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doi: 10.1002/eqe.444pmid: N/A
The effects of the spatial variability of the ground motion on the response of bridge structures are investigated in this study. Following a well‐established convention, the phenomenon is represented as the combined effect of three causes: the loss of coherence of the motion with distance, the wave‐passage, and the local site conditions. Since the nature and amount of non‐synchronism vary within ample limits a statistical approach is adopted. A parametric study is carried out on a representative set of bridges subjected to carefully selected combinations of the factors inducing spatial variability. The investigation has shown that the phenomenon affects the response considerably and, hence, the level of protection of these structures. It is observed that for all bridge types considered, the ductility demands at the base of the piers in the presence of spatial variability increase in the majority of cases. Further, for a given bridge type, the probabilities of failure vary by more than one order of magnitude depending on the combination of the parameters. Attention has been focused on a parameter representing the ratio between the maximum curvature ductility demand and the same quantity for the case of fully synchronous motion. This parameter has been used to correct the conventional synchronous design procedure by increasing the available ductility. The re‐analysis of all the cases with a modified ductility capacity shows that the procedure is effective in reducing the fragilities to the values corresponding to synchronous input. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.441pmid: N/A
This paper presents a performance‐based seismic analysis and design of a large suspension bridge, the new Tacoma Narrows Parallel Crossing in the State of Washington. The scope of the project included establishment of design criteria, extensive analysis and validation of the design. The analysis was performed using detailed three‐dimensional models that included geometric and material non‐linearity. The target post‐earthquake level of service was verified using stress, deformation and ductility criteria. In the absence of well‐established criteria, which relate the structural response of tower shafts to specific levels of performance, capacity analyses were performed to demonstrate that the design fulfills the performance objectives. The seismic analysis and design of this bridge was reviewed throughout the design process. An independent check team also performed separate analysis and validation of the design. Thus, this bridge constitutes an example of a large‐scale design project where the performance‐based seismic design procedures underwent rigorous assessment. This work demonstrated that the performance‐based approach for seismic design is an appropriate way for designing earthquake‐resistant structures. Further data that relate the structural response with the performance objectives are necessary. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.445pmid: N/A
Following the Loma Prieta earthquake, Caltrans (California Department of Transportation) started a multi‐year seismic vulnerability assessment and retrofit project of all major bridges in California, including the San Francisco–Oakland Bay Bridge (SFOBB) East Bay Span. First, a retrofit of the East Span Steel Truss structure was evaluated. However, owing to the high cost and the questionable reliability in terms of performance of the retrofit and owing to its difficult implementation under full traffic, a replacement structure was determined to provide a seismically more reliable alternative, resulting in the SFOBB East Span Replacement Project. The new bridge consists of four distinct structures: (1) the Oakland landing or touchdown structures; (2) a segmental concrete box girder crossing called the Skyway; (3) a self‐anchored suspension (SAS) signature span; and (4) a series of multi‐cell post‐tensioned concrete box girder bridges providing the transition to the tunnel on Yerba Buena Island. The new bridge will feature parallel roadways and will be built next to the existing bridge, which will be dismantled after the new bridge is opened to traffic. This paper focuses only on the seismic safety aspects in the RC elements of three of the four segments of the new SFOBB East Span Bridge, namely the Skyway, the SAS bridge and the Oakland touchdown. Analytical and numerical models were developed to assess the seismic behavior of the Bay Bridge and a large‐scale proof‐testing program was conducted at the Charles Lee Powell Structural Research Laboratories at the University of California, San Diego. In the framework of this program, two steel shear links at 100% scale were tested, two concrete piers of the Skyway at 25% scale, a 25% scale model of the West Anchor Pier W2, and two column 35% scale piers of the Oakland Touchdown were also tested. The large‐scale tests on all structural elements of the new East Span of the San Francisco–Oakland Bay Bridge with expected inelastic performance characteristics proved that the design fully meets all performance requirements. The large‐ or full‐scale tests were able to identify performance issues, resulting in design improvements and considerable cost savings. Alternative detailing and construction procedures were validated, leading to a more economic construction of the new bridge. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.446pmid: N/A
This paper presents results from testing and the associated analytical studies of steel shear links and orthotropic bridge decks to support the design of the new East Span for the San Francisco–Oakland Bay Bridge. Cyclic testing of full‐scale built‐up links showed that the specimens were able to reach an inelastic rotation more than twice that which would be produced from a 1500‐year Safety Evaluation Earthquake event. Nevertheless, brittle fracture occurred before the inelastic design rotation capacity, as specified in the AISC Seismic Provisions, was developed. Based on a parametric study, a modification to the welding details was proposed, which proved to be effective in preventing this type of fracture in a subsequent testing program. Monotonic testing of two reduced‐scale orthotropic bridge deck panels, one stiffened with closed ribs and another one with open ribs, also showed that these specimens could develop a compression capacity greater than that which would be produced by the design earthquake. The post‐buckling behavior was associated with the buckling direction and the type of ribs. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.440pmid: N/A
