Special Issue on Recent Advances in Seismic Design, Analysis, and Protection of Highway Bridges

Analysis and design of protection systems for highway bridges have witnessed extensive research activities in the last two decades. Researchers worldwide have studied highway bridges both analytically and experimentally and have documented the performances of highway bridges during recent earthquake events. Several damage mechanisms in both structural and geotechnical aspects of highway bridges have been recorded and analyzed in detail. Retrofit methods such as strengthening, base isolation, and other energy dissipative devices have been proposed, tested, and implemented in full-scale bridges. This special issue presents a spectrum of work from leading researchers highlighting the state of the art in the analysis, design, and protection of highway bridges.

Estimation of seismic risk is a central element in earthquake engineering. Integral to this aspect are fragility curves, which relate the conditional probability of reaching or exceeding any damage state as a function of the hazard [e.g., peak ground acceleration (PGA)]. Estimation of seismic risk is then undertaken by combining the seismic hazard and fragility curves using the total probability theorem. Papers in this special issue range from the estimation of fragilities using incremental dynamic analysis to using fragility curves as a measure to evaluate the performance of seismic protection systems and soil-structure interaction effects.

Tehrani and Mitchell, in the paper “Seismic Risk Assessment of Four-Span Bridges in Montreal Designed Using the Canadian Bridge Design Code,” assess the risk to 15 continuous four-span bridges in Montreal. All these bridges were designed according to the 2006 Canadian Highway Bridge Code and detailed with flexure-dominated failures. By combining the seismic hazard to the Montreal region with the fragility curves for different limit states calculated using the incremental dynamic analysis, the authors conclude that the probability of exceeding these limit states is reasonably low.

In the paper “Seismic Fragility Relationships of a Cable-Stayed Bridge Equipped with Response Modification Systems,” Barnawi and Dyke present fragility curves developed using time-history analysis for a benchmark cable-stayed bridge subjected to 20 synthetic ground motions. The bridge was evaluated with various response modification systems, namely passive damper, an active control system, and a semiactive control system using magnetorheological (MR) dampers. Based on a comparison of their fragilities, the authors conclude that MR dampers are an effective response modification strategy for cable-stayed bridges.

Wang et al., in the paper “Influence of Soil-Structure Interaction and Liquefaction on the Isolation Efficiency of a Typical Multispan Continuous Steel Girder Bridge,” present results showing the effects of the soil-structure interaction and soil liquefaction on the fragility of base isolated and unisolated bridges. Using typical multispan continuous steel bridges from the central and southeastern United States as a test bed, they demonstrate that the soil-structure interaction and liquefaction adversely affects the efficiency of base isolation (although liquefaction provides some natural isolation).

When highway bridges cross fault rupture zones, their dynamic behavior is considerably more complicated because of the incoherent ground motions at their foundations. Saiidi et al., in the paper, “Shake Table Studies and Analysis of a Two-Span RC Bridge Model Subjected to a Fault Rupture,” study the effects of incoherent ground motions occurring when a bridge crosses a fault on the damage type and location of a two-span bridge using shake table tests. These tests, conducted at the University of Nevada, Reno, involved supporting three-column bents on three individual shake tables and commanding incoherent motions using this setup. They conclude that incoherent motions affect both the type and location of damage compared with coherent input. Furthermore, they conclude from their analysis using OpenSees that existing analytical models are capable of predicting experimental observations from their tests.

Isolation systems are regarded as being extremely effective for protection of bridges during strong ground motions. It was possible to examine the performance of one such isolation system during the recent earthquake in Japan. The full-scale performance of a cable-stayed bridge during the 2011 Tohuku earthquake is described by Siringoringo et al. in the paper “Seismic Response Analyses of the Yokohama Bay Cable-Stayed Bridge in the 2011 Great East Japan Earthquake.” This 860-m three-span steel truss double-deck bridge was densely instrumented (85 vibration sensors at 36 locations measuring displacements and accelerations) prior to the event, which allowed the authors to gain a rich body of information regarding its performance. The authors conclude that the link-bearing connection, which essentially isolates the bridge in the longitudinal direction, performed well and as intended. However, the relatively large transverse displacements of the tower and girders caused the existing gap distance to be exceeded, resulting in pounding. Nevertheless, the bridge did not sustain damage during the event, because the design hazard exceeded the actual event.

In the paper “Novel Isolation Device for Protection of Cable-Stayed Bridges against Near-Fault Earthquakes,” Ismail and Casas evaluate the performance of a roll-n-cage isolator that uses rolling to achieve isolation, gravity for recentering, and metallic yield dampers or damping. They present results on the Bill Emerson Memorial Bridge in Missouri, subjected to three near-fault ground motions. Their results demonstrate the effectiveness of this novel isolation system.

Although isolation systems are ideal in many applications, low-cost alternatives to retrofit a large number of seismically deficient bridges in the midwestern United States are being investigated by infrastructure managers. Steelman et al., in the paper “Experimental Behavior of Steel Fixed Bearings and Implications for Seismic Bridge Response,” present experimental results for a new concept being studied by the Illinois DOT. This is called the quasi-isolated case, where fixed bearings in bridges are deliberately allowed to act as fuses during an earthquake and slide on the substructure, thereby limiting the damage to the substructure. Two cases are studied, weak anchors and weak pintles, where the dimensions are proportioned such that either the anchors or the pintles failed. The results confirm the advantage of the quasi-isolated concept, with the weak anchors being the preferable option both from a performance and a predictability point of view.

Innovative repair methods for damaged structural members are also of great interest in practice. Rutledge et al., in the paper “Repair of Reinforced Concrete Bridge Columns Containing Buckled and Fractured Reinforcement by Plastic Hinge Relocation,” present a new repair technique to restore strength and deformation capacity of RC bridge columns using plastic hinge relocation. Using carbon fiber-reinforced polymer (CFRP) sheets to strengthen the damaged section of columns, the plastic hinge can be relocated to a higher location. They demonstrate this method through experiments and provide relevant design considerations when implementing this technique.

The use of sensors and measurement technologies has grown rapidly in the recent decade. With this development comes the ability to reduce uncertainties in the design parameters and to predict failures through nondestructive evaluation. Rahimi et al., in the paper “Inverse Estimations of Dynamic Stiffness of Highway Bridge Embankment from Earthquake Records,” present an interesting optimization approach called the self-optimizing inverse method (Self-OPTIM). In their approach, two parallel simulations, one using boundary conditions derived from acceleration and one from velocity, are used to construct the objective function. They show that this approach is effective and superior to existing methods in inverse estimation of model parameters. They demonstrate their approach for inverse estimation of frequency-independent spring and dashpot parameters, representing the effects of embankments, using accelerations at the boundaries and internal degrees of freedom.

Iranmanesh and Ansari, in the paper “Energy-Based Damage Assessment Methodology for Structural Health Monitoring of Modern Reinforced Concrete Bridge Columns,” present a new dissipated energy damage index that accounts for the accumulated damage in RC columns. This new index, unlike conventional approaches that require more involved measurements, relies only on the curvature in the plastic hinge area to assess the health of the structure. For demonstration, the authors perform tests on $1/6$, $1/8$, and $1/10$ models in a hybrid-testing framework, where only the column experiencing damage is physically tested, and the remaining elements are numerically modeled.

This special issue has covered very important aspects in all three components of seismic bridge design. Ten high-quality papers highlight important advances and challenges in this area. The guest editors thank Prof. Anil Agrawal, Chief Editor, for providing them with the opportunity to develop this special issue. They would also like to thank the reviewers for providing timely and constructive comments during the peer review process and the ASCE staff members for helping the guest editors during the peer review process.