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Truss Strength


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Grade level: middle school

Background:
Old Faithful Lodge was an architectural triumph of its time. One of its most stunning features is its great hall lobby, where the pitched ceiling stands 76 feet above the floor. This incredible structural feat is made possible by the use of trusses to support the weight of the roof and the additional weight of snow in the wintertime. Natural wood beams would not be strong enough to hold this weight, so the architect, Robert C. Reamer, used steel beams. To retain the lodge's aesthetic appeal, Reemer had the steel beams covered with hollow wood logs so that they add to the lodge's rustic charm.

In this lesson, students will gain an appreciation for the structural achievement of Old Faithful Lodge by building their own roofs with and without trusses to test the strength of each.

Objectives:
    Students will:
  • Build two roofs, one with a truss and one without.
  • Compare the strength of the two roofs.
  • Draw conclusions about the value of trusses for support strength in buildings.
  • Research trusses to learn how they help with structural support strength.

National Standards:


http://www.nap.edu/readingroom/books/nses/html/overview.html#content

National Science Education Standards Standard A:
    Science as Inquiry
  • All students should develop abilities necessary to do scientific inquiry
Standard B:
    Physical Science
  • All students should develop an understanding of motions and forces
Materials:
  • Great Lodges of the National Parks: Grand Lodges . Video clips of the following segments:

    Start with: Start of program
    End with: Old Faithful Inn has become a symbol of the Park itself, reflecting the wilderness that is Yellowstone.

    Start with: Reemer was careful in the way he oriented the building...
    End with: Any of us are free to sit in a chair, sit back, enjoy ice cream, listen to the sights and sounds of the building.

    Start with: When Robert Reemer designed the building...
    End with: ... I definitely do things differently now.

  • Great Lodges of the National Parks: Canyon Lodges (PBS video)
  • Plastic drinking straws, at least 7 " long. You will need 31 straws for each pair of students.
  • Tag board
  • Scissors
  • Straight pins
  • Sticks of modeling clay; approximately 10 pounds per group, clay may be shared among groups to save in cost.
Preparation:
  • Make a set of roofs for the class to use as models.
  • Print a set of directions for each pair of students (see procedure below).
Procedure:
  • Tell the class that they are about to view video segments on Old Faithful Lodge at Yellowstone National Park. Ask if anyone has ever been to Yellowstone or if anyone has stayed in the lodge. If they have, ask them to describe the Park and lodge to the class. If no one has been there, ask what they know about the Park and then fill in the gaps. Tell them that as they watch the video clips, they are to pay particular attention to the special construction features that made this lodge an architectural triumph. Have them listen for the answers to the following questions: How high is the ceiling of the lobby? What did the architect do to get enough support to hold up the massive roof? What structure was needed to hold up the roof? What materials did Robert Reemer, the architect, use for this system of support?'
  • View the video clips with the class. Observe the images of the great hall to gain a perspective of its massiveness. Elicit answers to the questions from above.
  • Tell the class that they are going to explore truss strength by making their own roofs out of straws, pins, and tag board.
  • Divide the class in pairs and explain that they will make one roof without trusses and one roof with trusses. Show your previously made roofs and tell them that they can look at these when they need to. Hand out the written directions for each roof. Caution the students to be careful with the pins because it is easy to stick yourself when pinning the straw pieces.

  • To Make a Roof Without Trusses:
  • Cut 2 straws 5 " long. Pin the straws together at their ends to form a V. Pin a third 7 " straw to the two ends of the V to form a triangle. This forms the frame of the roof. Repeat procedure two more times so that you have three triangles.
  • Place two parallel 7 " straws on the table so that they are 7 " apart. Pin the ends of the long leg of a triangle at each of the ends of the parallel straws so that the bases of the triangles form a square with the parallel straws.
  • Pin the third triangle halfway between the other two triangles so you have a frame for your roof.
  • Cut two 8" x 5" pieces of tag board. Pin these to the slanted sides of the frame to form the roof.

  • To Make a Roof With Trusses:
  • Cut 2 straws 5 " long. Pin the straws together at their ends to form a V. Pin a third 7 " straw to the two ends of the V to form a triangle. This forms the frame of the truss. Place the long base of the triangle on the table and stand the triangle vertically. This will be the final position of the truss when the roof is complete.
  • Cut a straw 3 " long. Pin one end to apex of the triangle using the same pin that you used to pin together the apex. Let this new piece hang vertically and pin it at the midpoint of the base of the triangle.
  • Cut 2 straws 2 " each. Mark the midpoint of the slanted sides of the truss. Pin one of the new straws through this midpoint and turn it so that its other end rests on the base next to the bottom of the 3 " vertical straws. Pin this end to the base of the triangle. Repeat this procedure to pin the other 2 " straw to the other side of the truss.
  • Repeat the first 3 procedures so that you now have 3 separate trusses.
  • Place two parallel 7 " straws on the table so that they are 7 " apart. Pin a truss at each of the ends of the parallel straws so that the bases of the trusses form a square with the parallel straws.
  • Pin the third truss halfway between the other two trusses.
  • Cut two 8" x 5" pieces of tag board. Pin these to the top of the trusses to form the roof.
  • Pass out sticks of clay to each pair. They will pound the sticks flat on the tables to form a thick pancake.
  • Lay the clay pancakes one at a time over the roof without trusses until one of the straws fails. Write how many pancakes it took to make this happen.
  • Do the same with the truss roof. This roof should be able to withstand more weight.
  • Subtract the weight of the clay on the roof without trusses from the weight of the clay on the truss roof. Calculate the percent difference in weight the truss roof can hold compared to the weight the non-truss roof can hold by dividing the difference between the two weights by the weight that the non-truss roof held.
  • Have students try to explain how the truss helps with load. Write their ideas on the board.
  • Direct them to the resources on trusses to find how trusses help with structural support strength and how load is distributed by trusses. Tell them that although these resources are about bridges, the same design principles apply to truss roofs. They will write a paragraph to explain these principles.
Assessment Suggestions:
    Assessment may be based on the:
  • Ability to follow written instructions.
  • Ability to work cooperatively with partner.
  • Ability to conclude which roof has more strength.
  • Accuracy of math calculations.
  • Understanding of the structural strength of trusses.
Extensions:

Books:

Darling, D., 1991, Experiment! Spiderwebs to Skyscrapers: The Science of Structures

Dunn, A., 1993, Structures Series (Bridges)

Gordon, J.E., Structures : Or, Why Things Don't Fall Down

Harris, D.W., Truss Fun

Hooker, S., Ragus, C., Salvadori, M.G., The Art of Construction : Projects and Principles for Beginning Engineers and Architects

Johmann, P.A., Reith, E.J., Bridges: Amazing Structures to Design, Build, and Test

Kaner, E., 1997, Bridges

Lafferty, P., 2000, Eyewitness Books: Force & Motion

Salvadori, M.J., Why Buildings Stand Up: The Strength of Architecture