additionally,  he recommends installing a reinforced area where people can safely take cover in the event of a tornado.

Building for Earthquakes
Similar to design considerations in building for strong winds and tornadoes, construction in earthquake-prone regions requires its own unique guidelines. The basic objective of the seismic provisions in building code has been to protect human lives by helping buildings withstand intense seismic activity without collapse, even in the event of significant structural damage. 

An understanding of ductility is required to accomplish this goal in building design. Ductile structures are capable of sustaining large amounts of damage without significant degradation in strength or stability. Steel is a highly ductile building material, as it can allow significant damage to the structure(such as beams bending and buckling) without collapse. The damage can help reduce the earthquake loading on the building by changing its response characteristics. With this understanding, engineers are tasked with controlling the locations of damage in a building as load is applied to ensure it can withstand prescribed levels of force. 

Tom Murray, a professor emeritus in structural steel design at Virginia Tech, emphasizes the importance of prequalification of moment connections for intermediate and special moment frames. In order to be prequalified for special moment frames, bolted unstiffened and stiffened extended end-plate moment connections must attach beams to columns while attaining a measured flexural resistance of 80 percent of the ultimate strength of the beam at a story drift of 0.04 radians. 

Since 2001, the Metal Building Manufacturers Association (MBMA) has funded research to better understand metal building frame behavior when subjected to earthquakes.  In 2005, the MBMA began working with the University of California San Diego (UCSD) to study the effects of earthquake loads and frame behavior in depth. Six years later, full-scale shake table testing was completed on three metal buildings at UCSD. Reports on the first two tests have been received and show rafter lateral torsional buckling (LTB), along with damage at the base plates and knees. Ten component tests that further explore rafter LTB behavior have been evaluated to document the limits of this form of ductility. The results from these experiments are helping researchers connect the dots between metal buildings’ strong performance in earthquakes and the future tools to be used in designing, detailing and manufacturing these buildings. Additional efforts to understand the ductility and response characteristics of metal buildings for different configurations are planned for 2014 and beyond.  



Jerry Hatch is manager of engineering development for NCI Building Systems and chairman of the Metal Building Manufacturers Association Technical Committee. For more information, email jerry.hatch@ncigroup.com or visit www.ncilp.com