An Introduction to

Structures and Materials

in Design Practice

Session II

Paul Conder SFU Surrey

Review from Session I

Why is glass fiber strong, but a wine glass so brittle ?

Why do boats hulls sometimes break in the middle ?

Why shouldn’t you be alarmed when an airplane’s wing flexes as it takes off ?

Why are portholes round ?

The four horsemen of structural collapse:

Four different ways forces can act on an object:

Compression

Tension

Shear

Torsion

Now that we have seen how forces can act on structures,

let’s take a look at how different structures react to applied forces...

Structural System #1: Tensegrity

A term coined by Buckminster Fuller (tension + integrity), but the idea has been around for centuries:

Counteract a tensile force with a compressive force to create a rigid, light, closed system.

Examples: Bow and arrow Bicycle wheelGuitarSling seat seating panel.Tendon, muscle and bone combination.

Tension structures can be very light for the amount of load they can carry.

Structural system #2: Post and Beam

Or – roofs, tables and chairs…

Connections are critical due to stress at joints.

Ways of alleviating stress at joints: additional members to tie posts together,rigid wood joinery.

Examples: Japanese temple, Table frame, .03 chair before and after design in wood. (Notice how the runner between the back legs could be eliminated when the substructure was revised.)

Footings are critical at base of posts and walls.

This leads to things like battered walls (walls that flare outwards at bottom) and feet on table legs.

Structural system #3: The Truss

Compare how a square frame and a triangulated frame behave when under load.

The triangulated frame is far more stable.Its joints can be hinged, it is a very rigid structure overall.

A truss is made up of several members, often in a triangular pattern, that build up a rigid section with each member being placed in either direct compression or tension.

There are examples of trusses and triangulation in nature:collar bone, back and rib cage – (including soft tissues in tension)

And in architecture:Railroad bridges, John Hancock building.

And in product design:A Bike frame, Several pieces of furniture, Microsphere Chair leg, Ikea kitchen shelving, Car suspension, sailboat rigging.

Structural system #3: The Truss (continued)

Compare how a square frame and a triangulated frame behave when under load.

The triangulated frame is far more stable.Its joints can be hinged, it is a very rigid structure overall.

A truss is made up of several members, often in a triangular pattern, that build up a rigid section with each member being placed in either direct compression or tension.

There are examples of trusses and triangulation in nature:collar bone, back and rib cage – including soft tissues in tension)

And in architecture:Railroad bridges, John Hancock building.

And in product design:A Bike frame, Several pieces of furniture, Microsphere Chair leg, Ikea kitchen shelving, Car suspension, sailboat rigging.

Structural system #3: The Truss (continued)

Compare how a square frame and a triangulated frame behave when under load.

The triangulated frame is far more stable.Its joints can be hinged, it is a very rigid structure overall.

A truss is made up of several members, often in a triangular pattern, that build up a rigid section with each member being placed in either direct compression or tension.

There are examples of trusses and triangulation in nature:collar bone, back and rib cage – including soft tissues in tension)

And in architecture:Railroad bridges, John Hancock building.

And in product design:A Bike frame, Several pieces of furniture, Microsphere Chair leg, Ikea kitchen shelving, Car suspension, sailboat rigging.

Structural system #3: The Truss (continued)

Compare how a square frame and a triangulated frame behave when under load.

The triangulated frame is far more stable.Its joints can be hinged, it is a very rigid structure overall.

A truss is made up of several members, often in a triangular pattern, that build up a rigid section with each member being placed in either direct compression or tension.

There are examples of trusses and triangulation in nature:collar bone, back and rib cage – including soft tissues in tension)

And in architecture:Railroad bridges, John Hancock building.

And in product design:A Bike frame, Several pieces of furniture, Microsphere Chair leg, Ikea kitchen shelving, Car suspension, sailboat rigging.

Structural system #3: The Truss (continued)

Compare how a square frame and a triangulated frame behave when under load.

The triangulated frame is far more stable.Its joints can be hinged, it is a very rigid structure overall.

A truss is made up of several members, often in a triangular pattern, that build up a rigid section with each member being placed in either direct compression or tension.

There are examples of trusses and triangulation in nature:collar bone, back and rib cage – including soft tissues in tension)

And in architecture:Railroad bridges, John Hancock building.

And in product design:A Bike frame, Several pieces of furniture, Microsphere Chair leg, Ikea kitchen shelving, Car suspension, sailboat rigging.

Structural system #3: The Truss (continued)

Compare how a square frame and a triangulated frame behave when under load.

The triangulated frame is far more stable.Its joints can be hinged, it is a very rigid structure overall.

A truss is made up of several members, often in a triangular pattern, that build up a rigid section with each member being placed in either direct compression or tension.

