Seismic Displacement Demands on Skewed Bridge Decks Supported on Elastomeric Bearings

The unseating of decks is one of the most prevalent failure modes of bridges after earthquake events, as observed in the 2010 Chile Earthquake. Damaged bridges in Chile often had skew angles and were supported on elastomeric bearings. Similar bridge construction practices with decks supported on elastomeric bearings are also common in the central and eastern U.S. (CEUS). The seismic displacement demands on skewed bridges are more complicated than those on bridges without skew angles due to the coupling of translational modes with the rotational mode of vibration. The study presented in this article seeks to understand the seismic response of skewed bridge decks supported on elastomeric bearings. The scope of the study is limited to one- and two-span bridges, which constitute a large portion of bridge inventory in the CEUS. The vibration modes of skewed bridge decks are derived in closed form and the modes are compared when the gaps between the bridge deck and the abutment are open and when one of the gaps is closed due to seismic excitation. Nonlinear response history analyses are carried out to understand the effects of vertical ground motion, skew angles, aspect ratios, and different ground motion types on the seismic displacement demand in these cases. Amplification factors that approximate the increase in the displacement demand due to the skew angle are proposed.     

IDENTIFICATION AND VERIFICATION OF SEISMIC DEMAND FROM DIFFERENT HYSTERETIC MODELS

The goal of this paper is to develop a modified Bouc-Wen hysteretic model from cyclic loading test data for reinforced columns, including the behavior of stiffness degradation, strength deterioration, pinching and softening effects of RC members. Seismic demands on this inelastic single degree of freedom system when subjected to both near-fault ground motion and far-field ground motion excitations were examined.

The cyclic loading test of reinforced concrete columns was experimentally observed and a system identification computer program was developed to solve each control parameter of the hysteretic model. A least-squared method for identifying parameters of the model is proposed in this paper. The hysteretic constitutive law produces a smoothly varying hysteresis such as the control-parameters for strength deterioration, stiffness degradation, pinching and softening effects. Two implementations of (1) flexure damage and (2) shear damage were conducted to provide better understanding of hysteretic behavior of RC structural members. A pseudo-dynamic experiment was also developed to verify the model parameters.

Based on the developed hysteretic model, the seismic demand of this inelastic model was investigated by using both near-fault ground motion data and far-field ground motion data as input motion. An R-μT inelastic response spectrum from different hysteretic models was generated.     

Response Sensitivity of Highway Bridges to Randomly Oriented Multi-Component Earthquake Excitation

In two-dimensional and single axis three-dimensional finite element analyses, the ground motion incidence angle can play a significant role in structural response. The effect of incidence angle for three-dimensional excitation and response is investigated in this paper for response of highway bridges. Single-degree-of-freedom elastic and inelastic mean spectra were computed from various orientation techniques and found indistinguishable for combinations of orthogonal horizontal components. Probabilistic seismic demand models were generated for the nonlinear response of five different bridge models. The negligible effect of incidence angle on mean ensemble response was confirmed with a stochastic representation of the ground motions.     

Impact of Vertical Ground Excitation on a Bridge with Footing Uplift

Ground motions recorded in the epicentral region of an earthquake often have a strong vertical component with dominant high frequencies. Damage to bridges in near-source regions due to strong vertical ground motion has been reported. The beneficial effects of footing uplift on structural performance in form of reduction of seismic response of structural members have been confirmed in previous research. The uplift of bridge piers has been utilised in a very limited number of bridge structures, e.g., the South Rangitikei railway bridge in New Zealand. However, the near-fault seismic behaviour of bridges with footing uplift has been even less addressed. In this study shake table investigations were carried out on the response of a single-span bridge model with footing uplift subjected to simultaneous vertical and horizontal excitations. Near-fault ground motions recorded in the Canterbury earthquake sequences of 2010 and 2011 were used. The experimental results show that inclusion of vertical ground motions produce stronger axial force in the pier and larger bending moment in the deck. Concurrent horizontal and vertical excitations may also cause more frequent footing uplift than the solely horizontal excitations.     

Sensitivity of the Earthquake Response of Tall Steel Moment Frame Buildings to Ground Motion Features

The seismic response of two tall steel moment frame buildings and their variants is explored through parametric nonlinear analysis using idealized sawtooth-like ground velocity waveforms, with a characteristic period (T), amplitude (peak ground velocity, PGV), and duration (number of cycles, N). Collapse-level response is induced only by long-period, moderate to large PGV ground excitation. This agrees well with a simple energy balance analysis. The collapse initiation regime expands to lower ground motion periods and amplitudes with increasing number of ground motion cycles.     

