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Bridge Design Investigation

Bridge Design Investigation

Introduction: (Initial Observation)


Bridges are among the most important structures that engineers may design. Knowledge of forces and the way they are distributed allows engineers to design bridges with the least amount of materials and the highest possible strength. What factors to they consider? How the forces are distributed?

Without the knowledge of bridge design, one may try using the highest amount of strong materials in order to make s strong bridge. Many bridges collapse because they cannot resist the pressure of their own weight. In this engineering investigation project you will discover the rules by your own initial experiments and then use your knowledge to make a bridge that is light and strong. In your demonstration you will show that your bridge can carry loads that are many times more than its own weight.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

This is an engineering project. An engineering project does not require defining variables, question and hypothesis. In this project guide we have offered some questions, hypothesis and variables in order to conform with the requirements of experimental projects. For an engineering project you may ignore them and only focus on the calculations.

Information Gathering:

Find out about bridge structure. Read books, magazines or ask professionals who might know in order to learn about the effect of design elements in the strength of a bridge. Keep track of where you got your information from.

Start gathering information by learning about different types of bridges. You need to know some of the most important bridge types and you need to know why you are selecting truss bridge for your research. This is a sample of information you may find:

Important Bridge Types

Arch Bridge
Arch bridges are usually made of steel or stone.
Arches use a curved structure which provides a high resistance to bending forces. Both ends of an arch are fixed in the horizontal direction. Thus when a load is placed on the bridge, horizontal forces occur in the bearings of the arch. These horizontal forces are unique to the arch and as a result arches can only be used where the ground or foundation is solid and stable. The roadway may pass over or through an arch.

Cable Stayed
A typical cable stayed bridge is a continuous girder with one or more towers erected above piers in the middle of the span. From these towers, cables stretch down diagonally (usually to both sides) and support the girder.
Steel cables are extremely strong but very flexible. Cables are very economical as they allow a slender and lighter structure which is still able to span great distances.

A girder bridge is perhaps the most common and most basic bridge. A log across a creek is an example of a girder bridge in its simplest form. In modern steel girder bridges, the two most common girders are I-beam girders and box-girders.

The suspension bridge allows for the longest spans.
A typical suspension bridge is a continuous girder with one or more towers erected above piers in the middle of the span. The girder itself is usually a truss or box girder though in shorter spans, plate girders are not uncommon. At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables.

The truss is a simple skeletal structure. In design theory, the individual members of a simple truss are only subject to tension and compression forces and not bending forces.
Thus, for the most part, all beams in a truss bridge are straight. Trusses are comprised of many small beams that together can support a large amount of weight and span great distances. In most cases the design, fabrication, and erection of trusses is relatively simple. However, once assembled trusses take up a greater amount of space.

Use this link for more bridge types. Some bridges are a combination of two or more types. Examples are Truss Arch Bridges and suspended truss bridges. As you will see the skeletal structure of truss bridge exists in many other bridges and many other structures including buildings and large equipment such as tower cranes. This is the reason you choose truss bridge for your project.

Then visit a local truss bridge and make observations, focus on the design and try to identify specific features and design elements.

While visiting the bridge pay attention to the size and the shape of construction elements (steel pieces) and how they are connected to each other.

All Triangles
While looking at a truss bridge you will see that it is formed by many triangles. Even in the places you can identify a rectangle, another cross bar is dividing it into two triangles.

Think of a triangular based pyramid made of 6 same-length rods. Although all the joints may be movable, the whole structure remains solid and all angles remain unchanged.

Now think of a cube made of 12 rods and the same movable joints. Such cube is very flexible and with a slight force it can bend in any direction and all angles can change.

Because of the rigidity of triangular structures, they are used in larger structures such as truss bridges.

Learn more about truss bridges.

There are different possibilities for the relation of the travel surface or deck to the rest of a truss bridge structure.

Examples of the three common travel surface configurations are shown in the truss type drawings below.

In a Deck configuration, traffic travels on top of the main structure; in a Pony configuration, traffic travels between parallel superstructures which are not cross-braced at the top; in a Through configuration, traffic travels through the superstructure (usually a truss) which is cross-braced above and below the traffic.

Distribution of Forces in a Truss Bridge:

When loads are applied to a truss only at the joints, forces are transmitted only in the direction of each of its members. That is, the members experience tension or compression forces, but not bending forces. Trusses have a high strength to weight ratio and consequently are used in many structures, from bridges, to roof supports, to space stations.

Hear is a simple Bridge Design program you can use to see how the forces are distributed in different bridge elements. When you know the type of force (Compression or Tension) and the amount of force on each element, you can use it to decide on the necessary strength for that element. Note that a truss bridge is as strong as its weakest element. Make sure to read the instructions first and then try the program.

Go To the Bridge Design Simulator

More to learn:

Before starting with your project, you must have a good understanding of the tensile strength and compressive strength. You must also be able to measure the tensile/ compressive strength of any materials you may use to construct your bridge.


Girder: A girder is a large support beam used in construction, normally of iron or steel. Girders often have an I beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams.

Plate Girder: An I beam made of steel plates.

Wood blocks are always cut along the grain in order to provide the maximum strength to smaller wooden boards or wooden sticks. If the wooden stick is being used as a column and it is under compression, it may experience a combination of bending and compressive stress due to some lateral bending. So it is important to use short and straight sticks where an element is under compressive force.

