1994 Northridge Earthquake - Building Codes Changes

Building Codes Changes

With each major earthquake comes new understanding of the way in which buildings respond to them. Advances in the technology associated with testing systems, design and seismic modeling software, structural connections, structural forms, and seismic force resisting systems have accelerated dramatically since Northridge. The City of Los Angeles is stringent in their review of the seismic safety of everything built in the city. There is an array of building forms and systems that are no longer legal to build. An example is the previously popular "soft-story" multifamily apartments. These buildings typically look like a three-story box on a narrow lot, where the upper two floors overhang the lower floor and are supported on pipe columns so cars can be parked underneath. Because the ground level is soft relative to the upper floors, as pictured above, the upper portion can sway and fall onto the carport below. Today, no wood floors are allowed to extend more than an additional 15% beyond the shear wall or other lateral-load resisting element of the floor below. This typically results in overhangs of only up to three or four feet, compared to the 20 to 40 feet that was previously built. If an architect still wants this type of design, the structural engineer may specify that the previously flimsy pipe columns be replaced with a laterally stiff steel "moment frame." This can also mitigate the problem of the soft-story structure by stiffening the soft ground floor.