Session 2315

Use of a Small-Scale Models Testing Laboratoryto Teach Structural Dynamics

Anant R. KukretiUniversity of Oklahoma

Abstract

This paper describes the development and use of a unique teaching laboratory that wasestablished to enhance the teaching of structural dynamics to civil engineering seniors and firstyear graduate students. The laboratory was developed by extending an existing Small-ScaleStructural Behavior Laboratory, which was primarily focused on statically loaded structures,with the addition of dynamic models, excitation equipment, and faster data acquisition systems. An "erector set" concept was followed in the development of test models, so that students canvary model properties without fabrication of new components. Equipment and procedures weredeveloped for experimentation on free vibrations of one and two degrees of freedom systemswith various types and degrees of damping, response of these systems to base motion, andeffects of various base isolators on these models. Two undergraduate students assembled,debugged, and conducted these tests and generated a step-by-step manual to make theexperimental work easier for subsequent students. Lessons learned in the development of thislaboratory and its use by students, and the future use planned are also addressed in the paper.

I. Introduction

Recent experiences in teaching structural dynamics to civil engineering students suggest thatexposing students to physical models of dynamic systems may significantly enhance theirunderstanding of the topic. Many students of this generation appear to be focused on themathematical modeling of dynamic systems and do a good job of solving the mathematicalproblems, but lack a clear understanding of the physical implications of the solutions which theyobtain. It is believed that if students could be motivated to study physical models along with thecorresponding mathematical models, they would develop a much deeper understanding of thedynamic solutions, and also gain an appreciation of the implied assumptions of themathematical models and how important deviations from these assumptions may be. In order toexpose students to a positive experience in working with physical models of dynamic systems,the following wish list was developed:

1. At least some of the experiments should be more than just passive demonstrations, sothat the students can be motivated to explore answers to their own questions about thebehavior of dynamic systems.

2. Sufficient equipment and room need to be available so that multiple groups of studentscan work simultaneously, keeping the individual group size to a small number. P

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3. Modern data acquisition techniques should be used so that much of the drudgery ofexperimental data analysis can be automated, the students can also learn about modernmethods of data acquisition, and visualize the response results on a monitor screen inreal time.

4. Experiments to be conducted should include single and multiple degree of freedomsystems.

5. The effects of various types and degrees of damping devices should be demonstrated.

6. Experiments should be conducted with both free and forced vibrations.

7. The effects of aseismic devices, such as, base isolators, on model response should bedemonstrated.

8. Student-designed experiments should be made as feasible as possible.

In order to accommodate the desired number of student groups in available space and with areasonably modest equipment budget, it was decided to utilize small-scale models in thislaboratory. An existing Small-Scale Structural Behavior Laboratory previously developed todemonstrate statics principles was extended to include materials and equipment for dynamicsexperiments. This paper presents information about the process of developing the dynamicslaboratory and its use in teaching structural dynamics.

II. Available Resources Used

The existing Small-Scale Structural Behavior Laboratory included six student workstations, each built around a "model testing table." Funding for establishing this laboratory wasobtained from a National Science Foundation (NSF) Instrumentation and LaboratoryImprovement (ILI) grant, matched by internal funding from the University of Oklahoma. Thesemodel testing tables have tops of 4 in. thick bar-type steel bridge grating reinforced with steelchannels at each end and legs of 3 in. square steel tubing. Each model testing table is equippedwith a light weight reaction frame consisting of predrilled aluminum beam and columnmembers, various electronic (strain gaged) load cells (1 kip capacity), hydraulic load cells (10kip capacity) and hand-operated screw jacks for applying static loads, and Linear VariableDisplacement Transducers (LVDTs) for measuring displacements. Thus the models testing tablewith reaction frame have been designed to provide a closely spaced network of holes and anchorpoints to enable mounting of model structures, and load and displacement measuring equipmentat any desired location. Three micro-computer controlled data acquisition systems are available,with each system being shared between two model testing tables. Each of these systemsincludes a relay-type multiplexer and an integrating type voltmeter which sacrifices samplingspeed in order to gain very fine resolution of small voltage signals and good noise exclusion. Software in this laboratory was written specifically for ease of use in static testing and to givereal-time graphical display of selected data, but could not sample at sufficiently small predefinedtime intervals to be usable for the dynamic testing envisioned. This software also did not

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include any of the data analysis functions such as Fourier Transforms which are needed indynamic test data analysis.

