International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.III/ Issue I/January-March, 2012/46-49
Research Paper DEVELOPMENT OF LOW COST SHAKE TABLES AND
INSTRUMENTATION SETUP FOR EARTHQUAKE
ENGINEERING LABORATORY C. S. Sanghvi
1, H S Patil
2 and B J Shah
3
Address for Correspondence 1Applied Mechanics Department, L.D. College of Engineering, Ahmedabad, Gujarat, India
2Department of Applied Mechanics, S V National Institute of Technology, Surat, Gujarat, India 3Applied Mechanics Department, L.D. College of Engineering, Ahmedabad, Gujarat, India
ABSTRACT For the development in the field of earthquake engineering, experimental study is required. To study the effects of
earthquake, laboratory facilities are needed. The development has reached to a stage where earthquake simulation is
achieved in laboratory. Shake table is used to provide earthquake simulation and to test the prototype and scaled model of
the structure. In order to reproduce actual earthquake data, a six-degree of freedom electro-hydraulic shaking table is
essential. They are very expensive and require high maintenance and operational costs. There exists a need to develop
suitable teaching and learning aids to augment the classroom teaching. One of the most effective ways to achieve this is to
develop simple experimental setup with suitable shake table. Development of shake table for the Earthquake Engineering
laboratory to test models is a challenge. Single translation (horizontal) degree of freedom shaking tables is useful for
laboratory testing to study behavior of structures. From this perspective, low cost uni-axial shaking tables were designed &
fabricated at L.D College of Engineering. These low cost shake tables will be used to study behavior of structure through
models under harmonic as well as random excitation. The cost of shake table is very high and it is difficult for the institutes
to acquire such facilities. Based on this fact, an effort has been made to fabricate two low cost shake tables with required
specifications to test models in Earthquake Engineering Laboratory along with a LVDT based instrumentation setup. The
instrumentation setup comprises of LVDT and Data Acquisition System. Response of models studied through shake table
testing. Shake table with servo motor control & shake table with 1.0 HP motor is costing around Rs 3, 50,000/- & 80,000/-
respectively. The cost of instrumentation for such set up is only Rs 20,000/-. This effort will fulfill the basic need of the
Earthquake Engineering laboratory in form of low cost shake table & required instrumentation, to study behavior of
structure through models by shake table testing.
KEYWORDS: Earthquake Engineering laboratory, shake tables, experimental study, simulation, Single degree of
freedom, Accelerometer, Data acquisition, Uni-axial Shake table
1. INTRODUCTION The development in the field of earthquake
engineering requires experimental study. Laboratory
testing of components and structures as physical
models is an effective way to study the complex
phenomena. Correlation of results from laboratory
experimentation and analytical modeling will
increase the confidence of the researcher. A Shake
table can be used to test the model of the structure
which may be scaled or prototype to seismic shaking.
2. REQUIREMENT OF UNI-AXIAL SHAKE
TABLE
In order to reproduce actual earthquake data, a six-
degree of freedom shaking table is essential. Shake
table is a very complex electro-hydraulic system,
which is very expensive and requires high
maintenance and operational costs. As per Clause:
6.1.1 IS 1893-2002 [3], the random earthquake
ground motions, which cause the structure to vibrate,
can be resolved in any three mutually perpendicular
directions, i.e. two horizontal and one vertical
direction. The predominant direction of shaking is
usually horizontal. The vertical direction is ½ to 2/3rd
of the horizontal vibration. The self weight of
structure, i.e. gravity loads, compensates the effect of
vertical accelerations. Movement of shaking table
means application of strong ground motions
(accelerographs) to model of the structure to study
their behavior. Simulation of earthquake ground
motion in all six directions of consideration (i.e ±X,
±Y, ±Z) is complicated and costly. The effect of
horizontal ground motion is significant on structure
compared to the vertical motion which is almost 1/2
to 2/3rd of the horizontal acceleration. Thus, ground
motion consideration is left to major two orthogonal
horizontal directions.
Seismic analysis of structure means to provide
equivalent distributed lateral force acting at various
lumped level of structure above ground based on the
ground motion at that site. Thus, instead of getting
into complex nature of analysis, the behavior of
structure is analyzed when horizontal ground shaking
occurs. Horizontal shaking of shake table is
representing the horizontal shaking of ground. By
changing the orientation of test model on shake table
will give behavior and reading for the other
orthogonal direction too. Thus, uni-axial shake table
serves the purpose. It is always a challenge to
develop a low cost shake table with good instrument
set up for Earthquake Engineering Laboratory. Uni-
axial shaking tables were designed and fabricated at
the institute laboratory. One shake table is of 8 ft x 4
ft in size with amplitude variation of 0 to 100 mm
and frequency varying from 0 to 4 Hz. Second shake
table is of 5 ft x 3 ft in size with amplitude variation
of 0 to 50 mm and frequency varying from 0 to 25
Hz. This two shake tables will satisfy the basic
requirement for testing models falling in this
frequency range. The instrumentation setup of this
Shake table comprises of LVDT and Data
Acquisition System. A single degree of freedom
model was tested on this shake table to evaluate the
performance of Shake Table and instrumentation
setup.
