Unidirectional Ground Motions Research Articles

Shaking table tests and numerical study of a sliding isolation bearing for the seismic protection of museum artifacts

Isolation bearings are a seismic strengthening technique widely applied in building structures. However, sliding isolation bearings are seldom used for the seismic protection of artifacts housed in museums. The purpose of this study is to investigate the ability of the designed sliding isolation bearing to protect museum artifacts during earthquakes considering floor acceleration amplification. To better explore the effect of the magnification of the floor acceleration response on the seismic performance of the sliding isolation bearing, a three-story reinforced concrete frame structure was constructed, and a sliding isolation bearing-showcase-artifact system was placed on the floor of the structure. The seismic effectiveness of the proposed system was then evidenced by carrying out shaking table tests. It was observed from the tests that the sliding isolation bearing can effectively reduce the peak input acceleration, thus ensuring the safety of the artifact. Moreover, a seismic fragility analysis method is developed to evaluate the seismic performance of the sliding isolation bearing in the museum-showcase-artifact system subjected to unidirectional ground motions. Furthermore, the influences of the horizontal stiffness and the sliding displacement limitation of the bearing on the protective effect of artifacts are discussed based on the seismic performance assessment of the sliding isolation bearing.

Kinematic response of single vertical and batter piles to bidirectional ground motions in liquefiable soil

Kinematic responses of single fixed head vertical and batter piles in liquefiable soil under bidirectional ground motions have been studied with fully-coupled three-dimensional nonlinear dynamic analyses. Pressure dependent multi-yield constitutive model has been adopted for modeling the liquefiable sand behavior. The variable permeability of saturated sand during the process of liquefaction process has also been considered. The analyses are carried out for combinations of bidirectional earthquake loading conditions. For this, combination of longitudinal and transverse ground motions as well as combination of longitudinal and vertical motions are considered. The responses of the soil-pile systems under liquefying ground conditions, subjected to these bidirectional ground motions, are compared with the unidirectional loading conditions. Further, the effect of sloping ground on the response of the piles under bidirectional ground motions is also investigated. The developed sectional forces in both vertical and batter piles are lesser under bidirectional motions in comparison to unidirectional ground motion in both level and sloping grounds. However, the pile displacement is more in case of sloping ground under bidirectional ground motions comparing to unidirectional motion.

Effect of bidirectional ground shaking on structures in the elastic and post-elastic range: Adequacy of design provisions

During an earthquake, structures are generally excited to multiple ground motion components: two orthogonal horizontal components and one vertical component. However, the interaction of the deformations along two principal orthogonal directions in the presence of various levels of axial force in columns in post elastic range can be successfully captured if nonlinear dynamic analysis can be carried out using the bidirectional hysteresis model. On the other hand, the seismic codes prescribed simple combination rules to capture this effect in an equivalent sense even without consideration of axial force in columns. Though these rules may be valid in the elastic range, they may not be applicable in the post-elastic range as mentioned in a few seismic codes. Surprisingly, these codes did not specify any user-friendly provision needed by design offices. Hence, the applicability and efficacy of these rules in the post-elastic range is needed to be compared with that obtained from detailed nonlinear tine history analysis under orthogonal pair of bidirectional ground motions obtained from a number of chosen seismic acceleration data for performance-based design. In this study, idealized low-rise buildings with small, medium, and long periods were investigated, considering the presence of axial force in columns. Most of the current design codes and standards suggest that the addition of response of 30% of ground motion in other direction if added to the response of unidirectional ground motion in the considered direction of analysis or the resultant responses obtained by square root of sum of square from the unidirectional analyses carried out separately in both principal directions (referred as SRSS response) may predict the response with reasonable accuracy. The results obtained from the current study makes an attempt to exhibit the trends of post elastic range response for the structures. Further, it shows that seismic codes options significantly underestimate the post-elastic range seismic demand though they are proved to be adequate to predict elastic range response. This paper may prove helpful in improving code provisions.