A Base Isolation System for Developing Countries Using Discarded Tyres Filled with Elastomeric Recycled Materials

This article studies the performance of economic base isolators using tyres filled with elastomeric recycled materials. The research was conducted to analyze base isolators to be used in developing nations, where the application of conventional elastomeric rubber bearings due to economic reasons is limited.

The tested isolators are made of kart tyres filled with different recycled elastomeric materials and aggregates. Dynamic and static tests proved acceptable vertical to horizontal stiffness ratio of the bearings and shake table tests showed an excellent enhancement of the base isolated structural response compared to the corresponding fixed base structure.     

Smart Lead Rubber Bearings Equipped with Ferrous Shape Memory Alloy Wires for Seismically Isolating Highway Bridges

Shape memory alloys (SMA) can substantially improve the damping capacity and re-centering capability of elastomeric isolators. The objective of this study is to assess the seismic performance of smart lead rubber bearings (LRBs) equipped with double cross ferrous SMA wires. Hysteretic shear response of SMA wire-based LRB is determined using finite element method. The seismic response of a multispan continuous steel girder bridge isolated by SMA-LRB is evaluated. Hybrid SMA-LRB bearing exhibits a significantly lower shear strain demand (up to 46% reduction) and a higher energy dissipation capacity (up to 31% increase) compared to the LRB.     

Choices and Criteria for Seismic Strengthening

Risk assessment is affected by large uncertainties, depending on hazard, structure, damage, and loss analysis. Crucial problems and choices may refer to: (a) hazard parameters, including the definition of appropriate ground motion levels and of their probability to occur; (b) level of knowledge about materials, geometry, detailing; (c) assessed damage and failure modes; and (d) resulting potential for step changes in performances.

The cost of attaining a high level of knowledge may significantly reduce the remaining resources, it is therefore important to favor resilient solutions with a creative adoption of appropriate strengthening strategies.

In this framework, this article discusses the possible criteria for the mitigation of seismic risk and some of the alternative choices that may be adopted for strengthening, with reference to:

(a)?the modification of damage and collapse modes strengthening individual elements or locallyincreasing the deformation capacity;

(b)?the insertion of additional systems resisting to horizontal actions;

(c)?the introduction of base isolation, with the objective of capacity-protecting the existingstructure;

(d)?the reduction of displacement demand by added damping or introducing tuned masssystems.

Alternative strengthening choices lead to different protection levels and imply different performances that are, in general, represented by non linear or step functions of a cost parameter of the intervention. From these considerations, conceptual “structure driven” strengthening criteria, based on a logical use of resources, are discussed.     

CONCEPTUAL DESIGN OF ISOLATION SYSTEMS FOR BRIDGE STRUCTURES

Abstract

In the first part of this paper the objectives of an isolation system for a bridge structure are discussed, in relation to modelling options and modification of the traditional capacity design principles. A displacement-based design approach is then presented, using a linear equivalent single degree-of-freedom model. The preliminary design of an isolation system for existing bridges is based on the definition of a “structure regularity” which allows the estimation of whether the response of the real structure will be similar to that predicted in the preliminary design phase. The efficiency of the approach is shown in designing the isolation system for a highly irregular bridge.     

Inelastic Displacement Demand of Strength-Degraded Structures

This article presents findings from parametric studies involving nonlinear time-history analyses of inelastic systems with and without strength degradation. Results showed that estimates based on the equal-displacement and equal-energy propositions can be exceeded significantly by the inelastic displacement demands in the acceleration and velocity-sensitive regions of the response spectrum. The displacement demand behaviour is sensitive to the strength degradation and the frequency properties of the ground shaking. With a modest strength reduction factor of 2, the inelastic displacement demand would typically be constrained by the Peak Displacement Demand as indicated on the elastic displacement response spectrum for 5% damping.     

