Vertical Ground Motion Research Articles

Seismic fragility analysis of RC bridges in high seismic regions under horizontal and simultaneous horizontal and vertical excitations

Although Himalayan highways generally pass close to the active seismic faults, near field excitation effects are not considered in designing bridges. The effect of strong vertical shaking is known to cause several anomalous and remarkable consequences in bridges. To this end, we present a comparative seismic vulnerability analysis of representative RC bridge from Nepal Himalaya, considering horizontal only and simultaneous horizontal and vertical excitations. Fragility functions for longitudinal and transverse directions considering horizontal only and simultaneous horizontal and vertical excitations are constructed for representative RC bridge from Asian Highway-2 (AH-2) in Nepal that generally follows the same alignment as that of one of the very active seismic faults in the region. Analytical fragility functions are also compared with the existing empirical fragility functions. The results depict that the horizontal response parameters, such as displacement and displacement ductility, are not considerably affected by the vertical ground motion, but the effect is found to be increasingly significant for higher damage states. The sum of observations highlights that the transverse direction of the bridge will be more prominent while considering vertical excitation effects together with the horizontal excitation rather than the horizontal excitation only.

The Case of Strengthening the Base of a Deformed Retaining Wall

The vertical component of ground motions is known to cause severe damages to certain structures. Long span cable-stayed bridges are among those vulnerable structures due to their high mass and cables that take no compression. In this paper, the vertical earthquake effects on several cable-stayed bridges have been investigated using 3-D nonlinear time history finite element analyses. Different main span lengths, pylon shapes and pylon/deck connection types are considered. The results indicates that the vertical ground motion component should be considered in the analysis of such bridges and affects many key elements of the bridge.

Seismic performance of a wind turbine tower under near-field vertical earthquake excitation

For researching the influence of vertical component of near-field records on the seismic response of wind turbine towers, firstly, shaking table test of a 1/20 scaled 2.0 MW wind turbine is carried out. Second, the finite element (FE) model of the scaled tested model is established by ABAQUS. Finally, the dynamic stability is analysed based on the validated FE model. Moreover, the sensitivity of seismic response to different V/H ratios (peak vertical acceleration to horizontal acceleration) of near-field records is analysed through experimental and numerical studies. The results show that the near-field earthquakes have a greater effect on the acceleration and displacement response. The vertical acceleration, vertical displacement and the axial force of this tower are sensitive to the vertical ground motions and the V/H ratios. The larger contributions of vertical component of near-field ground motions lead to smaller safety factors, which is particularly more unfavourable for the dynamic stability of this tower. The middle and lower section of this tower is the dangerous position for local buckling. The influence of the near-field vertical ground motions and a reasonable value of V/H ratios should be better understood and studied, so as to provide guidance for structural seismic design.

Influence of vertical ground motion on the seismic response of underground structures and underground-aboveground structure systems in liquefiable ground

Vertical ground motion components are often observed during earthquakes, which could affect the seismic performance of underground structures. This study investigates the influence of vertical ground motion on the seismic response of underground structures and underground-aboveground structure systems (UASS) in liquefiable ground. Both centrifuge shaking table tests and numerical simulations are conducted. The centrifuge shaking table tests provide basic insights into the seismic response of underground structures under simultaneous horizontal and vertical shaking, and also serve to validate the numerical simulation method. Numerical simulations are conducted using solid–fluid coupled FEM with a constitutive model for soil liquefaction analysis. The analysis results show that although the influence of vertical ground motion on the shearing related motions and internal forces of underground structures is limited, its influence on the axial force of the structure is significant, especially for the center column. The inertia effect of the aboveground structure in UASS can amplify the influence of vertical ground motion via structure-soil-structure interaction. For UASS, the horizontal and vertical relative positions of the aboveground structure and the underground structure also affect the influence of vertical ground motion.

