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Development of economized shaking platforms for seismic testing of scaled models

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  • 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN – 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME ENGINEERING AND TECHNOLOGY (IJARET)ISSN 0976 - 6480 (Print) IJARETISSN 0976 - 6499 (Online)Volume 3, Issue 2, July-December (2012), pp. 60-70© IAEME: www.iaeme.com/ijaret.html © I A E M EJournal Impact Factor (2012): 2.7078 (Calculated by GISI)www.jifactor.com DEVELOPMENT OF ECONOMIZED SHAKING PLATFORMS FOR SEISMIC TESTING OF SCALED MODELS Wani Ahmad, Singh Amarpreet, Iqbal Sana, Lal Nawaf, Bhat Javed ADDRESS FOR CORRESPONDENCE Ahmad Wani is B.Tech Civil Engineering, National Institute of Technology, Srinagar, currently with Structural Erection Dept., National Thermal Power Corp., Ltd., Mouda, Nagpur, India. Email: wani.ahmed@yahoo.com Amarpreet Singh is B.Tech Civil Engineering, National Institute of Technology, Srinagar, currently with Structural Design Dept., RITES, Ltd., Gurgaon, India. E-mail: amarpreet89@gmail.com Sana Iqbal is B.Tech Civil Engineering, National Institute of Technology, Srinagar, currently with Civil Construction Dept., National Thermal Power Corp., Ltd. , Mouda, Nagpur, India. Email: sanaiqbal9@ymail.com Nawaf Lal is B. Tech Electrical Engineering, National Institute of Technology, Srinagar, currently with Bharat Petroleum Corporation Ltd., Faridabad, Haryana. E-mail: nawaflal@yahoo.com Dr. Javed Ahmed Bhat is Associate Professor, Civil Engineering Dept., National Institute of Technology, Srinagar. Email: bhat_javed@yahoo.com ABSTRACT Earthquake shake tables have been successfully used in the experimental assessment of dynamic behavior of structures While six-degree of freedom shake tables reproducing actual earthquake data employ expensive electro-hydraulic actuators however the uni-axial servo-motor controlled earthquake simulator, with the incorporation of the cost-effective lead screw as a linear actuator, and capable of supplying a frequency sweep, while maintaining a constant acceleration, is an apt arrangement to precisely evaluate the natural frequency, mode shapes, energy dissipation, etc. of scaled models in an economic manner. 60
  • 2. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME This paper describes the development of such a shaking platform at the Dynamics Laboratory of National Institute of Technology, Srinagar, India, at a very economic cost, primarily aimed at providing teaching facility or aid of structural dynamics, and organize local earthquake awareness programs for the persons associated with construction organizations in the seismically prone Kashmir region, India.Keywords: earthquake awareness, frequency sweep, lead screw linear actuator, uniaxial shaketableI. INTRODUCTION Nearly sixty percent of the people killed by disasters in the last decade have died due to earthquakes. Asia is the worst hit continent in terms of human losses, where during the last decade alone, disasters claimed eighty percent of all fatalities [1]. Earthquakes pose a significant threat to India also because of falling of almost 59% of its geographical area in earthquake vulnerable zones. The most clearly observable impacts of an earthquake are the loss of human lives and property, economic & social losses and environmental degradation. Over the last century, about 75% of fatalities attributed to earthquakes have been caused by the collapse of buildings [ 2]. The Kashmir region has been struck by numerous earthquakes in the past, including the 2005 earthquake (October 8, 2005, magnitude Mw 7.6 which claimed 73,000 lives, left 70,000 injured, 270,000 buildings were destroyed, and 180,000 buildings damaged). It ranks among the worst natural disasters in the history of the Indian subcontinent and Pakistan [3]. All these natural disasters highlight the importance of research in the field of earthquakeengineering [4]. There have been numerous organizations who have been working in this areaat national and international level to promote awareness about earthquakes among the youth, soas to attract research in the field of earthquake engineering. Following the Indian Oceantsunami in 2011, and thus realizing the destructive power of earthquakes and continuing needfor scientists, engineers and technologists to help protect society from their effects, internationalprojects were aimed at raising awareness among young people worldwide of the globalimportance of earthquake engineering skills [5]. Such projects involve awarenessprograms & earthquake engineering competitions, conducted