Building and Testing Structures

A Workshop from the Annual NC-Technology Students Association Leadership Conference

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Overview of the Workshop
Instructions and Downloadable Worksheets
How Coach Was Used for Testing (+Downloadable Project with Data)
Structure that held the most weight + DATA
Images from the Workshop
Video Clips from the Workshop
Hints, Tricks and Tips for Educators (Classroom Lesson Plans)
Links and Information
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Overview of the Workshop

Harris Educational was asked by the NC Department of Public Instruction and NC's chapter of TSA (The Technology Students Association) to conduct a workshop for top NC Technology students at this year's annual TSA Leadership Conference in Greensboro NC. The emphasis of the workshop was to be "how to improve a final product" and also on "Structures and Design" an area currently under redesign in the NC Technology Curriculum. Four one hour workshops were held for groups of 20 to 30 students in Middle and High School.

Traditionally students build structures from various construction materials such as balsa wood and then destructively test those structures by adding weight until the structure collapses or fails. In high school students may sometimes also determine the efficiency of the structures by dividing the weight of the structure by the amount of weight it can support. In our workshop we wanted to incorporate the idea of "Engineering Measurement" so that students could first test their structures using sensors to create quantitative data. They could then move on to destructive testing and see how the experimental data related to how the structure failed during destructive testing. Our workshop only lasted one hour and so obviously we did not have time to go very deeply into all aspects of such a project. At the bottom of this webpage are classroom lesson plan ideas on how to do this activity with your technology or physics class over a three day period.

Measurement and data collection was accomplished using CoachLab probeware systems, a force sensor, and two "flex" sensors that I built for $12.00 each.

I choose "Tech Card" construction materials provided in the U.S. by The Science Source {see below for contact information} due to their inexpensive nature, the fast pace at which they may be assembled, and the fact that students don't need special training or tools to work with these materials.


Tech Card Base	Tech Card C-Beam and L-Beam

We then established rules and conditions for the workshop, discussed the five forces that effect structures, tested a structure that I designed, and then designed, constructed, tested to gather data, and then tested by destructive means.

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Instructions and Downloadable Worksheets

The following guidelines were used as instructions for the activity in this workshop. At the bottom of this webpage we also include details on how to conduct this exercise under real classroom conditions over a three day period. Also included is a PDF format copy of the 2-page worksheet that was given out at the workshop.

Your Task:

1) Design an optimized structure that can hold as much weight as possible without failing or collapsing.

2) Build your structure using the materials provided.

3) Test your structure using a combination of data logging (gathering data from sensors) and destructive testing.

4) Determine the efficiency of your structure. {Did not perform in the workshop due to time constraints}

Materials Provided

Tech-Card Structural Design Elements Including

8 C-Beam “Box Girders” (2.5 cm x 2.5 cm x 25 cm)
16 L-Beam “Beams” (2.5 cm x 2.5 cm x 25 cm)
2 Base Plates “Plates” (2.5 cm x 7.5 cm x 12.5 cm)
Several (40+ Brass Rivets) {We found that students actually needed about 100 rivets to complete their designs}
1 Pair of scissors

Note: Tech Card Holes are 4 mm diameter at 2.5 cm intervals.

Rules and Requirements

You must use two base plates as a weight bearing “floor” for the top of the structure. These two plates must be joined together along their widest side (along the 12.5 cm side) using no more than three rivets for the joint.
The structure must be at least 47 cm tall. (i.e. two beams and/or girders joined by no more than one hole-overlap)
Joints between elements may only be made using the supplied rivets. Tape, Glue, String, Rubber Bands, or other methods are not allowed.
Beams may not be laminated… i.e. several components overlapping one another to form a composite structure. Exceptions: Two L-beams may be made into a box girder, or an L-beam and a C-Box Girder may be combined to make a full box.
No joint may overlap by more than one set of holes.
You may only work with the materials provided.

