Arches, beams, and columns are key structural elements
A structural element is simply a specific type of part in a structure. Structural elements help the structure stand up and hold itself together. Arches, beams and columns are key structural elements.
Structural elements - parts of a structure - are put together to hold the load of a structure. A structure must be designed to withstand the forces upon it. Those forces include the structure's own weight, the weight of things on and in the structure, and external forces like wind and earthquakes.
Structural elements can be under compression, tension, or torsion
Tension, compression, and torsion are three different ways that structures respond to the forces, pushes and pulls, exerted on them. Structural elements can be under tension (stretching out), compression (pushing down or in) and torsion (twisting). Compression comes from a pushing force, such as when you sit on a chair and the chair seat compresses down under your weight. Tension comes from a pulling force, such as when you stretch out a rubber band. And torsion is a twisting action, such as a swing might do in the wind. Sometimes a particular structural element may be under more than one of these forces.
A column is a vertical structural element that absorbs a compressive load
Vertical means that the element faces upward toward the sky. A compressive load means that the load on the top of the column, such as a roof, pushes down on it with its weight.
A column absorbs a compressive load, and transmits the load from the ceiling or roof to the foundation of the structure or other structural elements below. For example, in a simple structure with four columns in each corner, the weight of the roof will be transmitted down the four columns to the foundation.
A beam is a horizontal structural element between supports
A beam is a horizontal, or sideways, structural element between supports. You can see some beams circled in the image at the right. The supports are the vertical columns or walls supporting each end of the beam. The job of a beam is to transfer its load to the supporting columns or walls.
A beam may respond to the loads upon it by bending. When a beam bends down because of the load upon it, the top part of the beam is under compression (squished), while the bottom of the beam is under tension (stretched). Beams and columns have been used together in contruction since ancient times - as you may have guessed the age of some of the structures in the beams and columns images at right.
An arch is a curved structural element that is under compression
An arch is a curved structural element that spans a space. Arches have also been used since ancient times. An arch is under compression, but not tension.
The bottom left arch at right is a very old arch. This arch is a gate in a medieval city wall in France. Medieval means that it is from the Middle Ages, the time period before the Renaissance. The Middle Ages ended about 1500 A.D.
The stones at the top of the arch are smaller on the inside of the arch than they are on the other side. These stones are called "voissoirs". The top, center stone in the arch is called the "central voissoir", or "keystone". Once the keystone is in place, the stones will push against each other to maintain the arch.
Another useful structural element is a truss
A truss is a structural element that uses bars joined together at joints. The bars are held together at the joints by metal plates and bolts. You may be able to see triangle or pyramid shapes when you look at something using a truss design. You may see trusses in bridges - such as the bridges in the pictures - as well as in roofs, cranes, and vehicles like aircraft and spacecraft.
The triangle shape formed by the trusses is stable and strong, which is why trusses are often used for such important structures as bridges. Because of the triangular shape used, trusses can withstand forces more effectively than other shapes made from the same material. Trusses made from wood were used in structures a very long time ago.