Horizontal Seismic Excitation Research Articles

A comparative study between simplified and nonlinear dynamic methods for estimating liquefaction potential

This paper estimated the liquefaction potential of a saturated soil deposit subjected to a horizontal seismic excitation at its base using the total stress approach. A comparative analysis between the simplified and the nonlinear dynamic methods was used to verify to what extent the simplified method could be reliable. In order to generalise the reliability of the simplified method for any value of the maximum acceleration for the used earthquakes, a correction for the maximum acceleration less than 0.3g was proposed based on the comparison of safety factor values determined by the dynamic method illustrated by the equivalent linear model with lumped masses and the simplified method for a given profile of soil subjected to 38 earthquakes. The nonlinear behaviour of soil was represented by two hyperbolic models: Hardin and Drnevich, and Masing. To determine the cyclic resistance ratio (CRR), the cone penetration test (CPT) based method, the standard penetration test (SPT) based method, and the shear wave velocity based method were used. The safety factor was calculated as the ratio of CRR/CSR, where CSR represents the cyclic stress ratio. The results of the proposed correction have given smaller values of the safety factor compared to the nonlinear dynamic methods for the maximum acceleration less than 0.3g. In other words, by considering this correction, the most unfavourable case is always given by the modified simplified method.

Shape memory alloy reinforced concrete frames vulnerable to strong vertical excitations

Reinforced concrete (RC) framed buildings dissipate the seismic energy through yielding of the reinforcing bars. This yielding jeopardizes the serviceability of these buildings as it results in residual lateral deformations. Superelastic shape memory alloys (SMAs) can recover inelastic strains by stress removal. This paper extends previous research by the authors that optimized the use of SMA bars in RC frames considering the horizontal seismic excitation by addressing the effect of the vertical seismic excitation. A steel RC six-storey building designed according to current seismic standards is considered as case study. Five different earthquake records with strong vertical components are selected for the nonlinear dynamic analysis. The results were used to evaluate the effect of the vertical excitation on the optimum locations of SMA bars.

Bayesian nonlinear structural FE model and seismic input identification for damage assessment of civil structures

A methodology is proposed to update mechanics-based nonlinear finite element (FE) models of civil structures subjected to unknown input excitation. The approach allows to jointly estimate unknown time-invariant model parameters of a nonlinear FE model of the structure and the unknown time histories of input excitations using spatially-sparse output response measurements recorded during an earthquake event. The unscented Kalman filter, which circumvents the computation of FE response sensitivities with respect to the unknown model parameters and unknown input excitations by using a deterministic sampling approach, is employed as the estimation tool. The use of measurement data obtained from arrays of heterogeneous sensors, including accelerometers, displacement sensors, and strain gauges is investigated. Based on the estimated FE model parameters and input excitations, the updated nonlinear FE model can be interrogated to detect, localize, classify, and assess damage in the structure. Numerically simulated response data of a three-dimensional 4-story 2-by-1 bay steel frame structure with six unknown model parameters subjected to unknown bi-directional horizontal seismic excitation, and a three-dimensional 5-story 2-by-1 bay reinforced concrete frame structure with nine unknown model parameters subjected to unknown bi-directional horizontal seismic excitation are used to illustrate and validate the proposed methodology. The results of the validation studies show the excellent performance and robustness of the proposed algorithm to jointly estimate unknown FE model parameters and unknown input excitations.

A pseudo-static method for seismic responses of underground frame structures subjected to increasing excitations

The determinant factor for the seismic response of underground structures is the deformation of surrounding soil, which is different from above-ground structures. Considering that the existing deformation-based seismic design methods for underground structures are based on elastic hypotheses, this study proposes an improved Finite Element method called New Pseudo-Static Analysis (NewPSA) to predict the nonlinear behavior of underground frame structures subjected to increasing horizontal seismic excitations. A new method for one-dimensional(1-D) seismic analysis of soil layers is proposed to calculate the distributions of displacements and shear stresses along the depth in the free-field with increasing bedrock motions. Pseudo-static Finite Element analysis is then carried out by imposing the distributions of body forces derived from the 1-D analysis. A series of comparisons were made to validate the applicability of the proposed method. Its prediction capacity was also compared with that of a well-known method. The proposed method is able to predict the nonlinear performances of underground structures under horizontal earthquake loadings. Particularly, similar to the push-over analysis of above-ground structures, the method has the potential to predict the seismic capacity of underground frame structures subjected to design seismic loadings.