Dynamic tests of the collapse-prevention performance of a low-ductility low-rise steel concentrically braced frame

In past earthquakes, low-ductility steel concentrically braced frame (CBF) buildings have been observed to retain reserve capacity after brace fracture. The failure mechanisms and earthquake ground motion-sensitive characteristics of the reserve system of low-ductility CBFs are not understood due to limited dynamic experimental data. In this study, a series of shaking table tests is presented to examine the reserve capacity of a scaled two-bay, three-story, low-ductility CBF model in which a pair of square hollow section braces are arranged in a chevron pattern. The model structure was repeatedly subjected to two unidirectional ground motions with successively increasing magnitudes in a stiff soil site and a soft soil site. The experimental phenomena of elastic response, brace buckling behavior, brace reengagement behavior, reserve system response and collapse state were observed. The braces of the low-ductility CBFs were vulnerable to local buckling under dynamic loads and resulted in a soft story where braces failed, while braces and frames in the other stories had no further damage. The reserve stiffness, rather than energy dissipation or reserve strength, was the most important factor in the collapse-prevention capacity for the reserve system of the tested low-ductility CBF, and the lateral stiffness of the columns was the main source of reserve stiffness. Flexible reserve systems with small lateral stiffness were never significantly excited at their natural periods by hard soil site-specific ground motions, while with such systems, a greater risk of potential collapse appears to be posed due to earthquakes at soft soil sites than at hard soil sites for the test model.

Pounding Analysis of Isolated Girder Bridge under Nonpulse and Pulse-Like Earthquakes

Pounding damages are frequently encountered in concrete continuous girder bridges during strong earthquakes. In the evaluation of pounding responses, the simplified lumped mass model with the contact element are usually adopted. However, the pounding responses calculated based on the simplified model are often significantly different from the real results, because the simplified model fails to consider the details of pounding processes and ignores concrete damage. In addition, the seismic parameters (e.g., peak ground acceleration, peak ground velocity) affects the pounding responses greatly. To further enhance the aforementioned considerations, this study presents the longitudinal pounding analysis of an isolated continuous girder bridge subjected to the unidirectional ground motions based on a multiscale simulation scheme. The pounding between the bridge and side abutments was simulated with a contact algorithm, and the concrete damages were considered. Thirty-three nonpulse and thirty-three pulse-like real ground motions were selected as seismic excitations to investigate the effects of seismic properties on pounding responses. Two pounding patterns were recognized and found to be related to the V-shaped velocity segment of the seismic input. Correlation coefficient and semipartial correlation coefficient were used to analyze the relationship between the seismic parameters and corresponding pounding responses.

Seismic Analysis of Steel Cylindrical Liquid Storage Tank Using Coupled Acoustic-Structural Finite Element Method For Fluid-Structure Interaction

A seismic analysis of ground-supported, three-dimensional (3-D) rigid-base steel cylindrical liquid storage tank is investigated, using a coupled acoustic-structural finite element (FE) method for fluid-structure interaction (FSI). In this method, the contained liquid in the tank is modelled using acoustic elements and the cylindrical tank is modelled using shell elements. The impulsive and convective terms are estimated separately by using the appropriate boundary conditions on the free surface of the liquid. The convergence and validation studies of the proposed FE model are conducted by comparing the results reported in the literature. The parametric studies are performed for rigid and flexible tanks for the varying slenderness of the open roof tanks. The sloshing displacement and base shear time history responses are evaluated for the 3-D tanks subjected to harmonic unidirectional ground motions. Further, the results are compared with the commonly used two and three lumped-mass models of the tank. Moreover, the seismic response quantities of the tank subjected simultaneously to the bi-directional horizontal components of earthquake ground motion are also investigated using the 3-D FE model, and the response quantities are compared with the lumped-mass models. The results obtained from the 3-D FE model and lumped-mass model are in close agreement. The average percentage difference in the 3-D FE and lumped-mass models for maximum sloshing displacement prediction is 15 percent to 20 percent and that for the base shear is about 4 to 10 percent, in the case of the uni-directional harmonic ground motions. It is concluded that the sloshing displacement is not affected by the tank flexibility, but the impulsive hydrodynamic pressure and the impulsive component of the base shear increases with the tank flexibility.