The linear quadratic regulator (LQR) control algorithm is at the heart of many modern control design methods. There have been numerous publications dealing with LQR control and earthquake engineering. However, very few papers discuss the hysteretic loops produced by the LQR control force, although in the earthquake engineering field, hysteretic loops have been the key part of seismic designs. This paper shows the importance of investigating the hysteretic loops produced by the LQR control force for seismic response, and proposes a method to reproduce the hysteretic loops with a much simpler algorithm. The investigation was carried out on a cable‐stayed bridge model controlled either by the LQR algorithm, viscous damper, or the proposed method. The results show that the proposed method is capable of reducing seismic response better than the viscous damper case and is similar to the LQR case. The practical applicability of the proposed method is also investigated by using a variable‐orifice oil damper as the controlling device. The proposed method needs only displacement response at the device location, and therefore fewer sensors are needed than for the LQR algorithm. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.443pmid: N/A
A complete renovation of the Pan‐American Highway from La Serena to Puerto Montt, Chile, has been recently completed, including the construction of several long‐span bridges, with different structural systems. Most of them have some sort of seismic protection, such as high‐damping rubber bearings, friction bearings or energy dissipation devices. Ambient vibration tests have been conducted on seven of these bridges to obtain their natural frequencies, modes of vibration, and equivalent damping, under small amplitude vibrations. In two of them local accelerometer networks have been installed for continued monitoring of their seismic behavior and a number of earthquakes have already been recorded. The seismic records show clearly the beneficial effect of the isolation and energy dissipation devices in the longitudinal direction. Analytical models with different degrees of complexity have been developed to reproduce the recorded behavior under ambient vibration and moderate earthquakes. This paper presents some of the data obtained and summarizes the research done by the authors over the last 10 years at the University of Chile on the seismic protection of bridges. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.439pmid: N/A
The California Department of Transportation has nearly completed a $5.5 billion seismic retrofit program to retrofit strengthen over 2200 bridges on the state highway systems so they conform to the latest seismic hazard and performance criteria. Various unique solutions were developed and implemented to achieve the goals of the program. These techniques included the use of conventional steel and reinforced concrete jackets on bridge columns, advanced fiberglass and carbon fiber composite jackets, seismic isolation bearings and dampers, and seismic isolation silos. All of these techniques were designed to control the performance of a bridge by modifying or tuning its structural characteristics. This is much easier to achieve on a new bridge than on the retrofit of an existing bridge. Ideally a bridge should be designed with no deck joints and no bearings, with monolithic columns to superstructure framing, and with all columns the same length. While this ideal design is not achievable on many bridges, there are modifications that can reduce the vulnerability to damage during an earthquake. The structural control techniques illustrated are: hinge restrainer cables and extenders; fewer deck joints; column and foundation design to control the location of plastic hinge zones; conservative shear key design; pier rocking; steel jackets and carbon shells for external confinements; seismic isolation silos; shock transmission dampers; rubber–lead core isolation bearings; and inverted pendulum isolation bearings. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.442pmid: N/A
In this study the employment of buckling‐restrained braces (BRBs) as energy dissipation dampers is attempted for seismic performance upgrading of steel arch bridges and the effectiveness of BRBs to protect structures against strong earthquakes is numerically studied. With buckling restrained, BRB members can provide stable energy dissipation capacity and thus damage of the whole structure under major earthquakes can be mitigated. Cyclic behaviour of such members is addressed with a numerical simulation model, and a strength design method for BRBs is proposed. BRBs are then placed at certain locations on the example steel arch bridge to replace some normal members with two schemes, and the effect of the two installation schemes of BRBs for seismic upgrading is investigated by non‐linear time‐history analyses under various ground motions representing major earthquake events. Compared with the seismic behaviour of the original structure without BRBs, satisfactory seismic performance is seen in the upgraded models, which clarifies the effectiveness of the proposed upgrading method and it can serve as an efficient solution for earthquake‐resistant new designs and retrofit of existing steel arch bridges. Copyright © 2005 John Wiley & Sons, Ltd.
doi: 10.1002/eqe.448pmid: N/A
Braced piers of steel truss bridges having braces with latticed built‐up cross‐sections may suffer significant damage during earthquakes due to the buckling, rapid strength degradation, and fracture of the braces. One retrofit alternative for such a bridge pier is to supplement the existing structural system with a system of energy dissipation devices (or structural fuses) that will yield and dissipate energy prior to existing brace buckling. These devices depend on the global shear displacements of the pier to yield and dissipate energy. However, relatively tall and slender bridge piers are also subject to large overturning displacements. This paper investigates and quantifies the effectiveness of using supplemental systems relying on devices used to control pier shear displacements to retrofit braced steel bridge piers, including the effect of overturning displacements on the effective ductility of such systems. A closed form equation for the total ductility of the retrofit pier, as a function of the shear displacement ductility, is derived and expressed graphically. A simple preliminary design procedure that incorporates these effects is proposed and an example design is presented. Copyright © 2005 John Wiley & Sons, Ltd.
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