How buildings work
by Chris Woodford. Last updated: November 27, 2019.
Amazing buildings make amazing cities.
But what makes amazing
buildings so.... amazing? Apart from being
lovely to look at and wonderful to work in, an amazing building is
quite often the product of very clever engineering. In other words,
it's built not just on rocks or earth but on cutting-edge science and
technology. Amazing buildings can withstand earthquakes
and plane crashes. They can heat themselves using little more than the Sun's
gaze. They use advanced materials in very advanced ways so you never
have to paint the woodwork or clean the windows. Let's take a closer
look at some of the science hiding inside the places where we live,
work, sleep, and breathe!
Photo: Steel skeleton: You might look at a building and think the walls hold it up, but a modern building is just as likely to be supported by a hidden steel framework. In this partly constructed community center, a web of giant, interlocked, steel girders acts like a skeleton, resting on concrete foundations. Bricks are being built around the outside of the steel frame to give an attractive, traditional appearance, but they're largely cosmetic: the majority of the forces holding the building up are going to be supported by the steel inside.
How gravity works against buildings
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
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.
Artwork: 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.
Photo: Question: How do you build deep foundations for a tall building
without digging away tons of earth? Answer: Use a foundation drill like this.
These amazing drills can sink foundations over 30m (100ft) into the ground.
Some can drill holes about 2.5m (8.2ft) in diameter! Find out more in our main article about
How a building supports its own weight
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 drought 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 stories of a building are
going to be under much more pressure (the force acting per unit of
area) than the top stories, because they have to support more weight.
So if you built the lower stories 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 stories 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 stories up
How buildings balance forces
Buildings in the real world are not like towers
made of LEGO® or sandcastles. Those structures are usually made of
material, whereas a real-world building is mostly empty space. Not only
that, but the "empty space" inside a building usually has to support
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
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
channeled through the various parts of the structure.
Artwork: There's more than one way to balance the weight of a building. Instead of
chaneling it through heavy vertical walls and horizontal floors, Richard Buckminster Fuller's famous
geodesic domes distribute force evenly through an outer "skin" of interconnected triangles. This
creates an uninterrupted interior space much more cheaply and using much less material.
As he pointed out in his 1954 dome patent, you need 23kg (50lb) of wall and roofing material to
shelter 900 sq cm (one square foot) of floor space, but you can achieve the same end with only 0.35kg (0.78 lb) of geodesic dome. That works
out at abut 600 times less building material! Furthermore, Fuller claimed his domes were strong enough to withstand
winds of 240 km/h (150mph). Artwork courtesy of US Patent and Trademark Office
from US Patent 2,682,235: Building Construction
by Richard Buckminster Fuller, published June 29, 1954.
To make a building that is both strong and hollow, we need to put
horizontal and vertical structures together to do different jobs. For
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 story 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.
The force of the walls pushing down on the floor is exactly balanced by an equal force when the
floor pushes up on the wall. If that weren't the case, and the two forces
weren't exactly balanced, either the walls or the floor would be moving.
The fact that buildings and structures don't move tells us that the forces
acting on them must indeed be balanced—and that's why we call these sorts of constructions
If you've ever seen a building being demolished by a crane with a
wrecker ball (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: A floor, wall, staircase, or any other structure has to be supported to stop it
collapsing, but that doesn't mean it must be supported equally in all places. Though we tend to think of beams as
needing support at both ends, if a beam is strong enough, you can support it at one end only. Any downward force you put
on the beam travels down its length and is balanced at the single, supported end. A structure like this
is called a cantilever and it was used to great effect in the long, reinforced concrete terraces of this famous building,
Fallingwater, designed by architect Frank Lloyd Wright. Photo by Jack E. Boucher courtesy of US Library of Congress.
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 skin made from steel or glass.
“A building is not just a place to be but a way to be.”
Frank Lloyd Wright
Tension and compression forces in buildings
Artwork: Left: The vertical wooden beam is in compression: it's being squeezed by the weight
pushing downward and by the ground pushing back up. Right: An identical wooden beam, laid horizontally on top of two
vertical beams, is in compression at the top and tension at the bottom, while the vertical beams that support it are both in 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 story of your house. Suppose I
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.
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.
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 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: Buildings have to cope with ever-changing loads
from things like wind and the weight of the people inside. When architect Daniel Burnham
completed his famously tall and thin Flatiron building in 1902, some people believed
it would blow over in the wind. It earned the nickname Burnham's folly as a result.