New approach to seismic-resistant design and structural torsion mitigation

A new approach to seismic-resistant design of structures and mitigation of structural torsion is introduced and investigated. It is based on the controlled adaptation of all the structural columns of a single storey to partially decouple the superstructure from the moving ground. In addition, it allows to maintain adequate vertical rigidity together with adding notable horizontal flexibility; reduces storey drift; provides a resistance to lateral loads and to minor vibration; generates a self-restoring mechanism; incorporates a self-stopping mechanism besides ensuring the structural stability under extremely severe earthquakes. The new approach is not intended to form any plastic hinges or rely on them to provide damping, therefore, supplementary damping is provided using a new displacement-based bracing system designed by the author. A set of multi-storey asymmetric structures of 6- to 16-storey are considered to numerically assess the proposal’s efficiency under near-fault and long-period earthworks. Using nonlinear time history analysis, the 3D structures are examined under unidirectional ground motions applied along the most unfavourable horizontal direction, to produce the worst torsional responses that are to be mitigated using the new approach. It was found that the new approach has reduced significantly the structural responses including torsion. The base storey was found the optimum level for applying the new approach to ensure the highest efficiency. Furthermore, it was found that the proposed approach provides all the advantages of seismic isolation plus an additional benefit, that is, the new proposal is suitable for both short- and long-period structures. Simultaneously, the new proposal provides all the good characteristics of supplemental energy dissipation, plus an extra function, as it reduces inter-story drift, additionally.

Reliability and fragility assessment of the mid- and high-rise wood buildings subjected to bidirectional seismic excitation

Often the estimation of reliability and fragility curve for a structure subjected to earthquake loading is carried out for unidirectional ground motions, even though the multicomponent ground motions can affect the seismic responses. A simple procedure to estimate the reliability of 3D bisymmetric buildings subjected to the bidirectional orthogonal ground motions is proposed in the present study. The procedure considers that the responses of a 3D bisymmetric building can be approximated by those of an equivalent nonlinear inelastic two-degree-of-freedom (2DOF) system. The parameters of the equivalent system are identified using the capacity surface of the buildings obtained through nonlinear static pushover analysis or incremental dynamic analysis. The procedure is applied to estimate the reliability and fragility of mid- and high-rise wood buildings subjected to bidirectional ground motions. A comparison of the estimated reliabilities and fragilities by considering uni- and bi-directional ground motions is given, showing the importance of considering the bidirectional ground motions. The results indicate that the failure probabilities under the bidirectional ground motions are greater as compared to those obtained under the unidirectional ground motions. Therefore, for assessing the reliability of building under the seismic ground motions, the consideration of bidirectional ground motion can be important for seismic risk modelling and emergency preparedness.

Implications of bidirectional interaction on nonlinear seismic response of steel piers

The seismic performance assessment of bridges by nonlinear time history analysis using unidirectional spectrum-matched accelerograms is generally adopted in Japan's specifications. However, the simultaneous action of two horizontal components is known to induce a potentially higher inelastic response in steel piers relative to that under unidirectional excitation. In this background, a rising concern within research communities is the bidirectional effect on the risk of bridge failure or damage under a given seismic intensity during the service life of a bridge. In this study, the difference in the risk of steel pier damage/failure between unidirectional ground motion and bidirectional ground motion is assessed using seismic fragility. Fragility curves are constructed by subjecting a single steel pier with various geometric designs to a large number of earthquake ground motions using the incremental dynamic analysis methodology. The study indicates that the damage state of collapse is not changed by bidirectional excitations, whereas the use of bidirectional excitations in less damaging states reduces the bridge seismic performance and induces higher fragility. The response difference is quantified by introducing an equifragility factor. This factor is calculated as the ratio of intensity of the unidirectional input to the bidirectional input, which causes the same fragility caused by the unidirectional counterpart. The constant characteristic value of the equifragility factor provides useful information for considering the bidirectional effect in seismic design.

