INTRODUCTION TO CIVIL ENGINEERING AT UNIVERSITY OF FLORIDA USING TRUSS BRIDGE CONSTRUCTION FOR FRESHMEN

Transcription

1 INTRODUCTION TO CIVIL ENGINEERING AT UNIVERSITY OF FLORIDA USING TRUSS BRIDGE CONSTRUCTION FOR FRESHMEN Fazil T. Najafi 1 and Sarah R. Jayasekaran 2 Abstract Civil engineering is a professional engineering discipline that deals with the design, construction and maintenance of the physical and naturally built environment, including works such as bridges, roads, canals, dams and buildings. Civil engineering is the oldest engineering discipline after military engineering. Civil engineering is the application of physical and scientific principles, and its history is intricately linked to advances in understanding of physics and mathematics throughout history. At the University of Florida (UF), an engineering overview course, Introduction to Engineering, EGN 1002, is offered to freshman engineering students. A session in this course provides an overview of Civil and Coastal Engineering. It is in this session that a truss bridge laboratory exercise is conducted. The purpose of the laboratory is to familiarize students with the concept, theory and practical side of various areas within Civil and Coastal Engineering and is focused on aiding students in the process of making an informed decision for their future careers. This paper is focused on the procedure used for conducting the laboratory as well as how the collected resulting data can be utilized in real-life projects. Index Terms Bridge, compression, freshmen, tension. INTRODUCTION Engineering has been an aspect of life since the beginning of human existence. Civil engineering is the application of physical and scientific principles, and its history is intricately linked to advances in understanding of physics and mathematics throughout history. Because civil engineering is a wide-ranging profession, including several separate specialized sub-disciplines, its history is linked to knowledge of structures, materials science, geography, geology, soils, hydrology, environment, mechanics and other fields. Students taking this introductory engineering course are introduced to civil engineering concepts through participation in a laboratory in which they work on a truss bridge model. The truss bridge laboratory portion of the course lasts two hours within a three-hour class, and it allows students to be part of some elementary lab work as well as a hands-on project. The lab involves three teams of four to seven students from different engineering disciplines building a truss bridge from basic materials. The teams are given criteria such as cost, efficiency, and strength of the project and are asked to build their bridge within these parameters, following a truss bridge design of their choice. Each team is provided with the same quality of materials, and all materials are assigned a certain cost based on their properties. Teams are given a time limit of 30 minutes to complete the task of building the truss bridge of their choice. After completion of the projects, the class is relocated to the Structures lab, called The Pit, for testing of the projects. Each team s truss bridge is placed on a holding structure, as shown on Figure 1, and loaded with weight. The team with the combined score of highest weight to cost score wins. The lab introduces students to civil engineering concepts and creates a better understanding of compressive strength, stress, strain, neutral axis, tension, and compression. Students also gain an understanding of how engineering theory and concepts can be applied to real world projects and that engineering is applying science to solve problems [1]. WEE2011, September 27-30, 2011, Lisbon, Portugal. Editors: Jorge Bernardino and José Carlos Quadrado. 1 Fazil T. Najafi, Professor, University of Florida, Department of Civil and Coastal Engineering, P.O. Box , Gainesville, Florida, U.S.A., fnaja@ce.ufl.edu 2 Sarah R. Jayasekaran, Graduate Student, University of Florida, Department of Civil and Coastal Engineering, P.O. Box , Gainesville, Florida, U.S.A., srajkumari@ufl.edu 796

