ANANT R. KUKRETISchool of Civil Engineering and Environmental ScienceUniversity of Oklahoma

ABSTRACT

In this paper development of the equipment and experiments forteaching behavior of structures to undergraduate Civil Engineer-ing students is presented. A description of the laboratory andequipment used is presented, followed by a description of the ex-periments that are presently conducted. All experiments are doneusing small-scale models that can prefabricated and assembledeasily by the students. The role this laboratory can play in enhanc-ing undergraduate and graduate curricula is also discussed.

I. INTRODUCTION

The mathematical knowledge acquired by today’s engineeringstudents must somehow be augmented by a sound understandingof the basic physical phenomena involved and an appreciation ofreal behavior or performance. Recognizing the importance and thecrucial role that experimental testing plays in providing a meaning-ful structural engineering experience to the undergraduate students,the Small-Scale Structural Behavior Laboratory (SSBL) was estab-lished in the School of Civil Engineering and Environmental Sci-ence (CEES) of the University of Oklahoma (OU) from a NationalScience Foundation’s Instrumentation and Laboratory Improve-ment grant, which was matched by OU. In this paper the develop-ment and the impact of the SSBL, which is basically used as ateaching laboratory, are presented.*

II. DESCRIPTION OF THE LABORATORY FACILITIES

In the SSBL equipment and experiments have been developedfor teaching behavior of structures subjected to static loads to un-dergraduate engineers. All experiments are done using small-scalestructures that are relatively easy to fabricate and handle, and placeminimal demands on the capacity of loading and measuring de-vices. The SSBL is built around six “model testing tables,” which areconstructed from steel bridge grating. A light weight aluminum re-action frame with pre-drilled holes is bolted to the table top. Thus,the models testing table and reaction frame have been designed to

provide a closely spaced network of holes and anchor points to en-able load application at any desired location. At each table mechan-ical supporting devices and load, displacement and strain measuringdevices are provided, such that their use provides not only a hands-on experience, but also an opportunity to use modern computerizeddata acquisition equipment to obtain real time feedback of test re-sults. The equipment and supplies for each table are stored in a cab-inet provided next to the cabinet. Thus each “work station” consistsof the following three units:

• the model testing table• the reaction frame• the equipment cabinet.Six such work stations are provided. Some experiments such as

column tests on reinforced concrete members, exceed the capacityof the loading devices provided for the model testing table. The useof a separate testing frame is preferred rather than to attempt tomake all tables with sufficient capacity. Two such separate testframes are provided in the laboratory. Also a lightweight, portabletesting frame system, called the “demonstration frame,” that can bemoved to any part of the laboratory is provided. A video camerarecorder/player, VCR and a TV monitor are also provided whichcan be used to record and display the experiments as they are con-ducted.

The main equipment provided in the laboratory includes thefollowing items:

• Mechanical Loading Devices— two “S” shaped 1,000 lb. ca-pacity load cells are provided at each work station, each witha 6 in. travel machine screw actuator with fabricated base.

• Hydraulic Loading Devices - two 5,000 lb. capacity single-acting solid plunger cylinders at each work station; two50,000 lb. capacity single-acting solid plunger cylinders forcolumn test frames; and three 10,000 psi rating hand pumps,each with hydraulic hose, coupling and tee-cone fittings, and10,000 psi capacity pressure transducer connected to eachpump are provided, one of each to be shared by two workstations.

• Displacement Measurement Devices—one mechanical dialgage with 0.001 in. sensitivity and 1 in. range, two ± 0.1 in.travel Linear Variable Displacement Transducers (LVDTs)and one ± 1 in. travel linear potentiometer are provided ateach work station; and 10 wire potentiometers are providedfor the column test frames.

• Manual Strain Measurement Devices—three strain indica-tor boxes and three switch and balance units with 10 chan-nels, one of each to be shared by two work stations, onestrain gage installation tester for the laboratory, and a straingage installation slide presentation portfolio for student ref-erence are provided.

