Research

New Contact Models with Nonlinear Damping for Pounding Simulations

S. Muthukumar and R. DesRoches

Earthquake ground motion can induce out-of-phase Pounding damage from Bridge vibrations between adjacent structures due to differences in dynamic characteristics, which can result in impact or pounding of the structures if the at-rest separation is insufficient to accommodate the relative displacements. In bridges, seismic pounding between adjacent decks or between deck and abutment can result in localized deck damage, bearing failure, damage to shear keys and abutments, and even contribute to the collapse of bridge spans. This study investigates pounding in bridges from an analytical perspective. Pounding is simulated using contact force-based models such as the linear spring, Kelvin and Hertz models, as well as the momentum-based stereomechanical method. In addition, a Hertz contact model with nonlinear damping (Hertzdamp model) is also introduced to model impact.

The primary factors controlling the pounding response are identified as the frame period ratio, ground motion Proposed Impact Model with Energy Dissipation effective period ratio, restrainer stiffness ratio and frame ductility ratio. Pounding is most critical for highly out-of-phase frames. Impact models without energy dissipation overestimate the stiff system displacements by 15%-25% for highly out-of-phase, elastic systems experiencing moderate to strong ground excitation. The Hertzdamp model is found to be the most effective in representing impact. Traditional column hysteresis models such as the elasto-plastic and bilinear models underestimate the stiff system amplification and overestimate the flexible system amplification due to impact, when compared with stiffness and strength degrading models. Strength degradation and pounding are critical on the stiff system response to near field ground motions, for highly out-of-phase systems. Current design procedures are adequate in capturing the nonlinear hinge response when the bridge columns are elastic, but require revisions such as the introduction of time dependent reduction factors, and a frame design period to work for inelastic situations.

Selected Publications and Presentations

Muthukumar, S. and DesRoches, R. (2005) "Effect of Frame Restoring Force Characteristics on the Pounding Response of Multiple-Frame Bridges," Earthquake Spectra , In Press , January, 2005.

DesRoches, R. and Muthukumar, S. (2004). "Implications of Seismic Pounding on the Longitudinal Response of Multi-Span Bridges - An Analytical Perspective," Journal of Earthquake Engineering and Vibration , Vol 3 , No. 1 , pp. 1-9 .

DesRoches, R. and Muthukumar, S (2002). "Effect of Pounding and Restrainers on the Response of Multiple-Frame Bridges," ASCE Journal of Structural Engineering , Vol. 128 , No. 7 , pp. 860-870, July 2002.