Evaluation of equipment fragility curve in isolation building considering the vertical seismic ground motion

Seismic isolation structures have been introduced in a wide variety of fields as devices to enhance safety during earthquakes. In the nuclear field, although many studies have been conducted for introducing seismically isolated nuclear reactors, this has not been realized yet. To ensure higher reliability and safety of seismic isolation systems, probabilistic risk assessment, which consists of seismic hazard analysis, seismic fragility analysis, and system analysis, plays an important role. This paper aims to investigate the effect of the ultimate characteristics of isolation devices and vertical ground motion on the damage risk of isolators and equipment on the building. As a result, in the case of the isolation layer, it was confirmed that the horizontal response dominates the risk since the effect of vertical motion tends to be small. On the other hand, when the natural period of equipment is approaching either a horizontal or vertical period of the isolation system, the risk of equipment is tended to be underestimated. Thus, the consideration of the effects of vertical motion in addition to horizontal motion and the ultimate characteristics of isolators should be required for the risk assessment of seismic isolation systems.

Behavior of Reinforced Concrete Frames on Flexible Foundations Subjected to Both Horizontal and Vertical Ground Motions

This paper investigates the behaviour of combined horizontal and vertical accelerations on the seismic response of reinforced concrete resting on shallow foundations. Five and nine-storey buildings are considered to represent low- and medium- rise buildings. The building was assumed to be founded on shallow foundation with soil class corresponding to soft soil deposits, the soil of foundation was modelled by an equivalent massless spring coupled with a gap. The accelergrams used in this investigation in the nonlinear range are the horizontal and vertical components of the New Hall earthquake. Although most building codes postulate that SSI generally decreases the force demand of buildings, and increases the deformation demand, it was found that the inclusion of the vertical ground motion with SSI, the horizontal displacement are decreased compared to the case with only the horizontal component.

Characterisation of design spectra for vertical ground motion

Design recommendation on vertical ground motion offers dual challenges, namely the spectral shape and conditional peak-ground-acceleration. One way of recommending the vertical spectra is through V/H spectrum that can be multiplied with design horizontal spectrum. Seismic code recommendations do not explicitly account for the possible effects of magnitude (except EC8 Part-1) and epicentral distance. This paper is aimed to assess the possible effects of magnitude, epicentral distance and average shear wave velocity (representing the soil sites) on resulting V/H spectra. About 6,000 strong motion events are considered for this purpose. Resulting V/H and vertical spectra conditioned to the horizontal spectra are compared with the seismic code recommendations and prior-art. Also explored is the feasibility of possible recommendations on V/H spectra contingent on magnitude, epicentral distance and average shear wave velocity. Finally, the framework is assessed with respect to a strong motion event which was not included in the developmental database.

Characterisation of design spectra for vertical ground motion

Characterisation of design spectra for vertical ground motion

Effect of Ground Motion Orientation on Seismic Responses of an Asymmetric Stress Ribbon Pedestrian Bridge

The stress ribbon bridge uses ribbon in high tension to transfer loads and exhibits geometric nonlinearity under dynamic earthquake excitations. A typical double‐span asymmetric stress ribbon pedestrian bridge was introduced as a prototype, and nonlinear time history analysis was performed to investigate the effect of ground motion orientation on the structural responses. Non‐pulse‐type and pulse‐type ground motions were considered, and the influence of vertical ground motions was investigated. Numerical results showed that the stress ribbon bridge’s vertical and transverse displacements were sensitive to the excitation of the vertical ground motion. No single orientation led to the most critical values in all response indexes, and the critical orientation was approximately independent of the vertical ground motion. The negative bending moment of the ribbons, the pier top displacement, and the pier base moment at the transverse direction were sensitive to the ground motion orientation. Checking the responses resulting from different directions is necessary for a comprehensive estimation of the seismic performance of the stress ribbon bridge.

Dynamic-Response Analysis of the Branch System of a Utility Tunnel Subjected to Near-Fault and Far-Field Ground Motions in Different Input Mechanisms

There are few studies on the dynamic-response mechanism of near-fault and far-field ground motions for large underground structures, especially for the branch joint of a utility tunnel (UT) and its internal pipeline. Based on the theory of a 3D viscous-spring artificial boundary, this paper deduced the equivalent nodal force when a P wave and an SV wave were vertically incident at the same time and transformed the ground motion into an equivalent nodal force using a self-developed MATLAB program, which was applied to an ABAQUS finite element model. Based on near-fault and far-field ground motions obtained from the NGA-WEST2 database, the dynamic responses of a utility tunnel and its internal pipeline in different input mechanisms of near-fault and far-field ground motions were compared according to bidirectional input and tridirectional input, respectively. Generally, the damage to the utility tunnel caused by the near-fault ground motion was stronger than that caused by the far-field ground motion, and the vertical ground motion of near-fault ground motion aggravated the damage to the utility tunnel. In addition, the joint dislocation of the upper and lower three-way joints of the pipeline in the branch system under the seismic action led to local stress concentrations. In general, the branch system of the utility tunnel had good seismic performance to resist the designed earthquake action and protect the internal pipeline from damage during the rare earthquake.

