Protecting Base-Isolated Structures from Near-Field Ground Motion by Tuned Interaction Damper

This paper deals with the combined use of a low-damping base-isolation system and a semiactive control system referred to as a tuned interaction damper (TI damper). The TI damper generates friction-type forces (rigid-plastic behavior) through interactions between the primary isolated structure and an auxiliary structure. Because of its energy-dissipation nature, a base-isolated structure controlled by a TI damper is inherently stable, and as a semiactive control device, its operation requires only minimal external power. The efficacy of the proposed hybrid system is examined through a numerical simulation for a five-story scaled building subjected to near-field ground motions. A sensitivity analysis on the parameter dependence of the structural response on control force limit, stiffness ratio, and frequency ratio is presented. By tuning these parameters to optimal values, the performance of the base-isolated structure equipped with a TI damper can be enhanced. Based on the numerical simulation results, it is concluded that a TI damper is capable of suppressing the base drift of base-isolated structures subjected to near-field earthquake ground motions while maintaining the superstructure interstory drift and accelerations at small levels.     

“Smart” Base Isolation Strategies Employing Magnetorheological Dampers

One of the most successful means of protecting structures against severe seismic events is base isolation. However, optimal design of base isolation systems depends on the magnitude of the design level earthquake that is considered. The features of an isolation system designed for an El Centro-type earthquake typically will not be optimal for a Northridge-type earthquake and vice versa. To be effective during a wide range of seismic events, an isolation system must be adaptable. To demonstrate the efficacy of recently proposed “smart” base isolation paradigms, this paper presents the results of an experimental study of a particular adaptable, or smart, base isolation system that employs magnetorheological (MR) dampers. The experimental structure, constructed and tested at the Structural Dynamics and Control/Earthquake Engineering Laboratory at the Univ. of Notre Dame, is a base-isolated two-degree-of-freedom building model subjected to simulated ground motion. A sponge-type MR damper is installed between the base and the ground to provide controllable damping for the system. The effectiveness of the proposed smart base isolation system is demonstrated for both far-field and near-field earthquake excitations.     

Response Spectrum Analysis of Suspension Bridges for Random Ground Motion

The response spectrum method of analysis for suspension bridges subjected to multicomponent, partially correlated stationary ground motion is presented. The analysis is based on the relationship between the power spectral density function and the response spectrum of the input ground motion and fundamentals of the frequency domain spectral analysis. The analysis duly takes into account the spatial correlation of ground motions between the supports, the quasi-static component of the response, and the modal correlation between different modes of vibration. A suspension bridge is analyzed under a set of important parametric variations in order to (1) compare between the responses obtained by the response spectrum method of analysis and the frequency domain spectral analysis; and (2) investigate the behavior of suspension bridges under seismic excitation. The parameters include the spatial correlation of ground motion, the angle of incidence of the earthquake, the ratio between the three components of ground motion, the number and nature of modes considered in the analysis, and the nature of the power spectral density function of ground motion. It is shown that the response spectrum method of analysis provides a fair estimate of responses under parametric variations considered in the study.     

Probabilistic Critical Excitation Method for Earthquake Energy Input Rate

Since earthquake ground motions and their input effects on structures are very uncertain even with the present state of knowledge, it is desirable to develop a “robust” structural design method taking into account these uncertainties. Approaches based on critical excitation methods have been proven to be promising for such robust structural design. A new critical excitation method is developed here in which the mean earthquake energy input rate is chosen as a measure of criticality. The earthquake energy input rate is closely correlated with the story deformation and this supports the suitability of the energy input rate as a criticality measure in the case where the deformation is crucial in the design. The ground motion is described as a uniformly modulated nonstationary random process. The power [area of power spectral density (PSD) function] and the intensity (magnitude of PSD function) are fixed and the critical excitation is found under these restrictions. The key for finding the new random critical excitation is the interchange of the order of the double maximization procedures with respect to time and to the PSD function. Examples for a specific envelope function of the ground motion are presented for demonstrating the validity of the proposed method. Extension of the proposed method will be discussed for a more general ground motion model, i.e., nonuniformly modulated nonstationary models, and for a more general problem for variable envelope functions and variable frequency contents.     