III. Additional Resources Procured

Additional resources were procured to extend the capabilities of this laboratory to includedynamic testing. Funding for these equipment purchases was obtained from a separate NSF ILIgrant, matched by internal funding from the University of Oklahoma. Equipment acquired forthis addition included data acquisition systems, displacement and velocity transducers,accelerometers, signal conditioners, electric shakers with table attachments, model kits,electronic balance and other specialized parts.

The original data acquisition computers (80286-based PC-AT compatible machines) werereplaced with moderately priced 80486 DX2-66 machines. The new 80486-based computersretained the traditional PC-AT expansion buss, so the existing interface cards could betransferred to the new computers. The data acquisition system upgrades consisted of adding afaster data acquisition card to the computers, in addition to the GPIB interface card which wasretained to utilize the low-level scanners for static testing. Cards were selected with 12 bitAnalog to Digital (A/D) converters and multiplexers capable of 16 single ended or eightdifferential input channels. A particular brand of card was purchased because our departmentalready had several of this particular card in other laboratories, and the ability to borrow theseother cards for troubleshooting and emergency replacement was considered important. Thesecards can sample at a rate of 30,000 channels per second into computer memory using DMAchannels. Custom interface panels with a BNC connector for each channel were constructed. These panels also include a stereo guitar plug wired into each input channel circuit, such that amatching plug with a resistor and capacitor soldered inside it’s housing can be inserted to add anR-C filter to that channel. By producing a series of plugs with different values of resistors andcapacitors, filters with different time constants can be tried until an optimum condition ofresponse time versus noise reduction is obtained for each channel.

A comprehensive data acquisition software package was purchased from the manufacturer of theA-D cards to ensure software and hardware compatibility. This package is menu-driven andincludes the capability to acquire data at a preselected sample frequency, store data in files,analyze data using such functions as Fourier Transforms, and display plots of the raw andanalyzed data. It was hoped that this commercial software would come with a comprehensiveand self-explanatory Users Manual so that student training and assistance in the use of thesoftware would be minimized. Unfortunately, that was not the case.

Signal conditioning for some of the transducers was necessary due to the limited resolution ofthe 12 bit A/D converter on the faster data acquisition card. To avoid the high cost oflaboratory-grade signal conditioner/amplifier assemblies, a fixed-gain amplifier was assembledfrom individual modules mass-produced for industrial process control. These modules weremounted along with the required power supplies inside a metal enclosure. It should be notedthat one must be careful to find and specify these signal conditioner modules with sufficientbandwidth for dynamic signals, as a large majority of such units have very low bandwidth.

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Connections for transducers which only need DC power are mounted on the opposite side of thesame metal enclosure, so that the corresponding power supply module could be mounted in thesame enclosure and powered from the same AC power cord.

Transducers purchased for the dynamics experiments included LVDTs, velocity transducers andaccelerometers. LVDTs were purchased with internal modulator-demodulator circuits so thatthey could be powered from a simple DC power supply and the resulting DC signal could beinput directly into the A-D card. A model with ± 250 mm (1 in.) working range was selected. The velocity transducers are essentially linear generators, with a magnet moving inside a coil ofwire. These require no external power and generate sufficient voltage that amplification is notneeded. A model with ± 500 mm (2 in.) working range was selected. Mounting brackets forLVDTs and velocity transducers were fabricated as required. Accelerometers are used tomeasure the acceleration of the mass to which they are attached. An accelerometer with aworking range of 5g and frequency range of 0 to 350 Hz was selected. A L-bracket wasfabricated to attach an accelerometer to the model component, e.g., a floor mass.

