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Bridge Construction Theory
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1. |
There are many different ways to build a bridge and it can be quite a challenge to come up with a good design. However, to a large extent, the bridge you will be designing and building has been simplified somewhat, and a number of typical design decisions have already been made for you. Consider that:
| A. |
You won’t have to spend time deciding on materials; they are already chosen; |
| B. |
The bridge dimensions are set, both maximums and minimums; |
| C. |
The loading is simple (vertical force on the centre); and |
| D. |
The bridge is simply supported (it sits on the supports; it is not attached nor touches the supports in any other way.) |
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2. |
Design Principles. Minimise weight; maximize strength. Don’t add anything to your bridge which doesn’t make it stronger. What is meant by stronger? Well, we want to make it stiffer, to make it resist bending and breaking.
| A. |
Start with a simple beam. Use a popsicle stick. Rest it between supports then push down on its centre with your finger. What is happening in the beam when a load is applied? What is it about the shape of the beam that makes it strong or weak? Consider standing the stick on its edge rather that on its flat side. Notice how much stiffer it is. Now if the stick could remain standing on its edge without falling over, imagine how much weight it could carry. Have someone hold the stick on its edge while you try to break it by pushing down on it with you finger. Does the stick bend downwards? Does it try to bend sideways? From this little exercise consider that the same material is used in each case, both sticks are the same weight and the same shape; the only difference is in the orientation of the stick (beam) and the requirement for lateral support to allow the one stick to rest to rest on its edge. Imagine if both stick designs were entered into a contest how, through shape alone (i.e., by correct orientation), one stick bridge could easily win over the other. This little exercise may appear overly simplistic, yet it is from this fundamental principle that successful bridge designs will win the bridge building contest. |
| B. |
Check beam under load. What will cause the beam to break [engineers commonly use the term "to fail" since word like ‘to break’ can be limiting as a bridge may fail even though it isn’t broken. (For example, bridges in the contest will fail if they bend (buckle) too much and touch the hydraulic ram.) Does it come apart when pulled (under tension)? When pushed (under compression)? Does it bend too easily? You will find that for a beam (like a popsicle stick) under a vertical load (like your finger pushing down on top) the upper wood fibres of the stick are being pushed together (‘in compression’) while the fibres in the lower part of the stick are being pulled apart (‘in tension’). |
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Progress to a built-up structure. Analyse various polygons (e.g., a triangular shape, a square shape, a pentagon) made from the popsicle sticks. Assume you have pin joints to make your analysis simpler: Notice that the triangle is the only one that keeps its shape under load, the others fold together and collapse. Apply a load to the top of the triangle and see what is happening in each popsicle stick. Notice how the upper sticks will tend to be in compression (just like with the previous case with a single stick beam) and the lower stick will tend to be in tension. Now we can begin to see how a bridge structure is designed. So far we’ve only looked at the bridge in two dimensions. However, must ensure that the structure is stiff in all three dimensions. In a way, you will have to build a separate bridge-like element sideways (laterally) in order to prevent your structure from bending and twisting. Be sure the lateral supports don’t get in the way of the hydraulic cylinder during testing. |
| D. |
Study the materials used. A popsicle stick can withstand a certain amount of pulling (tension) and pushing (compression) forces. Try to build a structure that makes the best use of the strengths of the material. The glue joints also need consideration. Glue a couple of sticks together with an overlap of a cm or two. How would you break them apart (Pull? Push? Twist? Bend?). From this you will discover the weak points and the strong points of the glue joint. |
| E. |
Study the strength to weight ratio. Seventy percent of the bridge evaluation is based upon its "Strength Factor." Notice in the attached chart (below) how the Strength Factor rapidly increases as the bridge weight is reduced for a given load of 20 kg. A 1.0 kg bridge would have to support a load of 163 kg to equal a 0.35 bridge supporting 20 kg! |
| F. |
Study Bridge Designs. We’ve looked at the simple beam. Check out other bridge shapes. Notice that some bridge types require them to be anchored to supports while others simply rest upon the supports. Try to combine whatever characteristics apply, given the limitations outlined in the contest rules
(eg. materials, dimensions, type of support, loading). |
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Design Process.
| A. |
Plan- Study bridge designs (library, Internet, local bridges). Build a small practise bridge and experiment with it. (How much tension can a popsicle stick withstand? How much tension can my glue joint withstand? How much compression? How much bending occurs? How can I make my design stiffer using a minimum amount of sticks and glue?) Talk with your friends, teachers, and parents. Be accepting of all criticism and advice. Learn what works and what doesn’t work, and why. Look at the good points, the less desirable points, and the alternative ways of doing things. Plan out your bridge design on paper. Try to do as much as possible prior to construction. |
| B. |
Organize- Determine what order pieces should be glued together. Provide time for glue to dry between stages. Ensure glue has ample time to cure before testing. If working as a team, assign various tasks to each member. |
| C. |
Other- Protect bridge from damage prior to testing. Ensure all bridge building rules are met. |
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4. |
One final word of advice: have fun learning and experimenting
with bridges, and have fun experiencing the world of
engineering!
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