System identification of the Vincent Thomas suspension bridge using earthquake records

Abstract

SUMMARYThe Vincent Thomas Bridge in the Los Angeles metropolitan area, is a critical artery for commercial tra c ow in and out of the Los Angeles Harbor, and is at risk in the seismically active Southern California region, particularly because it straddles the Palos Verdes fault zone. A combination of linear and non-linear system identiÿcation techniques is employed to obtain a complete reduced-order, multiinput-multi-output (MIMO) dynamic model of the Vincent Thomas Bridge based on the dynamic response of the structure to the 1987 Whittier and 1994 Northridge earthquakes.Starting with the available acceleration measurements (which consists of 15 accelerometers on the bridge structure and 10 accelerometers at various locations on its base), an e cient least-squares-based time-domain identiÿcation procedure is applied to the data set to develop a reduced-order, equivalent linear, multi-degree-of-freedom model. Although not the main focus of this study, the linear system identiÿcation method is also combined with a non-parametric identiÿcation technique, to generate a reduced-order non-linear mathematical model suitable for use in subsequent studies to predict, with good ÿdelity, the total response of the bridge under arbitrary dynamic environments.Results of this study yield measurements of the equivalent linear modal properties (frequencies, mode shapes and non-proportional damping) as well as quantitative measures of the extent and nature of nonlinear interaction forces arising from strong ground shaking. It is shown that, for the particular subset of observations used in the identiÿcation procedure, the apparent non-linearities in the system restoring forces are quite signiÿcant, and they contribute substantially to the improved ÿdelity of the model. Also shown is the potential of the identiÿcation technique under discussion to detect slight changes in the structure's in uence coe cients, which may be indicators of damage and degradation in the structure being monitored. Di culties associated with accurately estimating damping for lightly damped long-span structures from their earthquake response are discussed. The technical issues raised in this paper indicate the need for added spatial resolution in sensor instrumentation to obtain identiÿed mathematical models of structural systems with the broadest range of validity.

System identification applied to long‐span cable‐supported bridges using seismic records

“…This paucity is primarily due to the limited number of seismic records since not until recently have long-span bridges been instrumented with permanent sensors, and not all the currently instrumented bridges are located in seismically active areas. Among the few studies of the SI applied to long-span bridges, the Vincent Thomas suspension bridge has been the subject of extensive research using seismic records from the 1987 Whittier Narrows and the 1994 Northridge earthquakes [12][13][14].…”

Dynamic Testing of Alfred Zampa Memorial Bridge

“…In another study by Huang et al (1999), impact forces in the longitudinal and transverse directions were generated by the sudden braking of a truck at a specified location on the bridge, whereas vertical impact forces were generated by simply letting the rear wheels of the truck drop from the top of a concrete block. Earthquake excitation (when -2-Journal of Structural Engineering, ASCE, ST/2006/025227, Revised Version: 06/07/2007 measured) provides a natural source of forced vibration for bridge and other civil structures (Smyth et al 2003). …”

Obtaining refined first‐order predictive models of linear structural systems

“…This choice stems from two considerations: (1) the time-histories of the recorded longitudinal and lateral accelerations show that the corresponding structural response vanishes quite rapidly, and that the ÿnal parts of the signals are dominated by a high noise-to-signal ratio, and (2) as discussed in detail by Smyth et al [30], most of the other channels are suspected to show signs of non-linear response, mostly in the form of banging of structural components. Instead, all the recorded time-histories of the main span vertical accelerations are clearly dominated by the structural response with a smaller noise-to-signal ratio, and the vertically dominant motion of the main span can be modelled as linear.…”