Model excitation equipment purchased included long-stroke and low-frequency electromagneticshakers (electro-seis by APS Dynamics, Carlsbad, California) with power amplifiers andoptional table attachments. Separate function generators capable of sine, square, and sawtoothforms were purchased to drive the systems. These table attachments depend on the linearbearings and associated rails in the electro-magnetic shakers, and as such have limitedoverturning moment stiffness. Since these units were purchased, the company has startedadvertizing a table with separate bearings and rails to be used with these shakers. The newertable designs appear to be more rigid in resisting overturning moments.

A collection of parts was constructed in an effort to create a dynamic model "erector set." Partsproduced include story masses of one inch thick steel and aluminum plates which have endsmachined square with the top and bottom surfaces. Holes are tapped into these ends formachine bolts which are utilized with steel bar straps across the ends to clamp vertical strips ofspring steel to the masses. By utilizing 250 mm (1 in.) wide spring steel strips of 16 mm (1/16in.) thickness and different heights (150 mm (6 in.), 230 mm (9 in.) and 330 mm (13 in.)),models can be produced with varying stiffness and natural frequencies. Also, multiple storymasses can be clamped to tall spring columns to produce multi degree of freedom models. Sufficient numbers and varieties of these parts were produced so that a wide variety of dynamicmodels can be assembled from pre-manufactured parts. Matched equal length sets of fourspring steel strips were produced in various lengths for different heights of specimens. Construction of the "erector set" model parts required routine machining with no significantproblems. The flat spring steel used for the model columns was obtained from a local springmanufacturer.

Three types of diagonal damping devices were manufactured to add inter-story damping to themodel specimens. A viscous damper was produced from brass rod and tubing, with the idea thatdifferent levels of damping could be obtained by filling the annulus between the rod and tubewith fluids of different viscosity. A friction damper was also manufactured from a rod sliding ina tube, but instead of filling the annulus with fluid, a teflon brake pad is held in a housing

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attached to the tube and pressed against the rod by an adjustable spring with a setscrew. Ayielding damper was produced by a solid rod and an extra wide clevis at one end. By utilizing alow strength metal such as solder for the clevis pin (which is subjected to bending due to theextra width of the clevis), damping can be produced by material yielding. Each of thesedamping devices is fitted with strain gages to produce a load cell to measure the damping forceduring experiments. These strain gages require the use of the signal conditioning equipment.

Various sizes of cylindrical rubber mounts, 250 mm (1 in.) in height and diameters ranging from250 mm (1 in.) to 380 mm (1.5 in.), were purchased for use as base isolators. These rubbermounts are commonly used to isolate vibrations from electrical motors. Also, molds weremachined from flat teflon plates so that students could cast their own isolators. It was found thatcertain two-part polyurethane joint sealers (used in concrete slab expansion joints) work well inmanufacturing these devices. Molds were prepared for following two types of such baseisolators: (1) a base isolator made of layers of very thin steel plates and polyurethane betweenthem, and two thicker end steel base plates; and (2) a similar base isolator with a lead pluginserted into the center to provide vertical stiffness, more horizontal stiffness against lateralloads, and more energy dissipating capabilities. Each of these base isolator is mounted belowthe four columns of the model frame by bolting it’s end base plates to the frame floor mass andthe shaker table top. Early attempts to produce base isolators from silicone caulking wereunsuccessful due to the extremely long time required for thick masses of the caulk to cure.

Other specialized equipment procured at each workstation includes pairs of electromagnetspowered by a common cord and controlled with a common switch. Electromagnets with a ratedpull of 310 N (70 lb) were used. These electromagnets are utilized to hold free vibration two-story model specimens in a pre-determined deflected position, and then release the two-storymasses at the same time. An electromagnet is also used to deflect a one-story mass by somearbitrary amount, and then release the mass to measure free vibration characteristics.