Effect Mechanism of Connection Joints in Fabricated Station Structures

The seismic response of a fabricated subway station is a complex structural connection problem that depends on the mechanical properties of the joints. In order to obtain the optimal joint distribution of a fabricated station structure under earthquake action, three finite element models of a single ring structure of fabricated subway stations assembled with seven, five, and four prefabricated components were proposed. Seismic wave characteristics, peak acceleration, and coupled horizontal and vertical seismic components were considered to study the seismic response of the fabricated subway station structure with different forms of the joint distribution. The dynamic time history method was used to analyze the seismic response in three aspects: structure plastic strain, interlayer relative deformation, and internal force. The damage indexes and residual strength indexes of the joints were offered based on the concrete damage index to evaluate the joints’ damage degree. The results showed that the joints of the vault or bottom plate had little influence on the seismic response of the fabricated station structure. The sidewall joints had the obvious seismic response and the most severe damage under horizontal ground motion or coupled ground motion, which were the weak joints of the fabricated station structure. The existence of vertical ground motion aggravated the damage degree of sidewall joints, making the damage occurrence time of sidewall joints earlier and the damage end time extended. On the premise of meeting the mechanical load and site requirements, an assembly scheme with fewer prefabricated components can be selected.

A Stochastic Model for Simulating Vertical Pulseless Near-Fault Seismic Ground Motions

ABSTRACTThe vertical near-fault seismic ground-motion component can cause significant structural deformation and damage, which can be evaluated from time history analysis using actual or synthetic ground-motion records. In this study, we propose a new stochastic model for the vertical pulseless near-fault ground motions that depends on earthquake magnitude, rupture distance, and site condition. The proposed model is developed based on the time–frequency characteristics of 606 selected actual vertical record components in strike-slip earthquakes. The use and validation of the model are presented using simulated records obtained by two simulation techniques. For the validation, the statistics of time–frequency-dependent power spectral acceleration estimated from the simulated records using the proposed stochastic model are compared with those from the actual records and the ground-motion models available in the literature.

The effect of vertical near-field ground motions on the collapse risk of high-rise reinforced concrete frame-core wall structures

According to the observations of past earthquakes, the vertical ground motions have had a striking influence on the engineering structures, especially reinforced concrete ones. Nevertheless, the number of studies on their aftermath is insufficient, and despite some endeavors done by researchers, there is still a shortage of knowledge about the inclusion of vertical excitation on the seismic performance and the collapse probability of RC buildings. Hence, the variation in the collapse risk of three high-rise RC frame-core wall structures when they undergo bi-directional ground motions is discussed. In this paper, incremental dynamic analyses are carried out under two circumstances, including the horizontal (H) and the combined horizontal and vertical (H+V) earthquakes, and the seismic fragility curves are derived. The inter-story drift ratio corresponding to the onset of collapse has also been defined. The buildings collapse risk under the two circumstances is obtained from the risk integral. Results indicate that in the H+V state, structures meet the collapse criteria for lower intensity measures. Thus, the collapse risk increases as the structures are subjected to bi-directional seismic loads, and the consideration of this effect leads to a more accurate evaluation of buildings seismic performance.