to act as precursors for youngminds to research in the earthquake engineering field. Similar programs are the need of thehour in seismically prone areas of developing countries like Kashmir (India), which lack basictechnological knowhow and basic instrumentation facilities for carrying out research work inthis direction. Although, some efforts have been made at NIT Srinagar by performing acomparative study on different types of construction practices using a Shaker without any recordof natural frequencies, mode shapes or displacements [7]. The problem of the determination of the response of structures to the prescribed excitingforces in theory can be formulated and solved in very general terms, even for situationsinvolving plastic deformations. To make any practical use of the analysis, howeverrequires that quantitative information be available on such basic structural dynamic propertiesas natural periods of vibration, mode shapes, energy dissipation, and yield limits. Suchdynamic properties depend in turn on many details of material behaviour and structural 61
  • 3. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEconfiguration that are not amenable to a fully analytical treatment. Direct experimental determination of such dynamic characteristics is thus a necessity [6]. Shake tables which reproduce actual earthquakes, consist of rectangular platformsdriven in upto six-degrees of freedom using servo-hydraulic actuators. Such shake tablesare too expensive for construction and operation. Thus, to facilitate teaching aid andorganizing awareness programs using scaled models, smaller-sized unidirectional shake tablesare preferred especially in the developing countries [4]. Sensing the need, a uniaxial servo-motor operated shaking platform was developed for theDynamics laboratory at National Institute of Technology, Srinagar at a very economic cost tofacilitate quality teaching of structural dynamics and earthquake engineering, as well asorganize awareness programs associated with the construction organizations in the local vicinity. Inthe present study an effort has been made to develop a uniaxial servo-motor operated shakingplatform. An important aspect about this project which needs to be highlighted is that the shake tablehas been fabricated at a cost which is a very economical, as compared to the costs of similartables which have been developed earlier [7]. II. SHAKE TABLE COMPONENTS The shake table developed at NIT Srinagar mainly consist of a wooden board of size 1.5ft x1.5ft (or 0.5m x 0.5m) as shaking platform over which scaled down models are mounted forunidirectional seismic assessment. The other features of this uniaxial shake table are as under:  High precision computer controlled brushless DC (Direct current)motor (Rating: 400 W, 3000 rpm) as vibration inducer (or earthquake like motion simulator).  Lead screw assembly serving as a linear actuator.  Amplitude range: + 125 mm.  Frequency range: 1 Hz to 8 Hz.  Acceleration range: 0.1g to 0.5g.  Payload capacity: 25 Kg  Has the potential to simulate the time history of any previous earthquake.The whole shake table is mounted over a steel plate of size 1016 mm X 580 mm. Four boxsections (Fig. 1) act as vertical supports for the angles overwhich the table slides. Lead screw,motor and associated assembly is mounted separately over a channel section to avoid theproblem of misalignment (as the poorly aligned assembly can develop undesired components offorces in transverse/downward direction causing extra load on the lead screw and the motorshaft). The nut of the lead screw transmits power to the center of table through a proper arrangementwhich comprises of a box section; square in shape that fits over the nut and a 6mm thickplate which connects table to the box section. The box section also serves to provide a verticalclearance of about 10mm, to easily by pass, the unwanted vertical shocks. The second part of the shake table is the data acquisition system, alsodeveloped at NIT Srinagar, which comprises of a dc power supply (35volt, 25A), acomputer running the MATLAB software, a DC tachogenerator, a 5ᾨ-16A rheostat, a data-acquisition board (PCL card), an optical isolation circuit, a differential amplifier circuit,a 15 volt dc supply and the Shake Table itself. The table has been successfully used for 62