TIPS

You may cut girders or beams to a different length, or at any angle.
You may cut along the length of a beam to make it into two straps (one dimensional) or along the length of a girder to make it into three straps.
You may use ALL the materials given to you, or as few as possible as long as you use both base plates and the structure is 47 cm tall.

Free Downloadable Worksheet

Click Here to download a PDF copy of the worksheet that we used in this workshop

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How Coach Was Used for Testing

CoachLab is a system for measurement and control that is very useful in Technology and Science experiments. In this workshop we set up CoachLab to record data from a force sensor and two flex sensors. The force sensor is capable of measuring force in the metric unit "Newton" The force sensor is used just like a spring scale is used to measure force except that data can be collected and observed on a computer in real time. The force sensor is placed underneath the tower and connected to the top center of the tower via a string that passes through a hole in the center of a plywood base that supports the base of the tower.

Students identify two beams in their tower and make a prediction as to whether or not the beam will bend when weight is applied to the tower. (i.e. they identify what they consider to be the strongest and the weakest points on their tower). They then attach "flex" sensors to these beams using paper clips in order to record the amount of flex that the beam undergoes and correlate that flex with the amount of force being applied to the tower at that same moment in time.

The force sensor is a standard zero to 50 Newton force sensor that is available from CoachLab dealers or from Vernier Software. The Flex sensors are sensors that I built myself for about $12.00 each. If you would like to learn more about how I built the flex sensors please click here.

Free Downloadable Project Files for Coach (works only with Coach software)

To download a WinZip archive file that contains the Coach project files used in this experiment click on the statement below and then choose the "Save to" option vs. the "Open from Location" option. Place the file on your desktop or another convenient location.

Click here to download TSA.zip (214 KB)

Once you have saved this file to your computer use a program such as WinZip to de-compress the files into a directory called "TSA.en" once you have created this directory you can move it to the Coach project directory either using Windows Explorer or by using the Project Manager in Coach Author Software.

The project files include data sets that were taken in our workshop. You do not need Coach Hardware or the Sensors used in this workshop in order to view the data. Simply click on "Ignore" if you get an error message saying that the CoachLab II Interface Panel was not found. (See this article for more information on that error message.)

Note: This activity can also be performed using CoachLab I, ULAB, TI-CBL, TI-CBL2, or Vernier LabPro

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Structure that held the most weight + DATA

This exercise was not held as a competitive event. All students who followed directions and completed their tower in the one hour time limit are considered winners. Due to time constraints, and the educational value of seeing what would happen to different designs some of the rules were kept fairly flexible in this workshop. Here then is information, pictures, and data (plus interpretations) of some of the structures that performed the best under stress and weight testing. {Note, full datasets can be downloaded above for Coach software}

The Tower that Held the Most Weight

Name: The Lightning Rod

Group: Group A Table 2 (3 Students)

Construction Type: Tubular Beam and Unitized Box

Max Sustained Force: 40.45 Newtons

Failure Under force Test: Cardboard Tearing

Max Sustained Weight: 25 Pounds

Failure Under Weight: Joint Failure at Corner Rivet

Why this tower worked well: When the tower collapsed it was due to failure of joints and tearing of material, indicating that the design of the tower was sound. All structures are only as strong as the materials that make them up, and the joints that join their parts together. The tubular beams in this tower carry weight directly from the top to the base. Side panels made up of L-beams folded flat into plates added a 'sheathing' effect to keep the building from wracking or torqueing. Top and bottom sections of the tower are joined by L-beams that unitize the structure into two boxes.

Data from "The Lightning Rod" structure under test {both data sets were taken simultaneiously}

Flex Sensor Data (from Coach Software)

Flex sensors were mounted to an upright L-beam that made up one corner leg of the structure and to one lateral unitizing plate that spanned across the bottom of the structure. The L-beam was expected not to bend, the unitizing plate was expected to bend.

No significant bending occurred during this test. At this zoom level the spikes in the graph are actually noise from fluorescent lights in the room.

