Experimental and Numerical Study of a Steel Bridge Model through Vibration Testing Using Sensor Networks

IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE)
e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 12, Issue 6 Ver. IV (Nov. - Dec. 2015), PP 83-89
www.iosrjournals.org
DOI: 10.9790/1684-12648389 www.iosrjournals.org 83 | Page
Experimental and Numerical Study of a Steel Bridge Model
through Vibration Testing Using Sensor Networks
Patel S G1
, Vesmawala G R2
1
Applied Mechanics Department, Sardar Vallabhbhai National Institute of Technology, Surat - 395007
2
Applied Mechanics Department, Sardar Vallabhbhai National Institute of Technology, Surat - 395007
Abstract: In recent years, vibration signatures are used to investigate modal parameters (i.e. modal
frequencies, mode shapes and damping etc.) from vibration response of the structures under the external
excitations. These identified parameters gives valuable information on the behaviour and performance of the
structures. In this paper, experimental study on a steel bridge model was carried out by using ambient
excitation method (i.e. remote car model). Mode shapes and its respective modal frequencies are extracted with
the use of sensor technology, signal communication and data acquisition technology. Along with the
experimental study, numerical, study is also carried out to evaluate the vibration properties of a steel bridge
model with the help of FEA software package (ANSYS) and the results are compared.
Keywords: Vibration test; Steel bridge model; Ambient vibration; Data acquisition; Finite element analysis.
I. Introduction
Vibration measurement of structure is a relatively new technique within civil engineering and other
engineering disciplines. Vibration measurement used to identify the vibration parameter which helps to
understand the real behaviour and condition of structure. The study is to evaluate the vibrational properties or
characteristic such as natural frequencies; Mode shape, Damping of the bridge structure with the help of the
vibration signature produced by the external excitation forces i.e. force or ambient excitation [1,2,3]. The
frequencies of vibration of the structure are directly depends on stiffness and mass of the structure, while the
mode shapes are related to the deflection of structures. Therefore vibration testing is good approach among all
other methods to obtain the data of the dynamic properties for structural monitoring and evaluation [4].
In this study, vibration testing on steel bridge model was carried out to study the interference between
sensing technology, data acquisition systems and computer. Therefore, the experimental set up is established to
study, how to measure the vibration signal, acquiring vibration signal, and evaluating vibration properties from
data obtained from vibration study. Hence, the steel bridge model is not prepared by considering the actual
bridge blueprints or prototype of the actual bridge. The force excitation method is used in this study with
moving car model for excitation of steel bridge model to perform the laboratory experiment
A complete vibration signal measurement system consists of Sensory system composed of sensors;
Data acquisition system; Data Communication system; Data processing system; Data Storage system and
Evaluation and verification system.[5] Following Fig. 1 shows flowchart of complete monitoring system.
Fig. 1 Flowchart of an effective complete monitoring system

Experimental and Numerical Study of a Steel Bridge Model through Vibration Testing Using…
II. Test Instrumentation
1.1 THE STEEL BRIDGE MODEL
The steel bridge model is prepared with mild steel plate to study the vibration measurement system to
evaluate vibration properties from acquired data from experiments. The mild steel plate having a thickness of 8
mm is selected to prepared bridge model and experiments are performed. The steel bridge models are of the size
1250 mm x 350 mm. A railing on steel plate is made with the help of the mild steel bar section of size 10 mm x
10 mm. The steel bridge model geometry and manufacturing is shown in following Fig. 2
Fig. 2 Geometry of the Steel Bridge Model adopted for study
1.2 MOVING CAR MODEL
The ambient excitation method is used to excite the steel bridge model by moving car model. A remote
control car having wheel spacing is considered for the study i.e. 150mm c/c. which is used to decide path of
moving car model over a steel bridge model as shown in figure. Weight of car model is not considered for the
study as excitation inputs are not measured in operational modal analysis.
1.3 SENSORS
Sensors are used in experimental study are uniaxial accelerometers. It measures acceleration in one
direction. As per the acceleration measurement in particular direction we have to use the accelerometer
accordingly. Sensitivity of sensor is 100mV/g. it’s having dynamic range ± 50g and frequency range is 0.5 to
10000 Hz. Curved surface magnet mounting pads were used for fixing the accelerometers at its predefine
location for experimental study.
1.4 DATA ACQUISITION SYSTEM
In experimental study, we have use NI compact DAQ i.e. NI cDAQ – 9174 and NI Data acquisition
(DAQ) Modules i.e NI 9234 module for acceleration measurement from National Instruments, USA. The NI
Compact DAQ system consists of a chassis of 4 - Slot, NI C Series I/O modules, and software. Chassis can
connect to a host computer over USB, Ethernet, or 802.11 Wi-Fi or operate stand-alone with a built-in
controller. With over 50 measurement-specific modules and 4-slot chassis, NI cDAQ provides a flexible,
expandable platform for sensor measurement system. The NI 9234 is a four-channel C Series dynamic signal
acquisition module for making high-accuracy sound and vibration measurements from sensors with NI cDAQ
systems. The NI 9234 delivers 102 dB of dynamic range and incorporates software-selectable AC/DC coupling
and IEPE signal conditioning for accelerometers. The four input channels simultaneously digitize signals at rates
up to 51.2 kHz per channel with built-in anti- aliasing filters that automatically adjust to our sampling rate. The
computer provides a processor (2.40 GHz), a system clock, a bus to transfer data into disk space to store data.
The processor controls how fast data is accepted by the converter. The system clock provides time information
about the acquired data.
1.5 LABVIEW SOFTWARE
The vibration data acquisition system software is designed in modularization using Laboratory Virtual
Instrumentation Engineering Workbench (LabVIEW), which is a platform and development environment for a
visual language from National Instruments. The programming language used in LabVIEW, referred to as
graphical language, is a dataflow programming one. LabVIEW is commonly used for data acquisition,
instrument control, and industrial automation on a variety of platforms. The designed software based on
LabVIEW can finish such the functions as data collection, data display, data transform, data memory, Time
record, FFT etc.[6]
III. Experimental Setup And Test Procedure
The steel bridge model is simply supported over a concrete block. The grid marking is done on the steel
bridge model to specify the coordinate & location for path of moving car model and the location of the
accelerometers as shown in Fig. 3. The Data Acquisition System is established with NI cDAQ – 9174 Chassis