3. SHAKING TABLES AT EARTHQUAKE
ENGINEERING LABORATORY
Shake tables prepared by L.D College of Engineering
(L.D.C.E) are Uni-axial Electro-mechanical Shaking
tables. These shaking tables are assembly of various
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.III/ Issue I/January-March, 2012/46-49
steel sections that forms a table on which a plate is
supported. The movement of this plate is considered
as shaking of ground due to earthquake. The term
Uniaxial means movement in one horizontal direction
only.
Specifications of low frequency shake table
� Uni-axial motorized electro-mechanical Shake
table
� Size 8 ft x 4 ft table platform for fixing model
� Operated manually and with motor as well
� Motor: Single-phase D.C motor with Electronic
panel board, frequency range 0 to 1500 rpm.
� Amplitude range of Shake table is from 0 to 100
mm
� Harmonic and periodic simulation
� Frequency of simulation - 0 to 4 Hz
� Payload capacity : 600 kg
Figure 1 Electro-Mechanical low frequency
Shaking Table
Mechanism of low frequency shake table
The Uni-axial shake table serves the purpose of
Laboratory testing of models for earthquake
simulation. The movement of L.D.C.E shake table is
in one horizontal direction. The top plate is connected
with a shaft (S). One end of the shaft (S) is assembled
with IS MC 70 35 5 block (B). This block (B) is
welded with bottom of the plate. The other end of the
shaft (S) in embedded into slider (L). The head of the
slider (L) is grooved into the shafting (T). The
distance of shaft end assembly from the zero mark of
Shafting (T) groove decides the amplitude of
shaking. The Shafting (T) is connected with an axle
that has a pulley (P). The pulley (P) is rotated by
motor attached with the belt. Thus, when the motor is
operated, the belt rotates the pulley (P). This helps
the axle to rotate and push the shaft forward and
backward. The shaft is connected with top plate on
the other end which moves the plate in horizontal
direction. The movement is eased by provision of
bearings block (R). Thus, the movement is smooth
without any jerks, as the angles (A) connected to
plate; slides on the bearing of bearing block (R). The
plate clamp (M) also helps by providing vertical
restraint to plate during higher frequency. The
movement of plate i.e amplitude of shaking remains
fixed during single instance of testing. The amount
by which the shaft in the groove of Shafting (T) is
shifted away from center, gives amplitude of shaking.
Figure 2 shows the arrangement of the whole
mechanism. The ratio of pulley (P) diameter to pulley
diameter of motor is 3.0, i.e the frequency of 1500
rpm of motor is reduced to 500 rpm. The mechanism
is such that at a particular instance of testing, the
amplitude remains constant and the frequency can be
regulated to provide variation in shaking.
Figure 2 Line diagram of Mechanism of Shake
Table
Thus, the excitation is harmonic in nature. The
response of structure to harmonic excitation provides
insight how the system will respond to other types of
forces.
50mm c/c13 mm Dia circle
50
50
575
575
87.587.5
1250
550 612.5550612.5
2500
ANGLE FRAME
Fig. 3 Plan View of Top plate of Shake Table
Showing arrangement for Model fixing.
Specifications of high frequency shake table
� Uni-axial motorized electro-mechanical Shake
table
� Size 5 ft x 3 ft table platform for fixing model
� Operated with motor as well
� Motor: Servo motor with Electronic control
panel, frequency range 0 to 9000 rpm.
� Amplitude range of Shake table is from 0 to 50
mm
� Harmonic and random motion simulation
� Frequency of simulation - 0 to 25 Hz
� Payload capacity : 500 kg
Figure 4 Electro-Mechanical high frequency
Uni-axial Shaking Table
Model is fixed on shaking table which represents
structure fixed to the ground. For making this
arrangement possible, the top plate is provided with
number of holes of 12 mm diameter at 50mm c/c
distance. This gives the flexibility of installing any
size of model and also helps in changing the
orientation of the model. This arrangement also helps
in fixing two models at a time to study the
comparative behavior of the same. Figure 3 shows
the Plan view of Top plate of shake table.
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.III/ Issue I/January-March, 2012/46-49
4. INSTRUMENT SETUP
Instrumentation setup is a data logging system which
records measurements continuously, in a number of
digital electrical devices. These data are required to
be processed to give data in the required format. This
data processing unit forms the data acquisition
system. Based on availability of LVDT as a sensing
unit, a 4 channel data acquisition system was
developed. This includes a LVDT and a circuit that
processes the data and provides in word format with
data logging rate of average 15 samples per second.
