DEVELOPMENT OF LOW COST SHAKE TABLE AND INSTRUMENTATION SETUP FOR EARTHQUAKE ENGINEERING LABORATORY

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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 LABORATORYC. S. Sanghvi

1, H S Patil

2and B J Shah

3

Address for Correspondence1Applied Mechanics Department, L.D. College of Engineering, Ahmedabad, Gujarat, India

2Department of Applied Mechanics, S V National Institute of Technology, Surat, Gujarat, India3Applied 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 isachieved 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 isessential. 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 todevelop 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 institutesto acquire such facilities. Based on this fact, an effort has been made to fabricate two low cost shake tables with requiredspecifications to test models in Earthquake Engineering Laboratory along with a LVDT based instrumentation setup. Theinstrumentation setup comprises of LVDT and Data Acquisition System. Response of models studied through shake tabletesting. 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 theEarthquake 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 earthquakeground 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 groundmotion 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 theground 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 tablewill 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 4ft 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 basicrequirement 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 instrumentationsetup.

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 Shakingtables. These shaking tables are assembly of various





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 directiononly.

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 earthquakesimulation. The movement of L.D.C.E shake table is

in one horizontal direction. The top plate is connectedwith 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 onthe 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. Themovement 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 pulleydiameter 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 ShakeTable

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.

50mmc/c13mmDiacircle

5 0

5 0

5 7 5

5 7 5

87.587.5

1 2 5 0

550 612.5550612.5

2500

A N GL E F RA ME

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 representsstructure 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 theorientation of the model. This arrangement also helpsin 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.





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 withdata 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 INSTRUMENTATIONSETUP

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 channelsof ADC. These pulses are given to the 2

ndorder 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’sserial 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-axialshaking 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 Mmc) 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

6Lateral 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 Kg11 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





7. RESULTS OF MODEL TEST LOGGED BY

LVDT:

LVDT based instrumentation setup is capable of

logging data in terms of displacement only. Theaverage 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 each1/15

thof second are plotted on Y. The maximum

displacement is at 9th

second i.e at 115th

and 117th

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 motionvalue 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 tableshaking 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 mmPeak 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 samedisplacement 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





.

Figure 10 FFT curve for Acc-2 i.e model response.

Table:II Comparison of parameters of model testing on low frequency shake tableParameters 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 tableParameters 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 lowfrequency 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 andinstrumentation 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” laboratoryexperiments;

• To provide an opportunity to use modern

engineering tools including sensors and

data acquisition/analysis equipment.

REFERENCES1. ASTM Designation A 276-87, Standard Specification

for Stainless and Heat Resisting Steel Bars andShapes.

2. Harry G. Harris, Drexel University and Gajanan M.Sabnis, Howard University, “Structural modeling andExperimental 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

DEVELOPMENT OF LOW COST SHAKE TABLE AND INSTRUMENTATION SETUP FOR EARTHQUAKE ENGINEERING LABORATORY

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