Responses of Buried Corrugated Metal Pipes to Earthquakes

This study describes the results of field investigations and analyses carried out on 61 corrugated metal pipes (CMP) that were shaken by the 1994 Northridge earthquake. These CMPs, which include 29 small-diameter (below 107 cm) CMPs and 32 large-diameter (above 107 cm) CMPs, are located within a 10 km2 area encompassing the Van Norman Complex in the Northern San Fernando Valley, in Los Angeles, California. During the Northridge earthquake, ground movements were extensively recorded within the study area. Twenty-eight of the small-diameter CMPs performed well while the 32 large-diameter CMPs underwent performances ranging from no damage to complete collapse. The main cause of damage to the large-diameter CMPs was found to be the large ground strains. Based on this unprecedented data set, the factors controlling the seismic performance of the 32 large-diameter CMPs were identified and framed into a pseudostatic analysis method for evaluating the response of large diameter flexible underground pipes subjected to ground strain. The proposed analysis, which is applicable to transient and permanent strains, is capable of describing the observed performance of large-diameter CMPs during the 1994 Northridge earthquake. It indicates that peak ground velocity is a more reliable parameter for analyzing pipe damage than is peak ground acceleration. Results of this field investigation and analysis are useful for the seismic design and strengthening of flexible buried conduits.     

Seismic Compression of Two Compacted Earth Fills Shaken by the 1994 Northridge Earthquake

Seismic compression is defined as the accrual of contractive volumetric strain in unsaturated soil during strong shaking by earthquakes. We document and analyze two case histories (denoted school site and site A) of ground deformation from seismic compression in canyon fills strongly shaken by the Northridge earthquake. Site A had ground settlements up to about 18 cm, which damaged a structure, while the school site had settlements up to about 6 cm. For each site, we perform decoupled analyses of shear and volumetric strain. Shear strain is calculated using one-dimensional and two-dimensional ground response analyses, while volumetric strain is evaluated from shear strain using material-specific models derived from simple shear laboratory testing that incorporates important effects of fines content and as-compacted density and saturation. Analyses are repeated using a logic tree approach in which weights are assigned to multiple possible realizations of uncertain model parameters. At the school site, predicted settlements appear to be unbiased. At site A, the analyses successfully predict the shape of the settlement profile along a section, but the weighted average predictions are biased slightly too low. We speculate that the apparent site A bias can be explained by limited resolution of the site stratigraphy, bias in laboratory-derived volumetric strain models, and/or uncertainty in the estimated earthquake-induced settlements.     

Performance and Analyses of Mechanically Stabilized Earth Walls in the Tecomán, Mexico Earthquake

This paper discusses the performance and analysis of four mechanically stabilized earth (MSE) wall bridge approaches shaken by the 2003 Tecomán, Mexico earthquake. Strong shaking during the earthquake caused varying degrees of permanent displacement in several of the MSE walls. Immediately after the earthquake a geotechnical engineering reconnaissance team made detailed damage surveys of each wall. Complete design and construction data were later obtained. The analyses indicate that pullout of the upper reinforcement layers was the mechanism most likely responsible for the seismically induced deformation of the MSE walls. The upper layers of reinforcement were vulnerable to pullout because of the low levels of confining stress and the limited number of reinforcing elements per unit width. While pullout was the principal deformation mechanism, other factors contributing to deformation include large peak ground acceleration, more than twice the design value, and possible directional effects in the ground motion, which directed significant energy to the direction perpendicular to some of the walls. The latter finding concurs with observations made during the reconnaissance, where a clear directional bias was observed in the MSE wall deformations. The applicability and validity of the pseudostatic and sliding block methods of seismic analyses are discussed in light of the observed performance.     