2 Holding structure 1 gap Plexiglass FIGURE 1 TYPICAL FRESHMAN STUDENT TRUSS BRIDGE WITH LARGE CRAFT STICKS IN TESTING FRAME. LABORATORY PROCEDURE The laboratory part of the lecture is a brief tutorial on basic engineering concepts as they apply to a truss bridge. Some concepts that are covered include, but are not limited to, tension, compression, neutral axis, stress, strain, buckling, and deflected shapes [2]. These concepts are related to students through demonstration with a foam board and drawings on a blackboard. The instructor conveys the purpose of the laboratory, which is, as a team, to design and build a truss bridge that will hold the most weight and cost the least. Finally, the instructor discusses the main failure modes of the bridge, so that students can apply the engineering concepts they have learned to design a more effective bridge. For example, the bridge is likely to fail at the bracing point, since these represent a weakness in the craft stick. Students are given 30 minutes to design and build the truss bridge out of craft sticks of two sizes and bolts. Each team will be given 29 short craft sticks and 27 long craft sticks with 20 bolts and nuts alongside craft tools, which include one screwdriver and one set of pliers. The time constraint forces students to form teams wisely and initiate leadership. This emphasizes the need for project management and timely delivery of materials [1, 3]. They are given a cost estimate sheet quantifying the cost of the materials as $0.75 for short craft sticks, $1.00 for long craft sticks, and $2.00 per nut-and-bolt combination. Once given the information, material, and time constraints, the students are free to do their work. Figures 1 and 2 represent large and small frames, respectively, created by students. FIGURE 2 TYPICAL FRESHMAN STUDENT TRUSS BRIDGE WITH SMALL CRAFT STICKS IN TESTING FRAME. Once the truss bridges are completed, the class will move to the Structures lab located on the bottom floor of Weil Hall. In the lab, the truss bridges are loaded into the holding structure, constructed of plexiglass and a wooden testing frame. Figures 1 and 2 are examples of truss bridges loaded in the frame. Once the truss bridge is placed in the frame, a bucket is attached to the truss bridge in which weight will be added. Figure 3 depicts a student placing a weight into the bucket. The truss bridge is then loaded with weight until failure. Figure 4 depicts a typical truss bridge that has failed and has been placed outside the frame for viewing. Figures 5a and 5b depict one team s truss bridge before and after being loaded with weight in the frame, respectively. Figure 6 shows three different examples of bridges built by different teams. Each bridge is photographed first in position on the holding structure and then held by one or more the of the team members. 797

3 a) b) FIGURE 3 TYPICAL FRESHMAN STUDENT TRUSS BRIDGE WITH SMALL CRAFT STICKS IN TESTING FRAME. LABORATORY RESULTS To calculate the scores for the truss bridge laboratory, the following equation is applied: Truss bridge failure load Final score 100 (1) Truss bridge cost The purpose of students calculating a final score for their truss bridge is to understand the relationship between cost and performance. They are shown that an engineer s job is to provide services that provide adequate strength for the least cost. For further analysis and study, data results of the truss bridge lab tests that were collected in classes during the period of September 2010 to December 2010 (see sample of the data in Table I). 1) FIGURE 5 MULTILAYER WARREN TRUSS BRIDGE: a) BEFORE LOADING; b) AFTER LOADING. 2) FIGURE 4 TYPICAL TRUSS BRIDGE FAILURE. 798

4 ANALYSIS 3) Collected data from the tests are analyzed by each student group and recorded according to findings. The first step in analyzing starts with finding the tension or compression in the broken section. The result is then compared with the standard point of overall data (see Table II). The comparison provides information on how much variation there is from the average and mean. A low standard deviation indicates the data points tend to be very close to the mean, whereas high standard deviation indicates that the data are spread out over a large range of values. As an example, the average failure load in the study was lb, with a standard deviation of lb. This means FIGURE 6 that most costs and loads (about 68 percent by normal EXAMPLES OF THREE BRIDGES BUILT BY STUDENTS distribution) have a failure load within lb of the mean AND BRIDGE TEAM MEMBERS HOLDING SAME. (21.77 lb lb) minus one standard (sigma) deviation, whereas almost all failure loads (about 95 percent) have a failure load within lb of the mean (8.01 lb lb) minus two standard deviations. The result shows that when compared, failure load results are widely dispersed and have TABLE I SAMPLE OF TRUSS BRIDGE LABORATORY EXERCISE DATA RESULTS Date Team Bolts Short Sticks Long Sticks Total Cost Failure Load (lb) Final Score (%) Structure Design A Warren 9/1/2010 B Warren C Warren D Howe A Howe 9/15/2010 B Warren C Warren D Pratt A Brown 9/22/2010 B Warren C Warren D Pratt A Pratt 10/7/2010 B Pratt C Howe D Warren 10/27/2010 A Warren B Warren A Warren 10/28/2010 B Pratt C Howe D Howe A Warren 11/17/2010 B Howe C Pratt A Pratt 11/18/2010 B Warren C Warren 12/1/2010 A Warren B Warren A Howe 12/8/2010 B Warren C Warren 799

