Vulnerable Structural Elements Columns Cap Beams Joints Footings Hinges and Supports Superstructure Abutments Insufficient Flexural Strength Inadequate Shear Strength Inadequate Anchorage

Footing Retrofit Measures Add a new reinforced concrete layer connect with existing concrete using dowel bars add new longitudinal and shear reinforcement add new piles Apply prestressing requires coring through the existing footing Introduce a pinned base condition Pinned base condition is possible for multi-column bent Investigat alternative paths – because adding shear reinforcement this region will be difficult

Footing Retrofit (Courtesy of Jacobs Civil Inc.)

Column-Footing Connections Adding shear reinforcement to the connection is difficult and costly Reduce average stress by increasing dimensions and/or prestressing Investigate alternative load path for improving force transfer through the column-footing connection

Vulnerable Structural Elements Columns Cap Beams Joints Footings Hinges and Supports Superstructure Abutments (a) Adding new piles, footing dowels and bottom reinforcement Insufficient Seat Length Bearing Instability

Hinges and Supports High-strength threaded rods Cables restrainers Pipe extenders

Cable Restrainer Retrofit Pipe Restrainer Detail (Courtesy of Georgia Institute of Technology ) Cable Restrainer Retrofit Pipe Restrainer Detail

Vulnerable Structural Elements Columns Cap Beams Joints Footings Hinges and Supports Superstructure Abutments Failure due to insufficient superstructure has not been observed; this outcome may reflectvulnerabilty Lack of Transverse Shear Keys Damage from Skewed Bridges Settlement

Enhance Superstructure Capacity Using locking hinges and movement joints Adding soffit slabs with additional reinforcement Using longitudinal post-tensioning Superstructure – expensive solutions

Abutments Add restrainers or pipe seat extenders Widen abutment seat Introduce isolation bearings Control lateral displacements by adding shear keys

DYNAMIC ISOLATION AND MECHANICAL DEVICES

Improving Seismic Behavior by Changing the Dynamic Characteristics of the Bridge Damping Systems Isolation Bearings Lock-up Devices

Damping Systems Reduce Force Demand and Displacement by Absorbing Energy Increase Damping to 30-40% of Critical Easy to Install (compared to common substructure retrofits) Typically Used on Large or Important Structures Energy Absorption Reduces Force Demand Large or important structures Substantial Stress Reduction - Greatly enhanced damping lowers both stress and deflection throughout a structure, whether the input is seismic or high winds. Easy to Model with Existing Codes - Taylor Dampers are completely viscous, linear in output and will simply and efficiently raise structural damping to 30%-40% of critical, vs. 2% for a typical undamped design. Easy Installation - A wide range of compact sizes are readily available, to reduce installation cost. passive dampers for extreme reliability, with no dependence on outside energy sources. Environmentally Proven Output - Thermostatically controlled, virtually unaffected by temperatures from -40°F to +160°F. Nonflammable inert fluid and stainless steel piston rods on all models.

Damping Systems Friction Metallic Viscoelastic Fluid-viscous

Friction Damping Systems Rely on frictional resistance caused by relative motion of interlaced steel plates (Courtesy of Oiles Corp.) Energy Absorption Reduces Force Demand Large or important structures 9. Oiles System The Oiles MS Damper is a box-shaped device that utilizes internally generated viscous shear forces to dissipate energy. The device is unidirectional and is designed to dissipate energy using the frictional resistance caused by the relative motion of interlaced steel plates acting in conjunction with entrapped fluid. The unit is not designed to carry compressive or dead weight loads; it is only intended to dissipate energy caused by the relative movement of the bridge and supporting structure. metallic hysteretic systems (Courtesy of Oiles Corp.)

