How buildings work: A simple introduction to structures from Explain that Stuff!

How gravity gets you down

All kids like building things! Whether we're stacking LEGO® blocks or playing cards in the living room, sticks in the forest, or sandcastles on the beach, we're all architects and builders at heart. Think back to the last time you made something in this way. What was the biggest problem you faced? One of the things that would have worried you was the possibility of your building toppling over once it reached a certain height. That's also true in the real world, where the number one problem any builder faces is keeping their structure upright.

The trouble is all to do with gravity: the magnetic-like force of attraction between any two objects in our universe. On Earth, we see gravity as a tendency for things to fall toward the floor, but gravity always work two ways. If you drop a pen, it does indeed fall toward the floor—but the floor also jumps up by a microscopic amount to meet it on the way! The force pulling your pen down toward Earth is exactly the same size as the force that pulls Earth up toward your pen.

Now gravity usually pulls things straight downward, but it can act in other ways too. Suppose you built a really tall brick wall. We can think of gravity acting on it in two different ways. We can see it as a collection of separate bricks, with gravity pulling on each one separately. Or we can think of it as a solid wall with gravity pulling on the whole thing, just as though all its mass were packed into a single point in its center. The place where an object's mass seems to be concentrated is called its center of gravity. For a simple brick wall, the center of gravity is slap bang in the middle of the central brick.

So what makes a wall fall over? If the center of gravity is over to one side (if we've not built the wall straight or if we've built it on sloping ground), the force of gravity acting down will produce a turning effect called a moment. If the moment is small, the mortar between the bricks can resist it and keep the wall upright. But if the moment is too large, the mortar will break apart, the bricks will topple, and the wall will collapse.

Why walls stay up and why they collapse. Left: If a wall is built upright or on flat ground, the center of gravity (blue dot) is directly above the center point of the wall's foundations (yellow dot), so the wall is stable. Right: But if a wall is built on sloping ground, the center of gravity is no longer above the center of the base. Now gravity (red arrow) creates a moment (green arrow) that tips the wall over. The higher the wall, the greater the mass above the center of gravity, the greater the turning force and the more chance the wall will collapse.

Now this doesn't just apply to single walls: it applies to entire buildings. If a skyscraper is 200 m (650 ft) tall and a gale blows it hard at the top, there's a huge turning force trying to tip the whole building over to the side. That's why tall buildings need deep foundations (where a significant part of the building is constructed underground to support the part that's above ground). If something tries to push the top the building to one side, the foundations effectively resist and push it back in the opposite direction! In other words, they help to counter the moment that would make a building topple to one side.

Weighty issues

It's not only sideways, toppling forces that buildings have to withstand. If you've ever picked up a brick or a piece of stone masonry, you'll know it's reasonably heavy. Now imagine how much all the bricks or blocks of stone in a skyscraper weigh. Add to that the weight of the floors and ceilings. And then, on top of that, the weight of all the office equipment, furniture, and people in the building. What you have is a gigantic lump of weight pushing straight downwards... which immediately raises two questions.

First off, why doesn't the whole building sink straight into the ground? Of course, if you build your skyscraper on quicksand or in the middle of a swamp, it might do exactly that! But most people build on reasonably firm earth (soil) or rock. There will be a certain amount of squeezing downward if you build onto earth, but once the soil is fully compressed (squeezed) it will be almost as solid as rock and further compression shouldn't be an issue. It is possible, however, if floods or draught make the earth too wet or dry, that the ground beneath the building could shift or sink. This problem is called subsidence and has to be tackled by pumping tons of concrete under a building to shore it up.

The other question is why the building does not collapse down on itself. You can probably see that the bottom storeys of a building are going to be under much more pressure (the force acting per unit of area) than the top storeys, because they have to support more weight. So if you built the lower storeys of a building from cardboard and the upper ones from brick, you'd run into problems quite quickly. But you might be able to build the lower storeys from brick and the upper ones from cardboard. And you could even build the lower ones from cardboard if you used some extra supports (such as steel pillars) to help support the weight of the bricks in the storeys up above.

