Contents of Curriculum Unit 01.05.04:

Upon completion of this unit, the students will be able to: (1) identify the engineering principles behind bridge building; (2) identify different types of bridges; (3) become aware of a process outlining how bridges are designed and built; (4) understand some of the physics important for designing, building and using bridges (5) understand some of the preliminary events that takes place before construction, and finally (6) draw and build a bridge according to specification using the principles of engineering.

The curriculum unit will be divided into three major sections. The first section will introduce students to the three major types of bridges- the beam bridge, the arch bridge and the suspension bridge. Even though most bridges combine attributes from more than one bridge design, all have as their most basic element one of these three bridge types.

The second section of the unit will discuss the engineering process in bridge design. The engineering process can be broken down into eight simple steps: (1) identifying the problem; (2) determining the constraints; (3) designing the bridge; (4) analyzing the design; (5) refining the design; (6) implementing the plan; (7) modifying the plan of implementation, and; (8) building the bridge based on modifications. The data that engineers need in order to design the bridge will also be covered in this section.

The third section of the unit will introduce teachers to one type of bridge building contests. The wooden coffee stirrer bridge contest presented in this unit can easily be adapted to using other materials such as popsicle sticks, straws, balsa wood or any other desired materials. I will cover the design constraints and specifications for each contest presented in this section. The materials that each group of students will use to complete the bridge, along with some helpful hints to complete the projects will be included in this section. Finally, an evaluation tool, or judging criteria will be presented to the instructor in order to determine if each group completed the assigned task and to assess their achievements in learning.

Introduction to the Different Types of Bridges

Beam Bridges

Another way engineers make the beam bridge longer or have more span and accommodate more weight is to allow the deck to set upon a truss system. A truss system is composed of triangles and can support heavy loads with its relatively small weight. The beam must be strong enough so that it doesn't bend under its own weight and the added weight of the traffic crossing it. When the load pushes down on the beam the top edge is pushed together or compressed, while the bottom of the beam is stretched or is under tension. The force of compression on the upper side of the beam causes it to shorten because of the load pushing the beam inward. The result of the compression on the on the topside of the beam causes tension in the lower part of the beam. Tension causes the lower part of the beam to lengthen. The middle of the beam bridge experiences very little tension and compression.

There are several different types of beam bridges. One type of beam bridge is the truss bridge. A truss bridge is a series of connected triangles that distribute the weight or load to each member of the truss. It consists of a top chord, bottom chord and web members. The truss bridge is lightweight, but very strong due to the open triangular members along its sides. Two special features about the truss bridge are that the members that make up the triangles or diagonals do not bend. Secondly, a truss bridge is more efficient because the individual members carry axial load and minimize any bending. Therefore it requires less material than a simple beam bridge. The members get pulled apart during tension and pushed together during compression. While the simple beam bridge consists of a solid web member to carry the load, the truss has a top chord that is in compression and the bottom chord typically in tension. The other members of the truss are in different stages of tension and compression. As heavy loads travel across the bridge it may deflect vertically in the middle due to the individual members of the truss reacting to the forces of compression and tension.

There are many different types of truss. The design, location and composition of the truss determine the type. For example, the Warren Truss, Pratt Truss and Howe Truss differ based on the arrangement of the triangles. Their identifying names are attributed to the engineers who invented each of these particular arrangements.

The cantilever bridge is another type of beam bridge. This kind of bridge is supported on two levers that are continuous over piers. A simple cantilever bridge consists of two cantilever sections with abutments on each side to act as counterweights. The downward force at the center of the cantilevered end of the bridge is counteracted by the weight of the continuous adjacent span anchored at the far end. The opposite ends called arms reaches out and meet in the center. There is also a pier that supports each cantilever arm. The cantilever bridge can be made even longer by adding an additional simply supported section to the middle of a cantilever bridge.

Arch Bridges

Suspension Bridges

In the suspension bridge each cable supporting a segment of the roadbed is vertically suspended from the primary drapped cable spanning between main pylon towers. The forces from permanent and moving loads push down onto the roadbed placing it in compression. The cables through tension, then transfer the forces to the towers, which carries the forces, through compression, directly into the earth where they are firmly imbedded. The tension cables running between the two anchorages support the forces. The cables stretch from the weight of the bridge and the traffic that travels from anchor to anchor. In addition to the cables, the anchorages are also under the forces of tension. Because they are firmly imbedded into the earth like the towers, the amount of tension exerted on them is resisted by the counter forces of the dead load. Most suspension bridges also have a supporting truss system underneath the bridge deck to help stiffen the roadbed and to provide a lateral stabilization of the roadbed. This extra support system resists wind and lateral forces and reduces the tendency of the roadbed to ripple and sway.

Suspension bridges come in two different types of designs; the elongated "M" shape and the "A" shaped design called a cable-stayed bridge. The two designs support the load of the roadbed in very different ways. The differences lie in the way the cables are connected to the towers. The cable-stayed bridge attaches all cables that support the roadbed to the tower and they alone carry the weight of the roadbed and the traffic. The series of cables are attached to the roadbed in two basic ways, using a running parallel pattern or a radial pattern. In the parallel pattern, the cables are parallel to one another and attached at different heights along the tower. Each cable carries a segment of the roadbed. In the radial pattern, each cable carries its section of the roadbed and they are attached to the tower at a single point. In the cable stay bridge, all segments of the roadbed must carry a horizontal compressive force to counter balance the equal force from the other side.