Metallic Damping Systems Use deformation of metal elements to dissipate energy Triangular Added Damping and Stiffness

Viscoelastic Damping Systems (Courtesy of Seoul National University, Seoul, Korea) Rely on controlled shearing of solids such as rubber or neoprene (Courtesy of SUNY Buffalo) Visco-elastic devices have an output that is somewhere between that of a damper and a spring. Under high level seismic inputs, the spring response dominates, producing a response that increases column stresses at any given deflection. This does not happen with Fluid Viscous Dampers. One of the most serious problems with visco-elastic devices is an unacceptable increase in force at low temperatures coupled with an accompanying overloading of the bonding agent used to "glue" the visco-elastic material to its steel attachments. At high temperatures, unacceptable softening or reduction of output occurs. This thermal variance from high to low temperature can be in the range of fifty to one.

Fluid-viscous Damping Systems Use forced movement of fluids within a confined cylinder Fluid Viscous Dampers are either Linear or Non Linear. A Linear Viscous Damper provides damping forces that are proportional to the relative velocity of the two end connections of the damper. A Non-Linear Viscous Damper provides damping force that is proportional to a specified exponent determined by the structural engineer. The structural engineer will specify their preference, depending on the amount of damping required. 11. Enidine System The Enidine energy dissipator is a telescoping piston/cylinder device that utilizes fluid flow through orifices to absorb energy. A silicon fluid is stored in two chambers separated by the piston head. Orifices are situated in the piston head, which allow the silicon to move back and forth between the two chambers. A fluid reservoir is located in one end to control the internal pressure and provide an additional silicon fluid source in the event of a leak during dynamic motion. The force generated by this device is a result of the pressure differential across the piston head and the fluid compressibility. The Taylor Deviceís energy dissipator is a telescoping piston/cylinder device that utilizes fluid flow through orifices to absorb energy. A silicon fluid is stored in two chambers separated by the piston head. Orifices are situated in the piston head, which allow the silicon to move back and forth between the two chambers. The force generated by this device is a result of the pressure differential across the piston head and the fluid compressibility. The unit is not designed to carry compressive or dead weight loads; it is only intended to dissipate energy caused by the relative movement of the bridge and supporting structure. Figure 2.1 consists of a photograph of one of the dampers submitted for evaluation. Figure 2.2 shows a dimensional drawing of a Taylor Devices damper, which consists of a damper cartridge, external guide sleeve, extender, and attachment clevices. The damper cartridge contains all fluid, and hydraulic components of the device. The external guide sleeve is used to increase buckling resistance of the damper, and also protects the piston rod of the damper cartridge from the outside environment. The sleeve guide is a simple non-sealing bushing, which scrapes potential contaminants from the damper cartridge. (Courtesy of MCEER) (Courtesy of Caltrans)

Total Seismic Demand Reduction Isolation Bearings Period Shift Damping Shift Total Seismic Demand Reduction In 1998, AASHTO approved the new Guide Specifications for Seismic Isolation Design Reduce effective stiffness of structure Increase Fundamental Period (away from the peak in the acceleration response spectrum) Reduce Force Transmitted to Supports Dissipate Energy

Isolation Bearings (Courtesy of Caltrans) Use unique properties of lead to absorb energy from seismic event while allowing movement from slow-acting forces Rubber bearing provides flexibility and acts as spring to re-center bearing Lead-Rubber Bearing – Benicia-Martinez Bridge, California USA

Isolation Bearings use concave dish to convert lateral motion to into vertical motion (horizontal energy to potential energy) reduce forces by lengthening structure’s period provide damping through friction gravity provides self-centering (Courtesy of Caltrans) Friction-Pendulum - Benicia Martinez Bridge, California USA

