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Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Construct a Truss Tower
Graphic retrieved from, http://www.world-city-photos.org/Paris/Eiffel_Tower/Pictures_of_Eiffel_Tower_black_and_white.jpg, on 5/11/2010
Graphic retrieved from, http://www.atlaso.com/images/bridge.jpg, on 4/09/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Construct a Truss Tower Performance Objective: Given VEX parts construct a truss tower
a minimum of 12” tall, and that will hold a weight ( > 5 lbs) without collapsing.
Enabling Objectives: define the following terms: force, compression, tension, and
shear identify the forces acting on an object explain how the geometric shape and orientation of an object
can affect the strength of that object or assembly explain the benefits of using certain geometric shapes over other
shapes in the construction of a structure design a truss bridge
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Forces acting on a structure
Force is a pressure on a body or structure If the force is great enough it will produce a
change in movement or in shape of the body or structure
Conceptually, a force can be thought of as a push or a pull acting on a structure or system
In constructing a truss tower, the forces acting on members are compression, tension, and shear
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Compression
Compression is a force or pressure that attempts to flatten or squeeze an object
Graphic retrieved from, http://library.thinkquest.org/06aug/02102/Forces%20acting%20on%20Bridges.html, on 3/27/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Tension
Tension is a force that attempts to pull apart or stretch an object
Graphic retrieved from, http://library.thinkquest.org/06aug/02102/Forces%20acting%20on%20Bridges.html, on 3/27/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Shear
Shear force attempts to cause the internal material of an object to slide against itself
Graphic retrieved from, http://library.thinkquest.org/06aug/02102/Forces%20acting%20on%20Bridges.html, on 3/27/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Interaction of forces
When weight is applied to the beam below it experiences compression, tension, and shear forces
compression
tensionshear
Graphic retrieved from, http://www.masstec.org/teched/mcasquestions.html, on 3/27/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Forces Applied to Structures
Various geometric shapes can be used in the construction of a structure Arcs, Rectangles, Triangles, etc.
Different shapes react to forces in different ways
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Arches Arches have a keystone in the
middle, that allow the sides of the arch to lean inward, applying a force to the keystone
Graphic retrieved from, http://upload.wikimedia.org/wikipedia/en/7/79/Gateway_Arch.jpg, on 4/09/2010
The Gateway Arch, St. Louis, MO
Graphic retrieved from, http://www.dkandsons.com/images/arch%20with%20keystone.JPG, on 4/09/2010
keystone
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Rectangles
Modular apartment buildings are constructed using rectangles
Graphic retrieved from, http://jouleconsultingengineers.co.uk/bigimages/640x480shed.jpg, on 4/09/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Triangles
Truss bridges use triangulation to support the weight or force Truss bridges
Graphic retrieved from, http://www.prhs.k12.ny.us/fac/HerskowitzM/images/cascade_teft_truss_bridge_476_1971_ampprn.jpg, on 4/09/2010
Graphic retrieved from, http://www.atlaso.com/images/bridge.jpg, on 4/09/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Combinations
What shapes are used in the Eiffel Tower?
Graphic retrieved from, http://www.thetravelpeach.com/europe-vacations/france/eiffel-tower.jpg, on 4/09/2010
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Strength and orientation of geometric shapes
Which shape and orientation gives the most structural strength when a force is applied?
bar angle channel
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Shape and Orientation
A beam placed vertically is stronger than if placed horizontally
horizontally the bar has very little strength when force is applied
vertically the bar has good strength when force is applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Bends add support Notice how this bar
flexes or bends when force is applied
An angled shape has strength in two directions due to the bend in the material, minimizing flex as a force is applied.
bendin bar
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Additional bends
Additional bends in the material create even more strength.
the strength is increased in this channel piece
bendin bar
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Strength of Geometric Shapes in structures
Which shape gives the most structural strength when a force is applied?
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Squares
What happens when you apply a force to the side of a square?
No Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Squares
The square collapses, because it has no strength from side to side.
Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Squares
What happens when your structure utilizes only square and rectangular shapes?
No Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Squares
The structure is not able to hold its shape, and it collapses.
Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Triangles
What happens when you apply a force to the side of a triangle?
No Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Triangles
When force is applied to the side, member A is in tension, while member B is in compression, this allows thetriangle to hold itsshape.
Force Applied
AB
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Triangles
What happens when you use triangulation in your square structure?
No Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Triangles When a force is applied, the structure will be
able to hold its shape and not collapse.
Force Applied
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Triangle vs. Square
Notice how the triangle did not lose its shape as the square did when force was applied to the side.
Using triangles in construction offers more structural support for an object than simple squares.
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
How does this apply to a robot?
Here is a robot that won a regional competition
What is good about the structural design? Why?
What could be improved? Why?
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Robot design
Identify various shapes and structures used in this design.
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
Robot design
Here are four possible triangles
Here are various structures
Bar with bendChannel
Available at: www.etcurr.com Lesson 2.1, Construct a Truss Tower
The end
Now don’t get bent out of shape over building your truss tower!