Civil Engineering Reference
In-Depth Information
Chapter 11
Sensitivity Analysis of the Lateral Damping of Bridges
for Low Levels of Vibration
A.R. Ortiz, C.E. Ventura, and S.S. Catacoli
Abstract
Damping is a dynamic property that has not been fully understood. In particular, damping in bridges requires
further study in order to gain a better understanding of the parameters that define its value and variation. In practice a
constant ratio of 5 % of the critical damping is commonly used in seismic analysis and design. However, this may not be
accurate for different levels of shaking. In addition, the experimental methods to estimate damping can provide different
damping ratios for the same structure, which increases the uncertainties on the damping estimates. This paper presents a
sensitivity analysis of the damping in bridges for different types of excitation and low levels of vibration. Data collected
from different sources is used to conduct the assessment, namely, scaled model tests, ambient vibration tests, and seismic
ground motions from instrumented bridges in California. A discussion of the results using several frequency and time
domain methods for damping estimation is presented.
Keywords
Lateral damping • Bridges • Low level shaking • Sensitivity analysis • Modal identification
11.1
Introduction
Damping is an important structural characteristic that modifies the structural response to dynamic excitation. In structural
engineering, damping plays an important role in the assessment of the structural demands due to wind and seismic loads that
historically had produced significant human and economic losses, as well as to service loads produced by humans or
machines.
An appropriate estimation of the damping ratio is connected to the correct assessment of the structural response.
Traditionally, bridge engineers have used a damping ratio of 5 % as representative of the lateral damping of standard
bridges under seismic loads. However, further studies are required to understand the sensitivity of damping ratio to different
parameters, such as: types and levels of ground motion, bridge's structural configuration and structural element's materials
[
1
]. In addition, the use passive devices as viscous dampers to improve the seismic performance of bridges has increased the
need to gain a better understanding of the damping ratio in bridges.
This paper initially presents the experimental results of a 3-DOF's lumped mass model used to explore the relation
between damping and the frequency and amplitude of the external loads. Then, data from bridges included in the ambient
vibration database of the Earthquake Engineering Research Facility (EERF) at the University of British Columbia are used
to estimate the damping ratio of bridges for low levels of vibration. Finally, the response of instrumented bridges in
California is used to calculate the damping ratio, especially for low levels of shaking.