Inelastic Displacement Ratios for Seismic Assessment of Structures Subjected to Forward-Directivity Near-Fault Ground Motions

This article presents results of a statistical study focused on evaluating inelastic displacement ratios (i.e., ratio of maximum inelastic displacement with respect to maximum elastic displacement demand) of degrading and non degrading single-degree-of-freedom (SDOF) systems subjected to forward-directivity near-fault ground motions. C_R spectra are computed for normalized periods of vibration with respect to the predominant period of the ground motion to provide a better ground motion characterization. This period normalization allows reducing the record-to-record variability in the estimation of C_R. An equation to obtain estimates of C_R for the seismic assessment of structures exposed to forward-directivity near-fault ground motions is proposed.     

The Baimoral Bridge of I. K. Brunel

Abstract

A considerable number of I. K. Bruner's railway bridges are still operational, varying in size from the great Royal Albert Bridge over the Tamar to a host of small masonry bridges for accommodation roads over or under his broad-gauge main lines and subsidiary routes like the Taff Vale Railway. There were, however, few bridges built by him apart from his railway works, and it could have been safely assumed that all such bridges which survive would have been clearly recognized as having been built by him. Yet this has not been so with one particular road bridge, for which Bruners responsibility has been either doubted or ignored, even though it survives in excellent working order. The bridge in question is that over the River Dee at Balmoral.     

The Impact of Flood-Induced Scour on Seismic Fragility Characteristics of Bridges

Earthquake in the presence of flood-induced scour is a critical multihazard scenario for bridges located in seismically-active, flood-prone regions. The present article evaluates seismic performance of four example reinforced concrete bridges when they are pre-exposed to regional flood hazards. Nonlinear time history analyses of the example bridges are performed for a suite of ground motion time histories in the presence and absence of scour expected from different intensity flood events. Fragility analysis is performed to develop seismic fragility curves of the example bridges for various scour depths. Results show nonlinear increase in bridge seismic fragility with increase in scour depth.     

SEISMIC RESPONSE CONTROL OF A CABLE-STAYED BRIDGE BY VARIABLE DAMPERS

Cable-stayed bridges exhibit unique responses under a strong motion. It is partly due to the complexity in their damping mechanism. Recently, the benchmark problem of a cable-stayed bridge was developed to clarify the effectiveness of various seismic control strategies. Due to the new development of magnetorheological dampers, the application of variable dampers in bridges becomes possible. In this study, the effectiveness of the nonlinear viscous damping force scheme and the two-step friction damping force scheme are investigated. It is found that the nonlinear viscous damping force scheme is effective to control the response of the cable-stayed bridge with less demand for the damping force capacity of a damper. In addition, the two-step friction damping force scheme shows the improvement over conventional friction damping because the energy dissipation of a damper can be increased.     

Robust Period-Independent Ground Motion Selection and Scaling for Effective Seismic Design and Assessment

A period-independent approach for the selection and scaling of ground motion records aimed at reducing demand variability is proposed for seismic response history analysis. The same set of scaled records can be used to study various structures at the same site regardless of their dynamic characteristics. The statistical robustness of the proposed and current approaches is compared through nonlinear inelastic dynamic analyses performed on single-degree-of-freedom systems and multi-story braced frames. The proposed approach leads to consistent response predictions with a limited number of records. This is advantageous for day-to-day structural design or assessment against code hazard-based seismic demand levels.     

A PROCEDURE FOR COMBINING VERTICAL AND HORIZONTAL SEISMIC ACTION EFFECTS

The vertical component of earthquake ground motion has generally been neglected in the earthquake-resistant design of structures. This is gradually changing due to the increase in near-source records obtained recently, coupled with field observations confirming the possible destructive effect of high vertical vibrations.

In this paper, simple procedures are suggested for assessing the significance of vertical ground motion, indicating when it should be included in the determination of seismic actions on buildings. Proposals are made for the calculation of elastic and inelastic vertical periods of vibration incorporating the effects of vertical and horizontal motion amplitude and the cross-coupling between the two vibration periods. Simplified analysis may then be used to evaluate realistic vertical forces by employing the vertical period of vibration with pertinent spectra without resorting to inelastic dynamic analysis.

Finally, a procedure is suggested for combining vertical and horizontal seismic action effects which accounts for the likelihood of coincidence, or otherwise, of peak response in the two directions.     

