The Science of Structures

When you read, you start with ABC.

When you sing, you start with do, re, mi.

When you build, you start with tension and compression.

Tension and compression are the key forces explaining how structures stay up and why they fall down.

Tension: the pulling force

Tension in structures isn't the same thing you feel when you forget to study for a test. Tension is a pulling force. It stretches materials. Link your hands together and pull. You feel tension. Stretch a rubber band. You see tension in action. The rubber stretches, and the band gets longer. It's in tension.

Look for materials in tension in: rope bridges, telephone wires, tents, suspension bridges, inflated stadium domes, steel cables supporting a full elevator, and hair when someone yanks on it.

Compression: the pushing force

Compression is a pushing force. It squashes materials. Put your hands together and push hard. You feel compression. Put a big marshmallow on the counter and push it down with your hand. As you push, the marshmallow gets shorter. It's in compression.

Look for materials in compression in: pyramids, telephone poles, arch bridges, elephant legs, tree trunks, and your little brother when you sit on him.

Tension and Compression

When a load is placed on a beam, as above, the top half of the beam shortens in compression. The bottom half lengthens in tension.

The different parts of a structure are either in tension, or in compression, or both. So the materials we use to build structures must be strong in tension, in compression, or both. Steel wires bundled together to make suspension bridge cables are one material strong in tension. A steel cable one centimetre in diameter can support 8,000 kilograms--the weight of two full-grown Indian elephants!

Stone would not be a good choice for a tension structure. Stone is, however, strong in compression. Think of the Egyptian pyramids, which are made of stone blocks, some weighing over a tonne. The blocks on the bottom support the weight of the upper blocks. The fact stone is strong in compression but weak in tension actually helped the Egyptians cut the huge limestone blocks. They drove wooden wedges into the limestone. The wedges were then soaked with water until they swelled up and split the limestone. The Egyptians used the strengths and weaknesses of stone to their advantage. Pretty smart!

What if we mix two materials, one strong in compression and one strong in tension? Embedding steel rods into concrete makes reinforced concrete, a material stronger in tension than ordinary concrete.


	Workers pour cement around steel reinforcing bars. The result will be a reinforced concrete floor. (Photo by Art Makosinski)

All structures--from spider webs to suspension bridges--have to stand up to the loads placed on them.

Live loads are the things a structure supports through regular use. Like "live" things, these loads can change and move. Live loads: snow, rain, people, cars, furniture, wind.

Dead loads do not move. The structure always has to support them. They are, well..."dead". Dead loads: walls, beams, arches, floors, ceilings.

Dynamic loads act suddenly upon a structure. Their effects can be quite disastrous unless the structure is designed to handle their force. Dynamic loads: earthquakes, tornadoes, tidal waves.

When engineers are designing a structure, they must plan carefully for loads. But sometimes it's hard to plan for every possible load situation: in 1945 a plane hit the Empire State Building while travelling at 400 km/h! The building withstood the force of this dynamic, dramatic load, but the plane's crew and some people in the building were not so lucky.


	Result of the 1994 earthquake in Northridge, California. (Photo courtesy U.S. FEMA)

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