Output-only modal parameter identification of civil engineer

Abstract. The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vi

Structural Engineering and Mechanics, Vol. 17, No. 3-4 (2004) 000-0001

Output-only modal parameter identification of

civil engineering structures

Wei-Xin Ren and Zhou-Hong Zong

Department of Civil Engineering, Fuzhou University, Fuzhou, Fujian Province 350002, P.R. China

(Received, 2003, Accepted August 20, 2003)

Abstract.The ambient vibration measurement is a kind of output data-only dynamic testing where thetraffics and winds are used as agents responsible for natural or environmental excitation. Therefore anexperimental modal analysis procedure for ambient vibration testing will need to base itself on output-onlydata. The modal analysis involving output-only measurements present a challenge that requires the use ofspecial modal identification technique, which can deal with very small magnitude of ambient vibrationcontaminated by noise. Two complementary modal analysis methods are implemented. They are rathersimple peak picking (PP) method in frequency domain and more advanced stochastic subspaceidentification (SSI) method in time domain. This paper presents the application of ambient vibrationtesting and experimental modal analysis on large civil engineering structures. A15 storey reinforcedconcrete shear core building and a concrete filled steel tubular arch bridge have been chosen as two casestudies. The results have shown that both techniques can identify the frequencies effectively. Thestochastic subspace identification technique can detect frequencies that may possibly be missed by thepeak picking method and gives a more reasonable mode shapes in most cases.

Key words:modal analysis; parameter identification; ambient vibration; spectra; stochastic subspaceidentification; engineering structures.

1. Introduction

Parameter identification through dynamic measurements is originally developed in more advancedmechanical and aerospace engineering disciplines (Ewins 1986, Ljung 1987, Juang 1994). There is aclear merit in trying to transfer this technology into civil engineering applications where we aredealing with problems which have a completely different scale, logistics and rationale, comparedwith mechanical and aerospace engineering counterparts. In the context of civil engineeringdiscipline, the encountered structures are complex and large in size such as buildings, bridges. So,the parameter identification technique developed should be compatible and more effective to use insuch large structures. Every single test on a new large object of infrastructure leads to somethingnew, unforeseen or unknown which is worthy of reporting.

Experimental modal parameter identification of civil engineering structures means the extractionof modal parameters (frequencies, damping ratios and mode shapes) from dynamic measurements. Professor

Abstract. The ambient vibration measurement is a kind of output data-only dynamic testing where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for ambient vi

2Wei-Xin Ren and Zhou-Hong Zong

These modal parameters will serve as basis or input to the finite element model updating, detectingand locating the possible damage in structures, long-term health monitoring of structures and thesafety evaluation of structures against different severe circumstances like earthquakes, wind loads. There are mainly three types of structural dynamic testing: (1) Forced vibration testing; (2) Freevibration testing; and (3) Ambient vibration testing. In the first method, the structure is excited byartificial means such as shakers or drop weights. By suddenly dropping a load on the structure, acondition of free vibration is induced. The disadvantage of artificial excitation methods is that traffichas to be shut down for a rather long period of time. This could be a serious problem for theinfrastructures intensively used. In contrast, ambient vibration testing is not affected by thedisturbances on the structures, because it uses the disturbances induced by traffic and wind asnatural or environmental excitation. The service condition does not have to be interrupted by usingthis technique. It represents a real operating condition of the structure during its daily use.

Basically, the modal parameter identification is carried out based on both input and outputmeasurement data through the frequency response functions (FRFs) in the frequency domain andimpulse response functions (IRFs) in the time domain. For civil engineering structures, the dynamicresponses (outputs) are the direct records of the sensors that are installed at several locations.However, the input or excitation level of the real structure in its operational condition is not easierto quantify. It is difficult to measure the input excitation forces acting on a real large structure.Although forced excitations such as heavy shakers and drop weights and correlated input-outputmeasurements are available in some cases, testing method, structural complexity, and/or achievabledata quality restrict these approaches to implement in practical applications. It is obvious that realoperating conditions of complex structures may significantly differ from those of controlledlaboratory environments. Due to this reason, the need to identify modal models under realoperational conditions often arises.

The output-only data dynamic testing has an advantage of being inexpensive since no equipmentis needed to excite the structure. The ambient vibration testing is a kind of output-only datadynamic test method. The modal parameter identification technique through ambient vibrationmeasurement …… 此处隐藏：36742字，全部文档内容请下载后查看。喜欢就下载吧 ……