Radial basis function neural network algorithm for semi-active control of base-isolated structures

Curved surface slider (CSS) is considered as an effective isolation device for structures subjected to earthquake ground motions. Due to constant frequency, CSS may encounter a resonance problem when subjected to near-fault earthquake ground motions. To overcome this problem, we propose CSS combined with a control device in this study. The control device consists of variable orifice fluid damper, and its damping coefficient is controlled by a radial basis function-based neural network algorithm. Numerical simulations are performed to evaluate the effectiveness of the proposed technique for only one-directional horizontal seismic excitations without any evaluation concerning the durability of CSSs. The results of the investigation demonstrate that the proposed technique is effective to reduce both the base shear and the sliding displacement of the isolated structure. In addition, the response predicted by the proposed technique is almost similar to the response of isolated structure with passive damper at optimum damping ratio.

A Nonlinear Model Inversion Method for Joint System Parameter, Noise, and Input Identification of Civil Structures

This paper presents a framework for nonlinear system identification of civil structures using sparsely measured dynamic output response of the structure. Using a sequential maximum likelihood estimation (MLE) approach, the unknown FE model parameters, the measurement noise variances, and the input ground acceleration time histories are estimated jointly. This approach requires the computation of FE response sensitivities with respect to the unknown FE model parameters (i.e., FE parameter sensitivities) as well as the FE response sensitivities with respect to the values of the input ground acceleration at every time step (i.e., FE input sensitivities). The FE parameter and input sensitivities are computed using the direct differentiation method (DDM). The presented output-only nonlinear FE model updating method is validated using the numerically simulated seismic response of a realistic three-dimensional five-story reinforced concrete building structure. The simulated building responses to a horizontal bi-directional seismic excitation is contaminated with artificial measurement noise and used to estimate the unknown FE model parameters characterizing the nonlinear material constitutive laws of the reinforced concrete, as well as the root mean square of the measurement noise at each measurement channel, and the full time history of the seismic base acceleration. The method presented in this paper provides a powerful framework for structural system and damage identification of civil structures, when the input excitations are not measured, are partially measured, or the measured input excitations are erroneous.

Experimental study on Y-shaped bridge under 3-dimentional earthquake ground motions

Based on similar theory and multi-dimensional seismic design theory, a Y-shaped bridge model with the scale ratio of 1:20 was made to conduct the seismic simulation shaking table test in this paper. Failure mode and dynamic responses of Y-shaped bridge under different seismic wave spectrum, different peak accelerations and different seismic excitation directions were studied. The experimental results show that, the piers mostly exhibit bending failure mode and the vertical ground motion has a great influence on the development of cracks in girders. Dynamic responses of the Y-shaped bridge are different and presented different laws when three different seismic waves were applied. Seismic wave is input along the direction of the branch straight girder, which is the most unfavorable earthquake input direction when calculating the peak acceleration at the top of piers. Multi-dimension seismic excitation should be considered when calculating the width of the expansion joint between adjacent girders. The horizontal bidirectional seismic excitation can satisfy the design requirements of strain response at the bottom of pier. The model structure would cause vertical vibration during its horizontal seismic excitation, and vertical seismic excitation exerts more significant influence on the vertical vibration of branch straight girder.

Study of seismic performance and favorable structural system of suspension bridges

By taking the Runyang Highway Bridge over the Yangtze River with 1490 m main span as example, structural response of the bridge under the horizontal and vertical seismic excitations is investigated by the response spectrum and time-history analysis of MIDAS/Civil software respectively, the seismic behavior and the influence of structural nonlinearity on the seismic response of the bridge are revealed. Considering the aspect of seismic performance, the suitability of employing the suspension bridge in super long-span bridges is investigated as compared to the cable-stayed bridge and cable-stayed-suspension hybrid bridge with the similar main span. Furthermore, the effects of structural parameters including the span arrangement, the cable sag to span ratio, the side to main span ratio, the girder height, the central buckle and the girder support system etc on the seismic performance of the bridge are investigated by the seismic response spectrum analysis, and the favorable earthquake-resistant structural system of suspension bridges is also discussed.

