Designing a strong bridge

Introduction: (Initial Observation)

Bridges are among the most fascinating products of engineering and physical science. While designing and building a bridge you have to overcome many difficulties and constraints.

In a bridge project you will be asked to design and construct a bridge that will hold the most weight for a given span. Now you are probably wondering where to start. What type of bridge is the strongest? (We get that question frequently from kids.) That question doesn’t really have an answer. There are too many variables involved for the question to be meaningful. For example, what are your material constraints? Certainly, it would not be of much use to you were I to tell you that stone arches were the strongest, when you were already instructed to build your bridge out of toothpicks.

The idea of “constraints” is an important one. Your teacher may have told you that you must build your bridge out of toothpicks, or Popsicle sticks, balsa wood, or maybe even spaghetti. The main reason that you were given the rules is to “constrain” you just like bridge engineers everywhere. Before a bridge project begins bridge engineers are told what the constraints are – what we can and can not do. It varies from job to job, and that’s what makes it fun.

Information Gathering:

The following is some general information that can help you design and construct your bridge. I suggest a balsa wood bridge and initial drawing on the paper. The main idea in designing the bridge is to redirect all forces to tensile and compressive forces. The reason that we do that is that most construction material have a high tensile strength or a high compressive strength, but they have little strength if you try to bend them. Converting weight force to tension and compression is done by forming triangles. A good on-line program to help you learn how weights distribute in a triangle is Build a Truss Program. This program allows you to build a truss bridge model and test it!

You can also click Bridges to see pictures of bridges made by other students.

Structure Bridges sells kits and videos and also has a tips page.

Form of Bridge

There are three major types of bridges: (See more details and diagrams)

• • the Beam bridge
  • the Arch bridge
  • the Suspension bridge

The biggest difference between the three is the distances they can each cross in a single span. A span is the distance between two bridge supports, whether they are columns, towers or the wall of a canyon. A modern beam bridge, for instance, is likely to span a distance of up to 200 feet, while a modern arch can safely span up to 800 or 1,000 feet. A suspension bridge, the pinnacle of bridge technology, is capable of spanning up to 7,000 feet.

What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to span a distance nearly 7 times that of an arch bridge? The answer lies in how each bridge type deals with two important forces called Compression and Tension:

• • Compression is a force which acts to compress or shorten the thing it is acting on.
  • Tension is a force which acts to expand or lengthen the thing it is acting on.

Beam bridges come in dozens of different styles. The design, location, and composition of the truss is what determines the type.

I will assume that you have been told that the bridge must span a certain distance and hold a certain weight (or the most weight compared to the other student’s bridges). Am I right so far? The first thing that we must do is to pick a material. That was probably done for you by your teacher. The next thing that we must do is to pick a bridge form or type.

For the type of project that you most likely have been given, the best design would probably be the Warren truss bridge as shown above. The Warren truss is a favorite of the railroads. Each truss (there are two, one on each side of the bridge) is composed of equilateral triangles. The bridge shown in the picture is a “Warren truss with verticals” so each equilateral triangle is divided into two smaller triangles. You can accommodate various span lengths by varying the length of your members (the sides of the triangles) and/or the number of panels (the number of triangles). If you decide to add panels you should add them to the center section where the chords (the upper and lower members) are parallel and the structure is the deepest.

To see samples of nicely constructed 34 inch-span Warren truss with verticals built by Ocean County College students click on the link. Some of these models support close to 200 times their own weight!

Another use of the Warren truss can be seen here in one of the spaghetti bridges.

The Importance of Connections

It is impossible to overstate the importance of connections to the strength of your bridge (or any structure for that matter). Really!
Look at the picture at the top of the page. Notice the “gusset plates” at the connections? This kind of detail is extremely important. Stresses flow like water. Where members come together there are stress concentrations that can destroy your structure. It would be very useful if you could visit a truss bridge and take a look at the connections there. If you are building the bridge out of balsa wood cut out and glue gusset plates similar to what Andy McConnell did in his well built bridge shown above. If you can use only toothpicks you may wish to glue little triangles of parts of toothpicks around the joints to make a kind of gusset “plate”.