Force Sensor Data (from Coach Software)

A steady pull on the force sensor between 0 and 4 seconds indicates a strong tower that is not bending or collapsing. Between 4 and 5 seconds the knot in the string connecting the force sensor to the tower started to slip but then tightened back to itself. From 5 seconds to about 9 seconds you can see that the force is not as regular indicating that the tower is under greater stress. A maximum force of 40.45 Newtons occurs at 10 seconds followed by a tearing of the cardboard supporting the force sensor at 10 to 13 seconds, and finally the force sensor was let go between 13 and 14 seconds.

More Pictures of the Lightning Rod (Click on the Thumbnail to launch the full sized picture in a new window)

Video Clips of the Lightning Rod under test can be found below

More Data Sets from Other Towers

Click here to view a webpage containing data from eight more towers tested during our workshop.

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Images from the Workshop

The following pictures depict towers under design, construction, data collection, and destructive testing. (Click on the thumbnails to launch the full sized picture in a new window.) These pictures show the wide variety of designs that were tried and the creativity of the students involved.

	This tower held 20 pounds
	A very unique design using overlapping arches to support the top platform
	This tower held 22 pounds {However, it did technically violate the rules of construction since the two base plates are not at the top of the structure}
	Another very good design that held a lot of weight
	The arch topped tower under construction
	An off center design linking a large base to a smaller top
	An arch topped design about to be weight tested
	A large open box design about to be weight tested
	Students, Tower, and their Teacher
	An interesting design about to be weight tested
	An asymmetrical design about to be weight tested
	The "Eiffel Tower" starting weight testing
	The top half of a pyramid design
	The bottom half of a pyramid design
	Students showing great teamwork building their design
	A group finishes their tower
	Thinking and communicating about design ideas
	Students working together to build their design
	The class hard at work building their towers
	Fudging on height by adding a upright (on the 22 Pound Tower)

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Video Clips from the Workshop

The following are video clips taken during the workshop. Each is available as an MPEG movie, or as a smaller "WMF" (Windows Media Format) movie. Click on the links to download and view each video clip.

Name	Description	Time	Size	Type	Download
25lb Tower Test	Testing the winning tower with CoachLab	1 minute 8 seconds	864 KB	WMF	Click Here
			6,143 KB	MPEG	Click Here
20lb Tower Test	Destructive testing the 20 lb tower	21 seconds	268 KB	WMF	Click Here
			1,855 KB	MPEG	Click Here
Not a True L	Using Coach to test a tower and identify the weak spots	29 seconds	376 KB	WMF	Click Here
			2,624 KB	MPEG	Click Here
Rip before Buckle	Destructive Testing of a tower	34 seconds	438 KB	WMF	Click Here
			3,072 KB	MPEG	Click Here
Breaking a Tower	Breaking a Tower by Destructive Testing	5 seconds	156 KB	WMF	Click Here
			447 KB	MPEG	Click Here
Using Coach to Measure	How we used Coach to measure	26 seconds	332 KB	WMF	Click Here
			2,304 KB	MPEG	Click Here

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Hints, Tricks and Tips for Educators

The following information includes lessons that I learned as an educator while performing this workshop, and a simple lesson plan for conducting this workshop in your own classroom over a three day period.

