Revisiting Earthquake Lessons - Impacts of the Northridge Earthquake on Design Ground Motions
Wednesday, May 1, 2019
Author: Ronald O. Hamburger, SE
This is one in a series of short articles revisiting lessons learned in past earthquakes. This series is being presented to share past lessons with newer members of our profession who were not there to experience them first hand. Contact Dave McCormick or Kelly Cobeen with your comments and suggestions.
Structure Type: All Structure Types
Earthquakes: 1994 Northridge
Lesson: The ground motion recordings obtained in the Northridge earthquake spurred the development of seismic micro-zonation, where the design forces on structures located near faults could vary substantially within a few kilometers, as well the development of near field Seismic Design Categories and many restrictions on structures designed in these categories. Because of the large database of motions provided by the Northridge earthquake and subsequent earthquakes it became possible to drastically improve the procedures used to estimate ground motions from earthquakes and improve seismic hazard maps.
At the time of the Northridge earthquake, the 1994 Edition of the Uniform Building Code (UBC) (ICBO, 1994) represented design ground shaking, and thereby the required lateral strength for structures, in a simple manner. The code designated five seismic zones ranging from 0 (no design requirements) to 4 (the most stringent seismic requirements). The zones were distributed over broad geographic regions, based on historic frequency of large damaging earthquakes in the region. Zone 4 covered coastal California and portions of northern Nevada while Zone 3 covered the balance of California, the portions of Oregon and Washington west of the Cascades and regions surrounding the Wasatch -Yellowstone basins and New Madrid fault zones. In zone 4, design ground shaking, defined as having a 10% chance of exceedance in 50 years, was assumed to produce a peak ground acceleration of 0.4g. Ground accelerations in other seismic zones were proportionately reduced (e.g. 0.3g in Zone 3, 0.2g in Zone 2, etc.). Seismic design forces were specified by reference to standard acceleration response spectra, indexed to the seismic zones and three reference site classes (Types 1, 2 and 3). Figure 1 illustrates the standard spectrum for a Type 3 site (the default soil condition) in Zone 4. The design force, for short period structures, was based on a 1g response acceleration (spectral amplification of 2.5), with design accelerations decreasing in proportion to structural period for structures having periods longer than about 1 second. These design requirements and the spectra themselves were based on a limited number of ground motion recordings that had been obtained over the past 50 or so years.
The January 17, 1994 Northridge earthquake produced an unprecedented number of new strong ground motion recordings including more than 60 recordings from instruments located within 20 kilometers of the fault rupture. Many of these records produced spectra with ordinates significantly exceeding those in the 1994 UBC Zone 4 spectra (Figure 2). More disturbing still, when these records were viewed in so-called Acceleration-Displacement Response Spectra (ADRS) format (Figure 3) it could be seen that they produced long period velocity pulses and large long period displacement demands, relative to the code spectrum.
At the time the earthquake occurred, the SEAOC Seismology Committee had just initiated work on developing code change proposals for what would become the 1997 UBC (ICBO, 1997) and later the 1997 National Earthquake Hazards Reduction Program (NEHRP) Provisions (FEMA, 1997) and 2000 International Building Code (IBC) (ICC, 2000). The Seismology Committee had already decided that this would be a relatively major code change cycle and had committed to converting Chapter 16 of the code from an allowable stress basis to a strength basis, to facilitate adoption by the pending IBC. However, the large number of structural failures observed in the Northridge earthquake, particularly in the northern San Fernando Valley where these unusual ground motion records were recorded, quickly convinced the committee to recommend far more-wide-reaching changes to the code. This motivation was reinforced, when 1 year later, the 1995 Kobe earthquake produced very similar ground motion records, as well as extensive damage. Dramatically, several security cameras captured the effects of these ground motions and velocity pulses on buildings and occupants making it politically possible to sell the need for these code changes to engineers, building officials and the public.