and Two Module NI – 9234. The head of the accelerometers are fixed with magnetic mounting pads and
signaling cables are connected at its output end. All accelerometers are attached on the edges of steel bridge
model at specified grid points. Accelerometers are attached with the second and third module NI – 9234 at each
BNC input slots. The computer system is attached with the NI cDAQ – 9174 with a USB connector. After
configuration of all instruments test were perform and data was recorded for the post analysis processes to
estimate the modal parameters of the model.
Fig. 3 Experimental setup for vibration measurement of the steel bridge model
Establish connection between accelerometers, data acquisition chassis (DAC) and modules with
computer. In LabVIEW, the graphical programming has to develop to create interface between data acquisition
system and computer. Vibration testing involves the steel bridge model with moving car at predefines location
path on the model. Moving car model generates motion waves, which are capture by accelerometers and
measures the resultant motion. Actual signals captures from accelerometers are recorded and analyzed in
LabVIEW to get the results. These results are used to get the natural frequencies, mode shape of the model.
IV. FE Modelling of Steel Bridge Model
The Steel bridge model geometry is generated in ANSYS as per dimensions of model adopted for the
experimental work. Model of 8 mm plate thickness are generated with properties were assigned as Density =
7850 Kg/m3
, Young’s modules (E) = 200000 MPa, Poisson’s ratio (υ) = 0.3. The origin of axes is shown in
figure, where the z- and x- axes are located in the plane of the bottom surface of the plate along the longitudinal
and transverse directions, respectively. The y-axis is perpendicular to the plate and positive upwards.
The steel bridge model is modelled using the SOLID 185 as model prepared from mild steel material in
study. Solid 185 is used for the three-dimensional modeling of solid structures. The element is defined by eight
nodes having three degrees of freedom at each node i.e. translations in the nodal x, y, and z directions. The
model of 8 mm thickness plate comprises 6281 elements. The total number of nodes included in the model is
4964 resulting in a system with 14892 degrees of freedom [7,8]. The model is illustrated in Fig.4
Fig. 4 Experimental setup FE modelling, meshing of the steel bridge model in ANSYS
All eight acceleration time histories obtain in experiment at specified location of sensors are taken into
the consideration for analysis of the model in ANSYS. Support condition is provided to the geometry of model
as simply support by imprint faces generated at bottom of the plate as per provided in experimental work.
Analysis of the steel bridge model is carried out using modal analysis performs in ANSYS. The results obtain
from analysis gives modal parameters such as mode shapes and modal frequencies.

