SPIDERWEBS TO SKYSCRAPERS: The Science of Structures - 3. A Choice of Materials
Figure 1. A steel worker balances on the framework of a building for the Los Angeles County Museum of Natural History.
Figure 2. Experimenting with the strength of bricks.
Figure 3. Bricks in a decorative pattern in a wall.
Figure 4. Pouring concrete to make a foundation.
Figure 5. Strengthened concrete.
Figure 6. A model cofferdam.
Figure 7. The Golden Gate Bridge is supported by thick steel cables.
In the past, people built structures using whatever natural materials were available, including animal skins, wood, mud, and stone.
But today, in addition to natural materials, we have a wide range of human-made materials. These include bricks, concrete, steel, and even more advanced substances, some of which were developed for use in outer space (see Figure 1).
New materials, because of their strength and ability to take on almost any shape, make exciting new types of structures possible. In fact, one of the main tasks or architects now is to select carefully which of the many available materials to use for each part of a new building.
Bricks and Superbricks
You will need:
What to do:
Break off three lumps of clay. Using the ruler and knife, mold and trim the first into a brick 4" long, ¾" wide, and ¼ high.
Cut off about 20 pieces of straw, each roughly ½" long. Combine these pieces thoroughly with the second lump of clay and make a brick the same size as the first.
Finally, mix about three pinches of sand with the remaining lump of clay before molding it into a third brick (see Figure 2). Allow all three bricks to harden overnight.
Poke two small holes in the top of the cup and pass the nylon thread through. Tie the thread in a loop about 2 feet long. Pass the thread over the middle of the plain clay brick and rest the ends of the bricks on two supports, such as two piles of books, as shown. Place the bowl under the cup.
Begin to slowly pour sand into the cup. Do this until the brick snaps under the load. Pour all the sand back into the cup and weigh it. Make a note of your measurements.
Repeat the experiments with the straw-and-clay brick and with the sand-and-clay brick. What do you notice? Can you explain your results?
Taking it further:
Try using other mixtures of straw and sand. What amounts of each produces the strongest brick? What happens if you mix bath straw and sand in with the clay? Experiment with other materials in the clay, such as strands of human hair.
You might also try to obtain different types of clay – from river banks or the ground (if there is clay soil in your area) or from craft stores. Which type of clay makes the best bricks?
Trials of Strength
In days gone by, the way to see if a new substance or design worked was to build an actual structure, such as a bridge, and hope that it stayed up! (see Figure 3) Today, however, new building materials are checked thoroughly in laboratories before being used in practice. Equipment is used to measure accurately the strength of a substance under different kinds of stretching, pressing, and twisting forces. Scientists do experiments on the material to find out how it is affected by high and low temperatures, water, and fire.
Often, a pure substance can be made stronger by adding other substances to it. For example, iron becomes tougher and more flexible when turned to steel by mixing it with a small amount of carbon. In a similar way, mud and clay can be made stronger by reinforcing them with sand, straw, animal hair, cow dung, or even blood. The particles of the added substance provide something for the clay particles to bind to and help to stiffen the finished brick.
The Rock That Pours
Cement is the great, all-purpose building material of today. It consists of a very fine powdered mixture of substances such as calcium silicates and calcium aluminates that are obtained from naturally occurring rocks. When sand and water are added to it, complex chemical reactions take place and the cement sets into a hard, solid mass called concrete.
Aside from being strong, concrete has the big advantage that it can be poured while in the form of wet cement. This makes it ideal for filling in the foundation holes at a building site or for being precast at a factory into any desired shape. The precast sections are then delivered to the building site for assembly.
Like clay, cement can be mixed with various amounts of sand, water, gravel, and other additives to give different types of concrete suitable for different jobs. For large structures, the concrete is given extra strength by molding it around steel bars up to two inches thick. The result is called reinforced concrete.
The steel bars are kept tightly stretched while the concrete sets, then upon being released they squeeze of "prestress" the concrete, giving it even more strength. Prestressed concrete will not crack even under exceptionally heavy loads (seeFigure 5).
Strong, Stronger, Strongest
You will need:
What to do:
[Ask a parent or teacher to help you with this experiment.]
Use the cardboard and tape to make a mold. Mix a small quantity of cement, sand, and water in the proportions recommended on the cement container, or ask the adult helper what would be a normal mixture to use. When ready, pour the wet mixture into the first compartment of the mold.
Make a second batch of cement, but this time add some gravel to make a stony mixture. Pour this into the second compartment. Make a third batch, without gravel, but with twice as much sand as before. Pour this into the third compartment.
Allow the mixtures to harden. Test the strength of each brick using the same method as in the experiment "Bricks and Superbricks."
Warning: Cement is harmful is swallowed. Wear old clothes and plastic gloves while doing this experiment. Clean up thoroughly afterward. Never wash unused cement down the sink – it will harden and block the drain.
Taking it further:
Cement is an excellent material for testing the strength of some of the shapes mentioned in this book, since it can be cast into any form. For example, mold could be made of beams and various types of arches. When hard, the structure could then be loaded either by putting weights on top or hanging them below. This kind of loading stretches the material by creating tension. More advanced students might also try subjecting concrete beams, arches, and other shapes to a force of compression. Loading a beam with heavy weighs along its length or squeezing the beam lengthwise in a vise are alternate ways of doing this. Safety goggles should be worn for such an investigation because of the risk of flying fragments when the concrete breaks.
Building on Riverbeds
Imagine that you had to build a bridge across a wide river. The design of the bridge calls for tall concrete columns, or piers, to act as supports at regular intervals. How can you put the piers in place without the river washing them away? The answer is by building a temporary dam called a cofferdam where the pier is to go.
Bridge builders sink long beams of hardened concrete, steel, or wood into the riverbed to make a square or circular wall that rises above the water level. Then they pump out all the water from inside the cofferdam, so that work can be carried out on the riverbed. The mud and soil is dug away until hard ground is reached, after which cement is poured into the hole to make a sturdy foundation. The pier is built up from the foundation until it rises above the top of the cofferdam. Then the cofferdam is removed and used in building the next pier.
Make Your Own Cofferdam
You will need:
What to do:
Cut cardboard strips and use staples to make a square and a triangle as shown. Try pulling the sides of these shapes. What do you notice?
Flatten out a lump of the clay so that it is wider than the base of the cup and about ½" thick. Stick the clay onto the bottom of the sink. Fill the sink to the depth of the cup. Cut the bottom out of the plastic cup and press this end firmly into the clay. Suck all the water out of the cup with the straw. Push one of the building blocks to the bottom of the clay. Gradually build up a column of blocks until it rises above the rim of the cup. Remove the cup so that the water rushes in around the column (see Figure 6).
Compare what you have just done with the description of how a cofferdam works in "Building on Riverbeds."
Steeling the Show
Very strong, yet lightweight, steel forms the tough skeleton of may large, modern buildings. Sometimes it is woven into thick cables and used to support suspension bridges such as the Golden Gate Bridge in San Francisco (see Figure 7). More often, though, it is made into I- or H-shaped beams, known as girders. These are then joined together to make a framework strong enough to hold the entire weight of a giant building. In fact, it was with the coming of steel frames that a new type of building became possible at the start of the twentieth century. That type of building, which is common now in large cities everywhere, is the skyscraper.