Energy capacity and seismic performance of RC waffle‐flat plate structures under two components of far‐field ground motions: Shake table tests

SummaryThe bidirectional response of a portion of a reinforced concrete (RC) waffle‐flat plate (WFP) structure subjected to far‐field ground motions is studied through shake table tests. The test specimen is a scaled portion of a prototype structure designed under current building codes and located in a region of moderate seismicity of the Mediterranean area. The specimen was subjected to a sequence of tests of increasing acceleration amplitude that respectively represented very frequent, frequent, design, and very rare earthquakes at the site. The test structure performed well (basically in the elastic domain) under very frequent and frequent earthquakes, approached the boundary between the performance levels of life safety and near collapse under the design earthquake, and collapsed under the very rare earthquake. Damage concentrated at column bases and at the transverse beams of the exterior plate‐to‐column connection. Columns dissipated about 10% of the total energy that contributes to damage, and the rest was dissipated by the exterior plate‐column connection. The total energy input on the structure until collapse under the bidirectional seismic action was very close to the value obtained in previous studies on a similar specimen tested under unidirectional ground motions. The capacity curve estimated from the experimental base shear vs top displacement relationship suggests it is best to use a behavior factor of at most q = 2 when designing WFP structures with the reduced‐spectrum force‐based approach.

Effect of the Static Coefficient of Friction of Curved Surface Sliders on the Response of an Isolated Building

ABSTRACT Effective implementation of the static friction of sliding isolators in object-oriented software for structural design has not yet been achieved, and use of the dynamic friction only for design is common practice. A modeling strategy to account for the contribution of static friction under unidirectional ground motion histories has been developed and used to assess its effects on the response of a building isolated with curved surface sliders. Under low-to-moderate intensity earthquakes, disregarding static friction can lead to unpredictable response of the isolation system and result in a non-conservative evaluation of accelerations and forces transferred to the superstructure.

Seismic incidence on base-isolated nuclear power plants considering uni- and bi-directional ground motions

The effects of bi-directional ground motions of the input seismic on base-isolated Nuclear Power Plants are investigated by comparing to the responses under uni-directional ground motion. For this aim, an analytical model is used to calculate the incidence angle (IA) of ground motions that vary from 0 to 360 degrees, with the interval of 15 degrees, and to show their effects on diverse engineering demand parameters. The observed responses corresponding to the displacement, the rotation and the element force are determined. In addition, the critical angles of the structure under two components with respect to one component are obtained. It is found that the maximum responses of the base-isolated structure for individual records may be decreased at some angles while increased at other angles. In case of bi-directional ground motions, the effect of IA is less considerable than uni-directional ground motion. Moreover, the critical responses under bi-directional seismic excitations are more adequate than uni-directional one. The displacement and shear force responses from one component are less than corresponding values from two components.

Estimation of Seismic Loss for a Portfolio of Buildings under Bidirectional Horizontal Ground Motions due to a Scenario Cascadia Event

Earthquake ground motions induced by a scenario event are spatially (partially) correlated and (partially) coherent. Simulated ground motion records can be used to carry out nonlinear inelastic time history analysis for a portfolio of buildings to estimate the seismic loss, which is advantageous as there is no need to develop and apply empirical ground motion prediction equations and the ductility demand rules, or to search the scenario compatible recorded records at selected sites that may not exist. Further, if the structures being considered are sensitive to the orientation of the excitation, multiple-component ground motion records are needed. For the simulation of such ground motion records, previous studies have shown that correlation and coherency between any pair of ground motion components need to be incorporated. In this study, the seismic loss of a portfolio of hypothetical buildings in downtown Vancouver under bidirectional horizontal ground motions due to a scenario Cascadia event is estimated by using simulated bidirectional ground motion records that include realistic correlation and coherency characteristics. The hysteretic behaviours of the buildings are described by bidirectional Bouc-Wen model. The results show that the use of unidirectional ground motions and single-degree-of-freedom (SDOF) system structural model may underestimate the aggregated seismic loss.

