Northridge Earthquake Example
On January 17, 1994, an earthquake with a moment magnitude of 6.7 hit the Reseda neighborhood in Los Angeles, California. Starting at 4:31 AM, Pacific Standard Time, the quake devastated the area for 10 to 12 minutes. As is often the case, the earthquake also produced aftershocks, but unlike most other earthquakes these aftershocks were both 6.0 Mw. The first came merely a minute after the original tremor, and the second came 11 hours later. With a death toll of 57 and over 5,000 injured, the destruction was devastating.
This amazing amount of destruction came as a shock to everyone. But an even greater shock came after a thorough analysis of the structural failures. Though some buildings may have been designed poorly, it is believed that most of the damaged structures did in fact adhere to building code. Of the specific buildings studied, several of these actually demonstrated strength greater than required by code. This of course, caused great concern and brought about major reevaluation of current building code.
This event highlighted the weaknesses of the basic structure design in standard use. Seismic loads are extremely rare, and so frame design of the time only accounted for common lateral loads, leaving support for seismic loads entirely to the elastic properties of the metal and quality of joint weldments. This is because when earthquakes do occur they apply significantly larger moments than the frames of that time were able to withstand. Research was initiated to provide an improved, less brittle, connection for the commonly used Special Moment Resisting Frames (SMRF). Other preventative methods included several different kinds of dampers, mechanisms that drain the harmonic motion of the building, and the development of new steels with greater elastic properties.