Project ’97 also changed the seismic design maps from a nominal hazard level of 10% probability of exceedance in 50 years to 2%-in-50-year ground motions factored by two-thirds. These changes resulted from Project ’97, a collaboration between the developers of the NEHRP Provisions (i.e., the Building Seismic Safety Council, BSSC, with funding from the Federal Emergency Management Agency, FEMA) and the USGS. The new maps became more directly based on the USGS National Seismic Hazard Maps. These parameters are more closely related to the seismic response of structures.
In the 1997 edition of the NEHRP Provisions, the seismic design maps changed to the short-period and 1-second spectral response acceleration parameters, S S and S 1.
#CASE CLICKER CODES FEBRUARY 2019 CODE#
Seismic zone map of the 1994 Uniform Building Code (redrawn from the original by Kenneth Rukstales of the USGS). For more information on pre-Northridge seismic design and hazard maps, a good reference is USGS Spectral Response Maps and their relationship with Seismic Design Forces in Building Codes.įigure 1. In turn, these seismic design maps were loosely based – with truncations, modifications, and approximations – on a USGS peak ground acceleration (PGA) fully probabilistic hazard map published in 1976. All of these maps were based – with some modifications, updates, and simplifications – on the A a and A v maps first introduced in the 1978 Tentative Provisions for the Development of Seismic Regulations for Buildings, also known as ATC 3-06. Even the seismic zone map in the UBC (1994 edition), shown in Figure 1, was derived from an A v map. Such maps were also used by two of the three model building codes of the time, the National Building Code (1993 edition) and the Standard Building Code (1994 edition). Changes to Seismic Design Maps in Model Building Codesīefore the Northridge earthquake, the NEHRP Provisions (1994 and preceding editions) provided maps of effective peak acceleration, A a, and effective peak velocity-related acceleration, A v. Geological Survey (USGS) National Seismic Hazard Maps. As described below, the changes to the NEHRP maps took advantage of another post-Northridge change: the modern generation of U.S. The NEHRP maps were – and continue to be – adopted into the International Building Code (IBC), which supplanted the UBC and other model building codes. More enduringly, generational changes were made to the seismic design maps in the NEHRP Recommended Seismic Provisions for New Buildings and Other Structures. The factors increased the design forces in Zone 4, already the highest seismic zone, by a multiplier as large as 2.0. From the 1994 to 1997 editions, acceleration- and velocity-related near-source factors were introduced. Consequently, the near-source design forces from the seismic zone maps in the Uniform Building Code (UBC) were increased.
Both measurements were within approximately 15 km of the source of the earthquake they were also near most of the damage described in other articles of this series.
The same is true of the peak ground velocity of 148 cm/s measured in Granada Hills. At the time, the horizontal peak ground acceleration of 1.8 g measured by a seismometer in Tarzana was the largest ever. The 1994 Northridge earthquake generated world-record ground motions.
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