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construction of high-rise buildings in regions of high seismicity is much more limited than that available in non-seismic regions. To protect the life and safety of occupants, U.S. building codes governing seismic design reflect a strong column, weak beam philosophy. Because the vertical columns are more critical to the stability of a structure than are the horizontal beams, engineers are required to design the columns

to be 120 percent as strong as the beams. As a result, in the event of strong earthquake shaking motion, the beams are damaged instead of the columns, so that the building will remain standing. One structural attribute that engineers have come to understand during the past 30 years as being critical to effective seismic design is ductility: the ability of a structural member, or

a connection between structural members, to bend in response to earthquake-induced forces while simultaneously continuing to support the loads it was designed to carry. Ductility a Key The ductilit y of concrete columns can be increased by including horizontal or transverse steel reinforcing as well as vertical steel. Lack of ductility in columns, beams, and connections has been blamed for the most serious damage to major buildings and transportation structures that occurred during recent major earthquakes. Non-ductile concrete and steel columns supporting the Hanshin expressway near Kobe – designed before 1971 when Japanese Building Standard Law was modified to require ductility in structural elements and connections – contributed to a spectacular failure of the elevated roadway. In response to failures of non-ductile columns on bridges and roadways during the Northridge earthquake, which shook the Los Angeles area on Jan. 17, 1994, and the Loma Prieta earthquake, which struck near San Francisco on Oct. 17, 1989, the California Department of Transportation has undertaken a retrofit program of non-ductile columns. Contractors are jacketing these columns with thin sheets of steel or carbon fiber materials to confine the concrete and increase column ductility. Detailing of connections with seismic forces in mind has also emerged as an important design consideration in recent years. After the two recent California quakes, for example, bridge engineers changed the requirements for the connection between adjacent concrete box girders that support bridges, increasing the seat width, or the area of overlap between the two sections, from about 6 in. to more than 20 in. (15 cm to 38 cm). Engineers have also begun to require much more substantial ties between separate structural members, such as box girders or beams. Structural members are now being linked by restrainers made of highstrength steel rods with steel plates at either end that are embedded in the concrete to keep the structural members from separating during an earthquake. This article was originally published on the Portland Cement Association’s Web site. For more information, visit www.pca.org.

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