This document provides information about different shapes used in structures and how they respond to loads. Rectangles bend and fail under heavy loads, while arches and triangles perform better up to a point. Arches spread apart when overloaded, and triangles eventually snap under too much tension. Bracing helps reinforce structures by resisting bending and spreading forces. The shape of a structure significantly impacts its strength and stability.

1. Forces Lab
Forces Lab | Materials Lab | Loads Lab | Shapes Lab
About This Lab
This lab simplifies the real-life forces and actions that affect structures, in order to illustrate key concepts.
Intro
Forces act on big structures in many ways. Click on one of the actions at left to explore the forces at work and to see real-
life examples.
Squeezing (Compression)
Compression is a force that squeezes a material together. When a material is in compression, it tends to become shorter.
Compression: See It In Real Life
The lower columns of a skyscraper are squeezed by the heavy weight above them. This squeezing force is called compression.
Stretching (Tension)
Tension is a force that stretches a material apart. When a material is in tension, it tends to become longer.
Tension: See It In Real Life
The weight of the roadway and all the cars traveling on it pull on the vertical cables in this suspension bridge. The cables are
in tension.
Bending
When a straight material becomes curved, one side squeezes together and the other side stretches apart. This action is
called bending.
Bending: See It In Real Life
The top side of the metal bar is pulled apart in tension, and the bottom side is squeezed together in compression. This
combination of opposite forces produces an action called bending.
Sliding (Shear)
Shear is a force that causes parts of a material to slide past one another in opposite directions. Ý
Shear: See It In Real Life
During an earthquake, parts of this roadway slid in opposite directions. This sliding action is called shear.
Twisting (Torsion)
Torsion is an action that twists a material.
Torsion: See It In Real Life
In 1940, the Tacoma Narrows Bridge twisted violently in strong winds and collapsed. The twisting force that tore this bridge
in half is called torsion. Ý

2. Loads Lab
Forces Lab | Materials Lab | Loads Lab | Shapes Lab
About This Lab
This lab simplifies the real-life conditions that affect structures, in order to illustrate key concepts.
Intro/Instruction
Forces that act on structures are called loads. All structures must withstand loads or they'll fall apart. In order to build a
structure, you need to know what kinds of external forces will affect it.
Dead Load
The weight of the structure itself is called the dead load. Anything permanently attached to the structure is part of its
dead load -- including the columns, beams, nuts, and bolts.
Live Load Failure Intro
The weight of the stuff on the structure is called the live load. Things that move around in or on a structure, like people,
furniture, and cars, are all examples of live load.
Live Load Failure
The beam failed because it could not support the heavy weight of the live load above it.
Live Load Success Intro
Thick Beam: The thicker a beam, the less likely it is to bend. Thick beams are used in structures that experience live and
dynamic loads.
Live Load Success
Thick Beam: The thick beam made this structure very strong. Now the beam won't bend from the heavy weight of the live
load on top of it.
Dynamic Load Failure Intro
Loads that change over time are called dynamic loads. Dynamic loads -- from wind gusts to pounding objects -- create
vibrations that can become bigger and more dangerous over time.
Dynamic Load Failure
The beam was vibrating too much from the dynamic load. This kind of vibration would be unacceptable to people occupying a
building or driving across a bridge.
Dynamic Load Success Intro
Thick Beam: The thicker a beam, the less likely it is to bend. Thick beams are used in structures that experience live and
dynamic loads.
Dynamic Load Success
Thick Beam: The thick beam absorbed the vibrations caused by the dynamic load and prevented the structure from bending
and galloping wildly out of control.
Wind Load Failure Intro
When wind blows on a structure, it is called wind load. Wind loads push horizontally on a structure.
Wind Load Failure
The structure collapsed because it couldn't withstand the strong gusts of wind.
Wind Load Success Intro
Cross-Bracing: Diagonal braces, usually made of steel, are used to strengthen and stabilize all kinds of structures.

3. Wind Load Success
Cross-Bracing: Cross-bracing is an excellent way to stiffen a structure experiencing wind load. When the wind blows, the
diagonal brace squeezes together and prevents the structure from flopping over.
Thermal Failure Intro
When a structure expands or shrinks with the temperature,it is experiencing thermal load. The temperature causes the
beams and columns to change shape and push and pull on other parts of the structure.
Thermal Failure
The intense sun made the beam expand, throwing the entire structure out of whack.
Thermal Success Intro
Roller Joints: Roller joints are used in structures that get really hot or cold. They give columns and beams the freedom to
expand and contract as the temperature changes.
Thermal Success
Roller Joints: Thanks to this roller joint, the beam can swell in the sun and slide over the column without damaging the
structure.
Earthquake Failure Intro
When the ground beneath a structure jerks back and forth during an earthquake, the structure is experiencing an
earthquake, or seismic load. Earthquake loads push and pull horizontally on a structure.
Earthquake Failure
That was more rattling and shaking than this poor structure could handle.
Earthquake Success Intro
Shear Walls: Solid walls of reinforced concrete or masonry -- called "shear walls" -- have great stiffness in the horizontal
direction. They resist loads that push or pull horizontally on a structure.
Earthquake Success
Shear Walls: Shear walls can handle being pushed, pulled, rattled, and shaken during an earthquake. They're a great way to
strengthen a structure prone to earthquake load.
Settlement Failure Intro
When the soil beneath a structure settles unevenly, it is called settlement load. Structures will sink and change shape when
they experience settlement load.
Settlement Failure
This structure is in bad shape -- literally!
Settlement Success Intro
Deep Piles: Heavy concrete pillars, or piles, are used to support structures on soft soil. The piles rest deep in the earth on
stable, solid soil and support the weight of the heavy structure above.
Settlement Success
Deep Piles: The massive concrete piles, sunk deep into the earth on hard, solid soil, keep the structure safe and sound where
it should be -- above ground!

