Intro Engineering and Architecture Tower Project

Transcription

1 1 Intro Engineering and Architecture Tower Project 100 Pts Note: Read the entire packet before attempting to start this project! Objective: Design and build a tower from balsa wood that can hold more than 25 pounds. Design Factors: Engineers design structures with enough strength to withstand the forces or loads that will be placed on them. All structures must contend with two types of forces: live loads (dynamic) and dead loads (static). Dead loads are permanent loads that do not change or move. The weight of the structure itself is a dead load. Live loads are those that move and change. Wind, snow, ice, cars, people, and thermal expansion and contraction are examples of live loads. Dynamic loads tend to produce greater forces than static loads and are more difficult to predict and calculate. Factor of Safety: Structures are designed with a factor of safety. If the structure is designed to withstand twice the predicted load, it will have a safety factor of 2. Engineers must balance safety against the cost of making the structure too strong. Thrust Lines: Thrust lines are imaginary lines of force caused by loads. They are transmitted through all parts of the structure to the ground. Diagonal or arch support members help disperse forces.

2 2 Elasticity All materials change shape under a load. Elasticity is the ability to of a material to return to its original shape and size when the load is removed. Elasticity enables materials to push or pull against the forces caused by loads. Types of Stresses Forces always push or pull to produce five types of stress: compression, tension, shear, torsion, and bending. Compression Compression is the tendency to push or squash a material. A material under compression is always shorter. Tension Tension is the tendency of a material to be pulled apart. Tension makes a material longer. Shear Shear occurs when a material is divided by two parallel but opposing compression forces. One part of the material slides past the other part because of the shearing force. Scissors cut paper using a shear force.

3 Torsion Torsion twists a material. A wrench tightens or loosens a bolt using torsion force. 3 Bending Bending can be demonstrated by placing a load on the center of a horizontal beam resting on two supports. The beam will bend downward placing the top of the beam in compression and the bottom of the beam in tension. Racking Racking is a kind of stress which distorts a square or rectangle, causing it to become a parallelogram. Strong Shapes Triangles or arches are used in the design of most structures because these shapes provide the greatest strength.

4 4 Arches have an advantage over triangles in that they support loads at any point along their curves. Arches have the disadvantage of pushing out at the base. Strong abutments are required to prevent the arch from spreading. Tower Engineering A tower is like a vertical beam that is partially stuck in the ground. The windward side of the tower is in tension. The leeward side is in compression. A tower is always under compression due to its own weight. A 1,000 foot tower is about 1 shorter than the total length of its parts because of compression. Loads place a structure under compression, tension, shear, torsion, and bending or a combination of these stresses. Uneven loads on the sides of the tower can subject the structure to torsion or twisting, causing a failure. Understanding how a tower might fail will help you design a strong tower. Substructures and Superstructures The part of the structure that is below the ground is called the substructure. The part of the structure above the ground is called the superstructure.

5 Sample Tower Designs 5

6 6 Tower Project Specifications 1. You and a partner will be given (6) 24 strips of 1/8 balsa wood for the project. This equals 144 total linear inches of balsa wood 2. Your tower may only be constructed of balsa and glue. You may not use any other materials such as tape, nails, or staples. 3. Have a plan. Measure twice, cut once as you will not be given more materials if you make a mistake. 4. Your tower must be constructed with four sides. Plan to use no more than 36 linear inches of balsa wood per side. 5. The tower must be 6 high and level on the top. 6. Each side of the tower must be 2-1/2 wide at the base. 7. Each side of the tower must between 2 and 2-1/2 wide at the top. 8. The tower must be completely open thru the center from the top all the way to the bottom of the structure. The tower in the picture below is open all the way thru the center from top to bottom. Tower Project Grading Points Total 50 Points Building the bridge to specifications. 50 Points Holds 25 pounds or more.

7 Tower Project Design and Building Procedure 1. Brainstorm ideas for the design of the tower. See page Get a sheet of graph paper and make two full size sketches of the side of the tower. Each grid space equals 1/8" on the graph paper. Make sure that it meets the design specifications on page 6. Show your sketch to Mr. Hatley Place a piece of cardboard under your graph paper. Get a piece of wax paper and place it on top of your graph paper. You will use this as a template to build the four sides of your tower. 4. Cut your balsa pieces to length and glue them together. A little dab will do you. Using excess glue will only add weight to the joints and make them weaker. Use pins to hold your pieces together while they dry. 5. When you have finished making all 4 sides, glue them together. You may be able to use small clamps. Use a square for accuracy. 6. Weigh your tower in grams and record it in the space provided. 7. See Mr. Hatley. Test the tower and record the critical load in grams. The critical load is the amount of force it takes to break your bridge. If the testing device reads in pounds, convert pounds to grams (multiply pounds by 453.6). 8. Calculate the engineering efficiency by dividing the critical load by the structural weight of the bridge.