5 TABLE II STANDARD POINT OF OVERALL DATA Overall Bolts Short Sticks Long Sticks Total Cost ($) Failure Load (lb) Final Score Average Mean Standard deviation Minimum result of each category Maximum result of each category significant high and low variances. It shows that failure load has a major effect on the overall final score. Table III shows the individual listing of highest and lowest cases of failure load, final score and total cost by date and team. In these tests, the highest failure load and final score was scored with the Pratt truss design (Figure 7), the lowest with the Warren truss bridge design (Figure 8). The result proves that failure load is a substantial factor in achieving a high/low final score. The overall winner of the bridge tests used the Pratt truss design for the structure. It has the highest final score among average final scores. The Pratt truss design has shown the highest average result in average failure load and final score among reliable data, which contains more than 10 cases (not all data are shown in Table I. Also, the average final score of the Pratt truss design is exceptionally high compared to the Warren and Howe truss bridge design by at least 12 points. The Pegram and Brown design has a higher average failure load than Pratt truss, but it could not be selected due to its low testing counting, which has low reliability in our study. This bridge-building and testing project provides the student a unique experience of engaging in an active engineering environment in various areas where the student may be a part of planning, management, teamwork, quality control, communications, and review of a project. In the end, the highest total score is awarded with the Design Team Winner Intro to Engineering mechanical pencil as a reward. TABLE III HIGHEST AND LOWEST CASES OF FAILURE LOAD, FINAL SCORE AND TOTAL COST Category Date & Team Bolts Short Sticks Long Sticks Total Cost Failure Load Final Score Highest Load/Score 10/07/2010 B Lowest Load/Score 11/17/2010 A Most Expensive 11/03/2010 B Least Expensive 10/06/2010 C a) 800

6 b) b) FIGURE 8 WARREN TRUSS BRIDGE: a) DESIGN WITH LOWEST FAILURE LOAD AND FINAL SCORE; b) BRIDGE AFTER FAILURE LOAD. research being conducted on temperature effects on bridge beams and the strength of anchor bolts constructed with certain adhesives. These are pointed out to further the point that civil engineering is a broad field and many different opportunities are available in civil engineering. CONCLUSIONS FIGURE 7 PRATT TRUSS BRIDGE: a) DESIGN WITH HIGHEST FAILURE LOAD AND FINAL SCORE; b) BRIDGE DURING LOADING. Following the laboratory session, the instructor informs the students about the purpose of the test and how the results are used to modify existing codes and provide future standards. This demonstrates another facet of civil engineering of which a student may have been unaware. Other tests being performed in the structures testing laboratory are noted and explained. Most notably, there is a) The Department of Civil and Coastal Engineering at the University of Florida provides a brief insight for freshman engineers into work involved in civil engineering and how it relates to daily life through a truss bridge laboratory exercise. Basic principles of civil engineering are discussed, put into practice in a student laboratory exercise, and demonstrated to the students through laboratory demonstrations. The brief introduction to tension, compression, neutral axis, stress, and strain allow students to begin to grasp the concepts that civil engineers apply in solving problems. The simple 30-minute laboratory truss bridge exercise provides data on the characteristics of each truss bridge design through its cost and performance. The distribution shows the range of each structure s capability. Of the various truss designs, the Pratt design claimed top choice due to its ability to hold the heaviest load and achieve the highest total score. These methods are effective at designing real-life civil engineering structures and could be used as pathfinders to an optimal truss design, which can be key to accomplishing a project within cost and load requirements. REFERENCES [1] Hoit. M.I., Ohland, M., and Kantowski, M., The Impact of a Discipline-Based Introduction to Engineering Course on Improving Retention, Journal of Engineering Education, Vol. 87, No. 1, January 1998, pp [2] Hoit, M.I., and Culpepper, K., Freshman Interdisciplinary Laboratory, Proceedings, ASEE Conference, Champaign-Urbana, June 1993, pp [3] Hoit, M., Lab Manual for Introduction to Engineering (EGN 1002), Department of Civil and Coastal Engineering, University of Florida, Jan