  • 4. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEobservation and measurement of various mode shapes and noting down their correspondingfrequencies. III. THE LEAD SCREW ASSEMBLY A lead screw, also known as a power screw or translation screw, is a screw designed totranslate turning motion into linear motion. Lead screws are manufactured in the same way asother thread forms. There are a number of advantages in using lead screw; some of them may besummarized as-Large load carrying capability, Compact, Simple to design, Easy tomanufacture; no specialized machinery is required, Large mechanical advantage, Precise andaccurate linear motion, Smooth, quiet, and low maintenance, Minimal number of parts, Mostare self-locking. The disadvantages are that most lead screws are not very efficient. Due to thelow efficiency they cannot be used in continuous power transmission applications. Theyalso have a high degree for friction on the threads, which can wear the threads out quickly. The lead screw assembly is an alternative approach used in the project in place of the ballscrew arrangement which is practically a friction less and an apt arrangement as a linearactuator. Various minor shortcomings in lead screw over ball screw arrangement couldpossibly come through by keen maintenance of the equipment. In the present study the pitch of the lead screw has been adopted as per the total run of the nutover threaded length in one second for the maximum amplitude. The motor has a capacityof 50rps while running on full speed. Maximum amplitude of the motion is 12.4425cm for afrequency of 1 Hz which means the motor has to complete one cycle of total run = 49.69cm ≈50cm in one second. For the desired amplitude pitch required comes out to be; 1 rotation =50cm / (50rps) = 1cm. As the motor would not be running at its full speed always since forsimulating simple harmonic motion it’ll have to change its direction of rotation every time andfor each extreme position the velocity of motion will be zero, thereby assuming an averagerotation speed of 35rps and with factor of safety as 1.05 the pitch was adopted as 15mm.Figure 1 shows the picture of the fabricated lead screw with variable pitches. This arrangement is equally suitable for the shaking assembly when used with the highprecision motor. Motor Lead Screw Supports Alignment assemblyFig. 1. Photograph shows the shake table foundation assembly along with the servo-motor fitted 63
  • 5. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME to the variable pitch lead- screw. IV. INSTRUMENT SETUP AND WORKINGA Brush less DC (BLDC) motor controlled by means of PI controller is used to drive the entireassembly. Limitations of brushed DC motors overcome by BLDC motors include lowerefficiency and susceptibility of the commutator assembly to mechanical wear andconsequent need for servicing, at the cost of potentially less rugged and more complexand expensive control electronics, thus facilitating the cost effectiveness. The speed of themotor can be easily controlled by the application of a DC voltage across its controlterminals, whereas for changing the direction of rotation a relay is provided which whenactuated causes reversal in the motor’s direction of rotation. By continuously varying thecontrol voltage in tandem with the signal to the relay, any desired motion of the motor can beachieved. The control of these signals is achieved by means of a PI controller which issuescontrol signals to the motor based on feedback mechanism and guides the motion of the motortowards the desired performance. Various instrumentation components are: A. DC Tachogenerator (Feedback mechanism) A small dc motor is used as a dc generator to measure the speed of the shake table assembly.The field circuit of the dc generator is fed from the same variable dc supply as the BLDCmotor. A 5ᾨ rheostat is used to feed a constant voltage of 1 volt to the field circuit. As the shaftof the generator rotates, it results in the generation of a dc armature voltage which isproportional to the speed of rotation, if the field voltage is kept constant. This DC armaturevoltage if then filtered and fed to the computer in a feedback loop. The dc tachogenerator has anadvantage over ac tachogenerator that the direction of the armature voltage reverses when thedirection of rotation of the shaft reverses. Hence the DC tachogenerator gives both themagnitude and the direction of speed, which is desired for our application. B. Differential amplifier The armature voltage from the dc tachogenerator is full of common mode noise and cannotbe directly fed to the PCL card for measurement of speed. The common mode noise is firstfiltered using a differential amplifier. In addition to the differential amplifier a 25uFcapacitor is also connected across the input of the differential amplifier to further reduce thenoise. C. Optical isolator circuit If the relay circuit and the speed control circuit of the BLDC motor are not isolatedfrom each other, it leads to unnecessary loading of the motor. Isolating the ground of thecontrol circuit from that of the relay circuit is achieved by means of an opto-isolator circuit.This simple circuit isolates the two grounds by means of optical isolation through the MCT2Echip. By using this chip the input and the output to the relay circuit are isolated. 64