A superelastic protective technique for mitigating the effects of vertical and horizontal seismic excitations on highway bridges

Vertical component of seismic excitations tremendously affects the performance of bridges during the earthquakes. Several conducted studies identified the lack of engineering attention to the vertical seismic excitation as the main reason of various considerable bridge damages during the past earthquakes. Thus, in this article, an innovative system with superelastic properties is proposed for retrofitting and also new design of the bridges which can simultaneously mitigate the effects of vertical and horizontal seismic excitations. In order to investigate the efficiency of the new system, an evaluation is performed through many nonlinear time history analyses on a three-dimensional model of a detailed multi-span simply supported bridge using a suite of representative ground motions of the bridge region. The analyses are conducted separately on the pertinent issues that affect the performance of the new proposed system. As a part of the study, to identify the sensitivity of the new system and evaluate the overall seismic performance, several assessment parameters are utilized. The results show that the proposed system is efficient for reducing bridge responses as well as improving nonlinear performance of the columns during vertical and horizontal seismic excitations.

Dynamic liquefaction of shear zones in intact loess during simulated earthquake loading

The 2010–2011 Canterbury earthquake sequence in New Zealand exposed loess-mantled slopes in the area to very high levels of seismic excitation (locally measured as >2 g). Few loess slopes showed permanent local downslope deformation, and most of these showed only limited accumulated displacement. A series of innovative dynamic back-pressured shear box tests were undertaken on intact and remoulded loess samples collected from one of the recently active slopes replicating field conditions under different simplified horizontal seismic excitations. During each test, the strength reduction and excess pore water pressures generated were measured as the sample failed. Test results suggest that although dynamic liquefaction could have occurred, a key factor was likely to have been that the loess was largely unsaturated at the times of the large earthquake events. The failure of intact loess samples in the tests was complex and variable due to the highly variable geotechnical characteristics of the material. Some loess samples failed rapidly as a result of dynamic liquefaction as seismic excitation generated an increase in pore water pressure, triggering rapid loss of strength and, thus, of shear resistance. Following initial failure, pore pressure dissipated with continued seismic excitation and the sample consolidated, resulting in partial shear strength recovery. Once excess pore water pressures had dissipated, deformation continued in a critical effective stress state with no further change in volume. Remoulded and weaker samples, however, did not liquefy and instead immediately reduced in volume with an accompanying slower and more sustained increase in pore pressure as the sample consolidated. Thereafter, excess pressures dissipated and deformation continued at a critical state. The complex behaviour explained why, despite exceptionally strong ground shaking, there was only limited displacement and lack of run-out: dynamic liquefaction was unlikely to occur in the freely draining slopes. Dynamic liquefaction, however, remained a plausible mechanism to explain loess failure in some of the low-angle toe slopes, where a permanent water table was present in the loess.

A parametric study on the axial behaviour of elastomeric isolators in multi-span bridges subjected to horizontal seismic excitations

This paper investigates the potential tensile loads and buckling effects on rubber-steel laminated bearings on bridges. These isolation bearings are typically used to support the deck on the piers and the abutments and reduce the effects of seismic loads and thermal effects on bridges. When positive means of fixing of the bearings to the deck and substructures are provided using bolts, the isolators are exposed to the possibility of tensile loads that may not meet the code limits. The uplift potential is increased when the bearings are placed eccentrically with respect to the pier axis such as in multi-span simply supported bridge decks. This particular isolator configuration may also result in excessive compressive loads, leading to bearing buckling or in the attainment of other unfavourable limit states for the bearings. In this paper, an extended computer-aided study is conducted on typical isolated bridge systems with multi-span simply-supported deck spans, showing that elastomeric bearings might undergo tensile stresses or exhibit buckling effects under certain design situations. It is shown that these unfavourable conditions can be avoided with the rational design of the bearing properties and in particular of the shape factor, which is the geometrical parameter controlling the axial bearing stiffness and capacity for a given shear stiffness. Alternatively, the unfavourable conditions could be reduced by reducing the flexural stiffness of the continuity slab.

Effect of Soil Structure Interaction on Torsional Response of Structure Supported by Asymmetric Soil Foundation System

The torsional response of a structure supported by asymmetric foundation was investigated in this study. Several types of the asymmetric soil foundation system were employed to analyze the effect of soil structure interaction on torsional response of the superstructure. It can be concluded from the study that torsional response would be generated for a structure supported by asymmetric soil foundation system under horizontal seismic excitation, and the generated torsional response of the superstructure changed with the degree of the asymmetry of the foundation.