Fragility-Based Performance Evaluation of Asymmetric Single-Story Buildings in Near Field and Far Field Earthquakes

Previous studies have shown that both distribution and intensity of response parameters in asymmetric buildings are dependent on their stiffnesses and strength distributions. The locations of centers of strength and rigidity relative to the center of mass provide suitable metrics for strength and stiffness distributions. In general, the proper locations of these centers are a function of earthquake ground motion characteristics and the level of building's nonlinear responses. In this article, using nonlinear response evaluation of single-story building models with a wide range of uncoupled torsional to lateral frequencies subjected to near field and far field earthquake excitations the proper location of building centers is studied . Diaphragm rotation, interstory drift, hinge plastic rotation, and ductility demand are selected as damage measure parameters. To compare the performance of models in each limit state, fragility representation of responses is used. It is concluded that proper configuration of building centers in a torsionally stiff building fundamentally depends on the chosen demand parameter. The proper configuration of centers in a torsionally stiff building for a specific demand parameter can converge the probability and distribution of related damages to those in the symmetric building counterpart. When the critical demand parameter for a case is identified, its corresponding arrangements of centers for a suitable seismic behavior may also be recognized. By rearranging the configuration of centers based on the attained configuration, the adverse effects of asymmetry can be avoided.     

Analytical Seismic Assessment of a Tall Long-Span Curved Reinforced-Concrete Bridge. Part I: Numerical Modeling and Input Excitation

The seismic response of bridges is affected by a number of modeling considerations, such as pier embedment, buried pile caps, seat-type abutments, pounding, bond slip and architecturally flared part of piers, and loading considerations, such as non-uniform ground excitations and orientation of ground motion components, which are not readily addressed by design codes. This article addresses a methodology for the nonlinear static and dynamic analysis of a tall, long-span, curved, reinforced-concrete bridge, the Mogollon Rim Viaduct. Various modeling scenarios are considered for the bridge components, soil-structure interaction system, and materials, i.e., concrete and reinforcing steel, covering all its geotechnical and structural aspects based on recent advances in bridge engineering. Various analysis methodologies (nonlinear static pushover, time history response to uniform and spatially variable seismic excitations, and incremental dynamic analyses) are performed. For the dynamic analyses, a suite of nine earthquake accelerograms are selected and their characteristics are investigated using seismic intensity parameters. A recently developed approach for the generation of non-uniform seismic excitations, i.e., spatially variable simulations conditioned on the recorded time series, is used. Methods for the evaluation of structural performance are discussed and their limitations addressed. The numerical results of the seismic assessment of the Mogollon Rim Viaduct are presented in the companion article (Part II). The sensitivity of the bridge response to the adopted modeling, loading and analyzing strategies, as well as the correlation between structural damage and seismic intensity parameters are examined in detail.     

Correlations Between Frequency Content Indicators of Strong Ground Motions and PGV

The frequency content of ground motions seems to be one of the most important parameters to explain the structural damage experienced during worldwide strong earthquakes. The frequency content of ground motions can be characterized by various stochastic and/or deterministic indicators: the frequency bandwidth indicator ? (Cartwright & Longuet-Higgins) related to the power spectral density function and, respectively, the control (corner) period T_c of the structural response spectra or the mean period T_M . Peak ground velocity (PGV) and the ratio PGA/PGV can be used as either damage potential parameters or frequency content indicators. A comparative analysis of stochastic and deterministic frequency content indicators and of PGV is applied to a set of 30 strong ground motion records having peak ground acceleration (PGA) from 0.2–0.8 g and recorded on 4 continents during the last 70 years.     

MODELLING OF THE GROUND MOTION AT RUSSE SITE (NE BULGARIA) DUE TO THE VRANCEA EARTHQUAKES

An approach, capable of synthesising strong ground motion from a basic understanding of fault mechanism and of seismic wave propagation in the Earth, is applied to model the seismic input at a set of 25 sites along a chosen profile at Russe, NE Bulgaria, due to two intermediate-depth Vrancea events (August 30, 1986, M _ω=7.2, and May 30, 1990, M _ω=6.9). Accordingly to our results, once a strong ground motion parameter has been selected to characterise the ground motion, it is necessary to investigate the relationships between its values and the features of the earthquake source, the path to the site and the nature of the site. Therefore, a proper seismic hazard assessment requires an appro-priate parametric study to define the different ground shaking scenarios corresponding to the relevant seismogenic zones affecting the given site. Site response assessment is provided simultaneously in frequency and space domains, and thus the applied procedure differs from the traditional engineering approach that discusses the site as a single point. The applied procedure can be efficiently used to estimate the ground motion for different purposes like microzonation, urban planning, retrofitting or insurance of the built environment.