IV. Experience in Construction of the Laboratory Enhancements Implemented

As stated in the previous section, a particular brand and model of data acquisition card wasspecified for compatibility with other cards already in use by our department. These cards haveproven to be quite reliable and adaptable to many data acquisition needs. The software packagewas purchased from the data acquisition card vendor to ensure compatibility with these dataacquisition cards. In retrospect, this particular brand and model of card is relatively popular,and is supported by many other data acquisition programs from independent software vendors. While we are not aware of any one particular software package which is most common for ourapplication, there may be other disciplines where a particular program is widely accepted aspreferable. If this were the case, then one should select the software first and then considerwhich A/D cards are supported by that software.

The signal conditioning systems assembled from industrial process control modules haveworked well. By careful selection and specification of models, units with sufficient signalbandwidth were obtained. One reason that this is an acceptable solution is that a small numberof devices actually need these signal conditioners and they can all utilize the same excitation

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voltage and amplifier gain. This results in a simplified setup for each experiment with fewercontrols to be mis-adjusted by the students.

Transducers procured for this lab addition include DC-DC LVDTs, velocity transducers, andaccelerometers. The DC-DC LVDTs have proven to be very durable and reliable instruments inother laboratories in our department, and the one’s selected with lower capacity appear to be thesame. We had no prior experience with the velocity transducers, but the one’s selected appearto be very durable and robust also. In selecting accelerometers, a wide range of types wereavailable. The use of small-scale models results in a desire for accelerometers with smallphysical dimensions and mass. Also, the large range of frequencies which are to be measuredlimits the type of accelerometer which can be used. This range of frequencies includes almoststatic conditions as well as higher frequencies than encountered in ordinary buildings because ofscaling effects. Conventional servo-accelerometers are good to zero Hertz (Hz) and can beobtained in reasonably small packages, but many require expensive external signal conditionersand/or have a relatively low maximum usable frequency. The most compact accelerometers arebased on quartz crystals, but these cannot be used for very low frequencies and require externalsignal conditioning. A recent development in accelerometers results from producing a tinypendulum along with strain gages and signal conditioning circuitry from a silicon chip. Theaccelerometers chosen were of this type from IC Sensors, Milpitas, California, and have a usablefrequency range from 0 to 350 Hz and require only DC power. To date we have had failure ofonly one accelerometer, which was inadvertently left on our servo-hydraulic driven shake tablein a research laboratory while the table was driven with a square wave signal during controllertuning. This no doubt resulted in severe overload of the accelerometer.

V. Experience in Development of Experimental Procedures for Use of the Laboratory

Two senior undergraduate students were employed to try out the equipment purchased andassembled, and write a manual for student use in this laboratory. The first task for thesestudents was to learn how to configure and use the data acquisition software purchased. Thisparticular software package is menu-driven, which means that as soon as the program is startedthe user is faced with many choices of operations to do. Unfortunately, the sequence ofoperations to properly initialize the program and set parameters for each data channel is notnecessarily self-evident. These two students spent a significant amount of time reading and re-reading the Users Manual supplied by the vendor before successfully operating the program. They then wrote a companion manual for this class specifying step-by-step instructions for eachexperiment they worked on. It is believed that this manual will make learning how to use theprogram much easier for subsequent students.

The first experiment conducted by the students was free vibration testing of a single degree offreedom model. The objective of the experiment was to explore the relationship between mass,stiffness and natural frequency. A single story model was erected on the model testing table,and first it’s horizontal stiffness was measured by applying a known horizontal static load to thetop floor mass using a hand operated screw jack and electronic load cell assembly, andmeasuring the horizontal displacement of the mass using a LVDT and the data acquisitionsystem. The experiment was repeated for different load levels. The load cell assembly was then

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removed and an accelerometer and velocity transducer was attached to the top story mass fo theframe. This mass was then displaced horizontally using an electromagnet, and then suddenlyreleased. The students measured the acceleration, velocity and displacement of the mass, andused the data analysis portion of the software to perform numerical integration to find the periodof vibration of the frame model (inverse of which is the natural frequency). The experiment wasrepeated by varying the height of the frame columns, and the students tested three heights,150mm (6 in.), 230 mm (9 in.) and 330 mm (13 in.), and compared the experimental values forstiffness and natural frequency with those obtained from theory. Their comparisons showed thatthe model results agreed with the theory within 11%. Thus the students successfullydemonstrated the relationships among the three important dynamic behavior parameters, mass,stiffness and natural frequency. The experiment can also be repeated to include the effect ofchange of the story mass. Another thing that was learned from this first test is that the verticallegs of the original model testing tables used for static tests are relatively flexible in thehorizontal direction, resulting in a sidesway natural frequency of the table similar to the modelthey were testing. The students observed how much this additional degree of freedom(horizontal motion of the table top) could affect their results, and eliminated this problem byclamping diagonal braces on the table legs.

