Engineering Structures 29 (2007) 2001–2017 www.elsevier.com/locate/engstruct Dynamic characteristics of a curved cable-stayed bridge identified from strong motion records Dionysius M. Siringoringo ∗ , Yozo Fujino Department of Civil Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 26 April 2006; received in revised form 5 September 2006; accepted 16 October 2006 Available online 6 December 2006 Abstract An assessment of dynamic characteristics of the 455 m Katsushika–Harp curved cable-stayed bridge is presented. Dynamics characteristics such as natural frequencies, mode shapes and modal damping ratios are obtained from seismic response of the bridge by employing a time-domain multi-input multi-output (MIMO) system identification (SI) technique. The technique makes use of base motions and superstructure accelerations as pairs of inputs–outputs to realize the coefficients of state-space system matrices. The SI results indicate the occurrence of many closely spaced modal frequencies with spatially complicated mode shapes. Fourteen global modes in the ranges of 0.45–2.5 Hz were identified, in which the girder motion dominated most of the modes. The tower modes were associated with girder modes and were characterized by the lowly-damped motion. Using identification results from six earthquakes, the effects of earthquake amplitude on modal damping ratios were observed. c  2006 Elsevier Ltd. All rights reserved. Keywords: Curved cable-stayed bridge; Instrumented bridge; Seismic response; MIMO system identification; Katsushika–Harp bridge 1. Introduction The availability of multi-channel permanent sensors allows regular full-scale dynamic tests that are essential for continuous monitoring of a bridge. In a seismically active region, such as Japan, this instrumentation provides an opportunity to use system identification (SI) techniques to explain bridge performance during earthquakes. By employing the SI technique, it is also possible to monitor any changes in the bridge behavior without the presence of visually observable damage. In the context of bridge monitoring, this excellent opportunity is beneficial to evaluate the adequacy of bridge seismic design code [1]. The general approach of an earthquake-induced SI is to use the input–output relation to recreate structural models that are capable of reproducing the actual responses. In one early study Beck [2] employed the output-error minimization method for a linear, time-invariant structural system with classical damping. McVerry [3] proposed a frequency domain ∗ Corresponding author. Tel.: +81 03 5841 6097; fax: +81 03 5841 7454. E-mail addresses: dion@bridge.t.u-tokyo.ac.jp (D.M. Siringoringo), fujino@bridge.t.u-tokyo.ac.jp (Y. Fujino). approach using transfer function to minimize the objective function of output error. Chaudhary et al. [4] improved it, for a more general problem of non-classical damping that includes the structural model in addition to the modal model. This method, while powerful and significantly insightful, requires prior information of structural properties that are typically unavailable and difficult to obtain, especially for large and complex structures such as cable-stayed bridges. Most conventional SI techniques were developed in the frequency domain due to the common practice of using frequency analyzer for data acquisition. These approaches offer advantages in incorporating soil–structure interaction into analysis, but often suffer from damping estimation especially when closely spaced modes are present. Compared to cable-stayed bridges with straight girder, the curved cable-stayed bridges are relatively few. Examples of well-known curved cable-stayed bridges are the La Arena Viaduct in Spain [5], the Safti Link Bridge in Singapore [6], the Rhine Bridge near Schahausen, Switzerland [7] and the twin curved cable-stayed bridge at the Malpensa airport in Milan, Italy [8]. In the analysis of a cable-stayed bridge under seismic action, the aspects of three dimensionality, multi-modal contribution, multiple-support excitations and modal coupling 0141-0296/$ - see front matter c  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2006.10.009