Response of a 2-story test-bed structure for the seismic evaluation of nonstructural systems

A full-scale, two-story, two-by-one bay, steel braced-frame was subjected to a number of unidirectional ground motions using three shake tables at the UNR-NEES site. The test-bed frame was designed to study the seismic performance of nonstructural systems including steel-framed gypsum partition walls, suspended ceilings and fire sprinkler systems. The frame can be configured to perform as an elastic or inelastic system to generate large floor accelerations or large inter story drift, respectively. In this study, the dynamic performance of the linear and nonlinear test-beds was comprehensively studied. The seismic performance of nonstructural systems installed in the linear and nonlinear test-beds were assessed during extreme excitations. In addition, the dynamic interactions of the test-bed and installed nonstructural systems are investigated.

Accidental eccentricity in multistory buildings due to torsional ground motion

Accidental eccentricity is aimed to account for any unforeseen factor that may attribute to the torsional response. These sources can be broadly classified into two components, namely, uncertainty in geometry and material properties, and torsional ground motion. Recently, conventional approach of shifting the center of mass was reported to be inappropriate when accidental torsion is contributed from the torsional ground motion and an alternative approach was proposed for the single story buildings subjected to unidirectional ground motion. The alternative approach is further investigated in this paper so as to include the multistory buildings under bidirectional ground motions, given the accidental eccentricity. Also developed is a framework for evaluating the minimum required accidental eccentricity due to torsional ground motion that can be used in developing the seismic code recommendations. Even though the number of response candidates based on which accidental eccentricity can be assessed is many for a multistory building, this paper presents a simple recommendation for the vertically regular buildings.

Practical Modal Pushover Design of one-way asymmetric-plan reinforced concrete wall buildings for unidirectional ground motion

This paper presents a new displacement based seismic design method called Practical Modal Pushover Design (PMPD). The method is applied to multistory one-way asymmetric-plan RC wall structures. PMPD combines concepts from Direct Displacement-Based Design with an inverse version (formulated herein) of Practical Modal Pushover Analysis (PMPA). PMPD generates designs which achieve peak deformations exactly equal to the governing deformation limits when analyzed with PMPA. An alternative method, Modal Pushover Design (MPD), which is, to some extent, an inverse version of Modal Pushover Analysis, is also discussed. MPD is computationally more demanding than PMPD, but has improved performance in cases where yielding may occur due to ‘higher mode’ response. Advantages of PMPD include explicit consideration of nonlinear, torsional and ‘higher mode’ effects. Iteration is limited to the response spectrum level, so multiple analyses of Multi-Degree of Freedom systems are not required. Capacity design principles are implemented directly from the start of the design process. A significant advantage of PMPD is that the engineer can have the same confidence in the structure’s seismic performance as he has in PMPA’s ability to predict the structure’s peak seismic responses. Therefore PMPD can be used for the seismic design of any structure for which PMPA is expected to provide acceptably accurate predictions of peak seismic responses. The effort required to carry out PMPD is similar to that required for PMPA. The only additional work consists of specifying a relative flexural strength distribution and executing a small number of iterations at the Single Degree of Freedom level.