The hardware and software components used for this
instrumentation setup are as follows:
Hardware [6]:
1. Atmel AT89C52 Microcontroller [4],
2. ADC0808CCN,
3. Vibration Sensor (LVDT),
4. MAX 232,
5. TL084 Quad Op-Amp
6. MCT2E Optoisolator
7. Power Supply (-5V to +5 V):
Power Supply (-15V to +15V)
8. Other circuit Components (resistors, capacitors
etc...). Software: Keil Version 3, Visual Basic 6
Figure 5 Main circuit and hardware component
for data processing
5. WORKING OF INSTRUMENTATION
SETUP
Three LVDT’s are considered for circuit design as
sensing component. Here vibration sensor is nothing
but a Linearly Varying Displacement Transducers is
having a stroke length of 100 mm. the LVDT gives
signal in range of 0 to 5 volts. This range is used for
providing displacement result of 0 to 100 mm range.
The circuit is designed to log 3 LVDT data and a
frequency counter data. 4 ADC (analog to digital
convertor) channels, 3 for the vibration sensor and
one for the frequency are used. This sensor will give
0 to 5 volts for 0 to 100 mm displacement. The
output of the sensors is given to 3 different channels
of ADC. These pulses are given to the 2nd order low
pass filter to convert it into DC (direct current). The
output of the filter is given to the ADC. Now the
channels of the ADC are rotated by the AT89C52
software. These channels are scanned by the software
program. The scanning time depends on the Visual
Basic (VB) software. Minimum scanning time is 1
ms. The output of the ADC is in hex. So it is
converted into BCD (binary coded decimal) by the
software. The appropriate range is set by the software
itself. i.e. 0 to255 is converted into (-50 to +50 mm)
for the sensors and (0 to 20Hz) for the frequency
input. Data are transmitted serially to the computer’s
serial terminal port using RS232 communication.
This data will be handled by VB program and log file
will be created for testing. Figure 6 shows software
screen of VB program showing screening of data for
sensor A.
Figure 6 Software screen of program developed in
VB
6. TEST MODEL AND SETUP FOR
PERFORMING EXPERIMENT:
The single degree of freedom model, made from
A304 stainless steel [1], was tested on uni-axial
shaking table and response at top of model was
measured using LVDT. To verify the results logged
by LVDT, uniaxial accelerometer was also installed
on model. Thus, the experimental setup consists of a
SDoF model, LVDT and its circuit, uniaxial
accelerometer and 16 channel vibration analyzer,
supporting stand for LVDT and shake table. (See
Figure 6). The model consists of a top plate 390 x
390 mm size supported by four 6mm square rods.
The rods are fixed in bottom plate of 490 x 490 mm
with check nuts. The model fabrication is done in
such a way where check nuts are used instead of
welding the model. This is done to avoid error during
welding and to achieve better response of model. The
natural frequency calculation for the model is given
in Table I.
Figure 7. Experimental setup including model and
instrumentation setup.
Table: I Natural frequency calculation for as
build model
1
Vertical rods
a) width(x) 6 Mm
b) Depth(z) 6 Mm
c) Ixx 108 mm^4
d) Izz 108 mm^4
2 Top Plate thickness 5 Mm
3 Density of steel A304 7407.41 Kg/m:^3
4 Elasticity E = 210000 M pa
5 Height of structure 490 Mm
6
Lateral Stiffness
K=4*(3EI/L^3) N/m 2313.32 N/m
7 Dimension in X-dir 390 Mm
8 Dimension in Z-dir 390 Mm
9 No of Columns 4 Nos
10
Mass of Structure
Top plate : 5.63 Kg
Vertical rods: 0.64 Kg
Extra for nuts/bolts: 0.28 Kg
11 Total lumped mass m 6.55 Kg
12
Omega ω
ω = (K/m)^0.5 18.79 rad/s
13
Frequency f
f = ω / 2*Pie 2.99 Hz
14 Natural period (T) 0.334 Sec
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.III/ Issue I/January-March, 2012/46-49
7. RESULTS OF MODEL TEST LOGGED BY
LVDT:
LVDT based instrumentation setup is capable of
logging data in terms of displacement only. The
average sampling rate is 15 S/s (Samples per second).
The testing was performed for 47 seconds. Figure 8
shows the data logged by LVDT in graphical format
as Displacement v/s Time.
Samples in terms of 1/15th of second for 720 samples
on X axis and values of displacement logged for each
1/15th of second are plotted on Y. The maximum
displacement is at 9th second i.e at 115
th and 117
th
sample. The value is +34.8 mm and -35.6 mm
respectively. The shake table amplitude during
testing was set to 15mm. Thus, maximum relative
displacement will be 35.6 mm less 15 mm to give
peak value of displacement at 9th second as 20.6 mm.