A novel insight into vertical ground motion modelling in earthquake engineering

AbstractRecent observations of failure and damage of buildings and structures under seismic action has led to an increasing interest for an in‐depth analysis of the vertical component of site ground motion. In particular, when dealing with saturated soils, the current engineering practice does not usually go beyond the simplified – formulation of the Biot's equations describing the coupled hydro‐mechanical behaviour, thus neglecting some terms of fluid inertial forces, despite the presence of more refined formulations, for example, the – formulation. Therefore, a theoretical and numerical validation of the – formulation as compared with the – formulation is proposed in this work, where the numerical simulations are compared with the analytical solution for the – formulation, which is also derived and illustrated in this text. The comparison between the two formulations and the analytical solution is provided for different levels of permeability and dynamic actions, which are representative of a wide scenario of site ground properties and seismic hazard in the vertical direction. In particular, the soil response is analysed in terms of acceleration and pore pressure time history, frequency content, acceleration response spectrum, and amplification ratio of acceleration. This study extends the discussion of the limits of applicability of the – formulation with respect to the rigorous solution of Biot's equations (obtained here with – formulation) to the context of a complex dynamic regime provided by the vertical components of real earthquake records, and paves the way for further investigations.

Loss assessment of rigid-frame bridges under horizontal and vertical ground motions

Previous investigations have attributed some reinforced concrete (RC) bridge piers suffered significant damages during large earthquakes due to vertical ground motion effects. This has motivated this study to investigate the use of concrete-filled steel tube (CFST) columns as a more resilient and sustainable alternative to resist combined horizontal and vertical ground motions. The classic performance-based earthquake engineering (PBEE) framework is used to examine the seismic performance of a rigid-frame bridge with construction options of circular RC, circular CFST, and square CFST columns. The hysteretic response of each column is calibrated to the results of hybrid testing of columns under combined horizontal and vertical ground motions. The incremental dynamic analysis (IDA) is conducted using a suite of 50 near-fault records. The results are used to compare the observed damage states and quantify the repair sustainability metrics including repair cost (economic), repair downtime (social), and repair carbon emissions (environmental), as well as the loss of resilience. The losses are integrated with the hazard curves to compute the expected annual losses, all defined as a set of decision variables in the PBEE framework. The results confirm the outstanding seismic performance of CFST columns and highlight their benefits in improving the resilience and sustainability of critical and post-disaster bridges that are prone to strong multi-directional ground motions.

Fragility and risk assessment for sliding artifacts in artifact-showcase-museum systems subjected to three-component ground motions

Vertical ground motions may increase irregularity of sliding response, while the subsequent impact on the sliding fragility of building contents is not clear. Moreover, seismic risk of structural collapse contributes to the total sliding risk of building contents, while it is usually ignored. This paper first introduces a seismic fragility and risk assessment method for the sliding artifact in the artifact-showcase-museum (ASM) system subjected to three-component ground motions. In this method, the square roots of the sums of the squares of peak ground accelerations and peak sliding displacements in two orthogonal horizontal directions are implemented as the intensity measurement and the engineering demand parameter, respectively. Incremental dynamic analysis using the nonlinear three-dimensional finite element model of the ASM system is performed to generate seismic fragility curves of sliding artifacts. The total sliding risk of the artifact is estimated as the sum of the sliding risk of the artifact on the premise of no structural collapse and the structural collapse risk. Then, the seismic fragility and risk of the sliding artifacts within a four-story frame structure and an existing museum is assessed when the systems are subjected to three-component ground motions, respectively. Effects of vertical ground motions, freestanding and restrained showcases, floor levels, sliding thresholds, structural collapse and different structures on the sliding fragility and risk of the artifact are investigated. Results show that the influence of vertical ground motions on the sliding fragility of artifacts in different structures may be significant or marginal depending on the ratios of peak vertical to horizontal floor acceleration. Moreover, the artifact in the freestanding showcase is less fragile than that in the restrained showcase regarding sliding failure. Furthermore, neglecting the structural collapse risk and estimating the sliding risk of the artifact by not removing the structural collapse data would underestimate the total sliding risk of the artifact.

Effects of vertical ground motions on the dynamic response of URM structures: Comparative shake‐table tests