Here is a connection detail of one of the spaghetti bridges.

The Properties of Balsa Wood

It is probable that your project will use balsa wood (Sold in hobby stores and some craft stores). The lightest of the commercially available hardwoods, it grows naturally in the humid rain forests of Central and South America. The best stands of balsa usually appear on the high ground between tropical rivers where there is plenty of rainfall and good drainage.

Balsa wood is so light because the cells of the wood are big and very thin walled, so that the ratio of solid matter to open space is very small. Woods typically have a gooey cement, called lignin, holding the cells together. In balsa, the lignin content is at a minimum. Only about 40% of the volume of balsa is solid substance. In a living balsa tree, the remainder of the volume is filled with water. That gives the tree, which can grow to 60 to 90 feet tall, the rigidity to stand. Each balsa cell is naturally pumped full of water until it becomes rigid much like a tire full of air. Green balsa wood must be kiln dried to remove most of the water before it can be sold.

If you are building a structure out of balsa wood (or any other wood for that matter) you should bear in mind that as a natural material its properties will vary considerably from piece to piece. Some of the variables involved include where and when it was grown, the orientation of the grain, the presence of irregularities, and the density of the individual sample.

You should select your pieces so as to eliminate any obvious imperfections.

You can use the following property values for design purposes:

¤ Low Density = 75 kg/m³ (0.0027 lb/in³); Medium Density = 150 kg/m³ (0.0054 lb/in³); High Density = 225 kg/m³ (0.0081 lb/in³)

All else being equal, you should select pieces of greater density because the strength increases more than the mass does. If you really want to optimize your project, you can click on the link to calculate the forces in your trusses and then select your highest density wood for the members with the highest stresses.

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.

The purpose of this project is to design and construct a bridge from balsa wood that spans 24 inches and holds 25 lbs or more. The weight of bridge by itself may not exceed 1 lbs. In order to design such bridge, we will first try to find out how much does the strength of balsa wood increases when connected in the form of an equilateral triangle?

Identify Variables:

When you think you know what variables may be involved, think about how you can 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.

While testing the effect of design factors on the strength, variables may be defined as follows:

Independent variable (also known as manipulated variable) is the simple design of a short bridge. Possible values are:

• • Simple design with straight sticks of wood placed side by side
  • Simple design with straight sticks of wood arranged in a right triangle form with a vertical divider.
  • Simple design with straight sticks of wood arranged in an equilateral triangle form with a vertical divider.
  • Any other arrangements that you may want to test.

Dependent variable (also known as responding variable) is the maximum strength at the center.

Constants are the total weight and material type.

Controlled variables are temperature and moisture.

Note: It is practically impossible to make different design bridges that use the same amount of material. To calculate the bridge strength based on a constant material weight, you can make the bridges with the amount of material that is needed and then divide the strength by the weight of material. In this way you will have strength per unit weight of material.

Strength per unit weight is a good scale for evaluating the design.

Hypothesis:

I think an equilateral triangle will have the highest strength and it will be about 100 times more than same amount of balsa wood connected side by side.

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.”

Experiment: Identify design factors that provide strength to the bridge.

Introduction: Before preparing your final bridge design, perform some experiments to see how does the strength of a balsa wood bridge changes based on the design.

Procedure:

1. Place a one foot balsa wood stick (of any size) between two supports and hang an empty plastic bucket with known weight in the middle of that. Start adding sand until the wood breaks. Record the final bucket weight that broke the wood.

2. Change the design by adding 2 more pieces of balsa wood to make a right triangle and add another piece as vertical. Glue all nodes, let them dry and repeat the sand bucket test again.

3. Make a new triangle design; but instead of a right triangle, make an equilateral triangle. Add the vertical pole, glue all the nodes, let them dry and repeat the sand bucket test.
Note that the vertical pole directs the force to the pinnacle. Almost like you are placing the bucket on the top of the triangle.

4. Make a new triangle design; but make this one a low height isosceles triangle. Add the vertical pole, glue all the nodes, let them dry and repeat the sand bucket test.