8. RESULTS OF MODEL LOGGED BY
ACCELEROMETER
In experimental setup for model testing, 4
accelerometers were used. Acc-1 was mounted on top
plate of shake table which provides an input motion
value for experimental model. Acc-2 was mounted on
top of experimental model to provide response of the
model due to shaking. Acc-3 was mounted on top of
LVDT stand to verify that the stand is stiff enough to
provide correct results of LVDT. Acc-4 was mounted
on base plate of model to verify that shake table
shaking and base plate shaking are same. Figure 8
shows graphical representation of all four
accelerometers [5]. As shown in figure 9, FFT curve
[5] analyzed for Acc-2 i.e for accelerometer mounted
on model to compare the results with LVDT results.
The displacement values of peak displacement for
Acc 1 to 4 obtained from FFT curve are as below: Peak displacement for Acc-1: 15.03 mm
Peak displacement for Acc-2: 33.83 mm
Peak displacement for Acc-3: 15.74 mm
Peak displacement for Acc-4: 14.68 mm
From the above values we can say that Acc-1, Acc-3
and Acc-4 are almost having same displacement
values. Thus, shake table amplitude, base plate of
model and top of LVDT wooden stand all have same
displacement value. As shown in figure 10, the
displacement value of model top is 33.83 mm. Thus,
relative displacement of model is 33.83 less 15.03 i.e
18.8 mm.
TIME (s)
Figure 8 Graphical representation of Displacement v/s Time logged by LVDT
Figure 9 Graphical representation of as acceleration v/s Time logged by accelerometers
International Journal of Advanced Engineering Technology E-ISSN 0976-3945
IJAET/Vol.III/ Issue I/January-March, 2012/46-49
.
Figure 10 FFT curve for Acc-2 i.e model response.
Table:II Comparison of parameters of model testing on low frequency shake table Parameters LVDT Acc-2 % diff. w.r.t Acc-2
Displacement, mm 20.6 18.8 9.57
Natural freq. Hz 2.99 3.15 5.08
Table:III Comparison of parameters of model testing on high frequency shake table Parameters LVDT Acc-2 % diff. w.r.t Acc-2
Displacement, mm 21.2 19.8 6.42
Natural freq. Hz 3.09 3.24 4.62
Maximum displacement of model sensed by LVDT
and Acc-2 accelerometer are 20.6 mm and 18.8 mm
respectively. The natural frequency calculated from
the physical properties of model was 2.99 Hz and
that sensed by accelerometer was 3.15 Hz. (see table
II). Thus, the values of displacement and natural
frequency are comparable and the nature of curve is
also matching (see figure 8 & 9).
9. CONCLUSION ON EXPERIMENTAL
STUDY:
1. The data sampling rate of accelerometer is
640 Samples/sec and the sampling rate of
LVDT is 15 Samples/sec. With this
sampling rate we are able to achieve
acceptable results with maximum variation
of 9.57%. The cost of LVDT based
instrumentation setup is around Rs.
20000=00 only while that of vibration
analyzer with accelerometer is 22 lacs Rs.
2. The cost of developing 8 ft x 4 ft size low
frequency uni-axial shake table is Rs
100000=00 along with LVDT set up which
is very simple, cost effective and yet
provides acceptable results of displacement
for the peak frequency of 3 Hz.
3. The cost of developing 5 ft x 3 ft size high
frequency uni-axial shake table is Rs
370000=00 along with LVDT set up which
is very simple, cost effective and yet
provides acceptable results of displacement
for the peak frequency of 25 Hz.
4. This low cost shake table and
instrumentation setup is the first step for the
development of earthquake engineering
laboratory which will provide the vision to
experience the subject of dynamics
practically. It will help:
• To develop understanding of dynamic
response of structures to undergraduate
students;
• To reinforce theoretical concepts
through the use of “hands-on” laboratory
experiments;
• To provide an opportunity to use modern
engineering tools including sensors and
data acquisition/analysis equipment.
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for Stainless and Heat Resisting Steel Bars and
Shapes.
2. Harry G. Harris, Drexel University and Gajanan M.
Sabnis, Howard University, “Structural modeling and
Experimental Techniques”, second edition, CRC
Press.
3. IS 1893 (Part 1): 2002 “Criteria for Earthquake
Resistant Design of Structures, Part-1 General
Provisions and Buildings (Fifth Revision)”, Bureau of
Indian Standards, New Delhi.
4. Mohammad Ali Mazdi, “The 8051 Microcontroller
and Embedded Systems”.
5. OROS 3-Series/NVGate User’s Manual - for V3.10 –
January 2006.
6. www.alldatasheet.com