• Strain Measurement by Data Acquisition System—four

July 1998 Journal of Engineering Education 215

Teaching Analysis of Structures Using aSmall-Scale Structural Behavior Laboratory

* Detailed information on the experimental equipment and procedures can beobtained from the author at the address given in the Author Biographies at the endof this issue.

bridge completion units at each workstation plus 12 spareswere fabricated to connect a strain gage to the data acquisi-tion system.

• Data Acquisition System - three PCs with 12 in. TTLMonitor, GPIB interface, printer, HP # 3497A data acquisi-tion control unit with a 20 channel multiplexor, 5 volt powersupply, and graphics, personal editor and spreadsheet soft-ware are provided in the laboratory. Each data acquisitionsystem is shared by two work stations, two of which arestored in permanent cabinets located on top of the equip-ment cabinet provided between two work stations and thethird one is stored in a mobile cart. At each station, thewiring to connect all instrumentation to the data acquisitionsystem is done by a 20 channel interface panel, which is con-nected to the multiplexor.

Software has been developed to collect and process the experi-mental data using the data acquisition system. This program iswritten in Fortran, with key subroutines written in assembly lan-guage. Immediately upon startup, the program reads a small textfile from the current directory which contains key words definingthe type of data acquisition hardware used. The program then setsinternal variables so that appropriate subroutines are called to inter-act with the data acquisition hardware. This file contains the fol-lowing information:

• Constant Channel Information - to input parameters whichremain constant throughout a test

• Manual Input Channel Information - to input data whichare not electronically collected

• Data Channel Information - to collect and convert to engi-neering units all the electronically transmitted data

• Derived Channel Information - data from any previously de-fined channels can be used in the mathematical operationsperformed to obtain the derived channel information

• Monitor Channel List - a list of channel numbers which willbe displayed as constantly updated text in real time as the ex-periment progresses

• Maximum and Minimum Value Channel Lists - lists ofchannels which are constantly scanned and the maximum orminimum values recorded are input here

• Graph Definitions—channels utilized to display x-y graphi-cal plots of test results.

Upon processing the setup file, the program reads the initial val-ues from each data channel which are subtracted from subsequentreadings, thus defining the reference zero for each channel. Theprogram then starts in “monitor mode,” where it scans only thechannels needed for the monitor channel list and displays the valuesas text on the left portion of the screen and the first graph on theright portion of the screen. From this screen the user may storedata, view other graphs, make screen printouts, or quit the programusing function keys. Upon stopping the program, the user isprompted for the name of the output file. In this file the data arestored as comma-separated variables, a universal format that can beimported by most spreadsheet programs.

III. EXPERIENCES WITH THE USE OF THE LABORATORY

The SSBL is used to teach an elective course CE 5723:Experimental Analysis of Structures. This course is a one-hour lec-

ture and two 2-hour laboratory classes per week. With the aid ofone experienced laboratory assistant (a graduate student), the stu-dents perform the following seven experiments during the firstthree quarters of the semester covering linear elastic behavior andnon-linear elastic behavior:

A. Calibration of a Load Cell and its Use to Study Stress-StrainRelationship for a Cable

In the first part of this lab, lead blocks of known weight are hungfrom a steel cable attached to a one kip capacity load cell, withweights added in increments. The objective of this experiment is tolearn the installation and operation of a transducer used to measureforce/load, and to learn the use of the Strain Indicator Box and dataacquisition software. In the second part of this lab the calibratedload cell is used to study stress-strain relationship of a flexible mate-rial (a nylon cable) both in the elastic and plastic regions.

B. Behavior of Cable Structures Under Increasing LoadsIn this lab following configurations of cable structures are exam-

ined: (i) a constant tension cable; (ii) a simple free hanging cable;(iii) a cable pretensioned with dead load; and (iv) a cable preten-sioned against a second cable. Effect of level of pretension and ini-tial sag on the behavior of simple hanging cables is observed andrecorded, particularly large changes in geometry exhibited by suchstructures when the configuration of load is varied, and how preten-sioned cable systems can be used to develop structures with accept-able load-displacement characteristics.

