Title

Metallic Dampers for Seismic Design and Retrofit of Bridges

Abstract

The purpose of this study was to develop an economic solution for the design and retrofit of continuous steel girder bridges in low occurrence seismic zones such as the Central and Eastern United States. Prior to 1975, the construction of such bridges exclusively used high rocker bearings and included two expansion joints at the ends of bridge decks for thermal expansion and contraction. They were designed with no seismic considerations. In this report, metallic dampers (steel rods) are introduced between the substructure and superstructure to improve the seismic performance of the bridges. Metallic dampers are used to provide restraint to the longitudinal movement of the bridges'The purpose of this study was to develop an economic solution for the design and retrofit of continuous steel girder bridges in low occurrence seismic zones such as the Central and Eastern United States. Prior to 1975, the construction of such bridges exclusively used high rocker bearings and included two expansion joints at the ends of bridge decks for thermal expansion and contraction. They were designed with no seismic considerations. In this report, metallic dampers (steel rods) are introduced between the substructure and superstructure to improve the seismic performance of the bridges. Metallic dampers are used to provide restraint to the longitudinal movement of the bridges’ superstructure under non-seismic loads and yield as fuse-like elements during a strong earthquake event. The scope of work included optimization of metallic dampers, experimental study of the cyclic behavior of full-scale dampers, experimental study of the dynamic behavior of high rocker bearings and dampers installed in a small-scale bridge system, and analytical development of a simplified procedure to account for pounding effect in the response spectrum analysis of highway bridges.

A total of four full-scale dampers, three with straight steel rods and one with linearly tapered rods, and an approximately 1/10-scale steel-girder bridge with various combinations of weight on the deck and different configurations of a small-scale metallic damper were tested in laboratory. Pounding effects on bridge responses were extensively studied with a simplified bridge model from which an equivalent damping concept has been developed based on the maximum deck displacement and mechanical energy criteria, respectively. The concept was validated with a number of linear and nonlinear analyses of a three-span bridge, A-237R in Southeast Missouri. The following conclusions can be drawn from the analytical and experimental investigations:

  1. No material stiffness degradation of dampers was observed in the test range and the hysteresis loop in the load-displacement plane was steadily developed. Metallic dampers can dissipate an appreciable amount of energy even at a low level of loading. For straight-rod dampers, it was recommended that 10% equivalent viscous damping be used in bridge design. The equivalent damping of the tapered-rod dampers rapidly increased with the applied load after the initiation of yielding.
  2. The performance of the dampers was consistent with respect to load and displacement. Under the same load, the tapered rods deformed considerably more than straight rods and revealed more uniform strain distribution along the steel rods, resulting in more energy dissipation.
  3. Metallic dampers were also effective as isolation units. Their engagement in the small-scale steel-girder bridge significantly reduced the strain on the bridge columns and the acceleration of the steel girders. These results ensure that, in the event of a destructive earthquake, damage will be localized to the dampers while the columns retain their structural integrity.
  4. Overturning of rocker bearings was not observed throughout the test program. Rocker bearings remained stable even when the bridge was subjected to a harmonic excitation of 0.54g at resonance.
  5. Pounding reduces the maximum deck displacements of highway bridges during a strong earthquake event. It is similar to damping effect. The equivalent damping, structural damping plus pounding effect, is strongly dependent upon the gap width of expansion joints and the dominant frequency of earthquake excitations. There is no conclusive functional relation between the equivalent damping and the frequency bandwidth of excitations.
  6. The equivalent damping based on the displacement criterion is significantly more accurate than that from the energy criterion. It can be used in the linear analysis (time history or response spectrum procedure) of highway bridges so long as the gap width of expansion joints is greater than 60% the deck displacement when pounding is ignored.
  7. Displacement-based design equations of equivalent damping are sufficiently accurate in representing pounding effect on the seismic responses of highway bridges with seat-type abutments. They are recommended for practical applications. Use of the equivalent damping concept simplifies the dynamic analysis of a geometrically nonlinear bridge system due to presence of expansion joints into that of an associated linear system.
superstructure under non-seismic loads and yield as fuse-like elements during a strong earthquake event. The scope of work included optimization of metallic dampers, experimental study of the cyclic behavior of full-scale dampers, experimental study of the dynamic behavior of high rocker bearings and dampers installed in a small-scale bridge system, and analytical development of a simplified procedure to account for pounding effect in the response spectrum analysis of highway bridges.

A total of four full-scale dampers, three with straight steel rods and one with linearly tapered rods, and an approximately 1/10-scale steel-girder bridge with various combinations of weight on the deck and different configurations of a small-scale metallic damper were tested in laboratory. Pounding effects on bridge responses were extensively studied with a simplified bridge model from which an equivalent damping concept has been developed based on the maximum deck displacement and mechanical energy criteria, respectively. The concept was validated with a number of linear and nonlinear analyses of a three-span bridge, A-237R in Southeast Missouri. The following conclusions can be drawn from the analytical and experimental investigations:

  1. No material stiffness degradation of dampers was observed in the test range and the hysteresis loop in the load-displacement plane was steadily developed. Metallic dampers can dissipate an appreciable amount of energy even at a low level of loading. For straight-rod dampers, it was recommended that 10% equivalent viscous damping be used in bridge design. The equivalent damping of the tapered-rod dampers rapidly increased with the applied load after the initiation of yielding.
  2. The performance of the dampers was consistent with respect to load and displacement. Under the same load, the tapered rods deformed considerably more than straight rods and revealed more uniform strain distribution along the steel rods, resulting in more energy dissipation.
  3. Metallic dampers were also effective as isolation units. Their engagement in the small-scale steel-girder bridge significantly reduced the strain on the bridge columns and the acceleration of the steel girders. These results ensure that, in the event of a destructive earthquake, damage will be localized to the dampers while the columns retain their structural integrity.
  4. Overturning of rocker bearings was not observed throughout the test program. Rocker bearings remained stable even when the bridge was subjected to a harmonic excitation of 0.54g at resonance.
  5. Pounding reduces the maximum deck displacements of highway bridges during a strong earthquake event. It is similar to damping effect. The equivalent damping, structural damping plus pounding effect, is strongly dependent upon the gap width of expansion joints and the dominant frequency of earthquake excitations. There is no conclusive functional relation between the equivalent damping and the frequency bandwidth of excitations.
  6. The equivalent damping based on the displacement criterion is significantly more accurate than that from the energy criterion. It can be used in the linear analysis (time history or response spectrum procedure) of highway bridges so long as the gap width of expansion joints is greater than 60% the deck displacement when pounding is ignored.
  7. Displacement-based design equations of equivalent damping are sufficiently accurate in representing pounding effect on the seismic responses of highway bridges with seat-type abutments. They are recommended for practical applications. Use of the equivalent damping concept simplifies the dynamic analysis of a geometrically nonlinear bridge system due to presence of expansion joints into that of an associated linear system.

Department(s)

Civil, Architectural and Environmental Engineering

Sponsor(s)

Missouri Department of Transportation

Report Number

RDT-01-005

Document Type

Report - Technical

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2001 National University Transportation Center at Missouri University of Science and Technology, All rights reserved.

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