Balancing forces

Buildings in the real world are not like towers made of LEGO® or sandcastles. Those structures are usually made of solid material, whereas a real-world building is mostly empty space. Not only that, but the "empty space" inside a building usually has to support the weight of people, office equipment, or factory machines. Having solved their first problem (how to make a structure that doesn't topple over), architects and builders immediately turn their attention to another problem: how to make a hollow building that can support its own weight and that of its contents and occupants. This comes down to understanding where the forces are in a building and how they are transmitted from one part to another—or, in other words, how gravity is channelled through the various parts of the structure.

To make a building that is both strong and hollow, we need to put horizontal and vertical structures together to do different jobs. For example, the outside walls usually play a vital part in keeping the building up, while the inside walls help to separate one room from another and the floors (which are often ceilings too) give us something to stand on. But it's not quite that simple when you start to think about forces. Imagine you're sitting on a sofa in the middle of the floor on the top storey of a large house. If there's no wall directly underneath the floor where you're sitting, what stops the sofa crashing through the floor? The total gravitational force acting downward (the weight of your body, the weight of the sofa, and the weight of the floor) is transmitted sideways through the structural members of the floor (which may be anything from simple wooden bars called joists to heavy metal ones known as girders) to the walls at the side. The force then channels down through the walls to the floor. According to Newton's laws of physics, the force of the walls pushing down on the floor is exactly balanced by an "equal and opposite" force when the floor pushes up on the wall!

If you've ever seen a building being demolished by a crane with a ball and chain, you will have noticed that buildings can stay up even with most of their walls knocked away. That's because some walls in a building are more important than others and not all of them support the building's weight. The main, structural walls are called load-bearing walls and they're usually built from solid brick or stone. Knock one of these out and a large chunk of your building will probably collapse. The other walls in your building may simply be cosmetic ones built from a lighter material such as plasterboard. You can easily remove these walls without affecting the building's ability to stay upright and keep its shape (which is known as its structural integrity).

Photo: Buildings can take more demolishing than you expect, especially if the walls are made from reinforced concrete, like this one. Picture courtesy of US Department of Energy.

When skyscrapers were first built, they had elaborate wooden frameworks inside them to support their weight—lots of internal walls to support all the force pushing down from above. Gradually, though, as people found they needed (and often preferred) wide open spaces inside buildings for offices and factories, architects found ways of getting rid of the internal walls. Having slender pillars or columns was one obvious way to do this. Another option was to have extremely strong outer walls and sturdy horizontal girders running through the floors and ceilings to carry the weight of the building across to this "outer skin". A third option was to have a strong central core, sturdy floors running out from it like the petals on a flower, and only a relatively light outer made from steel or glass.

Tension and compression

The parts of a building can behave in different ways when large forces act on them. Suppose, for example, you're back on the sofa in the middle of the floor on the top storey of your house. Suppose I reach in through a window with a crane and place a 50-tonne weight onto the floor right next to you. It's quite likely the floor will immediately collapse and you'll fall through the hole I've just made. But what makes the floor collapse? Obviously, the beams supporting the floor cannot withstand the weight we're subjecting them to—but how exactly do they break? And why does the floor collapse rather than the walls? The answer is all to do with tension and compression.

Suppose you have a wooden beam standing vertically. You can support lots of weight on top of it because there's something solid underneath transmitting the force of gravity directly to the ground. The more weight you put on the beam, the more you squeeze it. If you could measure the beam accurately, you'd see that it shrinks just a tiny bit with every extra bit of weight you pile onto it. When a beam is loaded like this, we say it's in compression: it's being subject to compressive or squeezing forces.

This vertical wooden beam is in compression: it's being squeezed by the weight pushing downward and by the ground pushing back up.

Now suppose you balance the same beam horizontally between two similar, vertical beams—much like balancing the floor of a house between the walls. If you pile weights onto the beam, it won't behave quite the same way as it did before. The entire beam will start to bend, but the top and the bottom will bend differently. The top of the beam will be squeezed (by compression forces) and it'll get slightly shorter, while the bottom will be stretched and it will become a bit longer. We say the bottom is in tension (it's stretching) and we call the forces that do this tensile forces.

