The Home of the Gladiators

The Colosseum is a vast arena in Rome, built between the 70s and 80s using stone, brick, and concrete. The Romans were the first to use concrete on a large scale. Arches were incorporated wherever possible throughout the 190 m (620 ft) structure to reduce its weight.

Suspension Bridge

Because steel is strong under tension, a roadway can be suspended from steel cables supported by towers. The result is a suspension bridge, essentially an inverted arch. Spans of over 2 km (1.2 miles) can be achieved this way. Thousands of years ago, early bridges were built similarly to cross rivers. A military suspension bridge from around 1900 (below) measures only 60 m (200 ft), as its lightweight wooden towers could not support a longer span.

Anchoring a Suspension Bridge

The cables of a suspension bridge distribute forces along their length, allowing towers to support long spans. To prevent excessive force on the towers, the cables pass over them and are anchored into solid rock on both sides. Complex anchoring systems are required to distribute loads and keep the cables securely in place.

Wood

Wood has evolved over millions of years to support leaves, flowers, and fruits above the ground. Once humans developed axes, they began cutting trees, and wood became one of the most widely used materials. It is used to make furniture and paper, and even concrete buildings rely on wooden molds during construction. Wood is a composite material made of long, strong cellulose fibers. This white substance, related to sugar, is found in all plants, but in trees it is reinforced by additional structural components, giving rise to different types of wood with distinct uses.

Different Woods

Each type of wood has properties that make it suitable for specific purposes. The most commonly used woods—soft and inexpensive—come from coniferous trees such as pine and fir, which grow in managed forests where each harvested tree is replaced. Conifers retain their leaves year-round and grow quickly in cold, shaded regions. Hardwood comes from slower-growing trees found in warmer, brighter environments. These woods are stronger and have finer grain but are more expensive. Some hardwoods, such as mahogany, are being harvested faster than they can regenerate, threatening both forests and wildlife.

Working with Wood

Wood is strong along the direction of its fibers (grain), but since these fibers are weakly bonded together, it can break easily across the grain. Unlike metals or plastics, wood is generally stronger in tension than in compression and is sensitive to moisture: it can swell or even decay in humid conditions. These characteristics must be considered in design. Many traditional objects, including furniture and musical instruments, are made of wood. A violin, for example, is carefully cut, shaped, and assembled so that the material itself contributes to the sound.

Plastics

Natural plastics, such as pine resin, have existed for millions of years, but around 1850 chemists began producing synthetic plastics. These materials vary widely: some are more transparent than glass, others stronger than steel; some are rigid, others flexible. They can be formed into fibers, tubes, sheets, or foams. The first synthetic plastic was invented by Alexander Parkes around 1855. It was later improved in the United States into celluloid, a flexible, transparent (but highly flammable) material that enabled the development of cinema.

These early materials initiated a transformation in material science that now influences nearly every aspect of modern life. Amber is a natural plastic—fossilized tree resin. When it formed, it was a sticky liquid capable of trapping small organisms, preserving them indefinitely.

Plastics are also known as polymers, from the Greek poly (many) and meros (parts), because their long molecules consist of repeating atomic patterns. Today, they can be engineered for almost any function. Most plastics soften when heated (thermoplastics), while others—thermosetting plastics—become harder. The first of these was Bakelite, invented in 1907 by Leo Baekeland.