  • 6. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 2 (a). Line drawing shows details of the box section used for force transfer between lead screw and table Fig. 2(b). Line drawing shows the lead screw connection with the table Fig. 2(c). Line drawing shows the variable pitch lead screw D.PCL Card Dynalog Micro Systems Pvt. Ltd. Provides AD-DA card – PCL 207, a high performanceanalog interface card for IBM/PC/XT/AT and other compatible computers. PCL 207 uses thehardware-based successive approximations method and provides 40,000 samples per second.The true 12-bit conversion gives an overall accuracy of 0.015 per cent reading +/- 1 bit.The output channel provides fast settling time at high accuracy. A multiplexer in theinput stage provides eight single-ended analog inputs. Standard output voltage ranges are userselectable whereas the input range is fixed. The PCL-207 data acquisition card uses thesuccessive approximation method for the analog to digital data conversion. The cardhas one channel of DAC conversion and 8 channels of ADC conversion. The card has 16registers for data acquisition and data output, the 16 registers have address in sequence to thebase address. The ADC card has 20 pins connector. The pins 1, 3, 5, 7, 9, 11, 13, 15 are foranalog to digital conversion. Pin number 17 is for digital to analog data conversion. The pins 2,4, 6, 8, 10, 12, 14, 16, 18, 20 are ground connections. The entire system is comprised of a dc power supply (35volt, 25 A), a PC running theSIMULINK software, a DC tachogenerator, a data-acquisition board (PCL card), an opticalisolation circuit, a differential amplifier circuit and the Shake Table itself. 65
  • 7. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Using SIMULINK, the user specifies the amplitude and frequency of the sine wave. The sine wave is the user- specified or desired command position of the stage. The voltage needed to move the stage at the desired sine wave position is calculated in SIMULINK through the PI control mechanism and sent through the analog output channel of the PCL card. The voltage appearing at the output channel of the PCL card is sent to the control terminals of the motor. The table moves back and forth at the position and frequency of the commanded sine wave. The resulting speeds of the motor are measured by the dc tachogenerator. The DC tachogenerator is connected to the PCL card and the signal can be displayed and processed further in SIMULINK. The BLDC motor has a feature that it doesn’t instantaneously start to follow a particular control voltage after starting; rather it takes some time to respond to the control signals. A ramp signal is provided to the motor for certain duration to determine its steady state characteristics. This simulation checks the response (speed) of the motor as the control voltage is increased. The ramp is kept within the limits of 0-4 volts with a saturation block. The analog input from the tachogenerator has a lot of noise, which is first filtered out by a digital filter after using a differential amplifier for filtering out the common mode signal and is then displayed on the scope. The stored values of voltage and speed are then plotted. In this simulation the relay is fed a constant signal keeping the motion unidirectional. After obtaining the speed v/s voltage characteristics for one direction, the relay is operated and the characteristics of the motor are now obtained in the other direction. From the two plots [Fig. 3(a). and Fig. 3(b)] it is evident that the two characteristics (forward and reverse are almost identical). Also the characteristics of the motor are linear in the range from 1.6 to 3 volts. Before 1.6 volts, the motor has a dead band and after 3 volts the motor enters a saturation region and the characteristics become non-linear. Fig. 3(a). Graph of motor characteristics (forward) 66
  • 8. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 3(b). Graph of motor characteristics (reverse) Fig. 4. Final control scheme using PI controller Fig. 5. Logic subsystem of the final schemeVI. FINAL CONTROL SCHEME In this final control scheme using PI controller, the control signal generated by the PIcontroller and the output signal sensed by the speed sensor are both fed to a control logicsubsystem. This control subsystem based on these two signals generates the final control signalwhich is fed to the control terminals of the motor and also the signal which triggers the relay. 67
