In this Introduction for Teachers:
Before you begin | Learning the Language | Structure is only part of a building | How to use these teaching materials | How to tell if your students are "getting it" | The "Basic Basics": The Vocabulary of Structural Principles | Structure is intertwined with other factors | You're surrounded

For students and teachers:
The "Basic Basics": Vocabulary of Structural Principles

All physical objects have a structure. You have one, and so does everything you can see or touch. (For that matter, things you can't see and touch, such as invisible gases, have a structure, too.) In this narrative about architecture, what we mean by structure is the part or parts of a building that actually hold it up and give it its shape. (Your skeleton provides the structure for your body. Imagine what you'd look like without one. (On second thought, don't.) The structural elements of a building -- the walls, the frame, the foundation -- are the parts that hold it together, as opposed to elements that enclose or decorate it. Structural elements can't be removed without damaging the strength and shape of the building.

You're probably inside a building while you're reading this. What's keeping the ceiling from falling in on your head, the floor from collapsing beneath your feet, and the walls from tumbling down into a pile of rubble all around you? (Unfortunately, these things do happen.)

The forces of gravity, wind, weight and other things that push and pull on a building are called stress. If all of the structural parts of a building aren't strong enough to withstand the stress applied to them, the structure will fail by cracking, crushing, or deforming (twisting out of shape). The weight of the building itself is called the dead load; it doesn't move or change. The weight of objects that go in and out of the building -- you, other people, furniture, fixtures and machinery -- is called the live load; it moves and changes with the life of the building.

The words tension and compression describe the two major kinds of stress that the parts of a structure undergo as a result of the load. No matter how complex the forces acting on the structure may become, each part of the structure is either in tension or in compression. Often, a structure is reacting to both tensile and compressive stresses at the same time. Neither tension nor compression is stronger than the other -- they're just descriptions of what happens to the material when it reacts to stress.

Compression involves pressing, pushing, and squeezing. (Find the word "press" in compression.) Feel compression in your arm muscles by pressing the palms of your hands together as hard as you can. Your muscles have contracted, bunched up, gotten shorter. Make fists and squeeze them tight; you'll feel compression in your hands, fingers and biceps.

Tension involves pulling and stretching. Feel tension in your arm muscles by stretching your arms out to the sides as far as you can. Your muscles have expanded, stretched out, gotten longer. Stretch a rubber band. When you stretch it (and only then), it's in tension. Tensile materials such as cables have to be stretched taut before they have any structural strength; when they're slack, they can't support weight.

Stand up and bend over to one side as far as you can. One side of you is stretching, getting longer. Is this side in tension or compression? The other side is getting shorter, and everything is getting squeezed together. Is this side in tension or compression?

Structure and building materials

The materials used to make a building are so much a part of the structure that it's tempting to mush the words together into structureandmaterials. Building materials, because of their own molecular structures, dictate a range of structural possibilities which in turn determine the form of the building. Conversely, the wish to construct a certain form automatically eliminates some materials that can't accommodate the shape or size needed for that kind of building. So structure and materials are in a chicken-and-egg relationship; they influence each other so much that it's hard to tell which one comes first in a building's design.

Masonry, which includes natural stone, adobe, brick, concrete and glass block, is strong in compression, which means that masonry materials can withstand incredible degrees of pushing, pressing or squeezing without being crushed. Masonry has been used to build structural walls since the beginning of civilization, when ancient civilizations built huge pyramids and columns by piling stones and bricks up on top of each other. But masonry units don't make good beams because they're brittle in tension and crack if they're used across a long span.

Wood and bamboo, because of their long, fibrous cells, are strong in tension, and so is steel. Each of these materials can be pulled and stretched under great stress, which means that they make good beams across open spans. Wood, bamboo and steel are also strong in compression and make strong walls and columns, just as masonry units do. Wood and bamboo are very light relative to their strength and easy to get in many areas of the world. Steel is much stronger, but it's also heavy, expensive, and requires modern technology.

Concrete is artificial stone that's just as hard as natural stone after it has cured. By itself, concrete is also just as brittle in tension as natural stone, so builders pour it around long, thin steel reinforcing bars (shortened to "re-bars") that can handle tensile forces without breaking. Builders use reinforced concrete for columns, beams, slabs, foundations, walls, arches, vaults and domes.

In this Introduction for Teachers:
Before you begin | Learning the Language | Structure is only part of a building | How to use these teaching materials | How to tell if your students are "getting it" | The "Basic Basics": The Vocabulary of Structural Principles | Structure is intertwined with other factors | You're surrounded