There are examples of trusses and triangulation in nature:collar bone, back and rib cage – including soft tissues in tension)

And in architecture:Railroad bridges, John Hancock building.

And in product design:A Bike frame, Several pieces of furniture, Microsphere Chair leg, Ikea kitchen shelving, Car suspension, sailboat rigging.

Structural system #4: The space frame

Multiple trusses intersecting each other to make up a slab-like fully triangulated open frame.

Eg the roof at Plaza of Nations, Fleetwood sofa, lighting rig column. – Great for really big light structures

Structural system #5: Geodesics

Another term originally coined by Buckminster Fuller.

Spherical and other 3d curved meshes can be created out of straight members, with all members being the same length.

Joints are often very complex in geodesic structures

Examples: Expo 67 dome, Science World, Soccer ball,

C-60 molecule, Radar domes in high north

Structural system #6: The Arch

An arch collects vertical loads and tuns them into lateral ones.

Voussoirs are constantly in compression, even though the arch can span a great deal of length.

Can even crack in three places. – NOT FOUR

Thrust lines, representing the sum of all forces applied to each stone in the arch, have to be contained within the body of the arch for it to remain stable.

The thicker the arch, the more variance in thrust line it can take.

http://web.mit.edu/masonry/interactiveThrust/examples.html

The more weight on an arch, the more stable it gets.

Outward force must be resisted for the arch to stay stable.

When a piece of string is draped downwards, it makes a similar shape to an arch, just upside down. - Gaudi used to use this method to analyze structures in his architecture

Not a parabola, not a circle, but a funicular shape.

Arches can be placed side by side, eliminating each other’s outward forces and lined up to create a vault.

Structural system #7: The Vault

Essentially, a vault is an extruded, or extremely deep arch.

Vaults can be seen frequently in architecture:

Churches, castles and wine cellars often feature vaults.

To balance the outward forces of the vault, walls either had to be built extremely thick, or flying buttresses were used.

Additional weight could be added to the top of the buttress to ensure that the thrust line from the load above would stay within the structure.

Structural system #8: The Dome

Basically, a dome is a rotated arch.

Examples: egg shells, igloos, your head, garbage can lid.

At about half way up the dome, the dome goes from being in compression (at the top) to being in tension (at the bottom)

This is why most ancient domes only used the top part of the dome - masonry won’t perform well in tension.

You can also put domes together to cancel out each other’s forces, and run tension members around to take tension load

By the way, the image of the igloo in the lower right isn’t really correct, …. Igloos are actually built up in a spiral from the base, like the other cartoon, not in successive rings.

As well, since the igloo is made of frozen water, which can take some tension, the igloo can be a “complete” dome.

Structural system #9: The Shell

Found all over the place –

Insect exoskeletons, reinforced concrete roofs, power tool casings, pretty well anything that is an injection molded clam shell, GRP furniture, car bodies.

Everything from the case of a pager, to a crab’s shell, to the roof of the TWA terminal are shells.

In their simplest form, they are domes that can withstand their own internal tensile forces and so are a closed system – but shapes can vary wildly and along with them their structural properties.

Shells are a great way to resist all four basic forces. They take on many of the best combinations of properties from all the above structural systems, while at the same time being able to completely enclose a product.

In product design, Shells tend to be made from materials that behave very well in both tension and compression. Like steel (car bodies), wood (Eames plywood furniture) or plastics (cell phone casings.)

Structural system # 9: Shells, continued

Injection molded and GRP components:

Examples of electronics components, power tools, Karim Rashid garbage can, Panton chair

Ribs and bosses increase stiffness and strength.

Fastening several components together to form box sections or other closed compound shapes greatly increases structural integrity.

Check out a base of an apple studio display CRTmolded structural elements can be beautiful,although they often aren’t.

Molding of ribs and bosses:Transitions are critical between the shell wall and the rib or boss

Structural systems #9: Shells Continued

Formed plywood structure examples:Eames ply wood chair and table.

Formed components are generally stronger and more rigid than flat components:

To increase stiffness and strength, add a fold or crease to build up effective depth.

Structural systems #9: Shells Continued

Creating closed shells is a very good way of resisting torsion.For example a ski, a palm pilot case.

Deep drawn components and welded box sections creating closed sections.

Like in car bodies for example.

Connections in building box sections are critical

To increase stiffness and strength, add a fold or crease to build up effective depth.

Important resources used in preparing this presentation:

Structures, (or Why Things Don’t Fall Down.) By J.E. Gordon, Penguin Books, 1978. ISBN: 0306801515

The New Science of Strong Materials (or Why You Don’t Fall Through the Floor.)By J.E Gordon, Princeton University Press, 1968.ISBN: 0691023808

An Introduction to

Structures and Materials

in Design Practice

Session II

Paul Conder SFU Surrey