Liquefaction and Soil Failure During 1994 Northridge Earthquake

The 1994 Northridge, Calif., earthquake caused widespread permanent ground deformation on the gently sloping alluvial fan surface of the San Fernando Valley. The ground cracks and distributed deformation damaged both pipelines and surface structures. To evaluate the mechanism of soil failure, detailed subsurface investigations were conducted at four sites. Three sites are underlain by saturated sandy silts with low standard penetration test and cone penetration test values. These soils are similar to those that liquefied during the 1971 San Fernando earthquake, and are shown by widely used empirical relationships to be susceptible to liquefaction. The remaining site is underlain by saturated clay whose undrained shear strength is approximately half the value of the earthquake-induced shear stress at this location. This study demonstrates that the heterogeneous nature of alluvial fan sediments in combination with variations in the ground-water table can be responsible for complex patterns of permanent ground deformation. It may also help to explain some of the spatial variability of strong ground motion observed during the 1994 earthquake.     

Evaluation of Seismic Performance for Tunnel Retrofit Project

A seismic retrofit program at Yerba Buena Island Tunnel is presented as a case history. The studies include evaluations of portal stability under earthquake excitation and performance of the tunnel liner as a result of seismic induced deformation. Due to different potential failure modes at the two portal areas, two separate evaluation techniques were utilized. Key block theory in conjunction with a Newmark type analysis was used to assess movements of a potential failure wedge at the west portal slope, while discontinuous deformation analysis was utilized at the east portal slope to evaluate a rotational mode of failure. To assess the performance of the liner subjected to design earthquakes, a two-step analysis procedure was adopted. The first step was to compute seismic induced deformations of the tunnel subjected to seismic wave propagation through the island rock without the presence of the liner. The second step, not reported in this paper, involved imposing the deformations of the tunnel onto the structural liner through spring elements that accounted for interaction between the liner and the surrounding rock. From the studies, performance of the existing tunnel supports was found to be acceptable.     

Simplified Procedure for Estimating Earthquake-Induced Deviatoric Slope Displacements

A simplified semiempirical predictive relationship for estimating permanent displacements due to earthquake-induced deviatoric deformations is presented. It utilizes a nonlinear fully coupled stick-slip sliding block model to capture the dynamic performance of an earth dam, natural slope, compacted earth fill, or municipal solid-waste landfill. The primary source of uncertainty in assessing the likely performance of an earth/waste system during an earthquake is the input ground motion. Hence, a comprehensive database containing 688 recorded ground motions is used to compute seismic displacements. A seismic displacement model is developed that captures the primary influence of the system’s yield coefficient (ky), its initial fundamental period (Ts), and the ground motion’s spectral acceleration at a degraded period equal to 1.5Ts. The model separates the probability of “zero” displacement (i.e., ? 1?cm) occurring from the distribution of “nonzero” displacement, so that very low values of calculated displacement do not bias the results. The use of the seismic displacement model is validated through reexamination of 16 case histories of earth dam and solid-waste landfill performance. The proposed model can be implemented rigorously within a fully probabilistic framework or used deterministically to evaluate seismic displacement potential.     

Summary of SAC Case Study Building Analyses

Summarized in this paper are the major findings from analytical studies of nine steel moment frame buildings conducted under Phase 1 of the SAC Steel Project. The buildings range in height from two to seventeen stories and most of them experienced damage to welded beam-column connections during the Northridge earthquake of 1994. Elastic response spectrum, inelastic static pushover, and elastic and inelastic time-history analyses were conducted using ground motion data representative of the Northridge earthquake to establish the loading∕deformation demands that the buildings experienced. The primary performance indices obtained from the analyses were demand-to-capacity ratios, interstory drift ratios, and inelastic hinge rotations. Maximum ratios of elastic member force demands to plastic strengths ranged between 1.0 and 2.0; maximum inelastic hinge rotations were 0.005–0.010 rad; and maximum interstory drift ratios were from 1 to 2%. These damage indices increased by 50%–150% under more severe ground motions recorded during the Northridge earthquake at the Sylmar site. Accuracy of the analyses is shown to be sensitive to a number of modeling parameters including finite joint size, joint panel behavior, composite beam action, strain hardening, second-order (P-Δ) effects, and three-dimensional response. Overall, there was only modest correlation between the frame performance indices and the observed connection damage, due largely to the fact that significant aspects of the connection fracture behavior are not captured in the frame analyses.     