4. Materials Lab
Forces Lab | Materials Lab | Loads Lab | Shapes Lab
About This Lab
This lab simplifies the real-life properties of a selection of materials, in order to illustrate key concepts.
Intro/Instructions
What you build a structure out of is just as important as how you build it! Different materials have vastly different
properties. Click on a material at left to find out more about it, and put it to the test.
Wood Properties
Type: Spruce (softwood)
Wood Pros+Cons
Strengths: Cheap, lightweight, moderately strong in compression and tension
Weaknesses: Rots, swells and burns easily
Wood Applications
Bridges, houses, two- to three-story buildings, roller coasters
Example: Son of Beast -- Cincinnati, Ohio
Wood Compression Message
You squeezed this block easily, but it took a lot of effort to make it break. Wood is cheap and pretty strong in compression.
That's why people build houses out of wood!
Wood Tension Message
It wasn't easy to break this block of wood because wood is strong when you pull it in the direction of its fibers. It would
have been three times easier for you to break this block if you'd stretched it from the top and bottom, across the direction
of its fibers.
Plastic Properties
Type: High-strength plastic fabric Ingredients: Long chains of molecules
Plastic Pros+Cons
Strengths: Flexible, lightweight, long-lasting, strong in compression and tension
Weaknesses: Expensive
Plastic Applications
Umbrellas, inflatable roofs over sports arenas
Example: Georgia Dome -- Atlanta, Georgia
Plastic Compression Message
Compared to steel, you squeezed this plastic block easily, but it took a lot of effort to make it break. The long chains of
molecules that make up plastic can be pulled and pushed in many directions without failing.
Plastic Tension Message
You stretched this plastic pretty far before it finally broke. The long chains of molecules that make up plastic can be pulled
in many directions without snapping. That's one of the reasons why circus tents are made of plastic fabric!
Aluminum Properties
Type: Aluminum alloy Ingredients: Aluminum with magnesium & copper

5. Aluminum Pros+Cons
Strengths: Lightweight, doesn't rust, strong in compression and tension
Weaknesses: Expensive
Aluminum Applications
Airplane wings, boats, cars, skyscraper "skin"
Example: Petronas Towers -- Kuala Lumpur, Malaysia
Aluminum Compression Message
It was pretty hard for you to break this aluminum block. That's because the magnesium and copper inside this block makes it
almost as strong as steel!
Aluminum Tension Message
It wasn't easy to break this aluminum block. That's because aluminum, when combined with metals like magnesium and
copper, is almost as strong as steel!
Brick Properties
Type: Ordinary brick Ingredients: Burned clay
Brick Pros+Cons
Strengths: Cheap, strong in compression
Weaknesses: Heavy, weak in tension
Brick Applications
Walls of early skyscrapers and tunnels, domes
Example: Original Thames Tunnel -- London, England
Brick Compression Message
You had to push this brick very hard to make it crumble. Bricks are very strong in compression. That's why early houses were
made of brick!
Brick Tension Message
You pulled this brick apart easily! That's because bricks are very weak in tension.
Concrete Properties
Type: Fine-grain concrete Ingredients: Cement, water, small stones
Concrete Pros+Cons
Strengths: Cheap, fireproof and weatherproof, molds to any shape, strong in compression
Weaknesses: Cracks with temperature changes, weak in tension
Concrete Applications
Early arch bridges and domes
Example: Pantheon - Rome, Italy
Concrete Compression Message
You had to squeeze this concrete block really hard to make it break. That's because concrete is very strong in compression.
Concrete Tension Message
You pulled apart the small stones and cement in this concrete block easily. That's because concrete is weak in tension.
Reinforced Concrete Properties
Type: Fine-grain concrete with high-strength steel Ingredients: Steel bars hidden in concrete

