Steeling the Show

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 temporar

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. Compare what you have just done with the description of how a cofferdam works in "Building on Riverbeds."

Steeling the Show

The Golden Gate Bridge is supported by thick steel cables

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. 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.

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. y 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.