C. Linear Elastic Bending Beam BehaviorIn this lab experiments are conducted to verify beam bending

theory, moment curvature relationship and beam deflections ob-tained from it. Following two experiments are conducted: (i) strainmeasurements at various depth levels of a cross-section and dis-placement measurements under increasing loads of a simply sup-ported steel beam subjected to symmetrical and unsymmetricalloading configurations, and (ii) to obtain the influence line forbending moment by applying a concentrated load at various equallyspaced locations one at a time, and using the strain gages mountedon the top and bottom surfaces of the beam at a cross-section in ahalf-bridge configuration to measure directly the moment pro-duced at the cross-section.

D. Study Principle of Superposition, Verify Maxwell’s Reciprocal Theo-rem, and Determine Experimentally Flexibility Influence Coefficients

In this lab these are studied for the following three types ofstructures, each of which are tested first under two equal point loadsapplied simultaneously and then by applying a concentrated loadone at a time at the previously loaded points: (i) simply supportedbeam; (ii) a cantilevered planar frame; and (iii) a cantilevered three-dimensional frame subjected to both bending and torsion.

E. Analysis of Planar TrussesIn this lab the effect of joint rigidity on secondary member

stresses (bending) is studied for a truss system. A simply supportedtruss, fabricated using members made of square hollow tubing andwelded joints, is tested under a mid-span joint load. One of the topand bottom chord members of this truss have strain gages mountedon the top and bottom surfaces of a cross-section a little away fromthe joint. For each load level the member strains and the displace-

216 Journal of Engineering Education July 1998

ment of the loaded joint are measured. The measured strains areused to calculate the axial force and bending moment generated inthe member, and the observed joint displacement is compared withthat predicted by theory.

F. Role of Shearing Deformations on Beam DeflectionsIn this lab a simply supported wide flange aluminum beam is

tested under a mid-span concentrated load and with varying spanlengths. The observed displacement is compared with beam dis-placement computed from theory considering both bending andshear deformations and only bending deformations.

G. Column Buckling and Elastic InstabilityIn this lab a rectangular steel column is tested to verify the

Euler’s formula for first and second buckling mode, study effects ofend restraints (pin-pin and pin-fixed) on column buckling behav-ior, and to develop an interaction formula for eccentrically loadedcolumns.

The list of aforementioned experiments is not intended to be thecomplete list for the future, but rather indicates the type of labora-tory work that at this time was thought to be well-suited for under-graduates. Detailed instructions for each lab assignment are pre-pared and made available to the students in the form of a LaboratoryManual. The models for the assigned experiments are pre-fabricat-ed and made available to the students, which enabled so manyexperiments to be completed during the semester.

In addition to the aforementioned seven experiments, two pro-jects are also conducted to evaluate the ultimate capacity and failuremodes, respectively, of model reinforced beams and columns. Inboth experiments, model concrete mix and model ribbed reinforc-ing steel bars are used. The test specimens are made using Type IIIPortland cement, and cured at elevated temperature (100°C) so asto gain full strength in 14 days. The beam and column models cho-sen are, respectively, 1:6 and 1:4 scale models of prototypes forwhich test results are made available to the students. The model re-sults are compared with those obtained from theory and designpractices, and also with the prototype results using laws of simili-tude and model analysis that they learn in the lecture classes.

IV. CONCLUDING REMARKS

The Small-Scale Structural Behavior Laboratory described inthis paper can be used in many different ways in the undergraduateand graduate curricula, including: 1) as an integral part of regularcourses in structural engineering, with all students performing se-lected experiments in one or more of their courses; 2) in a one-thirdlecture and two-thirds laboratory elective course, as done for CE5723; 3) for independent projects undertaken by students as theyencounter appropriate problems in design courses; 4) as a source ofdemonstration of structural behavior in lectures or recitations; and5) in sophomore courses designed to introduce students to conceptsof structural engineering and to provide them with an appreciationof what upperclass years will contain. The experiments that can beperformed in this laboratory are essentially unlimited and can bechanged to meet changing curricular needs at no additional cost inthe basic equipment provided in the laboratory.

ACKNOWLEDGMENTS

The writer would like to acknowledge financial support totaling$42,007 provided by the National Science Foundation (Award No.ENG-8852798) and equivalent matching funds provided by theUniversity of Oklahoma to establish this laboratory.

July 1998 Journal of Engineering Education 217