Journal of Bridge Engineering

This paper describes a new repair technique that involves the use of plastic hinge relocation to restore strength and deformation capacity of RC bridge columns. Summarized is the overall repair concept and experimental results that include the reversed cyclic testing of three large-scale bridge columns that were previously damaged, repaired using the proposed methodology, and then subsequently retested. To date, two different repair alternatives were executed using unidirectional carbon fiber sheets in the hoop and longitudinal directions, the latter anchored into the RC footing with 30-mm-diameter carbon fiber anchors. A method for predicting the force-displacement responses of columns repaired in this manner was also developed and found to give reasonable results. Also included in this paper are design considerations, which are carried out in the steps needed to design a repair system to relocate the plastic hinge in a column containing buckled longitudinal reinforcement. The responses show that the proposed hinge relocation technique is able to restore the lost strength and displacement capacity of damaged RC columns.

This paper investigates the near-fault (NF) seismic performance of a novel isolation system, referred to as the roll-n-cage (RNC) isolator, considering the cable-stayed Bill Emerson Memorial Bridge in Missouri. The RNC isolator is designed to achieve a balance in controlling bridge displacement demands, structural accelerations, and internal forces in the tower attributable to horizontal seismic forces. It provides in a single unit all the necessary functions of rigid support, horizontal flexibility with enhanced stability, and energy-dissipation characteristics. Moreover, it has two unique features that are built-in: energy-absorbing buffer and linear recentering mechanisms. Through a nonlinear time history analysis under three recorded NF earthquakes and three synthetic ground accelerations, the results show that the RNC isolator is a convenient isolation system in protecting cable-stayed bridges against NF earthquakes.

This paper presents the development of fragility curves of a benchmark cable-stayed bridge retrofitted with response modification systems. Fragility relationships are an accepted technique used for seismic risk assessment of structures. The example used herein is a benchmark cable-stayed bridge. Synthetic ground motions are used to excite the structure. Semiactive control is applied to the structure to evaluate and quantify its impact on response mitigation. The semiactive devices used herein are magnetorheological (MR) dampers. The performance of the structure retrofitted with MR dampers is compared with the structure retrofitted with passive, active, and no devices. Fragility relationships based on deck displacement, deck shear, and overturning moment are investigated for comparison. Analyses of peak and time history responses are used to support the accuracy of the fragility relationships. An investigation of the cables exceeding an appropriate tension range and control effort required for the control cases is also provided. The resulting fragility relationships show that MR dampers are effective at mitigating the effects of seismic excitation on the cable-stayed bridge.

Isolation has been identified as a popular retrofit measure for seismically deficient bridges. However, most isolation designs neglect soil-structure interaction (SSI) effects. The feasibility of isolation for bridges with SSI, especially in liquefaction-prone regions, requires further study. This paper investigates effects of SSI and liquefaction on the fragility of both an unretrofitted and an isolated coupled bridge-soil-foundation (CBSF) system. Results of the fragility analyses reveal that the failure probability of the isolated system is less than that of the nonisolated one for both stiff soils and soft soils, whereas SSI tends to simultaneously decrease isolation effectiveness. Results also show that liquefaction provides an effective natural isolation by reducing the curvature demands on the columns, although it increases the isolation bearing displacement and pile curvature. Therefore, SSI should be considered in the design of isolated bridges, as appropriate isolation in conjunction with the CBSF system could still offer a viable retrofit option by replacing the vulnerable steel bearings and reducing critical component demands, such as columns, even if liquefaction occurs.

The seismic performance of 15 continuous four-span bridges with different arrangements of column heights and diameters and designed using the 2006 Canadian Highway Bridge Design Code provisions were studied using incremental dynamic analysis (IDA). The seismic risk associated with exceeding different damage states in the columns, including yielding, cover spalling, bar buckling, and structural collapse (i.e., dynamic instability) was predicted. Some simplified equations were derived for Montreal, Quebec, Canada, to estimate the mean annual probability of exceeding different damage states in the columns using the IDA results. The results indicated that the probabilities of exceeding different damage states were reasonably low.

The objective of this study was to investigate the fault rupture effects on the seismic response of a bridge system crossing an active fault. A large-scale two-span bridge model supported on three bents, one on each of the University of Nevada, Reno (UNR), shake tables, was tested under earthquakes with incoherent motions that simulated fault rupture. The results were compared with those from an identical bridge model subjected to coherent ground motions in a previous shake table study. It was found that fault rupture substantially affected the damage type and location in the bridge bents. The most severely damaged bent in the current bridge was a relatively flexible bent near the fault. However, under coherent motions, the shortest bent experienced the severest damage. Linear static and nonlinear dynamic analyses of the test model revealed that existing analytical techniques are adequate in estimating the response of the most critical bent subjected to fault rupture.