According to the hardwoodinfo.com the compressive strength of basswood is 2220 up to 4730 lbf/in2. The tensile strength or shear parallel to grain is 600 to 990 lbs/in2. For example if you are using any length of 1/4″ x 1/8″ basswood, then the cross section area of your sticks are 1/32 square inch. To calculate the compressive strength of your sticks you can divide 2220 by 32. So your sticks can resist 69 Lbs of compressive force. The same way you can calculate how many pounds tensile force they can resist.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

In this project you will design a simple truss bridge and use your calculations to determine the strength of the bridge. You will then construct the bridge and test it to see how close is the actual strength to the strength you calculated. You will also calculate and report the strength to weight ratio of your bridge.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Any one of the design elements may be chosen as an independent variable (also known as manipulated variable). If you decide to make three similar bridges with 3 different materials, in order to compare their strength, then the type of materials is the independent variable.

Final strength of the bridge is the independent variable (also known as responding variable).


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. This is a sample hypothesis.

My hypothesis is that a basswood bridge will be stronger than a balsa wood bridge with same design and same size elements. My hypothesis is based on my measurements of the tensile strength of basswood and balsa wood.

To test this hypothesis you will make 2 identical bridges, one from balsawood and the other from basswood. You will then compare their strength by testing them.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Introduction: In this experiment you will design a truss bridge that spans 22 inches (Use Deck Configuration, so the actual structure will be below the roadway). You will calculate the total strength of the bridge to the best of your ability. You will then construct the bridge using basswood. you will finally test the actual strength of the bridge by distributing load on the bridge.


  1. Use the bridge designer program to design one truss structure. Your final bridge can utilize 2 or 3 of these structures under the roadway. The length of your bridge will be 22 inches (Between the fixed node and the rolling node). The roadway may extend a little more from each side.
  2. Calculate or measure the strength of balsawood or basswood sticks you want to use.

3. Add different loads to your bridge in the bridge design program and observe the distribution of forces on different elements of the bridge. Find the highest load that when distributed among the elements, no force will exceed the strength of wood sticks you want to use. That will be the single truss strength based on your materials.
4. Decide how many of such truss structures you want to use in your bridge. You may want to use 2, 3, 4 or 5 of such structures depending on the width of your bridge. Multiply the strength of a single truss by the number of truss structures you want to use. That will be the calculated strength of your bridge.

5. Construct the trusses for your bridge and then connect them to each other so that the connections also form triangles. This is necessary so the structure will not twist.

6. To have strong connections, use utility knives or special wood cutters and cut the woods in a way that surface contact will maximize in all nodes. Use strong glue and reinforcing plates in all the nodes of the truss structures.

7. To provide additional strength you may divide each triangle to multiple smaller triangles. You may also install additional connecting nodes between the truss structures.
8. Cover the roadway of your bridge with a sheet of cardboard, basswood or balsa wood. Whatever you use must be glued to the top of the bridge in order to maximize the strength. If you don’t glue it, then the roadway will not contribute to the strength of the bridge.
9. Measure and record the total mass of your bridge. You will need that to calculate the strength to mass ratio.
10. Make sure all the glues are fully dries and all the joints are strong before testing the bridge. Apply additional glue where needed.
11. Place the bridge between two tables or two stands that are exactly 22 inches away from each other. Start adding loads on the center of the bridge until it collapses. Record the final load before the bridge collapses. That will be the actual strength of your bridge.
12. Compare the calculated strength with the actual strength. Write what are the possible reasons for the difference.

Materials and Equipment:

This is a sample list of materials:

    • Basswood sticks 1/8″ x 1/2″, 20 feet
    • Utility knife or wood cutting tools (see below)
    • Wood Glue
    • Ruler or measuring tape

If you use strips of balsa wood or basswood in order to make a strong bridge, the angles you cut the strips will be quite important. You must cut the strips so that the contact points are perfectly aligned. Using a strip wood cutter you can cut strip woods in different angles. Strip wood cutter is available at MiniScience.com and the part number is MW1134.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.



Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List of References or


    • This website has tips for building model bridges from a former Science Olympiad participant, Garrett Boon:
      Boon, G., 2006. “Model Bridge Design,” Garrettsbridges.com [accessed November 30, 2006] http://www.garrettsbridges.com.
    • The PBS website, “Building Big” has a page on bridges that is a good place to start your background research:
      WGBH Educational Foundation, 2001. “Building Big: All About Bridges,” PBS Online [accessed November 30, 2006] http://www.pbs.org/wgbh/buildingbig/bridge/index.html.
    • This fascinating website has information on different types of steel bridges and how they are made:
      Matsuo Bridge Co., Ltd, 1999. “Bridges,” Matsuo Bridge Co., Ltd. [accessed November 30, 2006] http://www.matsuo-bridge.co.jp/english/bridges/basics.shtm.
    • On this website, you can learn about different types of bridges (arch, beam, suspension, and cable-stayed), and then play “Build A Bridge.” You’ll be given a site description, and you have to decide which bridge type would work best there.
      WGBH, 1997. “Super Bridge,” NOVA Online [accessed November 30, 2006] http://www.pbs.org/wgbh/nova/bridge/.
    • Wikipedia has a good article on suspension bridges:
      Wikipedia contributors, 2006. “Suspension Bridge,” Wikipedia, The Free Encyclopedia [accessed November 30, 2006] http://en.wikipedia.org/w/index.php?title=Suspension_bridge&oldid=58153872.
    • What can happen when the design is not quite right:
      Ketchum, M., 2000. “Mark Ketchum’s Bridge Collapse Page,” [accessed November 30, 2006] http://www.ketchum.org/bridgecollapse.html.
    • Check out this Science Buddies resource on building materials and forces:
      Pruitt, B., T. Bailey, and A. Tung, 2006. “Stress, Strain and Strength” Department of Mechanical Engineering, Stanford University (published by Science Buddies with permission) Stress, Strain and Strength.