Potential for Structural Failure in the Seismic Near Field

This study analyzes the possibility that large distortions and distortion rates due to wave-propagation phenomena within structures were responsible for unexpected cracking at connections of steel-frame buildings in the seismic near-field region during the Northridge (1994) and Kobe (1995) earthquakes. Since such internal wave propagation is characteristic of a structure with a continuous distribution of mass, the problem is studied by numerically simulating the structural response for both discrete and continuous models of a 20-story building, using ground motion time histories from the Northridge earthquake. The time histories are chosen from the far-field and near-field regions of the earthquake to determine if wave-propagation effects within the structure are especially significant in the near field. A truncated modal analysis is also performed using only the first vibrational mode to see if significantly lower response levels result. It is found that the continuous model gives higher response levels—indicating that wave propagation may have been a factor—but the discrepancy is not limited to the near field. Strain rates are higher from the continuous model than from the discrete model and much higher than from the truncated modal analysis, but the magnitudes are too low to be a significant factor in the observed damage. The explanation for the connection cracking may simply be high-intensity ground motion in the near field.     

National Program to Improve Seismic Performance of Steel Frame Buildings

This paper reviews the performance of welded steel moment frame buildings during the Northridge earthquake. Some of the studies being undertaken in the United States as part of the FEMA-funded SAC Steel Project are described. The intent of these studies is to devise improved methods for designing new steel frame structures; for inspecting, evaluating, and repairing seismic damage to these types of structures following a major earthquake; and for inspecting, evaluating, and retrofitting existing at-risk steel frame buildings. General observations resulting from these studies are highlighted and the overall format for the new design provisions is presented.     

Seismic Performance Assessment of Damage-Controlled FRP-Retrofitted RC Bridge Columns Using Residual Deformations

Despite the improved performance of fiber-reinforced plastic (FRP)-retrofitted bridges, residual deformations in the event of an earthquake are inevitable. Little consideration is currently given to these deformations when assessing seismic performance. Moreover, important structures are currently required not only to have high strength and high ductility but also to be usable and repairable after high intensity earthquakes. This paper presents a definition of an FRP-RC damage-controllable structure. An intensive study of 109 bridge columns, extracted from recent research literature on the inelastic performance of FRP retrofitted columns with lap-splice deficiencies, flexural deficiencies, or shear deficiencies, is used to evaluate the recoverability of such retrofitted columns. The residual deformation, as a seismic performance measure, is used to evaluate the performance of 39 FRP-retrofitted RC columns from the available database. Based on this evaluation, a requirement for the recoverable and irrecoverable states of FRP-RC bridges is specified. Finally, the Seismic Design Specifications of Highway Bridges for RC piers is adapted to predict the residual deformations of FRP-RC columns.     

Seismic Performance and Deformation of Levees: Four Case Studies

During the 1989 Loma Prieta earthquake the Pajaro River levees near Watsonville, Calif., spread laterally at multiple locations. Four of these locations are discussed in this paper. At one location, an industrial facility was also damaged and a dispute arose as to whether lateral spreading of the adjacent levee was the cause. Stability analyses were made of the industrial site for conditions before, during, and after the earthquake. To confirm the findings, analyses were also made of three other nearby locations where the actual deformation was documented and the subsurface conditions are well defined. The calculated levee deformations at the four locations are quite consistent with the observed movements (up to 60 cm). This experience provides increased confidence in the methods of analysis described, for the characterized subsurface conditions, and the range of ground motions experienced. Additional analyses made using the more recently developed multilinear regression lateral-spreading model (e.g., Youd et al. in 1999) yielded inconsistent results.     

