KidBridges

ENTRY 3

Today, our class and our teacher, Ms. T., took the subway from Flatbush to the High Street Brooklyn Bridge stop

. We started walking

from the Brooklyn side of the bridge.

First, we decided to figure out which parts of the bridge were in tension and which were in compression. Also, how do all the forces on the bridge stay in equilibrium when the traffic moves and stuff is always changing?

We tried to imagine ourselves as part of the bridge and figure out which parts of the bridge are in tension and which are in compression. We thought this wouldn't be too hard, but it was.

We thought that the cables

were in tension, but when we looked at them, Ms. T. showed us that the cables weren't just attached to the top of the towers

. They went over the towers and were attached to gigantic posts on each side of the river, one in Brooklyn and one in Manhattan. Ms. T. told us that the posts are called anchorages

(We already knew that from the web sites we looked at).

When we got part way across the bridge, we saw information plaques

near the first set of towers. They told about the anchorages too.

These anchorages are very, very heavy and are buried in the ground. The cables pull against these anchorages with tremendous force. They're in tension.

The cables are stretched over the tops of the towers

and they push down on them. That means that the towers are in compression. All the weight the cables support pushes down on the towers too.

We demonstrated

this, using our backpacks. The backpack straps pushed down on our shoulders like the cables push on the towers.

Do you know why the cables push down on the towers? It's because of gravity

. Gravity is the force that pulls everything toward the center of the earth. So, it also pulls the cables and whatever is attached to them down, onto the towers.

Ms. T.

Now we know the answer to our question, "Why doesn't the bridge fall down?". It's because all the different forces are in equilibrium. (I had to learn to spell that word right, and I did!)

When we got to the wooden part of the walkway we could feel vibrations

through our feet. Sheryl and I took our shoes off to feel the vibrations better. Then we noticed that some of the cables were vibrating too

. Ronetta wanted to know if some day all the vibrating could make the bridge break. Lucas said that this bridge was really old, over 100 years. Maybe it would get weak and collapse like old people. (It's almost like he wishes it WOULD fall down!)

Wow! Good questions often lead to more questions! Engineers know that vibrations have to be taken into account when they build a bridge. They use a great deal of math to make sure that vibrations don't make their bridges fail. But Lucas was right. Bridges do get old, just like people. The parts that are made of iron can rust. And some parts lose strength through a process called fatigue

(It's as though they get tired!). But don't worry. Bridge maintenance crews replace those old, worn-out parts.

Ms. T.

Then, we saw these guys wearing orange and climbing gear. Chris asked them what they were doing.

They told him that they were going to climb up the big bridge cables to check where the cables go up through the tops of the towers. They called that part the saddle. The saddle is supposed keep the steel cables from rubbing against the stone towers. We watched them climb up. They looked so tiny when they got up to the top and stood next to the American flag up there.

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