Seismic Design

Since the introduction of structural steel to building construction, in the late 19th and early 20th centuries, engineers have recognized that steel buildings and structures have performed extremely well compared with structures of other types of construction. One of the earliest and most dramatic examples of the ability of steel structures to withstand a strong earthquake occurred in the great San Francisco earthquake and fire of April 18, 1906. At that time, San Francisco’s urban center predominantly consisted of a mixture of light wood-framed and masonry bearing-wall construction. In addition, the city had approximately 30 high-rise buildings constructed with complete vertical load-carrying steel frames and infill masonry walls. The earthquake and fires that followed destroyed almost all of the timber and masonry buildings, but left the steel frame structures. Most of these steel frame structures, which were designed without any consideration of earthquake resistance, were repaired and restored to service, and more than 20 of these structures remain in service today. The observation of the outstanding performance of steel frame structures in the 1906 San Francisco earthquake led to the requirement in present-day building codes that tall structures must have complete vertical loadcarrying frames. Over the years, as California experienced many earthquakes, engineers repeatedly observed that steel frame structures performed in a superior manner relative to other building types. In part, this is why the urban centers of most cities in the western United States, including Los Angeles, San Francisco and Seattle, are composed of steel frame buildings. By the early 1990s, many engineers in the western United States believed that steel structures were inherently ductile and, as a result, essentially invulnerable to significant earthquake damage. This was reflected in the requirements of building codes of the era. Steel frame structures were permitted to be designed for smaller earthquake forces than buildings of other construction types. Also, relatively few limitations were prescribed on the types of configurations and detailing that could be employed in such structures, relative to the requirements for other types of construction. The magnitude 6.7 Northridge earthquake that struck the San Fernando Valley, just to the north of Los Angeles, on January 17, 1994, changed this perception. Following the Northridge earthquake, engineers began to discover that a number of steel frame buildings, including both moment frames and braced frames, had experienced significant structural damage, including buckling and fracture of braces in braced frames, and fractures of beam-to-column connections in welded steel moment frames. The damage sustained by moment frame structures was particularly alarming as it became evident that rather than behaving in a ductile manner, these fractures had occurred in a brittle manner. Although no steel frame buildings collapsed in the Northridge earthquake, just one year later, more than 50 steel buildings collapsed in the magnitude 6.8 Kobe, Japan, earthquake of January 17, 1995. These two events led to massive programs of research into the seismic behavior of steel frame structures, both in Japan and the United States. This research quickly fed into the building codes, and by 1997, the American Institute of Steel Construction (AISC) published a new edition of its Seismic Provisions for Structural Steel Buildings (AISC 341) that contained many new requirements affecting the materials, design and construction of steel structures intended to resist strong earthquakes. With the adoption of the International Building Code (IBC) throughout the United States, and that code’s broad requirements to design structures for seismic resistance, the design criteria contained in AISC 341 has become mandatory in many communities across the United States. In order to design structures to resist strong earthquakes, it is necessary to have an understanding of structural dynamics and the nonlinear behavior of structures. Structural steel continues to offer several economical and effective means for the design and construction of earthquake-resistant structures. This Facts for Steel Buildings presents an overview of the causes of earthquakes, the earthquake effects that damage structures, the structural properties that are effective in minimizing damage, and the organization and intent of seismic design requirements for steel structures in the United States today. More detailed information is available in the references listed at the end of the document.



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