Performance assessment of bridge structures instrumented with structural health monitoring systems requires development of deterministic methodologies producing quantitative information on different damage states. This aim can be achieved through defining engineering limit states that may be expressed by limiting values of quantities such as damage indexes. Displacement-based damage indexes for performance assessment of bridges do not consider the accumulated damage that results from plastic cycling. In more advanced approaches, the parameter that takes into consideration the effect of cyclic loading history and accumulation of damage is the dissipated energy. However, computation of dissipated energy for the purpose of structural health monitoring requires real-time measurement of lateral forces, which is beyond the capability of conventional sensing methodologies. This paper reports on the development of a methodology to evaluate the energy dissipation in reinforced concrete columns with circular cross sections based on the curvatures measured in the plastic hinge area. Integrating the introduced method in a normalized dissipated energy index provides the possibility of detecting and quantifying minor and moderate damage that is barely visible during visual inspection. The scope of the study included the evaluation of the proposed damage assessment method through hybrid simulation tests on $1/10\text{-}$, $1/8\text{-}$, and $1/6\text{-scale}$ hybrid models of a two-span bridge subjected to various amplitudes of near source ground motions of the 1994 Northridge earthquake. Fiber-optic sensors were employed for monitoring the deformation response of the columns in the plastic hinge area.

In this paper, the dynamic self-optimizing inverse method (Self-OPTIM) has been proposed to inversely estimate model parameters by using structural responses under dynamic loadings such as earthquakes. The proposed dynamic Self-OPTIM requires only acceleration records at the ground boundary support and a certain number of internal degrees of freedom. Unlike other signal matching approaches used in model updating, dynamic Self-OPTIM automatically minimizes an implicit objective function defined as a function of internal full-field stresses and strains. By use of the dynamic Self-OPTIM, dynamic stiffness model parameters of bridge embankments and a group of piles at the bottom of the central pier were successfully identified by using in situ earthquake records. The identified dynamic stiffness values of the embankment with abutment and piles are then compared with those estimated from the response of three-dimensional finite-element dynamic steady-state analysis and values from the literature. Validation tests with new earthquake records have also been demonstrated. Results from sensitivity analyses could reasonably identify critical parameters that mostly influence dynamic response under the given earthquake loading.

Yokohama Bay Bridge, with a total span length of 860 m, is the second longest span cable-stayed bridge in East Japan, and one of the most densely instrumented bridges in Japan. On March 11, 2011, the Great East Japan (Tohoku) Earthquake hit northeastern Japan with magnitude of $M_{\text{W}}$ 9.0, notably the largest earthquake in Japan’s modern history. Intensity $5+\left(\text{PGA}\hspace{0.17em}1.4–2.5\hspace{0.17em}\text{m}/{\text{s}}^2\right)$ of the maximum scale 7 of Japan Meteorogical Agency’s seismic intensity was recorded on the bridge site. This paper describes seismic response analyses of the bridge, system identification, performance evaluation of link-bearing connections (a seismic isolation system), and postearthquake field observation. Response analyses show that transverse vibration dominated the response of girder and tower, with a maximum girder displacement of 62 cm. The large transverse vibrations induced pounding between tower and girder on the tower–girder connections as shown by the periodic impulses on acceleration records. Meanwhile, longitudinal accelerations indicate that the link-bearing connections functioned properly during the earthquake. System identification reveals nonlinearity of the response as evidenced by variations in natural frequencies and mode shapes during large excitation. Despite these conditions, the bridge did not suffer any structural damages, since the ground motions experienced during the earthquake were less than the design and seismic retrofit ground motions.

Steel fixed bearings are commonplace structural elements for transmitting loads from superstructures to substructures, and they have typically occupied a role of elastic force transfer elements within the overall scheme of an earthquake resisting system (ERS). Recent revisions to design and guide specifications have acknowledged the possibility of bearings acting as fuses, but there is little research available to characterize bearing behavior for such design roles or the associated bridge response to be expected when bearings have fused. One design approach, adopted by the Illinois DOT (IDOT), applies capacity design principles and permits the bearings and superstructure to slide on the substructure. The intent of this design approach is to capture some of the beneficial aspects of conventional isolated systems, such as period elongation, reduction of force demands, and protection of substructures from large inelastic displacement demands, without incurring the additional design provisions and fabrication costs to satisfy the requirements for seismic isolation systems. To achieve this quasi-isolated bridge response, steel fixed bearings are used as fusing elements, where the steel pintles or anchor rods rupture, and the fixed bearing plates become free to slide on the supporting pier cap. A properly proportioned bearing will fuse prior to superstructure/substructure elements experiencing inelastic demands. The University of Illinois has been collaborating with IDOT to investigate the behavior of quasi-isolated bridge systems and to calibrate and refine IDOT’s ERS design and construction methodology. The research is composed of experimental testing to characterize fundamental bearing behavior, coupled with nonlinear global bridge modeling to evaluate limit state progression and estimate maximum displacement demands of the superstructure relative to the substructure. The cyclic response of full-scale steel low-profile fixed bearings demonstrates predictable sliding behavior, but based on current design procedures, these bearings are often overdesigned for use as fuses in quasi-isolated bridges. Consequently, a bridge, which in other respects may exhibit satisfactory quasi-isolated response, might also incur significant damage to the substructure unit where fixed bearings are provided. A parametric study of global bridge response demonstrates that the anchorage of fixed bearings to substructures could be reduced to limit the damage to the supporting substructure unit while incurring only a nominal increase in superstructure displacement demands.