Damped Sliding Isolation Bearing Isolation Bearings Disk Bearing Horizontal Spring Sliding Surface (Courtesy of R.J. Watson, Inc.) combine shear-inhibited disc bearing with a damping device to limit forces and displacements kinetic energy is dissipated through friction and converted into potential energy stored in horizontal springs The EQS transfers the energy of a moving mass (kinetic energy), such as a bridge deck during an earthquake, into heat and spring (potential energy). This is done through the MER which connects the superstructure to the substructure. The EQS can be adjusted at the direction of the design engineer, to achieve a wide variety of energy dissipation levels. This is because the EQS is designed to dissipate energy through friction. By adjusting the friction levels the amount of damping can be controlled. This extraordinary feature gives the engineer the ability to optimize the structural design. Unlike an elastomeric isolation bearing, the lateral effective stiffness of an EQS bearing can be varied in different directions to optimize the response of a structure. The EradiQuake System (EQS) isolator manufactured by RJW is a steel and composite sliding bearing. The isolator consists of a disk bearing supporting a bearing block which houses polyurethane springs (Mass Energy Regulator [MER]) and a polytetrafluoroethylene (PTFE) disk that slides on a stainless steel sole plate. The sole plate has a perimeter reaction plate (guide box) that transfers lateral load from the supported structure through the MER to the disk bearing shear pin that is connected to the masonry plate. During an earthquake, the sole plate slides on the PTFE disks and compresses the MER. The device is designed to reduce earthquake forces by allowing the structure to translate and by absorbing energy through friction. - - Damped Sliding Isolation Bearing ( ( ) )

Isolation Bearings (Courtesy of CERF) The FIP isolator is a composite unit consisting of three main elements: a pot bearing; a shock transmission unit; and one or two planes of elasto-plastic components. The isolator is characterized by the complete independence of the vertical load carrying element (pot bearing) from the elasto-plastic components (crescent moons) that control horizontal loads and dissipate energy. Vertical loads are transmitted through the guided pot bearing which slides on a dimpled, lubricated polytetrafluoroethylene (PTFE) and stainless steel plate interface. Transverse loads are transmitted to the backing plate through the guided pot bearing. During an earthquake, the shock transmission units engage the crescent moons, which subsequently undergo plastic deformation. The device is designed to reduce earthquake forces on the structure by allowing the structure to translate and by absorbing energy when the crescent moon deforms plastically. Guided Pot Bearing with Elasto-Plastic Energy Dissipaters - -

Steel Reinforced, High Damping Elastomeric Bearing Isolation Bearings (Courtesy of CERF) The Scougal isolator is a steel reinforced, high damping, elastomeric bearing. Steel load plates located at the top and bottom of the bearing distribute vertical loads and transfer shear into the bearing. Steel reinforcing plates provide vertical load capacity. The internal rubber layers provide lateral flexibility. A rubber cover surrounds the isolator and protects it from corrosion. During an earthquake, the rubber layers deform laterally. This device is designed to reduce earthquake forces and absorb energy by shear deformation of the rubber layers. - - Steel Reinforced, High Damping Elastomeric Bearing

Isolation Bearings Steel Rolling Bearing - - (Courtesy of CERF) The Ball-N-Cone (BNC) bearing manufactured by Tekton is an all steel rolling type bearing. A steel ball is sandwiched between two conical steel load plates. The ball is free to roll on both conical plates up to the perimeter edge. During an earthquake, the sphere rides up and down the constant slope of the two conical load plates. The device is designed to reduce earthquake forces by allowing the structure to translate laterally. The device does not provide damping. - - Steel Rolling Bearing

Lockup Device Control distribution of seismic loads to substructure (Courtesy of CERF) Control distribution of seismic loads to substructure Allow free movement for thermal and other slow-acting loads Do not absorb energy Control distribution of seismic loads to substructure Allow free movement for thermal, creep, shrinkage and other slow-acting loads Do not absorb energy Shock Transmission Units (STU) The Shock Transmission Unit (STU) is a hydraulic viscous device that does not absorb energy in the typical sense. In contrast to the Seismic Viscous damper, the purpose of the Shock Transmission Unit is to provide a very large resisting force to any sudden movement as in a seismic event. The STU allows for very slow movement of a large structure at very low velocities. Typically, this is a daily occurrence due to thermal expansion and compression of the structure. However, during a catastrophic event, the STU prevents all movement of the structure thereby transferring the force into the structure. Shock Transmission Units are designed to a customer specific force/velocity threshold. Below this value, the unit moves relatively freely with approximately 10% of the rated force. When the relative velocity of the end connections exceeds the design value, the unit locks up transmitting the force into the protected structure.