Study of seismic performance of cable-stayed-suspension hybrid bridges

By taking a cable-stayed-suspension hybrid bridge with main span of 1400 m as example, seismic response of the bridge under the horizontal and vertical seismic excitations is investigated numerically by response spectrum analysis and time history analysis, its seismic performance is discussed and compared to the cable-stayed bridge and suspension bridge with the same main span, and considering the aspect of seismic performance, the feasibility of using cable-stayed-suspension hybrid bridge in super long-span bridges is discussed. Under the horizontal seismic action, the effects of structural design parameters including the cable sag to span ratio, the suspension to span ratio, the side span length, the subsidiary piers in side spans, the girder supporting system and the deck form etc on the seismic performance of the bridge are investigated by response spectrum analysis, and the favorable values of these design parameters are proposed.

Seismic pressures on rigid cantilever walls retaining linear poroelastic soil: An exact solution

The dynamic response of a water-saturated linear poroelastic soil layer over bedrock retained by a pair of rigid cantilever walls to a horizontal seismic excitation is obtained analytically–numerically under plane strain conditions. Hysteretic damping in the soil skeleton may also be present. The problem is solved in the frequency domain and its exact solution is obtained analytically. This is accomplished with the aid of Fourier series along the horizontal direction and solution of the resulting system of ordinary differential equations to obtain the amplitudes of the soil skeleton displacements and the pore water pressure. Soil displacements and stresses, pore water pressure as well as wall pressures and resultant forces are explicitly presented. Their variation with frequency, hysteretic damping, porosity and permeability is numerically obtained and compared against an approximate solution, to assess the degree of validity of the assumptions.

Seismic enhancement of multi-span continuous bridges subjected to three-directional excitations

Considering the seismic ground motions as the excitations in only two principal horizontal directions of the bridges and ignoring the third vertical direction is a disregard for the seismic conditions of the region and the bridge distance from epicenter. Numerous cases of substantial damages have been reported among the bridges tremendously suffered from being exposed to the simultaneous three-directional seismic ground motions. Besides the significant compression and tension damages in the columns due to the presence of vertical excitation, it could lead to unexpected shear and flexural failures in the columns and other components as well. Because the axial force variation in the columns due to three-directional excitations, could affect the demands and capacities of the bridge’s components. With respect to this issue, several studies on the bridge damages during the earthquakes have urged researchers to offer efficient methods for bridges handling of the three-directional seismic excitations. Thus, this paper presents and evaluates a superelastic based system for designing as well as retrofitting the multi-span continuous (MSC) bridges that can cope with two- and three-directional seismic excitations. Efficiency evaluation of the proposed system is conducted through various nonlinear time history analyses on a three-dimensional model of a detailed MSC bridge using a suite of developed ground motions for the bridge region. Also, all the analyses are fulfilled based on variation of one influential design characteristic of the proposed system in order to achieve the optimal design. Several pertinent assessment parameters are used during the evaluation of the proposed system. Finally, the efficiency of the new system subjected to the vertical and horizontal seismic excitations is confirmed according to reduction of the bridge responses and improvement in nonlinear performance of the columns in comparison with the as-built bridge results.

Mitigation of Vertical and Horizontal Seismic Excitations on Bridges Utilizing Shape Memory Alloy System

Vertical seismic excitation has a tremendous effect on bridges and many researchers have pointed out bridges damages occurred during the past significant earthquakes which were direct results of ignoring vertical acceleration of ground motions. Many studies have emphasized the importance of extending practical methods to reduce effects of vertical acceleration of earthquakes besides effects of horizontal accelerations; but no practical method has proposed up to now. In this article, an innovative shape memory alloy system is proposed for bridges that can simultaneously controls effects of vertical and horizontal seismic excitations on bridge and reduce them. To evaluate the effectiveness of the shape memory alloy system, a nonlinear time history analysis is conducted on a detailed three-dimensional model of a multi-span simply supported bridge using a representative ground motion. The results show that the proposed new system is very effective for reducing effects of vertical and horizontal seismic excitations on bridges.