One of the most significant code changes was the introduction of “near-fault” zones into the code. These near-fault zones extended up to 15 kilometers on either side of active faults capable of producing large magnitude earthquakes including the Hayward, Newport Inglewood, San Andreas, San Jacinto and several others. Depending on site distance from these faults, short period design spectral acceleration values were elevated by as much as 50%, and long period values by as much as 100% relative to those required by the 1994 UBC. To make it practical to determine the distance of a site from one of these faults, SEAOC worked with the California Division of Mines and Geology and the International Conference of Building Officials to develop a series of large-scale maps (1/4” = 1 kilometer) showing the faults and 5-kilometer, 10-kilometer and 15-kilometer-wide bands surrounding these faults, superimposed over street maps. These were published in conjunction with the 1997 UBC (ICBO, 1998). In many ways, this opened the door for the use of the detailed ground motion contour maps, later adopted into ASCE 7-98 (ASCE, 1998) and the IBC.
In addition to increasing the amplitude of the design spectrum, SEAOC introduced a new minimum base shear equation, V=0.8ZNv(I/R)W into the code. In this equation, Z was the zone factor, taken as 0.4 in Zone 4; Nv was a near-field velocity-dependent coefficient, which ranged to as large as 2 near some faults and I, R and W were the occupancy importance factor, response modification coefficient and seismic weight, respectively. The intent of this new equation was to account for the effects of the large velocity pulses and long period accelerations and displacements seen in some records. This new base shear equation was later modified and adopted into the ASCE 7 requirements as Cs=0.5S1/(R/Ie), where S1 is the 1-second MCE spectral response acceleration, R and Ie are the response modification and occupancy importance factors.
The Seismology Committee was concerned that the impulsive nature of some near field ground motions would be particularly damaging to structures with certain irregularities including soft and weak stories. To encourage engineers to avoid such irregularities, the 1997 UBC only increased design forces on regular, short-period structures located on near-fault sites by 10% relative to that required under the 1994 UBC. However, as noted earlier, forces on irregular and long period structures on some sites were as much as doubled. Later, as the 1997 NEHRP Provisions were developed for ASCE 7 and IBC, new near-fault Seismic Design Categories (SDCs) E and F were developed and the most severe structural irregularities were prohibited in these SDCs.
In summary, the many ground motion recordings obtained in the Northridge earthquake, and the special characteristics and amplitudes displayed by some of these records spurred the development of seismic micro-zonation, where the design forces on structures located near faults could vary substantially within a few kilometers. Further, recognition that ground motions near faults could be substantially different than in far field locations, led to the development of near field SDCs and many restrictions on structures designed in these categories. Finally, as the large database of motions provided by the Northridge earthquake was supplemented by many additional records form the Kobe, Taiwan and Turkey earthquakes a few years later, it became possible to drastically improve the procedures used to estimate ground motions from earthquakes under the Pacific Earthquake Engineering Research Center’s Next Generation Attenuation (NGA) Program. That program continues to have major impact on the building codes and our design seismic hazard maps today.
ASCE, 1998, Minimum Design Loads for Buildings and Other Structures, 1998 Edition (ASCE 7-98) American Society of Civil Engineers, New York, NY.
FEMA 1997, NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, 1997 Edition, Federal Emergency Management Agency, Washington, DC.
ICBO, 1994. Uniform Building Code, 1994 Edition, International Conference of Building Officials, Whittier, CA.
ICBO, 1997. Uniform Building Code, 1997 Edition, International Conference of Building Officials, Whittier, CA.
ICBO, 1998. Maps of Known Active Fault Near-Source Zones in California and Adjacent Portions of Nevada, International Conference of Building Officials, Whittier, CA.
ICC, 2000. International Building Code, 2000 Edition, International Code Council, Country Club Hills, IL.
Figure 1 – 1994 UBC Spectrum, Zone 4, Site Class Type 3
Figure 2- Comparison of 1994 UBC spectrum and selected Northridge earthquake records
Figure 3- ADRS Comparison of selected Northridge Records and UBC spectrum