This horizontal wooden beam is in compression at the top and tension at the bottom, while the vertical beams that support it are both in compression.

We can keep piling load onto the beam for just as long as its internal structure can cope with these forces. At some point, the wood in the beam will splinter when the individual wood fibers can no longer cope with the tensile forces at the bottom. Then the beam will snap in two in the center, at the bottom, and the floor will collapse.

Like wood, concrete is good at withstanding compressive forces, but not so good at coping with tensile forces. Ordinary concrete is a superb material for making vertical walls, but it's much less effective for making horizontal floors because it's quite brittle: it will snap at a weak point just like wood if you pile too much weight onto it. You can make concrete much stronger by pouring it into a mold that contains a grid of rigid steel bars (often known as "rebar"). Concrete strengthened in this way is called reinforced concrete because the steel gives the concrete extra tensile strength and helps it to withstand tensile as well as compressive forces. Next time you see people constructing a huge concrete building, bridge, or other structure, take a look and see if you can see the steel reinforcement bars or rebar grid before the concrete is poured in.

Photo: Making reinforced concrete. These construction workers from the US Navy are spreading wet concrete from a truck onto a grid of steel reinforcing bars. When the concrete sets, the steel bars will give it added strength. Picture by Lt. Edward Miller, courtesy of U.S. Navy and Defense Visual Information Center.

Material world

Steel, concrete, and wood are three of our most versatile building materials—but there are lots of others, including composite materials and plastics. Architects and engineers use many different materials in their constructions and choose one material instead of another for a variety of reasons. Concrete is the material of choice for large structures such as tunnels and bridges, because it's strong, long-lasting, waterproof, fireproof, relatively inexpensive, and easy to mold into curved as well as straight shapes.

Suppose you were designing a skyscraper. How would you go about choosing the materials? First you'd need to know how many storeys high the building has to be. That's worked out by calculating how expensive the building land is, how much the building will cost to construct (an unknown, but you can guess roughly), and how much profit the owners want to make. Let's say you think the building will have to be 100 storeys high. You can now estimate how much it will weigh and how much weight it will have to support on each floor. So you can start to design some sort of a structure that will support that much weight for that height into the air. Probably you'll use steel and concrete for the structural parts of the building (where the weight will be supported), but you won't want to build a solid concrete block! So you might hide the structural parts in the center of the building and make the outer parts entirely of glass. But glass is heavy, so you'll need to factor its weight into your structural calculations too. And you'll need to figure out how the weight of the glass is going to be supported by the floor or ceiling of the storey it attaches to, or by the outer steel skin of the building.

You'll also have to think about keeping the building's occupants warm and comfortable. If you're making the facade from glass, it's going to absorb huge amounts of solar heat (something known as passive solar gain). That's great in winter, because it will help to reduce heating costs, but in the summer it could make the building unbearably hot. So maybe you'll want to use some sort of tinted or reflective glass that cuts down the solar gain a little? To figure all this out, you'll need to understand something about the science of heat and how it travels around inside buldings.

Photo: Passive solar gain: the large glass windows in this spacious wooden building help to absorb heat energy from the Sun. Picture by Donald Aitken courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

With the basic structure of the building decided, you'll turn your attention to the interior details. You might decide to make all the internal walls from steel panels that can be moved around as necessary to create flexible office space. Or maybe you want to use wooden floors or panelling to create a warmer and more friendly feel? Hopefully, you'll opt to use properly sourced sustainable wood supplies. For that, you'll need to understand why cutting down trees has an environmental impact and how that can be minimized.

You can see that every aspect of a building's design needs meticulous consideration. Making a building is not just a matter of coming up with something that looks good. It's about creating a structure that can survive all the strains of the modern world. For that, you need to be just as much of a scientist as an engineer!

The science of buildings

How gravity gets you down

Weighty issues

Balancing forces

Tension and compression

Cardboard constructions

Material world

Further Reading

Books you can read

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