Energy Dissipation in Surface Layer due to Vertically Propagating SH Wave

Vertical array data recorded during the 1995 Kobe earthquake are used to calculate the upward and downward energy flow based on one-dimensional SH-wave multireflection theory, from which the energy dissipation in a surface layer is evaluated as their residual. The dissipated energy thus evaluated in a liquefied site is found to reach about 70% of the upward input energy, which indicates that soil nonlinearity and liquefaction serve as effective energy absorbers. In contrast, more energy returns to deeper ground in sites without strong nonlinear behavior. Furthermore, the dissipated energy in the surface layer tends to increase nonlinearly in a convex shape with increasing equivalent damping ratio of the soil there. A simplified two-layer system indicates that the energy dissipation is influenced not only by the soil damping in the surface layer but also by the impedance ratio between the base and surface layers and the input frequency. The same convex relationship is also obtained in the two-layer system, indicating that the simplified system may reflect some important aspects of the energy dissipation mechanisms in the ground.     

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.     

Elastic-Plastic Seismic Behavior of Long Span Cable-Stayed Bridges

This paper investigates the elastic-plastic seismic behavior of long span cable-stayed steel bridges through the plane finite-element model. Both geometric and material nonlinearities are involved in the analysis. The geometric nonlinearities come from the stay cable sag effect, axial force-bending moment interaction, and large displacements. Material nonlinearity arises when the stiffening steel girder yields. The example bridge is a cable-stayed bridge with a central span length of 605 m. The seismic response analyses have been conducted from the deformed equilibrium configuration due to dead loads. Three strong earthquake records of the Great Hanshin earthquake of 1995 in Japan are used in the analysis. These earthquake records are input in the bridge longitudinal direction, vertical direction, and combined longitudinal and vertical directions. To evaluate the residual elastic-plastic seismic response, a new kind of seismic damage index called the maximum equivalent plastic strain ratio is proposed. The results show that the elastic-plastic effect tends to reduce the seismic response of long span cable-stayed steel bridges. The elastic and elastic-plastic seismic response behavior depends highly on the characteristics of input earthquake records. The earthquake record with the largest peak ground acceleration value does not necessarily induce the greatest elastic-plastic seismic damage.     

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.     

Exact Solution of Nonlinear Hysteretic Responses Using Complex Mode Superposition Method and its Application to Base-Isolated Structures

In this paper, the responses of the structures subjected to arbitrary ground motions are evaluated. With the problem formulated in a state-space form, an exact solution scheme capable of dealing with a variety of material cases is proposed, including cases when postyield stiffness exhibits strain-hardening, strain-softening, and elastic-perfectly plastic properties, respectively. The proposed method can provide much higher accuracy, and requires less computational effort than the traditional step-by-step integration solution technique. The reason for these advantages is discussed and the related formulas are provided. In addition, a new efficient approach is provided for evaluating dynamic response of nonlinear base-isolated structures by taking advantage of their characteristics. Two buildings excited by real earthquake and harmonic ground motions are considered in numerical examples to demonstrate the efficiency of the proposed method.     

Theoretical Model of Shield Behavior During Excavation. II: Application

Simulation of the shield behavior is carried out by applying the model of the loads acting on the shield to the shield tunneling work. The shield tunneling was carried out using an earth pressure balanced shield in sandy gravel layer entirely and the observed data were obtained by the real time measuring data system. To escape the errors of data, the filtering process is applied to the measured data prior to processing the simulation. The input ground properties are carefully determined from the two-dimensional finite element analysis, reverse analysis, and empirical values. The simulation results are compared with the observed data to verify the model performance. The results indicate that; (1) the ground loosening due to local collapse of the ground at the shield face and at the shield crown are necessary to take into consideration in order to simulate the actual shield behavior; (2) the force due to the deformation of wire brush affects the shield postures; and (3) the simulation results are in good agreement with the observed data.     

Control of food handling by cutaneous receptor input in squirrels

In a complementary neuroanatomical study by Brenowitz in 1980, it was shown that tree squirrels (Sciurus niger) have a higher relative density of mechanoreceptors in their glabrous forepaw skin than do ground squirrels (Spermophilus tridecemlineatus). The main purpose of this sudy was to test the prediction that tree squirrels would depend upon somatic sensory (cutaneous) input from their forepaws to a greater extent than would ground squirrels in food handling behavior. In addition, a series of more general questions about the sensory control of food handling was examined. First, using different sized food items, it was shown that food handling (rate of manipulation) is subject to sensory control, in general. Secondly, comparision of sham-operated groups with groups receiving median nerve (innervating the palmar surface) lesions showed that cutaneous input from the volar surface of the forepaw contributes to the sensory control in both species of squirrels. Thirdly, comparison of lesion effects in the two species showed that, as predicted, tree squirrels depend upon cutaneous input from their volar forepaw to a greater extent than do ground squirrels. Fourthly, by reanalyzing the above data it was shown that there is continued sensory feedback from food items rather than only an initial evaluation of them.     