The second experiment was free vibration testing of a two degree of freedom model. Theobjective of this experiment was to observe and compare the differences between the first andsecond natural modes of vibration of a two degree of freedom system. In this test the studentsfirst measured the story stiffness under static loading using for each floor a LVDT, a handoperated screw jack and electronic load cell assembly, and electromagnet. The experimentalprocedure was similar to the first experiment, except each floor mass was displaced one at atime, while the other was held in-place by an electromagnet. An elastic theoretical analysis wasthen performed to predict the two natural frequencies and corresponding mode shapes. Thestudents then used the electromagnets to deflect the model into a particular mode shape and thensimultaneously released both masses. A view of the experimental set up is shown in Figure 1. In this figure, the magnetic bases are shown at each floor mass level on the left side, and on theright side of each floor mass a LVDT and velocity transducer is mounted. Results from the firstmode shape were much closer to the prediction obtained from theory than for the second modeshape. The displacement time histories from the test using the second mode shape had moreaccidental excitation of the first mode shape than vice versa. This was attributed to lessaccuracy in deflecting the structure to the exact mode shape and also in slightly different timeswhen the two electromagnets released their respective masses. The possible time differencewould be a larger portion of the natural period for the second mode shape than the first.

The third experiment was to demonstrate the effects of damping on the free vibration response ofone degree of freedom models. In particular, the experiment was designed to observe the decayof free vibration with various degrees of damping, and to measure the damping parameters. Thestudents started by assembling and testing a model without damping devices, then addingvarious dampers and re-testing the model. For each damping device the degree of damping wasvaried, and the experiment was repeated for each damping level. For example, three differentfluids (water, mineral oil and glycerine) were used for the viscous damper, three differentsettings of the setscrew attached to the spring of the friction damper were tried, and two different

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Figure 1. A Two Degree of Freedom Free Vibration Experimental Set Up

materials (solder (the one used for welding) and plastic coated 14 gage copper wire) were usedfor the beam yielding damper. For each case, the students computed the damping coefficient byusing a logarithmic decrement method, and compared the measured response results and thecomputed damping coefficient for the different damping devices tested with varying amounts ofdamping. One unexpected result was that using water in the viscous damper resulted in moredamping than when the damper was emptied and then re-filled with glycerine. It was decidedthat friction between the rod and cylinder may be contributing to damping more than the shearforce in the viscous fluid. The lubricating qualities of the glycerine probably reduced the slidingfriction more than the higher viscosity increased the viscous damping. The tests on the frictionand beam yielding dampers gave the expected results. The solder beam dissipated more energythan the plastic coated wire, and resulted in a faster decay of the motion. Figure 2 shows asingle degree of freedom model with a damper. As shown in this figure, the top story mass hasan accelerometer mounted on the top surface of the mass, and a LVDT and velocity transducermounted on each side of the mass.

The fourth experiment was forced vibration of a single degree of freedom model in which thefrequency of the forcing function was varied and its effect on the model response was evaluated. The model was mounted on the shaker, to which a reference rigid frame was also attached tomount the LVDTs and velocity transducers. Each LVDT and velocity transducer tip wereattached to the side of the floor mass, on which an accelerometer was also mounted. Figure 3shows the view of a test model frame and the rigid reference frame mounted on the shaker withthe other equipment. Experiments with forced vibrations were limited to sinusoidal basemotions using the table attachments to the shakers. This was done because assembling differentmodels on the shake tables was easier than hanging the shaker at different heights so that it can

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Figure 2. A Single Degree of Freedom Model With a Damper

