Column base plate testing at the University of Houston.
Engineers in the construction industry work diligently to stay aware of the challenges before them when setting out to design and build safe, strong, and durable buildings for tenants and occupants. The protection of life safety, through the avoidance of earthquake-induced collapse, remains the primary goal of U.S. building codes.
The basic objective of the building code seismic provisions is to provide buildings with an ability to 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 and controlled degradation in strength to prevent development of instability and collapse.
Steel, in particular, is a tough and ductile building material, as it can allow significant damage to the structure (such as beams bending, buckling, and yielding) before a full collapse. This damage also has the effect of changing the response characteristics of a building, which can reduce the effects of cyclic loading. With this understanding, the engineer controls the locations of damage in a building while making sure that collapse does not occur prior to sustaining prescribed load levels.
In some cases, the damage used to control building collapse is near the beam-to-column connections. Several types of connections have been studied and understood in their role to control collapse. The Metal Building Manufacturers Association (MBMA) and its member companies are working to further connection technology as well as relying on technology currently available for seismic applications. At the 2013 NCI Engineering Seminar, Tom Murray, Ph.D., P.E, gave a presentation on the prequalification of moment connections for intermediate and special moment frames.
The presentation covered primarily connection prequalification limits and testing requirements for the “Bolted Unstiffened and Stiffened Extended End-Plate Moment Connections.” In order to be prequalified for the special moment frames, these 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. Other connections discussed were the reduced beam section (RBS) moment connection, the welded unreinforced flange – welded web (WUF-W) connection, the Kaiser bolted bracket (KBB), and the conxtech (CONXL) moment connection.
Seismic performance factors
First shake table testing at the University of California San Diego. The table is 60 feet wide, column to column, and was used to help understand how metal buildings behave under the stress of earthquakes.
The metal building industry currently relies heavily on the ordinary moment frame (OMF) and the associated seismic performance factors for frame design, detailing, and fabrication. The industry also produces intermediate moment frames to meet the requirements of specific projects. In the coming years, the frame types currently being developed for the metal building industry will become available with their documented seismic performance factors, design, and detailing requirements.
Since 2001, the MBMA has funded research to better understand metal building frame behavior when subjected to earthquakes. In 2005, the MBMA began working with University of California San Diego (UCSD) to study in depth the effects of earthquake loads and frame behavior. 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 lateral torsional buckling (LTB), along with damage at the base plates and knees. UCSD also has studied the fundamental period and the effects of mezzanines on low-rise metal buildings.
LTB is being explored as a form of ductility. Ten component tests that further explore LTB behavior have been tested 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 design, detailing, and manufacture of these buildings. Further efforts to understand the ductility and response characteristics of metal buildings for different configurations are planned for 2014 and beyond.
Metal buildings are particularly resilient in earthquakes because of their typically light weight and low height. They have a history of good performance in earthquakes and are considered a good material to use for construction in earthquake-prone regions. Demonstrating the positive attributes of metal buildings through research and calculations has been the task at MBMA for the last several years. In addition to the research conducted at UCSD, a study on related bracing requirements has also been performed at Georgia Tech University. Lightweight, low-rise buildings have performed well with thin elements. For taller and heavier buildings, the use of the prequalified connections that Murray described is currently applied on a case-by-case basis. They are a critical piece of the puzzle in understanding and extending the use of metal buildings for seismic implications.
Jerry Hatch is manager of engineering development for NCI Building Systems and chairman of the Metal Building Manufacturers Association Technical Committee. He has worked in the metal building industry for 19 years as a production engineer, engineering manager, and director of research and development. To learn more about NCI Building Systems, visit www.ncilp.com. Contact Hatch at email@example.com.