Other Forces That Act on Bridges

If not considered when designing a bridge resonance can also have a detrimental effect on a bridge. Vibrations can travel through a bridge in the form of waves. In order to prevent a resonant magnification of the vibration, the potentially destructive resonance is controlled. Methods to dampen such vibrations must be built into the bridge design in order to avoid any resonance frequencies. Interrupting the waves prevents the harmonics from growing in length and becoming destructive. The dampening technique used to prevent resonance waves from growing usually involves variable inertias. For example, if a bridge roadbed is made up in different sections of overlapping plates the movement of one section is transferred to the next section via the slightly varied weights and sizes. This simple technique will create enough friction to counteract the frequency of the resonant wave. This change in frequency will prevent the wave from building and create two different waves, thus eliminating the chance of the resonant wave strength to become destructive. An excellent example of a resonance wave destroying a bridge occurred in 1940 when the Tacoma Narrows Bridge was destroyed by a forty miles per hour wind.

Factors in Bridge Construction and Design

(1) A plan of the site is needed in order for the engineer to see all of the

obstacles that has to be bridged, such as rivers, streets, contour lines of valleys and the desired alignment of the new traffic route.

(2) The requirements of the bridge itself such as the width of the bridge, including the width of the lanes, safety rails, medians and walkways.

(3) Weather and environmental conditions such as length of flood periods, high and low tide levels, length of flood or drought periods.

(4) The topography of the environment.

(5) The soil and substrata conditions of the planned site based on the results from the data collected from borings and the soil mechanics data.

(6) Local conditions like the accessibility for the transport of equipment, materials and structural elements that must be used for construction.

(7) The environmental requirements regarding the aesthetic quality of the bridge which is about to be built.

(8) Health and safety requirements such as noise, vibrations and it compatibility with adjacent sites surrounding the bridge.

The Engineering Process

(1) Identify the Problem

Before the engineer can begin work, the problem or task that is going to be undertaken must be known.

(2) Determine the Constraints

Constraints are the limitations that must be considered before you begin designing your bridge. Even though they are not limited to the materials, size, and money, you have to consider them the problem or task.

Preliminary Design

____(A) Brainstorming- Once both the problem or task has been determined and the constraints have been identified the group needs to think of as many ways to possible to solve the problem. The ideas should be broad enough to allow for unique solutions to arise. The class, meeting in smaller groups, may find that rough sketches are extremely helpful to stimulate a lot of different ideas. Even though all of the ideas may not be good ones, but they may inspire another idea that may lead to a solution to the problem.

____(B) Focus- Once all reasonable ideas are listed and the sketches are drawn, the group should choose the best two or three ideas for further development. The rough sketches should be converted to scaled or measured drawings.

(3) Analysis of Design

During this step, the designs are studied based on their merit in relationship to strength, cost, market appeal, and manufacturability. Models, drawings and calculations can be presented at this stage. A decision should be made at this point on which design to use or rather to begin a new design.

(4) Design Refinement

This step begins after a design has been analyzed. Any problems or unresolved considerations with the design should have been made apparent by now. Each design team should attempt to rectify the problems by making improvements in the design. After the corrections have been made, then each team should go back and analyze the design once again.

(5) Implementation Plan

Once the final design has been approved, it must be translated from an idea on paper to the real thing. Before the plan can be implemented, plans need to be made outlining the construction process. The methods of construction together with the strategy for scheduling involved a list of the tools; machinery and materials needed to complete the project. A listing of the parts and the dimension of the project are drawn up. The order is which the bridge is to be built is also written down and the specifications are compiled.

(6) Modify for Implementation

Any obstacles that may arise during the building of the bridge must be analyzed in order to find out if it is a big enough problem to cause a change in the original plan.

(7) Implementation

Lesson Plans for Teaching This Unit

Introduction to Bridges (Day 1- 2)

Virtual Bridge Fieldtrip (Day 3)

Town Meeting (Day 4-5)

Bridge Fieldtrip (Day 6)

Simple Bridge Experiments and Activities (Day 7-9)

Introduction to the Bridge Building Contest (Day 10-11)

Meeting of the Companies (Day 12)

Blueprint of the Bridge (Day 13-14)

Bridge Building (Day 15-16)

Final Inspection (final class period)

Evaluate the model based on the rubric presented during day seven.

Hands-On Bridge Building Activities

Activity I: Strength Test of a Beam and Arched Bridge

Objectives

The students will build a beam and arch bridge. Then they will see which of the two bridges will hold the most weight.

Materials Needed

2-4 bricks per group, corrugated card board, weights

Procedures

(1) Place two bricks 20 cm apart.

(2) In order to make a higher structure, place the second set of bricks on top of the first layer of bricks.