A New Hybrid Control of mega-Sub Controlled Structure under Seismic Excitation

Mega-sub controlled structure (MSCS) is a new structural configuration presented recently and has received widespread attention . To further improve the structural control effectiveness of MSCS, in this paper, a new hybrid control method is proposed which employ active control and adjust the stiffness of substructure together to form a huge hybrid controlled system based on the particular conformation characteristic of MSCS. The control law and dynamic equation to assembling parameter matrixes for the MSCS subjected to horizontal seismic excitation are presented. The result indicates that, compared with the corresponding status without actuators, the new structure employing hybrid control method proposed in this paper has the ability to develop a high level of response control effectiveness when subjected to seismic excitations and does significantly further reduce the responses of structure; a much better control effectiveness will be obtained after parameters optimization.

The response of a two-degree-of-freedom dynamic sliding system subjected to uni-directional horizontal dynamic and seismic excitations

An investigation is presented that studies the response of a two-degree-of-freedom dynamic sliding system; that is a single-degree-of-freedom oscillator being fi xed on top of a rigid block that can slide on its supporting shaking table along a horizontal axis when subjected to unidirectional dynamic or earthquake excitations along this horizontal axis. This problem appears to be of interest in predicting the dynamic and earthquake response of superstructures supported on a large foundation block capable of horizontal sliding by means of seismic sliding isolators. Special mock-ups are tested at the shaking table of Aristotle University for this purpose utilizing horizontal simulated earthquake excitations based on prototype earthquake ground motion recordings. The numerical results, which were obtained by a computer software developed for this purpose, are compared with the corresponding experimental measurements. The measured acceleration and displacement responses of the mock-ups appear to be, in all the examined cases, in reasonably good agreement with the numerical predictions. However, in certain cases, the numerically predicted sliding displacement of the rigid block appears to have an offset that differs from the sliding response that was observed during the experiments. This is more pronounced when there is no spring linking the rigid block with the shaking table and must be attributed to manufacturing tolerances of the mock-ups. It is demonstrated that the developed software, although it tries to represent such a rather complex problem in a simplifi ed manner, can be useful at the preliminary design stage of structural systems resting on sliding isolators. Results from various experimental and numerical tests demonstrate that the oscillator’s response is generally made smaller by the sliding of the rigid block. This is true for a wide range of frequencies; however, there is a relatively narrow frequency window in which the oscillator’s response is amplifi ed. This frequency window has a peak value that is slightly higher than the oscillator’s eigen-frequency when it is considered to be a single-degree-offreedom system fi xed at its base.

Dynamic experimental investigation on the uplift response of liquid storage tanks under seismic excitations with different characteristics

A series of seismic table tests about the large steel cylindrical liquid storage tank models with floating roof were taken in this study. Horizontal and vertical direction seismic excitations were input respectively to the experimental structure system under different working conditions to test and analyze the seismic response behavior. The effects of various factors, such as the liquid surface height, the floating roof, the different wave amplitudes and frequencies, as well as their combined effects on the seismic dynamic response were obtained. The absolute displacement and the uplift response behavior law of the test models were obtained under different conditions. It was concluded that not only uplift value but also the absolute displacement of the tank bottom should be considered. And it should be noticed that a critical input frequency exists during the model response changing with frequency. In addition, important factors in experiment were described systematically in this article, for example, the experimental equipment selection, the experimental method, the tank model design, the laser displacement sensors, the data acquisition, the measurement point layout method, seismic excitation input method and so on. A reliable basis could be provided for theoretical analysis, structural design and computer numerical simulation research through this investigation.

The Dynamic Response Analysis of Piled Wharf System under Seismic Excitation

In order to study the behavior of piled wharf system under seismic excitation, numerical analyses are conducted by employing the Newmark’s time-integration algorithm. In the computational model, both nonlinearity of subsoil and the contact behavior between pile-soil interfaces are considered. The numerical results show that the horizontal displacement extremum of all piles in time history increase with the pile height under horizontal seismic excitation. Especially, the side pile of wharf system in the bank slope appears larger horizontal displacement compared with the other piles. Furthermore, the extremum variation of horizontal acceleration and bending moment of the side pile displays a non-monotonic characteristic. Based on computational results and theoretical analyses, several conclusion remarks are drawn, which can provide valuable and referable experience to some extent for related engineering practice and design.