Each group of students should be given a GO/NO-GO sheet describing the critical conditions for the towers they are building, particularly the height requirements and the base plate requirements.
The easiest and quickest way to attach the force sensor to the tower is by using a string from the top of the tower down to the force sensor. To do this cut a string that is at least 20 cm (about 8 inches) longer than twice the height of the tower. Thread that string through the center of the students tower and pull it out through two equally spaced holes on the roof plates. Tie the string together (to form one big loop) using a square knot. Place the tower over the hole in the testing platform and pull the loop of string down through the hole. Pull up on the top of the string until the force sensor hangs less than one inch (2.54cm) away from the bottom of the base of the tower and tie it into a double knot to take up the remaining slack.
Have students place the string into their tower themselves as part of the building process in order to save time when it comes to testing the tower. Leave the string in the tower when you destructively test the tower.
In order to be able to measure a higher (more accurate force) on the tower it is useful to place two large washers over the holes that the string goes through in order to help spread the force and keep the cardboard from ripping.
Students do best with about 100 fasteners. Our original worksheet called for 40 per group but 100 were generally used even by simple designs.
The flex sensors are most easily attached to beams in the towers by using ordinary paper clips and/or the smallest size "large clip" style report clips.
Ordinary hand-weights (i.e. dumbbells) can be used to test student designs. For one pound weights a quick solution is canned food. {this tip provided by a teacher in the workshop}
Students can become confused at the differences between "force" and "weight" as you test this tower, particularly since the force sensor gives readings in Newtons and the weights are done in either English pounds, or by metric kilograms. It is probably useful to review the concept of force and show how "Weight" is mass exerting a force due to gravity. 1 Newton is equal to 0.2248089 Pound Force
Students should name their towers after construction by writing on them with a felt tip pen. When results are saved using Coach software, or if pictures are taken the file names should correspond to the name of the tower.
Older and failed towers can be scavenged for their fasteners and for materials that are still structurally sound in order to save money. Broken pieces (and stressed pieces) can also be useful to keep in order to show future classes examples of how structures fail.
It is useful to build your own "flawed" tower. You can use this tower to introduce the five forces that effect structures and also show them how much better their designs turn out to be than your intentionally flawed design.

Lesson Plan Ideas for Tower Construction and Testing

Day 1: Discussion and Lecture [Structures, Forces, and Testing] 45 minutes to 1 hour

Introduce students to the basic concepts of structures using a conversation/lecture and use examples from the building you are located in, local landmarks such as bridges, and other familiar structures like the Eiffel tower.
Discuss the five basic forces that effect all structures. (Compression, Tension, Bending, Torsion, and Shear)
Discuss the difference between force and weight
Show the students a tower that you have constructed. Test the tower using Coach software and discuss the data. Ask students to describe how the design might be improved.
Close out the day by destructively testing the tower that you have constructed.
Assign groups, familiarize students with the requirements of their exercise, and get them started on the design process.

Day 2: Building their Towers [Cooperative Learning, Hands On Construction] 45 minutes to 1 hour

Give students their GO/NO-GO sheets and discuss the parameters effecting their tower designs
Hand out materials for students to work with including: (Tech-Card beams, fasteners, string, scissors, and any other materials needed)
Move from group to group as they build their structure and advise on NO-GO situations like lamination or height requirements. Answer any questions they have, and keep them on task.
Students should write their tower's name on the tower in a prominent location using a felt tip pen and then store their completed towers.
Clean up the room and return all unused materials.

Day 3: Testing the Towers [Data Collection and Destructive Testing] 45 minutes to 1 hour

Any students that have not completed their towers should finish the towers as you test the other towers.
Student groups should bring their tower to the front of the room for data collection. Quickly attach the flex sensors and force sensor to their tower and record the data. Save the data set under the name of their tower.
Move that group on to a destructive testing station. Have them destructively test their tower.
Discuss each tower as you test it and ask how they might improve their design.
As the towers are destructively tested ask the students how their data predicted the real failure of their tower.
Have students scavenge the destroyed towers for fasteners and for salvageable beams.
Clean up the room and return all unused materials.

Notes:

This exercise works fine with only one CoachLab interface and Computer but can also be used with multiple CoachLab systems with experiments running in parallel. In this case it may be best to print out the results of interesting towers and share them with the rest of the class as a homework assignment or handout the following day. Otherwise a projector or large computer monitor are useful for students to learn from each others designs.
This exercise can also be worked into a modular classroom (or round robin activity) by setting it up as one of many workstations. To adjust for a modular approach you may also wish to have students write lab reports, or build a second tower based on what they have learned from their first design and compare its tests with the original in order to see if they can improve upon their designs.

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