(3) Place a piece of corrugated cardboard between the two bricks.

(4) Gradually place weights in the center of the cardboard until it begins to sag in the middle.

(5) Record the amount of weight the bridge held before it began to sag.

(6) Rebuild your bridge as stated in steps one and two above.

(7) Place an arch between the two bricks by carefully bending a piece of corrugated cardboard in the form of the arch. Make sure that it fits snugly between the bricks that are 20 cm apart.

(8) Place a piece of cardboard on top of the arch between the two bricks.

(9) Gradually place weights in the center of the cardboard until it begins to sag in the middle.

(10) Record the amount of weight the bridge held.

Observations

(1) What type of bridge did you build at the beginning of the activity?

(2) What was the second type of bridge you built called?

(3) What effect did adding the arch to the bridge have, if any?

(4) Which bridge would you rather cross, why?

Activity II: Building a Strong Support

Objective

The students will build a tower with wooden coffee stirrers. Each tower will be tested to see which one will hold the most amount of weight. Be sure to point out the characteristics that made a tower weaker or stronger.

Materials Needed

Wooden coffee stirrers, glue, weights

Procedures

(1) Break the class down into groups of three or four.

(2) Have each group draw a blueprint of the tower that they plan to build.

(3) Once the instructor approves the tower, have the students build the tower.

(4) Place the towers on display in front of the class.

(5) Place weights on the tower until it begins to buckle.

(6) Record the weight that the tower held before it began to buckle.

(7) Have each group discuss why their bridge buckled, and any improvements that they could make in order to make their tower stronger.

Lesson Extension

Have each group redesign their tower with the recommendations they made to improve their tower. Test each tower to see if it is stronger than the first one they made.

Culminating Activity: Bridge Building Contest

When the contest is first announced the problem or task the students are about to undergo must be presented to the student. This could be done in a variety of ways. You can present the information to the class by showing the class a film, placing the information on the board, or design a rubric such as the one at the end of this section. The materials used can be change, but the criteria presented not only serve as explicit directions for the student, but it also a performance based evaluation that really assess the students work.

A list of the parameters the students must use before designing their bridges should be done as you determine the type of bridge the students will make. If you wish to consider the economic behind bridge building, like buying materials, you can copy this activity by letting the students buy or keep a record of the amount of materials they use in completing their bridge. For example, if your students are building a straw bridge, you can place a price on the straws and have your students keep record of how many straws were used. They can also be charged for all materials used while constructing their bridge. Have the students figure out how much their bridge is costing on the daily basis. The dimensions of the bridge should also be stated. For example, you could have the students build a bridge of straw with a span of 30 cm and a width of 5 cm. As an additional criteria you can limit the amount of straws each kid should use, or the amount of weight (load) the bridge should hold. A list of specific materials should be presented to the students based on the parameters of your contest.

Once the students have a full understanding of their limitations. Let the students meet in groups and decide the type of bridge they will build. Have them make a rough design of their bridge on construction paper. This will serve as their preliminary design. The next step in this process will call for the students analyze their design by drawing a more detailed drawing on graph paper. The students should design the bridge according to parameters presented during the introduction. Each student in the group should design a bridge, make sure they include a side, end and front view of the bridge. As a group, have the students decide on a design, or utilizing ideas from each of the individual bridges design a final blueprint of the bridge the group will construct.

Have the students write down the steps involved in building their bridges. Keeping a daily journal is a wonderful way of ensuring that all students are actively involved in designing and constructing the bridge. Whenever there is a structural change in the bridge design, the group has to update or draw a new blueprint of the bridge. All of this work should be kept in a portfolio under the group or company name.

Once the students begin constructing the bridge, they will make modifications which should be recorded in their journals. Make sure you check the students logs on the daily basis. Give daily observational grades in order to promote good work, and group working skills. You could also have students do weekly reports to the class of their progress. Through the use of the previously stated ideas, this would be a wonderful integrated unit of study for any middle school classroom.

Task:

The bridge must have a span at least 30 cm long, a roadbed at least 5 cm wide and a clearance of at least 10-cm in height. The distance between the bridge supports should be at least 10 cm apart, unless the bridge has only one tower or support structure. Draw the side, top and end view of the bridge along with the support structure according to the specifications. This will serve as the template, or blueprint that the firm will use to construct their model. Once the design is approve by the president of the firm, you will be given materials (according to the type of bridge the firm choose) to build the model.

The following materials are available for your group to use in order to build the model: coffee stirrers, wire, glue, string and hooks. Make sure the materials that are needed to complete your model are clearly shown on the firms' drawing. You will receive these materials once your design is approved. Your design and model will be evaluated using the rubric below.

A rubric is an evaluation tool that lets the student know exactly what is expected of them in an assignment. Before the students begin the project, the instructor should go over each of the criteria listed below. The possible points for each item listed should be told to the students. To make grading easier, the points should total one hundred. Once the students fully understand their task and how they will be graded everyone will have a full understanding of what is expected of them.

Rubric for the Bridge Design

Rubric for the Model

Notes

Additional Resources

Videos

Websites

Teachers' Bibliography

Students' Bibliography