V. Results & Discussion
In experimental results, spectrum graph shows different colour plot lines which represent the spectrum
produce due to individual accelerometers. The natural frequencies of the steel bridge model were identified
using spectrum graph, which is computed for the vertical signals obtained from car model during test. Thus, the
individual spectrum graphs have peaks corresponding to all modes [9] Therefore, the frequency values obtained
in all the graphs for all modes are varying. Exect prediction of the frequency value related to mode is not
possible but it can give the approximate range. Typical acceleration time histories and Spectrum graph
generated during experiment at each run in LabVIEW are shown in Fig. 5 and Fig.6 respectively.
Fig. 5Typical acceleration time history generated at each run in LabVIEW program
Fig. 6 Typical Spectrum graphs generated during experiment at each run in LabVIEW program
The very small first peak in all spectrum graphs corresponds to the fundamental mode with a natural
frequency[9], which is 28.978 Hz for first vertical bending mode. The second peak and other corresponding
peak at a frequency of near to 46.864 Hz ,64.109 Hz, 96.981 Hz, 104.064 Hz, 107.538 Hz, 134.651 Hz, 144.098
Hz, 148.078 Hz and 165.556 Hz respectively is more clearly find, indicating second and other correspond to
predominantly different modes. In total, 10 modes with natural frequencies below 200 Hz were identified as
shown in Fig. 7.
In analytical results, the first peak in spectrum graphs corresponds to the fundamental mode with a
natural frequency of 29.468 Hz for first mode represents bending. The second and third peak in spectrum graphs
at frequency 47.097 Hz and 64.305 Hz corresponds to torsional and bending mode respectively. The remaining
peaks are clearly defined peaks represents the remaining eight modes with a frequencies 97.082 Hz, 103.94 Hz,
107.37 Hz, 134.85 Hz, 143.99 Hz, 148.17 Hz and 165.63 Hz. corresponds to bending, tortional or combination
of both modes. Which is shown in Figure 8. The experimental and analytical results obtained are much closed as
shown in Table 1.
Table 1 Comparison of experimental & numerical values of Model Frequencies
Modes
Modal Frequencies
Experiment Values Numerical Values
1 28.978 Hz 29.468 Hz
2 46.864 Hz 47.097 Hz
3 64.109 Hz 64.305 Hz
4 96.981 Hz 97.082 Hz
5 104.064 Hz 103.94 Hz
6 107.538 Hz 107.37 Hz
7 134.651 Hz 134.85 Hz
8 144.098 Hz 143.99 Hz
9 148.078 Hz 148.17 Hz
10 165.556 Hz 165.63 Hz

Fig. 7 Mode shapes and Modal Frequencies for 8 mm plate model (Experimental)

Fig. 8 Mode shapes and Modal Frequencies for 8 mm plate model (Numerical)
VI. Conclusion
From the presented study, vibration properties such as modes and modal frequencies are evaluated for
steel bridge model under constant moving load without considering the ambient excitation (i.e. remote car).
Experimental results are evaluated by the data store during test by using LabVIEW program. These data are in a
form of time domain signals, after applying filters and single processing technique to reduce the disturbance in
signals such as white noise etc. and time domain data were process with applying fast fourier transform
technique to get spectrum graph in frequency domain. Each peak in spectrum graph represents corresponding
modes and modal frequencies.
First modal frequency is 29.978 Hz corresponds to bending mode. Other corresponding modal
frequencies are 46.864 Hz, 64.109 Hz, 96.981 Hz, 104.064 Hz, 107.538 Hz, 134.651 Hz, 144.098 Hz, 148.078
Hz and 165.556 Hz, which represents bending, torsional and combination of both were identified. Numerical

study carried out in FEA commercial software (i.e.ANSYS) with same parameters of steel bridge model adopted
for the experimental study. After performing numerical analysis, First modal frequency 29.468 were calculated
with bending mode. Remaining modes and modal frequencies are 47.097 Hz, 64.305 Hz, 97.082 Hz, 103.94 Hz,
107.37 Hz, 134.85 Hz, 143.99 Hz, 148.17 Hz and 165.63 Hz. corresponds to bending, tortional or combination
of both modes. It is found that results are having closed resemblance of modes and its modal frequencies
calculated with experimental as well as numerical studies.
REFERENCES
[1] O. S. Salawu and C. Williams, “Review of full-scale dynamic testing of bridge structures,” Engineering Structure, Vol. 17, No.2,
1995, pp.113-121
[2] Biggs, M. John, “Vibrations of simple span bridges,” Transactions, ASCE, Vol. 121, 1959, pp. 291-318.
[3] M. L. Silver and T. Venema , “Vibration measurement of steel transit structures,” Journal of the Structural Division, ASCE, V
101:st9, 1975, pp. 1855-1869.
[4] Fraser, E. Ahmed, H. Xianfei, and P.C. Joel (2010), “Sensor network for structural health monitoring of a highway bridge,” J.
Comput. Civ. Eng., ASCE, Vol.24, 2010, pp.11-24.
[5] H. W. Shenton, and P. J. Nicholas, “PC-Based data-acquisition system for structural monitoring”, J. Comput. Civ. Eng., ASCE,
Vol.3, 1989, pp. 333-347.
[6] Manual “LabView” (National Instruments, 2012).
[7] C.R.Farrar and T. A. Duffey , “Bridge modal properties using simplified finite element analysis,” Journal of bridge engineering,
ASCE, 1998, pp.38-46.
[8] ANSYS 14.0, “Modelling and meshing guide,” Release 14.0, (SAS, Inc., 2011).
[9] N. M. M. Maia, J. M. M. Silva, J. He, N.A.J. Lieven, R. M. Lin, G.W. Skingle, W. M. To, A. P. V. Urgueira, “Theoretical and
experimental modal analysis,” (Taunton, Somerset, UK : Research Studies Press Ltd, 1997)

Experimental and Numerical Study of a Steel Bridge Model through Vibration Testing Using Sensor Networks

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Experimental and Numerical Study of a Steel Bridge Model through Vibration Testing Using Sensor Networks