Seismic Energy Dissipation of Inelastic Structures with Tuned Mass Dampers

The energy transfer process of using a tuned mass damper (TMD) in improving the ability of inelastic structures to dissipate earthquake input energy is investigated. Inelastic structural behavior is modeled by using the force analogy method, which is the backbone of analytically characterizing the plastic energy dissipation in the structure. Numerical simulations are performed to study the energy responses of structures with and without TMD installed. The effectiveness of TMD in reducing energy responses is also studied by using plastic energy spectra for various structural yielding levels. Results show that the use of TMD enhances the ability of the structures to store larger amounts of energy inside the TMD that will be released at a later time in the form of damping energy when the response is not at a critical state, thereby increasing the damping energy dissipation while reducing the plastic energy dissipation. This reduction of plastic energy dissipation relates directly to the reduction of damage in the structure, and TMD is therefore concluded to be quite effective in protecting structures from suffering major damage during an earthquake. However, storing energy in the TMD is restricted if the structure becomes plastic at a small displacement level. In this case, the effectiveness of TMD diminishes, and the structural response becomes practically the same as those without TMD installed.     

Closed-Form Sensitivity of Earthquake Input Energy to Soil-Structure Interaction System

The input energy to a soil-structure interaction (SSI) system during earthquake shaking is taken as a structural performance measure and is formulated in the frequency domain. The purpose of this paper is to derive the closed-form expression of the sensitivity of the input energy to the SSI system with respect to uncertain parameters representing soil stiffness and damping. It is demonstrated first that the input energy expression can be of a compact form consisting of the product between the input motion component (Fourier amplitude spectrum of acceleration) and the structural model component (so-called energy transfer function). With the help of this compact form, it is shown that the formulation of earthquake input energy in the frequency domain is essential for deriving the closed-form expressions of the sensitivity of the input energy to the SSI system with respect to uncertain parameters in contrast to the time-domain formulation including inevitable numerical error and instability. This formulation is then extended to a multidegree-of-freedom superstructure model. Numerical examples support the fact that the closed-form expressions enable one to find in a reliable and efficient way the most critical combination of the uncertain parameters that leads to the maximum energy input.     

Effects of Seismically Induced Pounding at Expansion Joints of Concrete Bridges

This paper presents the result of a study on the effect of pounding at expansion joints on concrete bridge response to earthquake ground motions. An engineering approach, rather than continuum mechanics approach, is emphasized. First, the dynamic behavior of a damped multidegree-of-freedom bridge system separated by an expansion joint involving an impact is examined by means of the finite element method. Second, the sensitivity analysis of the stiffness in gap elements is performed. Third, usefulness of the analysis method for simulation of pounding phenomena is demonstrated and the effect of pounding on the ductility demands measured in terms of the rotation of column ends is investigated. Two-dimensional finite element analysis using a bilinear hysterestic model for bridge substructure joints and a nonlinear gap element for the expansion joint is performed on a realistic bridge with an expansion joint. The effects of the primary factors on the ductility demand such as gap sizes and characteristics of earthquake ground motion are investigated through a parametric study. The major conclusions are (1) the effect of impact most directly depends on the size of momentum (or pounding magnitude); and (2) the pounding effect is generally found to be negligible on the ductility demand for wide practical ranges of gap size and peak ground acceleration, but is potentially significant at the locations of impact.     

1D Time-Domain Solution for Seismic Ground Motion Prediction

A full time-domain solution for predicting earthquake ground motion based on the 1D viscoelastic shear-wave equation is presented. The derivation results in a time-domain equation in the form of an infinite impulse response filter. A solution in the time domain has several advantages including causality, direct modeling of impulsive and transient processes, and ease of inclusion of nonlinear soil behavior. The method is applicable to any arbitrarily layered silhouette presented as SH-wave velocity, damping coefficient, and mass density profiles for designated soil intervals. For nonlinear evaluations, an equivalent-linear formulation is incorporated and the standard modulus and damping degradation curves become part of the input set. Input motion can be either rock-outcrop or body-wave motions measured or estimated at the bottom of the geologic profile, and the output is the estimated ground motion time history. Application of the method to vertical array strong motion records from Garner Valley, and Wildlife Site, Calif., shows that predicted surface (and interval) ground motion is virtually identical to that measured. The differences between the results of linear and nonlinear analyses are negligible for most cases. A comparison of the time-domain model with SHAKE shows that SHAKE fails to accurately predict time histories in some situations, whereas the time-domain solution always yields satisfactory predicted surface ground motions.     