Performance-Based Seismic Vulnerability Evaluation of Masonry Buildings Using Applied Element Method in a Nonlinear Dynamic-Based Analytical Procedure

A thorough four-step performance-based seismic evaluation for a six-story unreinforced masonry building is conducted. Incremental dynamic analysis is carried out using the applied element method to take advantage of its ability to simulate progressive collapse of the masonry structure including out-of-plane failure of the walls. The distribution of the structural responses and inters-tory drifts from the incremental dynamic analysis curves are used to develop both spectral-based (Sa) and displacement-based (interstory drift) fragility curves at three structural performance levels. The curves resulting from three-dimensional (3-D) analyses using unidirectional ground motions are combined using the weakest link theory to propose combined fragility curves. Finally, the mean annual frequencies of exceeding the three performance levels are calculated using the spectral acceleration values at four probability levels 2%, 5%, 10%, and 40% in 50 years. The method is shown to be useful for seismic vulnerability evaluations in regions where little observed damage data exists.

Seismic Analysis of Reinforced Concrete Rib Arch Bridge

There are few researches on seismic response of reinforced concrete rib arch bridges at present; therefore, it is necessary to analyze seismic performance of this kind of bridges. Based on the engineering background of a three-span reinforced concrete rib arch bridge, a full bridge finite element model is built to analyze the structural dynamic characteristic and seismic response of the bridge. The internal forces and displacements of each key section is compared and discussed when the bridge is excited by horizontal unidirectional ground motion or the combination of vertical and horizontal ground motion. The structural seismic response calculated with different analysis methods is compared. The research results of this study can be used as a reference for the seismic design of similar bridges.

Dynamic Response of a Chevron Concentrically Braced Frame

Large-scale shake table tests were conducted at E-Defense, Japan, to examine the dynamic response of a steel concentrically braced frame. The specimen was a single-bay, single-story frame with a pair of square hollow structural section braces placed in a chevron arrangement. The gusset plates connecting the brace to the framing elements were provided with an elliptic fold line to accommodate out-of-plane rotation of the brace in compression. The specimen was subjected repeatedly to a unidirectional ground motion with increasing magnitude until the braces buckled and eventually fractured. The bracing connections performed as intended; the gusset plates folded out of plane, and no crack was observed in the gusset plate or in the critical welds. Consequently, the test results demonstrated excellent performance of the bracing connections. Elastic deformation of the beam prevented the braces from developing their full tensile strength. Yielding in the middle of the beam, which was predicted by monotonic loading analysis, did not occur. The specimen response was reproduced by a numerical model using fiber elements. This model was able to predict the occurrence of brace buckling and fracture and thereby accurately trace the dynamic behavior of the frame. © 2013 American Society of Civil Engineers.

Pseudodynamic response of torsionally unbalanced two‐storey test structure

AbstractTwo one‐way eccentric, two‐storey, one‐by‐one‐bay reinforced concrete (RC) structures are pseudodynamically tested under unidirectional ground motions. Theoretical considerations about the effect of torsional coupling on modal periods and shapes agree with modal results of the test structure, considering member stiffness is equal to the secant stiffness to yielding in skew‐symmetric bending. Modal periods of such an elastic structure are in fair agreement with effective periods inferred from the measured response at the beginning of a test of a thoroughly cracked structure and at the end of the test. A time‐varying stiffness matrix and a non‐proportional damping matrix fitted to the test results may be used to reproduce the measured response approximately by modal superposition and identify the role of the four time‐varying modes. Flexible side columns sustained very large drift demands simultaneously in the two transverse directions and suffered significant but not heavy, damage at lap‐splices. RC‐jacketing of the flexible side columns practically eliminated the static eccentricity between the floor centres of twist and mass as well as the torsional response. Inelastic time‐history analysis with point‐hinge member models, using as elastic stiffness the secant stiffness to yielding and neglecting post‐ultimate‐strength cyclic degradation of resistance in members with plain bars and poor detailing, predicted fairly well the response until the peak displacements and member deformations occurred. After that, it underestimated displacement peaks and the lengthening of the apparent period and missed the gradual drifting of the response towards a permanent offset. Copyright © 2007 John Wiley & Sons, Ltd.