Figure 3. A Single Degree of Freedom Model With Base IsolatorsMounted on a Shaker

be attached to the various models. The frequency of the shaker table excitation was varied usinga sweep/function generator, and the test was repeated for different forcing frequencies, which incomparison to the model frame natural frequency were either much less, about equal, or muchgreater. Characteristics of the electromagnetic shaker that limited these tests which could be runinclude: the displacement amplitude changes with input frequency for a fixed amplifier setting,and changes in shaker displacement amplitude due to different interaction forces of different testspecimens with the same input frequency and amplifier setting. These characteristics precluderealistic reproduction of motions with varying frequency content, such as earthquake ground

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displacements. They also limit comparison among different model specimens to those run withidentical amplifier and signal generator settings. Even then, the table motion will be somewhatdifferent for different models due to the model-table interaction. Considering these limitations,in the fifth experiment the table tests were used to test the same single degree of freedom modelused in the fourth experiment with various damping devices installed in it. The effects ofdifferent dampers on the response are compared to that obtained for the undamped model of thefourth experiment. While the results will not be rigorously quantitative, they give a reasonablequalitative representation of the differences in performance among the dampers tried. In thesixth experiment, rubber mount base isolators were mounted underneath each column of thesingle degree of freedom model frame and the table tests were repeated. The same qualitativecomparison was used to show the effects of various size base isolators.

Comparing the response results obtained for the different dampers and base isolators, it wasconcluded that the best damper was the beam yielding damper with a solder, and the bestcommercial rubber mount base isolator for the one story model frame tested was the one with250 mm (1 in.) height and 350 mm (1 3/8 in.) diameter. This damper and base isolator changedthe natural frequency of the one story model by 32% and 22%, respectively, and reduced theacceleration of the first floor mass by more than 50% in comparison to the model without thesedevices. Also, it was noticed that the acceleration of the top floor mass with the commercialbase isolator was 25% less than that of the model with the beam yielding damper.

VI. Student Experiences in Use of the Facilities Developed

Unfortunately, the first experience the two pilot students received was one of frustration with thesoftware and it’s Users Manual. We expect this problem to be greatly minimized with the moreexplicit manual generated by these students. On the other hand, these students developed asignificant sense of accomplishment when the experiment worked as planned and theexperimental results closely matched the analytical predictions obtained from theory. Thestudents developed a significant understanding of many of the implicit assumptions made in thetheoretical solutions and how inaccuracies in these assumptions can affect the analyticalprediction as compared to reality. They learnt how structural damping characteristics areexperimentally measured and computed, which are needed as input data to obtain analyticalresponse solutions from theory. As a secondary issue, the students learned about measurementtransducers and issues about data acquisition such as resolution and range of a system ofinstruments. The students learnt about the role of damping and aseismic devices in reducingunwanted natural and forced vibration effects on a building structure.

VII. Planned Use of the Laboratory

The proposed laboratory improvement is planned to teach a new course, Experimental DynamicAnalysis of Structures, which will be taken by senior undergraduates as a professional electiveand by first year graduate students. The course will be conducted as a one-hour lecture and twotwo-hour laboratory classes per week. With the aid of one graduate laboratory assistant, thestudents will perform eight experiments during the first ten weeks on the following three topics: P

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1. Measurement of dynamic parameters and free vibration response, which would includethe following three experiments:

(i) Measurement of natural frequency of single degree of freedom systems.(ii) Measurement of natural frequencies of multiple degree of freedom systems.(iii) Measurement of damping parameters and the effects on free vibration structural

response.

2. Measurement of forced vibration response, which would include the following fourexperiments:

(i) Measurement of structural response to harmonic excitation.(ii) Demonstration of various types of structural dampers on structural forced

response.(iii) Demonstration of vibration isolation technique to reduce effect of forced

vibration.(iv) Study of behavior of different height structures to earthquake ground motion.