6. Reinforced Concrete Pros+Cons
Strengths: Low cost, fireproof and weatherproof, molds to any shape, strong in compression and tension
Weaknesses: Can crack as it cools and hardens
Reinforced Concrete Applications
Bridges, dams, domes, beams and columns in skyscrapers
Example: Hoover Dam - Nevada/Arizona border
Reinforced Concrete Compression Message
You had to squeeze this block really hard to make it break. That's because concrete and steel are both very strong in
compression.
Reinforced Concrete Tension Message
It was hard to pull this concrete block apart because the steel bars inside make it very strong in tension. That's why some
of the tallest skyscrapers in the world are made of reinforced concrete.
Iron Properties
Type: Cast iron Ingredients: Iron with lots of carbon
Iron Pros+Cons
Strengths: Molds to any shape, strong in compression
Weaknesses: Weaker than steel in tension, breaks without warning
Iron Applications
Arch bridges, cannons, historic domes
Example: Iron Bridge - Shropshire, England
Iron Compression Message
It wasn't easy for you to squeeze this cast-iron block. Cast iron is strong in compression. That's why early arch bridges were
made of cast iron.
Iron Tension Message
It was easy for you to pull this cast-iron block apart. That's because cast iron is brittle -- it snaps without warning.
Steel Properties
Type: High-strength steel
Ingredients: Iron with a touch of carbon
Steel Pros+Cons
Strengths: One of strongest materials used in construction, strong in compression and tension
Weaknesses: Rusts, loses strength in extremely high temperatures
Steel Applications
Cables in suspension bridges, trusses, beams and columns in skyscrapers, roller coasters
Example: Sears Tower - Chicago, Illinois
Steel Compression Message
You had to push extra hard on this steel block to make it bend and break. Steel is stronger than any other material in
compression. That's why engineers choose steel beams and columns to support most skyscrapers.
Steel Tension Message
You had to pull this block incredibly hard to make it break because steel is stronger than any other material in tension.
That's why the cables in the Golden Gate Bridge are made of steel.

7. Shapes Lab
Forces Lab | Materials Lab | Loads Lab | Shapes Lab
About This Lab
These labs simplify the real-life conditions that affect structures (shapes), in order to illustrate key concepts. In the real
world, many variables affect the strength and stability of any give shape. The choice of materials, joints, load distribution,
and size and thickness of a structure all affect its ability to resist loads. For example, a triangle made of paper would
collapse much sooner than an arch made of steel -- an effect that was not demonstrated in this lab.
The shape comparisons in this lab depend upon the following conditions: each shape is of equivalent thickness, the joints are
hinged, and the live load is applied downward to the structure at a single point at its top and center.
Intro/Instructions
The shape of a structure affects how strong it is. Rectangles, arches, and triangles are the most common shapes used to
build big structures.
One Elephant on Rectangle
The weight pushes down on the rectangle and causes the top side to bend.
One Elephant on Arch
The weight presses down on the arch and is spread outward along the curve to the ground below.
One Elephant on Triangle
The weight causes the top two sides to squeeze together and the bottom side to pull apart.
Three Elephant on Rectangle
The weight caused the top side to bend too much, so it failed!
Three Elephant on Arch
The arch likes to be pushed and squeezed. The weight pushes this arch into a stable, tightly squeezed shape.
Three Elephant on Triangle
Unlike the rectangle, the sides of the triangle did not bend under the tremendous weight. This is why the triangle is still
standing.
Six Elephant on Rectangle
The weight caused the top side to bend too much, so it failed!
Six Elephant on Arch
The arch likes to be pushed and squeezed, but not this much! When an arch is pushed too hard, the sides spread apart and
collapse.
Six Elephant on Triangle
The triangle is still standing because the pulling force in the bottom side is balancing the pushing forces in the upper sides.
Nine Elephant on Rectangle
The weight caused the top side to bend too much, so it failed!
Nine Elephant on Arch
The arch likes to be pushed and squeezed, but not this much! When an arch is pushed too hard, the sides spread apart and
collapse.

8. Nine Elephant on Triangle
Even triangles have their limits! All this weight made the third side stretch so much that it snapped in half.
Push Rectangle
What happens when you push the side of a rectangle?
The rectangle is a wobbly, unstable shape. When you push the side, it flops into a slanted parallelogram. This happens without
any of the rectangle's sides changing length.
Push Braced Rectangle
Now when you push the side, the diagonal brace gets squeezed, preventing the rectangle from flopping over.
Push Arch
What happens when you push down on an arch that is not supported on both sides?
The force of the finger pushes the sides of the arch outward.
Push Braced Arch
As the arch tries to spread outward, external supports, called buttresses, push back on the sides of the arch and prevent it
from spreading apart.
Push Triangle
What happens when you push the side of a triangle?
The outer edge squeezes together, and the inner edge pulls apart. When one side experiences these two forces at the same
time, it bends. The weakest part of the triangle is its side!
Push Braced Triangle
When you poke the top of the triangle, the two sides squeeze together and the bottom side pulls apart. The triangle doesn't
bend because each side experiences only one force at a time. When used properly, triangles are the most stable and rigid
shapes used in construction today.