Wave Passage and Ground Motion Incoherency Effects on Seismic Response of an Extended Bridge

This paper investigates the implications of ground motion spatial variability on the seismic response of an extended highway bridge. An existing 59-span, 2,164-meter bridge with several bearing types and irregularity features was selected as a reference structure. The bridge is located in the New Madrid Seismic Zone and supported on thick layers of soil deposits. Site-specific bedrock input ground motions were selected based on a refined probabilistic seismic hazard analysis of the bridge site. Wave passage and ground motion incoherency effects were accounted for after propagating the bedrock records to the ground surface. The results obtained from inelastic response-history analyses confirm the significant impact of wave passage and ground motion incoherency on the seismic behavior of the bridge. The amplification in seismic demands exceeds 150%, whereas the maximum suppression of these demands is less than 50%. The irregular and unpredictable changes in structural response owing to asynchronous earthquake records necessitate in-depth seismic assessment of major highway bridges with advanced modeling techniques to realistically capture their complex seismic response.     

Site Response at Treasure and Yerba Buena Islands, California

A variety of methods are utilized to reinvestigate the physical relationship between the seismic response of Treasure Island (TI) and Yerba Buena Island (YBI) in California. These islands are a soil (TI) and rock (YBI) site pair separated by 2 km. The site pair has been used previously by researchers to identify soil response to earthquake shaking. Linear regime ground motions (MW4.0–MW4.6 and PGA: 0.014–0.017 g) recorded in the TI vertical array indicate a coherent wavefield in the sediments and an incoherence between the rock and sediments. Our analyses show that the greatest change in the wavefield occurred between the rock and soil layers, corresponding to a significant impedance contrast. The waveforms change very little as they propagate through the sediments, indicating that the site response is a cumulative effect of the entire soil structure and not a result of wave propagation within individual soil layers. In order to highlight the complexity of the site response, correlation analysis was used to demonstrate that the rock and soil ground motions were not highly coherent between the two sites. YBI was, therefore, shown to be an inappropriate reference site for TI. One-dimensional (1D) vertical wave propagation and inverse techniques were used to differentiate between 1D site response and more complex site behavior. Both 1D methods (vertical wave propagation and inverse transfer functions) proved incapable of capturing the site response at TI beyond the initial four seconds of motion. Finite difference waveform modeling, based on a two-dimensional velocity structure of the northern San Francisco Bay was needed to explain the linear site response at TI as horizontally propagating surface waves trapped in the bay sediments. A simplified velocity structure for the San Francisco Bay including a single 100 m basin layer (constant shear-wave velocity of 400 m/s) over a 1.5 km/s layer of Franciscan bedrock was able to trap energy in the basin and produce surface waveform ringing similar to that observed in the TI data. Due to surface waves propagating in the San Francisco Bay sediments, any 1D model will not fully characterize site response at TI. All 1D models will fail to produce the late arriving energy observed in the ground motions.     

Compaction Grouting Test Program for Liquefaction Control

Following the 1989 Loma Prieta earthquake a detailed analysis of liquefaction risk was carried out for an industrial site near the Pajaro River at Watsonville, Calif. A ground improvement project, by grouting, was proposed to prevent lateral spreading and a compaction grout test program was undertaken to validate and refine the project design. In phase 1 of the test program the grout hole spacing was 2.5 and 1.8 m (8 and 6 ft). In phase 2, a closer hole spacing was used, 1.5 and 1.2 m (5 and 4 ft.) The test program employed cone penetration tests and standard penetration tests measurements for evaluation before and after each phase. The results showed the relationship of ground improvement versus grout hole spacing, grout take, and grout pressure. The test program showed that a zone of susceptible soils at the site could be effectively improved to increase their strength and resistance to liquefaction and thus prevent lateral spreading during severe earthquakes.     

Seismic Energy Dissipation of Inelastic Structures with Multiple Tuned Mass Dampers

The ability to use multiple tuned mass dampers (TMDs) in improving inelastic structural performance to dissipate the earthquake input energy is investigated. Inelastic structural behavior is modeled using the force analogy method, which is the backbone of analytically characterizing the plastic energy dissipation in the structure. Both tuning period and placement of the multiple TMDs are studied to give the best structural performance in terms of plastic energy dissipation. Numerical simulations are performed to study the energy responses of structures with and without TMD installed, and the effectiveness of TMDs in the reduction of energy responses is also studied by using tuned mass spectra. Results show that the installation of TMDs gives the structure additional capability of dissipating a large amount of damping energy and at the same time reducing the amount of plastic energy demand and therefore reducing damage in the structure. More important, TMDs have the ability to draw the plastic energy dissipation at the lower stories and release it to the upper stories. This is particularly beneficial for structures that would otherwise suffer more damage at the lower stories than the upper stories. However, the reduction in plastic energy dissipation is quite sensitive to the earthquake vibration characteristics, and TMDs should not be used for structures with weak upper stories.     

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.