Record your results in a table like this:

Based on the results of your above four experiments, design your final bridge.

Note: To measure tensile strength, compressive strength and shear strength of material, you may use Newton scales. A complete set of Newton scales allows you to measure the strength with different forces up to 50 Newtons (5000 grams).

If you have access to a set of Newton spring scales, you will not need to use plastic bucket described above. You may simply pull the structure with a spring scale while reading it.

Make a Bridge using toothpicks: (Bridge Making Procedure)

Toothpick is one of the best material for building a bridge. While making a bridge using toothpicks, you face many of the challenges that engineers face while making a real bridge.

Connecting pieces together is an important part of making a bridge. Any chain is only as strong as its weakest link. Try different ways of connecting pieces together to see which one produces the best result.

The pictures in the right show how you may use wood glue and a piece of paper to connect two or more toothpicks in a line. Wood glue requires a few hours in warm and dry room to harden.

Paper is turned a few times around the tooth picks. Adhesive tapes are temporarily used to hold the tooth picks in line.

Another way of doing this is using a heavier piece of cardboard or heavy construction paper to make a joint.

First fold the piece around one toothpick and use some pressure to form it like a tube.

Then use a staple to hold the joint in its form.

Apply some glue to the ends of two tooth picks and insert them in the holes from two opposite sides in a way that parts of toothpicks overlap.

Leave it this way for a few hours to dry.

Other joints involving more than two toothpicks may require a backing paper and a cover as shown here.

While making such connections, if two toothpicks are in line, narrow sections will overlap. Otherwise it is better if you cut some or all of the narrow ends.

All these are options that you have in order to make strong joints.

In the construction of the bridge shown below, we will make all joints together.

To do that you can construct one side of the bridge on a table. You must use small pieces of tape to hold the toothpicks in place. You may also cut the sharp / weak ends of the toothpicks that are used to form triangles.

When the layout is complete and all pieces are in place, one by one you must slide a piece of cardboard under the joints, apply some glue and then cover it with another piece of paper.

Picture on the right shows two sides of the bridge waiting for joints to dry.

For the toothpicks that formed the main bars of the bridge and travel along the bridge, we should not cut the sharp ends. The joints get their extra strength by overlapping sharp ends.

For the toothpicks that join the main bars (showed in blue) we must cut the sharp ends.

Note that the top part of the bridge may need to be shorter because of the triangular formations on the sides.

When all the sides are ready, you can use tape to connect the sides together as shown in the picture.

If your bridge is going to be tested for strength, you must use glue for this part as well.

This picture will show you why the top part of the bridge may be shorter.

This is the completed bridge.

Fettuccine Pasta Bridge

Fettuccine Bridge Procedure:

Fettuccine pasta come in the form of flat – long strips. Fettuccine is a good choice of material for constructing a bridge because it offers the same challenges that engineers face while designing a real bridge with material such as steel and wood. A short piece of wood or a short piece of steel pipe may look strong; however for a long distance such as crossing a river, all these will show their flexibility and softness.

While designing your bridge from pasta, first hold a strip of pasta in your hand and break it. Repeat this test with different lengths of pasta and different directions. In which direction and what length does the pasta show the highest strength?

Also measure the length, the width, and the height of each strip.

First make the sides of the bridge. Each side is formed of two long parallel strip connected to each other using shorter strips. Connections must form triangles. Use wood glue to connect the pieces together.

Strips of pasta are not as long as your bridge, you will need to connect 2 or more strips to make a long strip. To do that, place two strip of pasta along each other, then glue a shorter piece (about 4 cm) over the joint. To make sure that the strips will not move around during construction, you may use tapes to secure them to your table or construction board.

The roadway and the upper level of the bridge must be made in a different orientation. To make the roadway, place the strips on a table so that they stand up on their smaller side. Use tape to hold them in that position temporarily.

Make sure that all overlapping joints are toward the inside. Outer sides must remain level so you can later connect them to the sides of the bridge.

The roadway must not be very wide. Excess wideness reduces the strength of your bridge. While connecting two sides, make triangles. If you really have time and want your bridge to be very strong, you may also divide each triangle to 3 smaller triangles. This will give additional strength to your bridge. You may also glue additional strips of pasta over the roadway.