Dynamic Testing of a Masonry Structure on a Passive Isolation System

Lightly reinforced and unreinforced masonry buildings have not performed well in earthquakes. Evaluation of past performance of masonry structures has led to more stringent design and construction requirements in the current building codes, and has raised concerns about the performance of existing lightly reinforced and unreinforced masonry buildings in future earthquakes. Base isolation has been shown to be effective in reducing damage to large building structures, and appears to be particularly effective in protecting stiff masonry structures. Using the base isolation principle, Kansas State University’s stiffness decoupler for the base isolation of structures (SDBIS) was designed to effectively reduce the acceleration and force transferred into a building superstructure during a seismic event. The sliding system uses a passive method to provide damping and to dissipate some of the kinetic energy to reduce relative displacements. In addition, the SDBIS system includes a self-centering element that will recover the majority of the induced displacement and provide resistance to overturning. In order to apply the SDBIS system to the masonry building industry, dynamic testes were performed to evaluate the structural response of a full-size one-story masonry model that was supported by the SDBIS system. Acceleration time-history results are presented for dynamic tests using the July 21, 1952 Kern County earthquake, Station 1095 Taft Lincoln School record, the May 19, 1940 Imperial Valley earthquake, Station 117 El Centro Array #9 record, the February 9, 1971 San Fernando earthquake, Station 279 Pacoima Dam record, and the January 17, 1994 Northridge earthquake, Station 24436 Tarzana Cedar Hill record ground motions. Test results show the system is effective when used with a masonry structure.     

Seismic Behavior of Steel Girder Bridge Superstructures

Recent earthquakes exposed the vulnerabilities of steel plate girder bridges when subjected to ground shaking. This paper discusses the behavior of steel plate girder bridges during recent earthquakes such as Petrolia, Northridge, and Kobe. The paper also discusses the recent experimental and analytical investigations that were conducted on steel plate girder bridges and their components. Results of these investigations showed the importance of shear connectors in distributing and transferring the lateral forces to the end and intermediate cross frames. Also, these investigations showed the potential of using end cross frames as ductile elements that can be used to dissipate the earthquake input energy. The paper also gives an update on specifications and guidelines for the seismic design of steel plate girder bridges in the United States.     

Probabilistic Performance-Based Procedure to Evaluate Pile Foundations at Sites with Liquefaction-Induced Lateral Displacement

Liquefaction-induced ground deformation has caused major damage to bridge and wharf structures in past earthquakes. Large lateral ground displacements may induce significant forces in the foundation and superstructure, which may lead to severe damage or even collapse. A performance-based earthquake engineering (PBEE) approach can provide an objective assessment of the likely seismic performance, so that agencies can evaluate bridge or wharf structures, compare retrofit strategies, and rank them within their overall system. In this paper, a probabilistic PBEE design procedure that incorporates findings from recent research on this problem is presented. The proposed approach can provide answers in terms that are meaningful to owners, such as expected repair costs and downtimes. The methodology is validated through its application to a well-documented case history. Results show that the proposed approach provides a good estimate of the seismic performance of pile-supported structures at sites with liquefaction-induced lateral displacement.     

Inelastic Damage Analysis of Reinforced Concrete Bridge Columns Based on Degraded Monotonic Energy

This paper summarizes the prediction of seismic damage of two existing bridges. The objective is to apply a damage index definition for reinforced concrete bridge columns under cyclic loading to existing bridge columns that might experience real seismic loading in the future, and to evaluate the ability of the damage index in describing the damage progression of the bridge columns during real seismic loading. Two existing bridges were selected from the Greater Vancouver Area in Canada. The first bridge, the Garneau Flyover, was designed in 1985 to ATC-6-1981 and is expected to have sufficient resistance to lateral earthquake loading. The second bridge, the Clydesdale Street Underpass, was designed long before ATC-6-1981 and is expected to show little or no lateral earthquake resistance. The damage index is applied to each of these structures, with columns modeled using the CANNY nonlinear structural analysis program. Shear and bond slip deformations were considered by making a simple modification to the column flexural properties. A series of nonlinear dynamic analyses were performed using records from the 1971 San Fernando, 1978 Miyaki-Oki (Japan), 1989 Loma Prieta, and 1999 Taiwan earthquakes fitted to the Vancouver firm ground spectrum. The calculated damage index provides a simple numerical indicator of the damage during an earthquake, easily computed from the results of a nonlinear dynamic analysis.     

