Earthquake Spectra

A simple procedure to identify site frequencies using earthquake response records from roofs and basements of buildings is presented. For this purpose, data from five different buildings are analyzed using only spectral analyses techniques. Additional data such as free-field records in close proximity to the buildings and site characterization data are also used to estimate site frequencies and thereby to provide convincing evidence and confirmation of the site frequencies inferred from the building records. Furthermore, simple code-formula is used to calculate site frequencies and compare them with the identified site frequencies from records. Results show that the simple procedure is effective in identification of site frequencies and provides relatively reliable estimates of site frequencies when compared with other methods. Therefore the simple procedure for estimating site frequencies using earthquake records can be useful in adding to the database of site frequencies. Such databases can be used to better estimate site frequencies of those sites with similar geological structures.

Recent studies have demonstrated the need to consider the ground motion frequency content in the development and use of R−μ−T relationships. Results from two different approaches to determining these relationships are unified in the present paper. Two bilinear R−μ−T/T_g relationships are recommended for most strong ground motions and structural systems. One is more accurate, while the other, more conservative relationship is used in FEMA 273, ATC-32, and the simple version of the N2 method. Both relationships are indexed by the characteristic period of the ground motion, T_g. Simple methods to determine T_g from smoothed design spectra and recorded ground motions are provided. Neither recommended relationships are applicable to the nearly harmonic ground motions that may be generated at sites containing soft lakebed deposits. An example illustrates the application of these relationships to a code design spectrum in both the acceleration-displacement and yield point spectra formats.

Multistory models are studied to assess design recommendations for torsionally unbalanced multistory buildings. Structural systems are assumed as shear beams with nonlinear lateral-resisting elements oriented along two orthogonal directions and subjected to a bidirectional earthquake ground motion. Five-story rigid-diaphragm models with mass or stiffness eccentricity are considered in the study. Accidental eccentricity is not included in the modeling and, therefore, its related recommendations are not assessed. Design recommendations studied here are based on the simple static procedure and include two values and an expression for the amplification factor α to compute story eccentricities, three values of the torsion-shear reduction factor δ, an additional lateral force F_t to be applied at the building top level, and a minimal story eccentricity for the building. Design recommendations to control ductility demands of torsionally unbalanced multistory buildings are given.

We compare the distribution of damage to housing caused by the 1995 Hyogo-ken Nanbu (Kobe) earthquake with those estimated for several source models proposed for this earthquake. This comparison aims at identifying source models that can provide loss estimates that are most appropriate for planning emergency response activities just after earthquakes and/or for preparing effective countermeasures for mitigation of future earthquake disasters. The results suggest the necessity of finite-fault slip models that can reproduce or predict accurately strong ground motion within a frequency range closely related to damage. The loss estimation just after earthquakes based on source models can be counted on in areas without dense strong-ground-motion observation networks. Even with the dense networks, source models will also be useful for accurate loss estimation in the immediate vicinity of earthquake source faults.

The design codes and standards (e.g., UBC, IBC, NFPA-13) estimate the amplitude of the seismic load in sprinkler-pipe braces, but they do not specify the number of cycles for which this load must be resisted by various components of pipe braces. Because the components can fail in low-cycle fatigue, the number of load cycles must be considered in establishing the strength of the brace components. The first part of this study deals with determining the number of cycles for which a component must resist its rated capacity. Strong-motion records from 18 strongly shaken buildings were incorporated into a low-cycle fatigue model to develop a test criterion for measuring the seismic strength of brace components. In the second part of this study, a series of tests were conducted to gain insight into the cyclic behavior of brace components. Finally, a test protocol was established to measure the seismic strength of brace components. With some modifications, the protocol can be applied to many other nonstructural components.

California's first major earthquake safety policy initiative was 1933's Safety of Design and Construction of Public School Buildings Act, popularly known as the Field Act for its author, Assemblyman Don C. Field, who became the Field Act's legislative champion. The foundation for its enactment a month after the 10 March 1933 Long Beach earthquake was laid earlier by the 29 June 1925 Santa Barbara earthquake, the Dam Act of 1929, the drafting of a proposed Building Code for California, the formation of the Structural Engineers Association of California, and general acceptance of California's earthquake risk. This paper reviews the roles played by the pre-Field Act factors and the politics of the law's enactment, including the central role played by the State Division of Architecture in preparing the legislation and supporting Assemblyman Field's efforts.

Installation of damping devices has been limited to diagonal or chevron brace configurations until the recent development of the toggle-brace configurations. These configurations magnify the effect of damping devices, thus facilitating their use in stiff framing systems. This paper introduces the scissor-jack-damper system that was developed as a variant of the toggle-brace-damper systems, with the added advantage of compactness. The effectiveness of the scissor-jack configuration is demonstrated through testing of a large-scale steel-framed model structure on an earthquake simulator. Experiments showed that despite the small size of the damping device considered, the scissor-jack system provided a significant amount of damping and substantially reduced the seismic response of the tested structure. Response history and simplified analyses produce results that are consistent with the experimental results.

We determined S-wave velocities (V_s) to about 40-m depth at 65 locations in the Memphis-Shelby County, Tennessee, area. The V_s measurements were made using high-resolution seismic refraction and reflection methods on the ground surface. We find a clear difference in the V_s profiles between sites located on the Mississippi River flood plain and those located to the east, mostly covered by loess, in the urban areas of Memphis. The average V_s to 30-m depth at 19 sites on the modern Mississippi River floodplain averages 197 m/s (±15 m/s) and places 17 of these sites at the low end of NEHRP soil profile category type D (average V_s 180-360 m/s). The two remaining sites are type E. V_s to 30-m depth at 46 sites in the urban areas east of the modern floodplain are more variable and generally higher than the floodplain sites, averaging about 262 m/s (±45 m/s), still within category D. We often observed the base of the loess as a prominent S-wave reflection and as an increase in V_s to about 500 m/s. Based on the two-way travel time of this reflection, during an earthquake the impedance boundary at the loess base may generate resonances in the 3- to 6-Hz range over many areas of Memphis. Amplitude spectra from four local earthquakes recorded at one site located on loess indicate consistent resonance peaks in the 4.5- to 6.5-Hz range.

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.