3. Field measurements and performance evaluation using accelerometers and a portabledata acquisition system.

For each of the experiments listed under the first and second topics, prefabricated “erector set”model parts, described earlier, will be utilized. The students will be required to assemble theseparts, install the necessary instrumentation (LVDTs, velocity transducers and accelerometers),and acquire and analyze the data using the automated data acquisition system and software. TheLaboratory Manual which explains the step-by-step procedure to use the software and conductthe experiments, which was developed by the two pilot students, will be made available. Thismanual also contains a detailed description of the theoretical background for each experiment toenable students to compare experimental results with those obtained from theory. These modelswill not be tested to failure, and will be reusable. After completion of an experiment, eachstudent will submit an individual laboratory report.

For the third topic on actual field measurements different experiments can be chosen, whichincludes floor vibration measurements in a stadium structure and aerobic exercise rooms,determination of natural frequencies of tall buildings using ambient vibrations, determination ofstructural response to vibrating mechanical equipment, and vibration of highway bridges due tovehicular traffic. Different experiments can be chosen in different years. For example, for thefloor vibration measurement experiment, accelerometers will be placed on several floors withdifferent levels of noticeable vibration and different types of construction. A portable dataacquisition system will be used to record the acceleration response of the floors to variousloadings such as a single heel drop and pedestrian traffic. The acceleration response will beanalyzed using the software supplied to the students to determine natural frequencies, anddamping ratios. Also, the maximum response to a standard loading (heel drop) will becompared to the perception of floor vibration. The type of floor system will be compared to thenoticeability of floor vibrations and all the measured parameters.

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During the last third of the semester, each group will be required to conceive, design andorganize two experiments. A list of topics will be provided, but the groups will be encouragedto use this list as a guide and develop their own problems. Once the group has selected theirtopic, they will be asked to conduct a literature search for related publications. This will helpthem to design their experiment and decide if special fabrications and equipment are needed. Ifso, the students will submit shop drawings for fabrication. Each group will be required to writea combined project report on the two experiments and make a class presentation of one of them. This will be coordinated during the last two weeks of the semester.

VIII. Concluding Remarks

One issue which must be addressed when adding a laboratory experience to any class is how tooptimize the student experience compared to the student and faculty effort required. Forexample, if students are asked to only reproduce pre-defined and pre-tested experiments, therewill be less effort required on everyone’s part but the student will miss out on the experience ofdesigning a unique experiment and learning about all the details required to make it work. Students often learn more when they design and conduct an experiment from scratch, butusually need more time and help to make things work. For each class an appropriate balance ofpre-defined versus student defined experiments or projects must be determined.

In experimental work, the more visual the feedback, the better. Many concepts such as physicalbehavior of dynamic models are easier to grasp when presented in graphical format. Examplesof this are mode shapes of two degree of freedom structures.

Well-trained laboratory assistants to assist the students in equipment set-up, software operation,and results interpretation can greatly enhance the students experience in laboratory classes. Obviously these assistants must constantly consider the first issue of how much to let thestudents find out for themselves and when to offer help to avoid too much student frustration.

In summary, it appears that a positive laboratory experience can be provided and that it certainlyappears to help the students understanding of dynamic behavior of structures.

IX. Acknowledgments

The author would like to acknowledge the financial support totaling $31,120 provided by theNational Science Foundation (Award No. USE-9250904), and matching funds provided by theUniversity of Oklahoma for the laboratory development presented in this paper. The authorwould like to acknowledge the assistance given by Dr. Benjamin J. Wallace, AssociateProfessor, School of Civil Engineering and Environmental Science, University of Oklahoma, inthe development of this laboratory. Some parts of the laboratory development described in thispaper were presented at the Thirty-first Midwest Section ASEE Conference, held at Tulsa,Oklahoma on April 10-12, 1996.

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ANANT R. KUKRETIAnant R. Kukreti is a professor of Civil Engineering and Acting Director of the School of Civil Engineering andEnvironmental Science at University of Oklahoma. He has won numerous teaching awards, which include theBurlington Northern Foundation Teaching Award, Regents Award for Superior Teaching, ASEE Midwest SectionOutstanding Teaching Award, and the ASEE Fluke Corporation Award for Innovation in Laboratory Instruction.

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