In this way your roadway will look more real and your bridge will be all pasta. Make sure you use enough glue where it is needed. Some may prefer to glue a cardboard or construction paper over the roadway.
The upper level or upper structure will look identical to the roadway.

After you make the roadway, the upper level and two sides, let them dry and be ready for the next step

To connect the sides to the roadway, first apply glue to one side of the road and then connect the side wall. You may use tapes and small clamps to hold these two pieces in position for about one hour until the glue is partly dry. You can then repeat the same with the other side.
By now your bridge has a roadway and two walls or side structures. Let it stay so the glue will dry.

This picture shows my construction table with the bridge. I am yet to attach the upper level.

To connect the upper level, apply glue to one side of the upper level and one side of the bridge wall. Insert the upper level in place and use tapes to temporarily hold the pieces together. You can later remove the tapes.

When the bridge is ready, do the final inspection. Remove extra tapes and apply more wood glue anywhere that you think may provide extra strength.

The bridge in this picture is 29 inches long (72 cm) and it feels very strong.

Materials and Equipment:

Balsa wood (or bass wood) and wood glue (Elmer Glue) are the main material that you will need for this project. You may also want to use toothpicks instead of balsa wood. Toothpicks used for the bridge described above is available at MiniScience.com.

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):

Report and Compare the results of your four experiments.

Calculations:

You can use the calculate the forces to see how forces are being distributed in your bridge.

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.

Conclusion:

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.

References:

Additional references for this project may be found in your local library. Search for books related to strength of material or bridges.

Question:

I am not sure where to start for my project. I have to build a bridge that is:

LENGTH 24″
WIDTH 7″
HEIGHT 8″
WEIGHT MAX 1/2LB (8 OUNCES)
AND I CAN ONLY USE
SODA STRAWS ( PAPER OR PLASTIC)
PLAIN SHEET CARDBOARD OR PAPER ( MAXIMUM THICKNESS 1/16″ ONE LAYER ONLY
COTTON STRING OR THREAD
WOOD ONLY IN THE FORM OF TOOTH PICKS OR POPSICLE STICKS
WHITE ELMER’S WOOD GLUE
ALSO BRIDGE MUST HAVE A ROADWAY WITH A MINIMUM OF AT LEAST 4″ OD VERTICAL AND 3″ OF HORIZONTAL CLEARANCE TO ALLOW FOR LOADING OF A 1″H X 2″W X 6″L WOODEN LOADING BRACKET
BRIDGE MUST BE SELF CONTAINED UNIT NOT ATTACHED TO A BASE
AND A 3/8″ HOLE MUST BR DRILLED IN THE CENTER OF THE BRIDGE TO ALLOW FOR A LOADING BOLT.
MY PROJECT IS DUE January 9TH 2004 BUT I KNOW IT GOING TO TAKE ME AWHILE TO BUILD IT, THIS IS MY FIRST TIME DOING A PROJECT LIKE THIS.
I DON’T UNDERSTAND WHAT I AM SUPPOSED TO DO

Answer:

This is what you need to do:

1. 1. Collect samples of different acceptable material and test them for strength and length.
   2. Experiment different methods of joining material to each other.
   3. Draw the bridge structure.
   4. Assemble the pieces.

To draw the bridge structure, draw a 24″ line and divide it to four 6″ segments. Build an equilateral triangle on each segment. Draw another line to connect the top of triangles to each other.

By now you should have a drawing like this. This can be any side of the bridge.

For the bottom of the bridge, draw two parallel line, 24 inches each and about 6 inches apart. Connect these two lines with multiple right angle lines.

The top of the bridge will look the same, but it is only 18 inch long. These two parts may need further reinforcement.

Then you use straws or tooth picks to construct these pieces. All completer pieces can be connected using cotton strings.

Depending on the strength of straw or toothpick that you use, any of these materials may need additional reinforcement. To do that, you can use additional straws or tooth picks to divide rectangles to triangles and triangles to smaller triangles.