Although it certainly does channel winds down into the streets around it, the famous
New York City landmark remains standing to this day. Photo by Carol M. Highsmith,
courtesy of the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.
Make way, heavy load!
Tension and compression aren't the only forces that buildings have to cope with.
From the tallest skyscraper to the simplest bridge, every static structure
also has to cope with varying loads. An office block will weigh far more when it's
full of people, computers, desks, and photocopiers than it does when it's empty,
and the people who build it have to take that into account. Similarly, bridges
have to cope with varying forces both from things that drive over them and the weather,
which can lead to bending and twisting (torsion) that might make them collapse.
Every static structure has to be able to cope with a mixture of dead loads (its own basic weight) and live loads (the weight it carries when it's occupied or being used), so working out what those are and how big they will be
is an important part of building design.
You can read more about this in our detailed article on bridges.
Photo: Top: Placed upright, a cardboard toilet roll can support three heavy books. Bottom: Placed flat, it can't even support one!
If you have a hollow cardboard tube (such as a kitchen towel holder
or empty toilet roll), you probably know that it's rather better at
withstanding some forces than others. Try it! If you place the tube
vertically, you can stand quite a lot of weight on the end. You could,
for example, put quite a lot of heavy books on top of the tube without
it showing the least signs of stress. The weight of the books will try
to squeeze the tube downward. In other words, the tube is in
compression. Placed upright, cardboard tubes are structurally very
sound because there are solid walls going all the way down from the top
to the bottom to support any weight on top. Also, because the walls
have a circular cross-section
(you get a circle if you slice through them), the forces are spread
through the structure: no part of any wall is loaded more than any
other. Cardboard tubes are so strong that one Japanese architect,
Shigeru Ban, has made a feature of them in lightweight temporary
buildings, such as emergency housing for refugees.
But suppose you try to make the floors of a building out of
cardboard tubes. You can probably see we're heading for trouble here
straight away! If you place a cardboard tube horizontally and try to
stand things on it, you'll soon squash it flat. That's because there's
only hollow, empty space between the place where you're applying the
force and the ground. The curved, cardboard walls are simply too thin
to channel the forces around them so the whole structure collapses. In
other words, cardboard tubes are not very good at withstanding
compressive forces when they're placed horizontally.
What this tells us is that some materials work well in buildings
when we use them in a particular way and they work badly if we use them
in other ways. In other words, it's important to understand the
properties of materials if you want your buildings to work effectively.
Choosing the best materials for a building
“No design is possible until the materials with which you design are completely understood.”
Ludwig Mies van der Rohe
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 bridges and tunnels, 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 stories 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 stories 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 story it attaches to, or by the outer
steel skin of the building.
“The materials of city planning are: sky, space, trees, steel and cement; in that order and that hierarchy.”
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). 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 energy and how it travels around inside buildings.
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
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
paneling 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 on places like
topical rainforests 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 history factor
Thanks to advances in science and engineering, today's buildings are very different from yesterday's.
Building would once have been a matter of trial and error: primitive structures were, quite literally,
nothing more than cunning piles of found materials designed to give shelter from the storm.
Today, as we've just seen, much more thought—and calculation—goes into buildings
and static structures like bridges. The Swiss architect Le Corbusier famously said that "a house
is a machine for living in"—a modern home is as sleek and well-engineered as a modern car.
That means modern buildings can't be easily compared with historic ones. People who lived 200, 500, or 1000 years ago didn't have the range of materials we have today, or the ability to source and transport materials over long distances. Nor did they have the scientific understanding of how materials behave when they're stressed and strained in different ways or subject to different kinds of environmental shock for years, decades, or centuries.
If you're trying to understand a building, you need to look through the eyes of the people who built it.
What problems were they trying to solve? What materials did they have? What other building techniques existed at that time that they could copy or develop?
Photo: Cathedrals old and new: two architectural solutions to the same problem.
1) Bath Abbey, a stone cathedral in England, can trace its history back to 675CE, but the building we see today has been reinvented quite a few times since then. 2) The Cathedral of Saint Mary of the Assumption (Saint Mary's Cathedral) in San Francisco, California is a more modern "solution" to the same "problem" made from precast concrete and dating from 1971. Credit: The Jon B. Lovelace Collection of California Photographs in Carol M. Highsmith's America Project, Library of Congress, Prints and