AbstractThis paper discusses the results of an experimental study aimed at evaluating the influence of the vertical ground motion component on the seismic performance of unreinforced brick‐masonry buildings. The research was motivated by post‐earthquake observations of significant structural damage in the vicinity of the fault, where horizontal and vertical ground motions are often strong and synchronized. Vertical accelerations can fluctuate gravity loads, which control the in‐plane lateral load capacity of masonry piers and affect the out‐of‐plane overturning stability of thin walls. Such phenomena seem not to be sufficiently explained in existing literature, while experimental evidence is undoubtedly missing. Here, the damage potential of vertical accelerations was investigated through a series of multidirectional shake‐table tests on full‐scale structures under simulated near‐source ground motions of increasing intensity. The experiments comprised three nominally identical building specimens subjected to the principal horizontal component alone, the horizontal component combined with the vertical one, and the full three‐component ground motion. The buildings included structural/nonstructural elements (e.g., gables, chimneys, and parapets) sensitive to gravity load variations due to their low axial loads. Two different sets of three‐component earthquake records were employed to assess the effects of both tectonic and induced seismicity scenarios. Overall, the vertical earthquake motion did not cause appreciable differences in the behavior of the buildings. Any influence on the strength and peak response of structural/nonstructural walls was marginal and non‐systematic. Data and observations from these experiments add substantially to our understanding of the vertical acceleration effects on masonry structures.

PGA vertical estimates for deep soils and deep geological sediments – A case study of Osijek (Croatia)

This paper analyzes vertical PGA strong-motion estimates for the city of Osijek. Osijek serves as a case-study area for low to medium seismicity regions with deep soil and deep geological sediments underneath the local soil. The new regional PGA attenuation equations for vertical ground motion are constructed for this purpose, as are the accompanying seismic micro-zoning maps. The vertical PGA values of different regional real strong motion are compared to the empirical PGA estimates. Finally, vertical to horizontal PGA estimation ratios are computed and examined. The results suggest that the vertical PGA design values for the examined area can be approximated as 0.61 of horizontal PGA values, which is a 37% higher ratio than the one given by Eurocode 8 for Type 2 spectra in 2004. When the recorded data from similar regions increases several-fold compared to what is now available, it will be possible to better calibrate the empirical scaling equations and to produce more credible vertical PGA estimates.

Effect of variable axial load on seismic behaviour of reinforced concrete columns

This study investigated the effect of variable axial loads (time-varying axial loads) on the seismic performance and failure mechanism of reinforced concrete (RC) columns with different constraints at the top. The contribution of horizontal and vertical ground motions to the variation in the axial force was revealed by evaluating the axial forces on the RC frame columns and bridge piers under the action of near-fault ground motions. The variation law of the axial force was arrived at using the fast Fourier transform (FFT). Based on previous experiments, 30 cases were designed to discuss the effect of various parameters, namely amplitude, frequency and phase of the variable axial load on the seismic performance of the column. The hysteresis behaviour of specimens under different variable axial loads in terms of hysteresis loops, skeleton curves, stiffness degradations, and energy dissipations were analysed. The results indicated that the frequencies of the variable axial load and lateral displacement of the RC column were constants related to the characteristics of the structure. In addition, variable axial loads were detrimental to the seismic performance of the RC columns, especially those with a large amplitude variation, high frequency, and an identical phase as the lateral drift. It is unsafe to design or evaluate the column based on a constant axial load. Furthermore, constraints at the top of the column strengthened the effect of the variable axial load on the seismic performance of the column, and this resulted in an opposite influence rule of the variable axial load.

The effect of ground motion vertical component on the seismic response of historical masonry buildings: The case study of the Banloc Castle in Romania

The current paper aims at analysing the effect of ground motion vertical component in the case of near-field excitations on the structural response of historical masonry buildings. In this perspective, the Banloc Castle in Romania has been considered as a reference case study. This historical masonry structure was damaged by the Banat-Voiteg earthquake that occurred in December 1991 in the homonymous city. To this purpose, a FEM model of the building, set up using the DIANA FEA analysis software, has been studied in the non-linear dynamic field considering a real ground motion record based on specific source-to-site distance, moment magnitude and focal depth. The effects of the impulsive earthquake have been evaluated considering the main engineering demand parameters, such as displacements and forces, focusing on two different scenarios, namely horizontal and horizontal + vertical ground motions. The obtained results have shown that the vertical ground motion sensibly modifies the structural capacity of the building since it produces a relevant modification of the in-plane walls behaviour and the stress conditions. Important considerations about the behaviour (or seismic response modification) factor, ductility and stress in terms of axial and shear forces have been provided. Moreover, numerical damage patterns have been compared to the real cracks detected. Finally, a set of fragility curves has been derived taking into consideration the near-field effects.