Tell Me About
Background Information for Structures and Shapes
Two structures with which children are familiar are bridges and towers.
Review this background information to introduce a few concepts to your class. You may also want to explore other activities related to our school program.
- Why do we need bridges?
- What shapes can bridges take?
- What shapes are the strongest?
- What is a truss?
- Does the bridge material make a difference in the strength of the bridge?
- The CN Tower
Why do we need bridges?
Bridges make it possible to cross from one shore of a river to another. In earlier times, bridges allowed trade between farmers, and the movement of goods across a natural divide.
Early man often placed stepping-stones in the narrow sections of small streams as a way to get to the other side. It is easy to see how a bridge might develop, using stepping-stones as a base for logs or planks. The earliest bridge was probably a toppled tree that had fallen across a nearby stream.
Are bridges always over water?
No, they can be over other roads, over railways, over deep gulleys. And they are not only for cars and people. Wooden trestle bridges were essential to the passage of the railway through the mountains in western Canada.
Bridges over rivers allow the water to pass easily through the "holes in the bridge."
Bridges allow traffic over obstacles (rivers, roads, ravines), without changing the obstacle.
What shapes can bridges take?
Three common shapes are:
The word "beam" comes from the old English word for tree. The first bridges were probably logs.
the bridge support. How high do you make the piers? That depends on what has to travel underneath.
the distance pier to pier. The span depends on the strength and weight of the beam. The thicker the material, the more weight it will hold. The thicker the material, the further the span. But, the thicker the material, the heavier the bridge, so the more it takes to hold it up. A very strong beam could be too heavy for the "span" and sag into the river. We can lighten the beam by using a truss.
Can we make the span longer another way?
There is a type of beam bridge called a cantilever. It is related to a lever.
Imagine if one end of a lever, balancing on a fulcrum, were anchored to the ground. The other side could hold considerable weight. Suppose there were two levers on each side of a gap, each with one end fixed to the ground, something like two diving boards extending from the opposite banks of the river. Then, a centre beam could be balanced across these "diving boards." The fixed levers would be able to hold up the centre beam. In this way, a cantilevered bridge can span a wide river.
Arches are made up of wedges and a "keystone," the wedge at the top. An arch bridge stands up with "push." The "push" is in the form of a curve. Arch bridges can be very wide because you can have arch after arch in a continuous line.
If the arch were very shallow, you would need to anchor the bottom wedges to the ground. This is usually done with a supporting wall.
To show the principle of compression or "push" to the class, pick up, from a shelf, five or six books by pushing against the end books and lifting. The books stay together because you are exerting a "push." Can you place another book on top of the row? It depends on how hard you exert the pressure against the end books.
Can you think of a three-dimensional structure that uses the same principle as the arch bridge?
Suspension bridges are really arch bridges turned upside down, but they hold together with tension or "pull" instead of "push," like the arch bridges. They can span much greater distances than any other type of bridge.
Suspension bridges are very flexible, which is not a problem, as long as the swaying of the bridge is not in rhythm with repetitive gusts of wind. Then the swaying could build and build, until finally the bridge disintegrates.
What shapes are the strongest?
With cardboard sticks and two pronged pins, build a square and a triangle.
Try pushing the sides together. The triangle resists, but the square collapses, unless the angles are braced. You could create a brace for the square by constructing a diagonal - which, in effect, is making two triangles.
The triangle is the strongest shape. Triangles are used to make a very strong form called a truss.
Another very strong shape is a tube. Think about bicycle frames and bamboo shoots.
Try this on your students.
Can you make a sheet of newspaper hold up a book?
Try folding it twice lengthwise and rolling it into a tube. This newspaper tube is strong enough to hold up a book.
What is a truss?
Beam bridges can span a greater distance with a thicker beam. But there is a limit, because too thick a beam will sag into the river. An alternate solution is to build a beam of triangles, a "truss," which is much lighter than a beam of similar thickness. A truss is extremely strong and stable.
Does the bridge material make a difference in the strength of the bridge?
Shape is important for strength, but so is the material. A steel beam will hold more weight that a wooden beam of equivalent size. Today, bridges are built of steel and reinforced concrete.
Reinforced concrete beams are created by placing steel bars into the concrete before it hardens. This skeleton of steel helps the concrete withstand the tension that develops when long, unsupported beams are subjected to bending under heavy weight loads.
Our own CN Tower
The CN Tower, in Toronto, at 553 metres, is one of the tallest free-standing structure in the world.
How was this tower built?
Construction engineers used a slip form. Vertical rods were put into place; a mould was built around these rods and the concrete was poured. When the concrete had hardened enough to support weight, the mould (slip form) was pushed vertically up the rods, and more concrete was poured. This process was repeated again and again until the desired height was attained.
Another famous tower
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