  • 9. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMESince there is noise present in the sensed speed signal, it cannot alone be used fortriggering the relay. Also the control signal alone cannot be used for triggering the relaysince even when the control signal reaches to zero speed, its not necessary that the motor isalso stationary. Triggering the relay when the motor is still in motion causes jerks in itsmovements. The control logic subsystem takes care of these constraints. It accepts both thecontrol signal generated by the PI controller and the signal from the speed sensor and basedon their values devises a scheme for the smooth functioning of the motor. This control scheme is now used for running the shake table assembly in a sinusoidalmanner from 1 Hz to 4 Hz. The reference signal and the output of the motor are compared ona scope. Also the various control signals generated at various stages of the control logic arealso displayed. Fig. 6(a). Response of the control scheme at various frequencies Fig. 6(b). Response of system at frequency of 1 Hz 68
  • 10. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEME Fig. 7(a). Photograph showing shake table during the model testing (response being recorded by 1g accelerometer) {view by overhead camera} MDF Model Wooden Table Tachogenerator Fig. 7(b). Photograph showing model testing in progressV. ECONOMIC ANALYSISA cost analysis of the project indicates that the total cost of the project, which includedestablishing the shaking platform, purchase and installation of the servo motor, a computer andcontrol items, as well as response recording instrumentation sums to a total of 1,00,000 INR,which is quite less as compared to the costs incurred by earlier educational shake tables [7].VI. RESULTS AND DISCUSSION The cutback in costs has been achieved by incorporating lead screw as a linear actuator toconvert translation into rotation, instead of the ball screw. Moreover, in contrast to theearlier projects [7], the idea of having two separate shake tables for two sets of frequencies hasbeen done away with, by the innovative incorporation of two different sets of pitch on thesame lead screw, thus amplifying its scope of simulating ground motion. Also, the shake tableis capable of providing a frequency sweep while maintaining a constant acceleration, for all 69
  • 11. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976– 6480(Print), ISSN 0976 – 6499(Online) Volume 3, Number 2, July-December (2012), © IAEMEaccelerations ranging from 0.1g to 0.5g, which is not a feature in the earlier attempts [7]. Theshake table is capable enough of simulating random earthquake accelerograms, within anacceleration of 0.5g. The shake table has proved to be very effective in finding various dynamic parameters ofscaled down models made by the use of MDF (Medium Density Fibre) as model material. Fewresults of the tested models are presented herewith; figure showing mode shapescorresponding to a three storey model representing a nine storey RCC building. (Calculation ofmode shapes has been done analytically and found in correlation with the experimentallyobserved values). Fig 8. Mode shapes (experimentally obtained) [found in correlation with the analyticalresults]. REFERENCES[1] United Nations International Strategy for Disaster Reduction (UNISDR), Press Release, UNISDR 2010/01; 28 January 2010[2] National Disaster Management Authority (NDMA), Government of India, http://ndma.gov.in/ndma/earthquake.html[3] Sung Jig Kim, Amr S. Elnashai, “Characterization of shaking intensity distribution and seismic assessment of RC buildings for the Kashmir (Pakistan) earthquake of October 2005”, In Engineering Structures 31(12):2998-3015[4] Baran, T., Tanrikulu, A.K., Dundar, C. and Tanrikulu, A.H. (2011), CONSTRUCTION AND PERFORMANCE TEST OF A LOW-COST SHAKE TABLE. Experimental Techniques, 35: 8–16. doi: 10.1111/j.1747-1567.2010.00631.x[5] Selling earthquake engineering to young people, Wendy Daniell 1 ; Adam Crewe 2; Proceedings of the ICE – Civil Engineering, Volume 158, Issue 2, 01 May 2005 , pages 73 – 79 , ISSN: 0965-089X, E-ISSN: 1751-7672[6] Robert L. Wiegel, Bruce A. Bolt; Earthquake Engineering, Prentice-Hall (1970) Nature, pg. 127.[7] C. S. Sanghvi, H S Patil and B J Shah, Development of Low cost shake table and instrumentation setup for earthquake engineering laboratory, International Journal od Advanced Engineering Technology, E-ISSN 0976-3945, IJAET/Vol.III/ Issue I/January-March, 2012/46-49 70