(Courtesy of Caltrans) Testing (Courtesy of Caltrans) Full-scale Dynamic Testing Stiffness Damping Force Degradation Large-scale laboratory testing Need to test new retrofit technologies The characteristic responses include stiffness, damping, EDC, and force degradation. The Need for Testing Bridge owners around the country were interested in opening the market to different manufacturers that offer a variety of isolation and energy dissipation systems, including elastomeric bearings, sliding systems, and others. There are certain advantages to each system; however, the performance of any device depends greatly on the proper design of all components and on the manufacturing quality. Due to the lack of an industry group that brings those suppliers together, there has been no uniformity of design standards, performance criteria, or even an agreement on prototype testing. In addition, while several of the available systems have undergone static and dynamic testing, the testing often consisted of full-scale pseudo-static tests or reduced-scale shake-table tests. Some dynamic characteristics can be established from static behavior, but a full-scale dynamic test is the only true test of the response of any of these systems to an earthquake. Full-scale tests are essential to develop the higher confidence levels among bridge designers that will lead to a wider application of seismic isolation in the United States.

Applications Fluid Viscous Dampers at Bridge Abutment (Courtesy of Caltrans) (Courtesy of Caltrans) Most common in structures with articulation in original design (limits application to concrete bridge retrofits) Limit forces to non-ductile substructure elements to below yield limit state Fluid Viscous Dampers at Bridge Abutment

(Courtesy of Caltrans) Applications (Courtesy of Caltrans) Most common in structures with articulation in original design (limits application to concrete bridge retrofits) Limit forces to non-ductile substructure elements to below yield limit state Seismic Isolation of San Diego Signature Bridge The San Diego-Coronado Bay Bridge extends 1.6 miles across San Diego Bay from downtown San Diego to Coronado Island. “The main toll structure consists of three long channel spans of orthotropic box girder construction and twenty-six shorter spans of steel plate girder. It encompasses a 90-degree, 1800-foot radius curve within the western approach.” The bridge was opened to traffic on August 3, 1969 during the celebration of the 200th anniversary of the founding of San Diego, the oldest city on the West Coast. In 1970, it received the Most Beautiful Bridge Award of Merit of the American Institute of Steel Construction. Fault lines lie under the west end of the bridge. The long length, height, flexibility and 90-degree curve of the bridge all contribute to the seismic response characteristics of this bridge. Installation Solution: To mitigate damage to the structure resulting from a seismic event, Enidine Viscous Dampers were retrofitted into the bridge piers. 20 Viscous Dampers were installed at the joints of the bridge pier and pile arrays. This configuration controls the movement of the structure during a seismic event, thus preventing excessive whipping by the bridge piers and spans. Fluid Viscous Dampers at Pier – San Diego Coronado Bridge, California, USA

Applications Lock-up Devices - Arthur Laing Bridge, Vancouver, Canada (Courtesy of Colebrand, Limited) Lock-up Devices - Arthur Laing Bridge, Vancouver, Canada

Applications Friction Pendulum Bearing, Kodiak, Alaska, USA Most common in structures with articulation in original design (limits application to concrete bridge retrofits) Limit forces to non-ductile substructure elements to below yield limit state (Courtesy of SUNY Buffalo) Friction Pendulum Bearing, Kodiak, Alaska, USA

Applications (Courtesy of HDR, Inc.) Most common in structures with articulation in original design (limits application to concrete bridge retrofits) Limit forces to non-ductile substructure elements to below yield limit state Friction Pendulum Bearings - Lake Natoma Crossing, California, USA

Applications (Courtesy of HDR, Inc.) (Courtesy of HDR, Inc.) Most common in structures with articulation in original design (limits application to concrete bridge retrofits) Limit forces to non-ductile substructure elements to below yield limit state Lead Rubber Isolation Bearings at Rte 242-I-680 Interchange, California, USA

SUMMARY Retrofit document provides an overview of seismic retrofitting techniques Highlights both experimental evaluation and field implementation Provides references to test results and design equations Retrofitted bridges performed well in the 1994 Northridge earthquake Most techniques summarized in the document have not been tested under large magnitude earthquakes Note that the Northridge was a moderate magnitude event

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