Temporal aN and aT in Earthquake Ground Motion at a Single Point

This paper describes a continued study on three-dimensional temporal characteristics of earthquake ground motions at a single point. Based on an instantaneous tangential and normal acceleration decomposition of ground acceleration trajectory, a ground motion can be partitioned into a finite sequence of staggered time intervals of acceleration and deceleration. A formulation is developed to estimate speed and angular changes over a partitioned interval in terms of rates of positive and negative tangential and normal acceleration. Based on these concepts, general ground motion properties, peak ground acceleration, peak ground velocity, and peak ground displacement are examined. Several Northridge earthquake records are studied in detail. It is found that the highest peak of ground acceleration in these records corresponds to a high peak of deceleration, and a velocity maximum often precedes the peak of acceleration.     

Failure Analysis of Modular-Block Reinforced-Soil Walls during Earthquakes

Several modular-block reinforced-soil retaining walls failed during the 1999 Ji-Ji (Chi-chi) earthquake of Taiwan. Similar walls showed distress during the 1994 Northridge, Calif., earthquake. The instability or failure of these walls offered an opportunity to validate the simplistic pseudostatic limit-equilibrium procedures. In this study, the Ta Kung Wall of the Ji-Ji earthquake is analyzed, and the Gould and Valencia Walls of the Northridge earthquake are revisited with an improved estimation of local site acceleration. The local acceleration was estimated by using simple attenuation relationships obtained through the earthquake records. The results of analysis indicate that these three walls had adequate internal stability under estimated site acceleration. The geosynthetic length was inadequate to resist compound modes of failure where the potential failure surface extends beyond the reinforced zone. The external stability was most critical in the presence of horizontal and vertical accelerations.     

Bayesian State Estimation Method for Nonlinear Systems and Its Application to Recorded Seismic Response

The focus of this paper is to demonstrate the application of a recently developed Bayesian state estimation method to the recorded seismic response of a building and to discuss the issue of model selection. The method, known as the particle filter, is based on stochastic simulation. Unlike the well-known extended Kalman filter, it is applicable to highly nonlinear systems with non-Gaussian uncertainties. The particle filter is applied to strong motion data recorded in the 1994 Northridge earthquake in a seven-story hotel whose structural system consists of nonductile reinforced-concrete moment frames, two of which were severely damaged during the earthquake. We address the issue of model selection. Two identification models are proposed: a time-varying linear model and a simplified time-varying nonlinear degradation model. The latter is derived from a nonlinear finite-element model of the building previously developed at Caltech. For the former model, the resulting performance is poor since the parameters need to vary significantly with time in order to capture the structural degradation of the building during the earthquake. The latter model performs better because it is able to characterize this degradation to a certain extent even with its parameters fixed. For this case study, the particle filter provides consistent state and parameter estimates, in contrast to the extended Kalman filter, which provides inconsistent estimates. It is concluded that for a state estimation procedure to be successful, at least two factors are essential: an appropriate estimation algorithm and a suitable identification model.     

Seismic Performance of RC Bridge Column under Repeated Ground Motions

Seismic performance of reinforced concrete bridge column under repeated earthquake ground motions is investigated through shake-table experimentation on a scale model. The specimen is subjected to a series of simulated ground motions at different levels of shaking intensity. The deformation and damage evolution of the test column is addressed in terms of selected mechanical quantities including the effective stiffness, hysteretic energy dissipation, residual displacement, and dominant vibration frequency. The test column, designed according to the AASHTO seismic design specifications, survived successive ground motions by virtue of its outstanding energy-absorption and ductility capacity. Analysis of the experimental data indicates that structural degradation of the column closely correlates with its decreasing effective stiffness and increasing hysteretic energy dissipation. The residual displacement measured at the column top after each shaking event increases with the growth of damage in the column. A frequency-domain analysis of the vibration response of the column during successive ground motions indicates that increase in the structural degradation of the column results in a decrease in the dominant vibration frequency of the column.